Chapter 6: Vibrational Spectroscopy of Adsorbates on Thin Metal Films

196

CHAPTER 6

VIBRATIONAL SPECTROSCOPY OF ADSORBATES ON THIN METAL FILMS J. Heidberg and H. WeiB

1.

INTRODUCTION

Vibrational spectroscopy has proven to be a powerful tool in the chemical analysis for determining the kind and concentration of molecular species and atoms on a surface. This is true even during reactions on a surface. A further goal of the study of adsorbate vibrational modes is a better understanding of their role in energy transfer and dissipation processes, as well as in chemical reactions on surfaces. For adsorbates on metal surfaces "surface selection rules" hold, which dependent upon the excitation mechanism determine the number of discrete vibrations likely to be observed in any spectrum. The selection rules predict the intensities of the vibrational modes, and, in case of metal films, these are expected to be affected either by the thickness of the metal layers or the diameter of metal particles. For example, if carbon monoxide is adsorbed on a single crystal metal surface, one usually observes the signal of the CO stretching vibration indicating that the molecule remains undissociated. In certain cases however, one observes also the vibrational modes of the atomic species C and 0, showing that dissociation occurs on the surface. Each of these vibrations (CO, C, 0) has its distinct frequency for each bonding site. The spectral intensities relate to the concentration and, according to the selection rules, to the surface site geometry of the various adsorbed species. Vibrations of CO molecules perpendicular to the surface plane are predicted to have significant intensity in the infrared absorption spectrum, contrary to those of CO molecules parallel to the surface, being much weaker or even not observable. On very thin films these distinctions are expected to be relaxed. Applications of vibrational spectroscopy include: (1) The qualitative chemical analysis by the important finger print technique and the estimation of the concentration of surface species e.g. in the

197

field of catalysis, adhesion, lub~ication. (2) Detection of su~ face sites, thei~ bond strength, symmet~y p~ope~ties and concent~ation. In situ employment of othe~ su~face analytical tools such as low ene~gy elect~on diff~action (LEED), which p~ovides the surface c~ystallog~aphic ar~angement, and Auge~ spect~oscopy, yielding the chemical elements in the su~face, can be appropriate to suppo~t the conclusions d~awn. (3) Detection of ~econst~uction and relaxation of the clean and cove~ed metal su~face, again in combination with othe~ techniques. (4) Determination of late~al inte~ actions between adso~bed molecules in dependence upon coverage, especially using the isotope mixing technique. (5) Detection of two-dimensional phase t~ansitions. (6) Measurement of su~face migration and energy bar~ie~ heights between diffe~ent su~face sites. The determination of the change of population of individual su~face sites in dependence upon the time and the subst~ate tempe~ature provides ene~gy bar~ie~s to su~face mig~ation. Fo~ nucleation and epitaxial g~owth of layers the surface mobility of atoms and molecules is essential, but often unknown. Alte~ations, fo~ example by lase~ radiation, can be studied by vib~ational spect~o scopy. A featu~e of vib~ational spect~oscopy, pa~ticularly inf~a ~ed spect~oscopy, is its non-int~usiveness, which means, the condition of the adso~bate is in gene~al not significantly changed in the measurement.

2.

Ove~view

In

~ecent

of Techniques in

Vib~ational

Surface

Spect~oscopy

years the awa~eness of the vibrations of surfaces and layers as well as the interest in the application of vib~ational spectroscopy has been growing, mainly due to a steady development of experimental techniques with sufficient ~esolution and surface sensitivity. The techniques may be classified according to the natu~e of the exciting probe: photons, electrons, neutral pa~ticles. adso~bed

As far as resolution and expe~imental compatibility are conce~ned inf~a~ed spectroscopy is unsu~passed /1/. The resolution can be < 1 cm- 1 and better, the ambient p~essure can be up to several bars. Sensitivity is the limiting factor. Mo~eove~, on surfaces of metals only those vibrational modes of

adso~bed

species can be

198

observed, which possess a component of their transition dipole moment normal to the surface (metal surface - normal dipole - selection rule; see fig. 1) /8/. The classical transmission technique as applied to a strong absorber, like CO on supported metal films and catalysts, requires surface areas of 0.1 m2. In infrared reflection absorption spectropscopy (IRAS) a monolayer CO on a single crystal metal surface has an absorption of the order 10- 3 to 10- 2. To observe the corresponding change in reflectivity the following conditions must be met: (1) Grazing incidence of the infrared light beam. (2) The incident light must have an electric field component polarized in the plane of incidence (p-polarized light). (3) the metal surface - dipole selection rule must be fulfilled. Infrared Fourier transform spectroscopy offers the multiplex advantage as the whole spectr.al range is simultaneously sampled. The duration of a single scan can be shorter than 1 sec, yielding the additional benefit of getting rid of source fluctuations. For correct data acquisition it must be guar.anteed that the resolution of the analog-to-digital converter is sufficient for the whole signal intensity range. The high throughput of a Michelson interferometer in FTS can in general not fully be used in flat surface work however, due to the need to reflect light off a sample surface over a narrow range of angles around the optimum grazing angle. The ultimate sensitivity to date is about 5 x 10- 3 % absorption.

Q):

=t= /i\ +

,, II:' \

I

I

'-' -

(2)+ ~ + I',-.... ' ,"',, " ',\ .... _,

e

+

1'"//#//&

+ --- ...... , '

-

....

14--- ,

Fig. 1: Molecular dipoles on a metal surface and their images Infrared emission spectroscopy for the study of surface vibrational modes suffers from low sensitivity in experiments employing room temperature spectrometers. Monolayer adsorption on high surface area dispersed catalysts, oKide growth of copper and molyb-

199 denu~, all with sample temperatures - 400 - 500 K and room temperature spectrometers were investigated 19,10/. A liquid nitrogen cooled Fourier transform spectrometer was used to obtain monolayer sensitivity on a flat metal surface at 300 K 111/. Recently emission signals from sUbmonolayer coverages of adsorbates (CO) on single crystal metals (Ni(100» over the frequency range from 400 to 3000 cm- 1 with a resolution of < 10 cm- 1 were measured 112/. The sample temperatures were ~ 250 K, the entire grating spectrometer used was cooled to 5 K. The ratio of the adsorbate emission to the substrate emission was 10- 4 to 10- 2.

With CO 2 laser infrared, excitation of surface electromagnetic waves (SEW) and the observation of the stretching vibration of H chemisorbed on W(100) was achieved 113/. The coupling of incoherent infrared light of thermal sources into SEW is weak. With tunable pulse lasers desorption can be induced by resonant excitation of internal vibrations of adsorbed molecules, and desorption yield versus excitation frequency curves can be measured. Highly resolved spectra are obtained, implying e.g. 13CO/ 1 2CO isotope resolution 1141. Metal films can be investigated by the attenuated total reflection technique 13,15/. The metal surface must be positioned close to the ATR crystal (e.g. silicon, KRS 5). The number of internal reflections for optimum sensitivity is approximately equal to the reciprocal of the fractional loss per reflection. For incidence, the p-polarized light is more strongly absorbed than the s-polarized light with the electric field vector oscillating perpendicular to the plane of incidence. When there are strains in the ATR crystal, polarization mixing may occur. With eight reflections a monolayer of benzoic acid on AI/A1 20 3-KRS5 was clearly detected 115/. The development and widespread application of surface vibrational spectroscopy by inelastic electron scattering, electron energy loss spectroscopy EELS, has provided strong influence in the field 116/. The resolution of EELS is about 30 cm- 1 (5 meV), the sensitivity 10- 3 of a monolayer, which is extremeley high. Overtones and combinations can often be observed. The accessible spectral range from 100 to 5000 cm- 1 is wide. It is possible to excite in-

200

tecnal and extecnal localized adsocbate vibcations as well as the collective vibcational modes of ocdeced sucface layecs and theceby detecmine surface phonon dispecsion celations. EELS can be applied only undec high vacuum. As mentioned, in infcaced spectcoscopy absocption occucs only if thece is a vibcational dipole moment change pecpendiculac to the metal sucface. This holds also foe EELS with the cestciction that the scattecing of electcons is in the speculacly ceflected dicection. Off-speculac, the dipole selection cule bceaks down and also vibcations pacallel to the sucface ace obsecved. Thus EELS offecs the advantage ovec IRS of being capable to detect vibcations parallel to the sucface and to distinguish between vibcations perpendicular and pacallel to the sucface. The long-cange Coulomb field of the moving electcons excites the dipole nocmal to the surface. The shoct-range intecaction between the adsocbate and electcons can excite also vibcations pacallel to the sucface. As fac as the basic pcinciples ace concecned, closely celated to IR spectcoscopy and EELS thece ace inelastic electcon tunneling spectcoscopy IETS and sucface enhanced Raman spectcoscopy SERS. The selection cules opecative in these techniques ace similac in that the vibcational tcansition moment of the adsocbate intecacts with the local electcic field. In inelastic electron tunneling spectcoscopy at liquid helium tempecatuce an electcon cuccent flows fcom a metal electcode thcough a thin isolating layec (-15 A), usually an oxide on which the molecules of intecest ace adsocbed, to another metal, at best supecconducting lead, when a voltage is applied /4,17,18/. The cuccent will be mainly due to elastic tunneling of electcons. Above a minimum voltage Vv = hvv/e an adsocbate vibration with fcequency Vv will be excited. A step in the curcent-voltage cucve is observed at this voltage Vv' The technique, applicable in the spectcal cange fcom 300 to 4000 cm- 1, is sensitive (0.1 monolayer) and has high cesolution, at 2 K about 10 cm- 1. Infcaced and Raman active modes ace observed, a featuce of the spectca is the stcength of CH vibcational modes. Tunneling spectcoscopy, at pcesent being a technique foe special systems, could become of basic intecest, if vibcationally inelastic tunneling in the scanning tunneling miccoscope would be detected. Vibcations of a single atom on a metal surface would become obsecvable.

201

Raman scattering is a very informative spectroscopy for investigating the vibrations in bulk phases. However, the Raman crosssection is very small causing only weak scattering when a laser beam is reflected from a covered metal surface. Nevertheless, spontaneous Raman scattering by molecules on single crystal metal surfaces, unroughened metal films and tunnel junctions has been observed, using detector arrays and multichannel techniques, saving not only measuring time, but also eliminating effects of fluctuations of the excitation laser /19/. On the other hand, surface enhanced Raman scattering SERS is a highly sensitive vibrational spectroscopy for special systems and surface conditions /20,21/. It is applicable to the detection of adsorbates, especially pyridin, on roughened surfaces of certain metals, such as Ag, Cu, Au and others. The Raman cross sections are enhanced by factors of 10 4 to 10 6 over the values for adsorbates on smooth single crystals. So far such high enhancements have not been accomplished under precisely controlled conditions. SERS has been applied to electrodes in electrochemical cells, metal-gas systems, colloidal suspensions, matrix-isolated metal clusters and rough films. Adsorbate vibrations and for clean Ag and Cu films phonon modes have been observed /22/. The mechanism of enhancement is difficult to describe. A large part of the special enhancement observed on rough films has been assigned to electromagnetic resonances, localized in pores (grain boundaries), which are excited by visible light. The other part of the enhancement has been attributed to resonant charge transfer excitations giving rise to the "chemical" Raman enhancement. Charge transfer is assumed to occur between localized surface defects and the adsorbate. SERS of ethylene and ethane adsorbed on Ag and Cu films is quenched by postexposure to 02' leading to submonolayer coverages of atomic oxygen without change of the original adsorbate coverage /22/. It is its high specificity which has prevented SERS from developing to a general purpose quantitative analytical tool as is infrared spectroscopy. It should be valuable, however, for characterizing certain adsorbates on noble metal films and processes thereon. Low frequency modes of adsorbates in finely dispersed metals can be probed by neutron scattering /23/. Neutrons are scattered relatively strong by hydrogen. Thus by covering a high surface area

202

material with hydrogen-containing adsorbates makes neutron scattering to a surface sensitive technique. Surface vibrations at high temperatures and pressures can be observed. Hydrogen adsorbed on palladium black, Raney nickel and platinum black, benzene on Raney nickel, and other systems have been studied.

Table 1

Techniques in vibrational spectroscopy of covered and uncovered thin metal films spectral range Icm- 1

technique IR transmission absorption

>300

IR reflection absorption enhanced Raman

>300

resolution Icm- 1

>100

-------------------_.

Raman

>400

sensitivity! surface monolayers area/cm 2

remarks highly dispersed metals, spectral range dependent on substrate flat metal surface

2

__ ._.__._-_._--_._----_._--------_._----1-10-2

10

>10-3

roughened Ag,Cu, Au and others; pyridin, ethen and others flat metal surface (Ag)

ele;;~---------;1-;;------;3;------1-0::-1-~-1-0::-4----1-;::-'----flat

u_______________

energy loss

1 - 10- 2

IR laser

10- 1

SU;f;~;~--

---

high vacuum

>450 < 1 nanosecond time >900 resolution ,resonant ___________________________________________________________J2hotodesoprtion IR emission

210 - 3000

<10

neutron scattering

<1000

50

For the given surface area. Not the potential of the techniques refer to actual measurements.

0.1

0.1 *

1

flat metal surface, highly dispersed metals, liquid nitrogen or liquid helium cooled spectrometer highly dispersed metals, hydrogen adsorbates

is indicated but the figures

203

3.

Vibrational Modes of Adsorbates. Selection Rules

3.1 Isolated Adsorbed Molecules An adsorbed molecule containing N atoms will have 3N vibrational modes. The (3N-6) vibrations of the free molecule (or (3N-5) for a linear free molecule) are called the internal vibrations. When the loss of molecular identity is small upon adsorption, the internal vibrations, to a good approximation, are little changed upon adsorption. Because of the adsorptive forces there will be in addition 6 (or 5) external vibrations which arise from 3 quasi-translational and 3 (or 2) quasi-rotational (vibrational) motions of the molecule (the numbers in brackets refer to linear species). The external motions are usually low in frequency, quite separated from the internal vibrations. There can also be other types of changes upon adsorption. A degenerate mode of a gas molecule may split due to the influence of a less symmetrical environment on the surface. Not only the vibrational frequencies can shift but also changes in the intensities can occur. First we want to understand the number of vibrational modes before studying their frequencies, intensities and line shapes. Whether these 3N modes are actually observed in a vibrational spectrum depends upon the symmetry of the adsorbate. This can be seen more clearly when an example, the system CO adsorbed on a metal surface, is considered. The CO adsorbed normal to a {100} face of a simple cubic crystal on top of a surface atom has C4v point group symmetry (see fig. 2). There are two stretching vibrations belonging to the totally symmetric A, representation, one of them being similar to the CO stretching vibration of the free molecule. The other arises from the translational motion of the whole molecule against the surface in z-direction and becomes a carbon-metal stretching mode. The remaining modes are degenerate pairs of E representation and derive from restricted translations parallel to the surface and rotations about the x and y axes. Under C2v site symmetry in the bridge position there are the internal A1 stretching vibration and the external A1 mode normal to the surface. Here the degeneracies of the parallel modes are removed, yielding two modes with the atoms vibrating in the plane of the bridge (B,) and two modes perpendicular to this plane (B2)'

204 on - top A, Vi

I I

0

bridge

cI /\ M

I

I

M

0

A, Vz

I

M

0-

0

c

c if)

M

B, v)

c-

/\

I

M

M

.lI4

0I

cI

M

M

OI

J

-c E

M

I /\

I

E

0

I

c A, Vi

t

A, Vi

M

B, V', 0 -

I

-c /\ M

B2 V5

M

0-

_cI

I M

B2 116

0I

c-

I M

Fig. 2: Localized normal vibrations of CO adsorbed on a metal surface: a) on-top of a surface atom and b) bridged to two atoms

According to the "surface - normal dipole - selection rule" /8/ only totally symmetric vibrations within the adsorbate which have components of vibrational dipole change perpendicular to the metal surface are observed: v, and v2. The parallel modes v3 to v6 are not observed. This is true for infrared reflection and transmission spectroscopy as well as for specular EELS. In off-specular EELS the dipole selection rule breaks down and vibrational modes parallel to the surface are detected. This dependence upon the scattering angle is shown in EELS of the system atomic hydrogen adsorbed on a tungsten W(100) surface at saturation coverage and 300 K: W(100)p(1 x 1)H.

205

The system is well characterized, both experimentally (substrate geometry, bonding site, surface homogenity) /24,25/ and theoretically (self consistent calculations) /26/. The saturated phase (8 = 1) is obtained by exposing a clean (reconstructed) W(100) surface to more than 2 Langmuir of molecular hydrogen. Dissociative adsorption occurs; the H-atoms chemisorb in bridge sites with C2v point symmetry. There are two H per unit cell, hence six vibrational modes corresponding to restricted translations along the x, y, and z axis. Fig. 3 shows the fundamental vibrational modes of W(100)p(1 x 1)H. The reduced mass of the oscillator is approximately equal to the mass of the H atom so that only the light H atom is moving against the rigid tungsten surface in this vibration. The frequencies of the W-H modes are well above the W phonon frequencies (~ 300 cm- 1). Only the symmetric stretching mode normal to the surface is dipole allowed. The v2 and v3 modes represent motions parallel to the surface and are not active under the dipole selection rule. (However the overtone 2v2 of the wagging

@ W-H-W

I I H@ I I W-H-W

:!)H

@ W-H-W

I

I

HG

GH

I

I

@

W-H-W @

W-H-W

W-H-W

I

I

H-

H-

I I W-H-W -+

W-H-W

I I I I w-..!:!.-W H

H

t

I I I t I W-H-W H

H

H

/'" ",W'7T7'WvI

@ H

/'"

'7W 77777","WT? v2

W-H-W

!l

I

I

H!

I

W-H-W

H-

/'" "7W~W?: v3

Fig. 3: Localized normal vibrations of H adsorbed on a tungsten surface in W(100)p{1 x 1)H geometry (from /28/). There are two H per unit cell, hence six modes. For vl only the in-phase mode is dipole-active, both v2 and v3 are two fold degenerate

206

mode corresponding to the out-of-plane bridge site vibration is allowed.) In fact only v1 (and 2v2) are observed by infrared absorption reflection spectroscopy 113,27/. In specular EELS only v1 has been detected, in off-specular EELS v1 and both v2 and v3 125,28/. At low coverage (0 - 0.1) hydrogen forms a c(2 x 2) ordered layer, the structure of which derives from a displacement of pairs of substrate W atoms which come into contact. The substrate W forms a "puckered" lattice, the modes v1 and v3 are dipole active and observed in specular EELS 125/. The IR spectra exhibit only v1 128/. Only if all of the possible vibrations are observed, the point group symmetry of the adsorbate can be determined unambiguously. The environment of a molecule on a surface can approximately be considered as a static surroundings whose only relevant property is its symmetry as seen by the molecule. Any dynamic interactions between the molecule and its surroundings are ignored. These assumptions characterize the "site symmetry approximation". Qualitatively the effects of a decrease of the point symmetry in going from the gas to the surface are of two types: (1) change in the selection rules and (2) splitting of degeneracies. A non-degenerate vibration may be inactive under the high symmetry of the free molecule but active under the symmetry of the adsorption site. Both effects have been observed in the physisorption of CO 2 on NaCl films: the totally symmetric v1 stretching vibration becomes infrared-active upon adsorption on highly adsorptive, evaporated NaCl films deposited at the low temperature of 77 K 129/, and the v2 bending vibration, being two-fold degenerate in the gas, splits upon adsorption on NaCl indicating parallel orientation of CO 2 on the surface 130/. Apparently, such effects have not yet been studied on metal surfaces. Slight perturbations of the gas phase species on metals are expected only in low temperature adsorption, as with acetylene on silver (110). In general chemisorption on metals leads to an appreciable change of the structure. Only after the structural type of the adsorbed species has been identified, e.g. in case of di-o-adsorbed or n-adsorbed ethene, a detailed analysis of the vibrational spectra can provide information about the specific site symmetries for the chemisorbed species. However, such conclusions are best drawn when diffraction shows uniform, ordered

207

adsorbed species. Especially at low and intermediate coverage adsorption at various sites may well bring up additional features in the spectra which may complicate a site group analysis.

3.2 Metal Particle Size Effect The "metal surface - normal dipole - selection rule" corresponds to the classical response of a metal surface to an electric field. An electric field normal to the surface has a reflected component which is almost in phase with the incident field, making the field strength at the surface nearly twice that of the incident field. On the other hand, an electric field parallel to the surface has a reflected component out of phase such, that the resultant of the incident and reflected field is nearly zero on the surface. Therefore only electromagnetic waves polarized normal to the surface can excite adsorbed dipoles. Around a small metal sphere, the electric field is also always normal to the surface just at the surface, but there is some deviation from the normal direction with increasing distance. If the wavelength of the unpolarized light is large compared with the radius of the sphere the normal and parallel components of the electric field, which are experienced by the adsorbed molecules, can be estimated. Accordingly the intensities of the vibrations with a parallel dipole moment and a perpendicular dipole moment, III and

11'

respectively, have been given by Ibach as

(~R

2

(1)

where d is the distance from the surface and R the radius of the sphere /31/. For molecules adsorbed on small metal particles the surface dipole selection rule is expected to be relaxed /55/. No relevant experiments are known. The resultant electric field around a metallic sphere, which is in a homogeneous electric field, is shown in fig. 4. The inducing homogeneous field and the field of the induced surface charges superimpose to the resultant field, which is enhanced over the inducing field at the poles and is zero at the equator of the sphere.

208

Fig. 4: Resultant

elect~ic

field

a~ound

3.3 Dynamic Vibrational Coupling in

a metallic

sphe~e

Adso~bates

molecules constitute in gene~al a coupled vib~ational system, where the inte~action between the vib~ations of the individual molecules may not be igno~ed. Splitting of modes, even of those being non-degenerate in the gas may occu~ if the unit cell of the adsorbate contains two o~ mo~e molecules whose vibrations then couple giving modes with different f~equencies and intensities. Translationally equivalent molecules, i.e. equivalent molecules in different unit cells, may also couple, but in this case the diffe~ent f~equencies are not in gene~al observable since only the in-phase motion fo~ all unit cells has significant intensity. As an illust~ation the coupling in an adlaye~ of N vib~ating CO molecules is considered. Treating the adlayer as an ar~ay of N coupled oscillators, N no~mal modes of vib~ation arise, having different frequencies and diffe~ent intensities. Each isolated oscillato~ is assumed to have the same f~equency Vo = 2065 cm- 1. modes of vib~ation of five such identical coupled osThe no~mal cillators in a linear a~ray and those of nine such osicillato~s in a square 3 x 3 a~ray a~e shown in fig. 5. The intermolecular coupling is assumed to be weak as compa~ed to the intramolecula~ /32/. Adso~bed

209

In the systems the in-phase vibpation is the most intense, but in both thepe is anothep vibpation having about 5 % of the intensity of the in-phase mode. Appapently fop most expepiments, this splitting can be ignoped and only the in-phase mode needs to be consideped.

LLLLJ

= 2075 em-' I = I 00

II

~

• 2086 em-' I = 1.00

II

~ 1

= 2070 em-' I : 0.00

II

1I

2064 em-' : 005

II :

1

~

~

2059 em-' I = 000

II :

2056 em-' I : 0.004

II :

4? II: 2062 em-' 1=0.04

~

= 2053 em-' 1: 0.0006

II

~ I : 0

¥P 2062 em-' I : 0

II :

~

2072 em-' 1 : 0 (2 - fOld degenerate) II :

~

2058 em-' 1 =0 ( 2 - fOld aegenerote ~ II :

Fig. 5: Vibpational modes, fpequencies and intensities of five pespective nine coupled molecules (fpom 132/)

3.3.1 Dynamic Vibpational

Cou~

in Adsopbates on Metal Films

Spectposcopic studies concepning collective vibpational modes have mainly be made about adsopbates on flat single cpystal faces, in opdep to minimize the contpibutions to the ovepall spectpum of the adsopbate on steps, kinks and othep defects, which inteppupt the pegulapity of the appay of oscillatops, being necessapy fop effective coupling.

210

However, on the basis of calculations it can be estimated that the dipole-dipole coupling effect occuring in adsorbates in square facelets of 10 nm edge length (containing about 10 3 co molecules) on a copper surface is practically not to distinguish from that of an infinite face (see also fig.6) 133/. The dipole-dipole coupling

2095

(00.00)=2111 ern"

,

(4x4)

.\-

~

(3x3)

E

S

cr:

ill CO ~ ::J

Z

ill

> «

\/

2085

.

(2x2)

./

2075

1

I·

~

2065

/

/

•

/'

/'

/'

__ -,"

-

(5x5)

/' / ' , - SQUARE ARRAY

/'

••••••••••••••••••• ••• " " LINEAR ARRAY

/--T-...,.....,--r-........... . .--.,. . .,. -,. -. , . . .,. . ~-.-.,. . . .~ 5

10

15

........,.....,.....,....-r-'

20

25

NUMBER OF OSCILLATORS

Fig. 6: Wavenumbers of in-phase vibrational modes for one-dimensional and two-dimensional islands of coupled oscillators (adapted from 132/)

is determined by a sum of pair interactions. Due to the form of this interaction which depends upon i j being the distance between the i-th and the j-th adsorbed molecule, the sums of pair interactions can be truncated at a much lower than infinite distance, without significantly changing the final results. In fact, coverage-dependent vibrational frequency shifts due to dynamic vibrational coupling between adsorbed CO molecules have been observed on Ag, Au and Cu films deposited at the very low substrate temperatures of 4 K (but no vibrational splitting due to dynamic vibrational coupling has been observed as yet) 134/.

Hi}, H

211

4. Vibrational Frequencies of Adsorbates The whole of vibrational frequencies has proved to be a powerful instrument for the determination of the structure of adsorbates, strengthened by comparisons made with metal organic complex compounds and matrix isolated species. A reliable and precise theoretical description of the vibration of even the simplest adsorbed molecule is not feasible. One could only speculate on a number of distinct mechanisms of frequency shifts, being presumably independent and having additive effects. Again we shall consider the isolated molecule on the surface first and then the effect of coupling between the adsorbed molecules.

4.1 Frequency Shifts of Isolated Adsorbates 4.1.1 Mechanical Renormalization

Binding one of the adsorbate atoms to a rigid or vibrating adsorbent atom results in vibrational frequency shifts of the adsorbed molecule, even if all its other structural and bonding properties remain unchanged. This can be recognized from the simplest model of masses and springs such as

M

The eigenfrequency

k

va M-

c of the internal adsorbate vibration ~

C - 0 ..

with M fixed is approximately ( 2)

where ( 3)

212

is the eigenf~equency of the f~ee CO molecule in the gas phase, k and ko a~e the fo~ce constants of the MC and CO bond, ~especti vely, ~ is the ~educed mass of the CO molecule and m the mass of the adso~bent atom M /4/. When the adso~bent is pe~mitted to vib~ate, the f~equencies of the vib~ational modes of the adso~bed molecule a~e reno~malized to v1' v2' and ~emain localized vib~ations as long as the inte~nal frequencies a~e well above the vib~ational frequency continuum of the adso~bent. This is the case for many systems of interest, with light molecules such as N2, CO, C6H6 adso~bed on heavy metals. For linea~ M-CO, conside~ing only vib~ations no~mal to the su~face, one finds f~equencies 000

(MCst~etch)

\11

vf [ 1

(eOst~etch)

\12

v~

[1

Mc + M0 +----- ] 2m

-f

+

(~)4

-f \12

Mo(M c + Mo) ] ---2mM c-

(4 )

, (5 )

where vf and v~ is the Me st~etch and CO st~etch frequency with M fixed, Mo' Me' m is the mass of 0, e and M, ~espectively /4,35/. v~ is equal to va' Notice the blue shift of the frequencies with ~espect to the f~ee molecule in the gas. The additional shift in eq. (5) a~ising f~om mobilizing the metal adsorbent, M, is negligible compa~ed with the effect due to bonding of C to M acco~ding to eq. (2). The external vib~ation v1 is much more influenced by the adso~bent vib~ation. Fo~ instance, fo~ Pt-CO, whe~e the Debye f~equency of Pt is 156 cm- 1, and the external f~equency (Mest~etch) v1 is measured to be 465 cm- 1, the adso~bent motion pushes the exte~nal f~equency vf upwa~ds by some 30 cm- 1 /4/. We should be awa~e that vf and v~ are not measurable. In p~inciple they may be infe~~ed f~om the stretching f~equencies of metal carbonyles M-CO, asserting that adsorption does not significantly alte~ the fo~ce constants. Furthe~mo~e, G~imley's adso~bent model is a semi-infinite system of mass points and sp~ings without su~ face modes. In case of thin films and small pa~ticles the theoretical pictu~e may change.

213

4.1.2 Self-Image Shift The simplest model of a metal adso~bent is a semi-infinite pe~fect conducto~, co~~esponding to a sha~p discontinuity in the dielect~ic constant E, changing from E + ~ in the adso~bent to E = 1 in the vacuum. Then, any dipole p at the distance z f~om the metal su~face induces a vi~tual o~ image dipole P1 symmet~ically located at a distance -z from the surface (see also chapte~ 2., fig. 1). Fu~ther, it is assumed that the inf~a~ed active adso~bate vib~a tion can be ~ep~esented by a point oscillator with a vib~ating dipole moment p and an angula~ eigenf~equency wOo Fo~ this model, the field of the image dipole P1 affects the adso~bate vibration such as to lower the eigenf~equency, i~respective of the orientation of the vibrating dipole p. The ~edshift ~w of the angula~ vib~ational f~equency upon adso~ption due to the self-image effect is app~oximately

(6) o~ ionic pola~izability and u e the of the adso~bate /4/. As an illust~ation we conside~ CO on Pt, the CO molecule being pe~pendicula~ to the su~ face. Setting z ~ 1 ~ and taking fo~ the pa~amete~s U v and ue the co~responding gas values, we obtain ~w/wo ~ 2 %, i.e. ~v = - 40 cm- 1. The~e a~e uncertainties in this estimation. The position of the image plane at the metal surface is uncertain because the electron density ~eaches out into the vacuum. Fu~the~mo~e, the CO adso~bed is ~eally not a point dipole and the pa~ameters u e and U v a~e inc~eased with respect to thei~ gas values by facto~s (1 - u e/4z 3)-1 and (1 - u e/4z 3)-2, ~espectively. However, expe~imentally f~om intensity studies it is found, that in general the self-image has no significant effect on U v and u e. By improving the simple model of the metal by means of a Thomas-Fe~mi dielectric function the self-image effect on the stretch f~equency also d~ops to a neglegible value. Thus the self-image plays a key ~ole in the metal surface selection rule, but appa~ently has no conside~able effect on the vibrational frequency. whe~e

elect~onic

Uv

is the

vib~ational

pola~izability

214

4.2 Characteristic Adsorbate Frequencies There exists a substantial literature on the vibrational frequency shifts of the species formed in the chemisorption of carbon monoxide, ethylene and acetylene on metal surfaces. Chemical structural information can be inferred from these shifts with the help of those of model metal cluster compounds. At present vibrational spectroscopy provides the best tool of studying these structures. In the CO adsorption on metals the CO stretching frequencies have certain characteristic values: for linear species with CO bonded to one metal atom VCO is in the range 2000 to 2130 cm- 1, for bridged species coordinated to two metal atoms 1880 < vCo/cm- 1 < 2000, while for CO coordinated to three or four metal atoms 1650 < vCo/cm-1 < 1880 /36/. According to the simple molecular orbital picture the bonding of CO to metal atoms is due to charge donation to the metal from the 50 orbital and back-donation of metal electronic charge into the 2n* antibonding orbital causing a weakening of the CO bond /37/. With higher coordination the back-donation of charge from an increasing number of metal atoms increases. In this picture, with higher coverage the CO frequency tends to increase since the number of metal atoms available per adsorbed CO diminishes resulting in a re-emtying of the 2n* orbital. This predicted increase in the CO frequency is in concert with experiment. In case of ethylene and acetylene most interpretable results refer to adsorption at low temperature « 250 K) under which conditions, with few exceptions, the surface species retain the formulae of H2C = CH 2 and HC = CH /38/. At temperatures> 250 K CH bonds of the adsorbed ethylene are usually broken, one of the species formed being the ethylidine species, CH 3CM3. Two types of spectra from low-temperature-chemisorbed ethylene are observed which can be assigned with considerable confidence to di-o-adsorbed ethylene and n-adsorbed ethylene:

M

215 adso~bed acetylene the CC f~equencies of the adso~bed species much lowe~ than those of the gaseous molecule, indicating bond orders between 1 and 2. These la~ge changes probably arise from the inte~action of the adsorbed hyd~oca~bon molecule with 3 or 4 metal atoms. The following structu~es a~e p~oposed /38/: Fo~

a~e

/:~

M - HC

M =

~:/ M

CH - M

B-type

A-type

The types of spect~a obse~ved, when ethylene and acetylene a~e sepa~ately adsorbed at low temperature are listed in table 2. There is a relationship whereby di-o-type spectra of ethylene go with A-type spectra of acetylene and ~-type spectra of ethylene with B-type spectra of acetylene, with two exceptions.

Table 2:

Correlation between spectra of ethylene and acetylene separately adsorbed at low temperature (from /38/)

metal/surface plane Ni fcc Pd fcc

111

(di-o) A ~

Fe bee

100 (d i r-o ) ,

A'

~

-----------------_._--B

Pt fcc

110

(di-o) B B

(di-o)

B ._---_._-_._---

(di-o)' A'

(di-o) A

(di-o)' and A' are considered to represent variants of (di-o) and A, respectively.

The corresponding frequency ranges are summa~ized in table 3. There is a coupling between the CC stretching modes and other modes, so that the estimated ranges of the CC-stretching frequencies are only approximate as derived from the spectra observed from the

216

C2H4, C2D4 and C2H2' C2D2 adso~bed species. The app~oximate CC st~etching f~equencies v(CC) show that the CC bond o~de~s a~e g~eate~ fo~ types ~ and B spect~a in ~elation to those of type di-o and A, ~espectively. The CH st~etching f~equencies v(CH) suppo~t this pictu~e as in hyd~oca~bon spect~a abso~ptions between 1 app~oximately 3070 and 2980 cm- a~e associated with sp2 hyb~idi zation (CC double bonds) and between 2980 and 2800 cm- 1 with sp3 hyb~idization (CC single bonds). The CC bond o~de~ can - in c~ude measu~e - be ~elated to diffe~ent deg~ees of back-donation f~om the occupied d-orbitals of the metal atoms into the anti bonding ~*-orbitals associated with the CC multiple bonds of ethylene and acetylene. Natu~ally othe~ facto~s, such as geometry, can also playa significant ~ole.

Table 3:

F~equency

~anges

v(

(from 138/) CH) Icm- 1

v(

CC) Icm- 1

v( CM) Icm- 1

Ethylene Type di-o spect~a Type ~ spect~a

3000 - 2910 3075 - 2990

1260 - 1150 1470 - 1390

470 - 390 315 - 250

Acetylene Type A s pe ct r a Type B spectra

2940 - 2850 3010 - 2950

1300 - 1190 ____1360 - 1210

540 - 420 < 400

4.3 Coupled Vibrations of Adsorbates Naturally, in an experiment vib~ations of a single adsorbed molecule cannot be observed. The surface coverage is finite and usually the vibrational spectra are ~ecorded as a function of coverage. The vibrational frequencies of adso~bates are found to depend upon coverage. Best cha~acterized are the vibrational spectra of ca~bon monoxide adsorbed on metals. In general, as the CO cove~age is increased up to the monolayer, the CO st~etching frequency shifts to highe~ values by 25 to 130 cm- 1, an exception being Cu(111)/CO, Ag (deposited < 150 K)/CO and polycrystalline Au/CO, where the frequencies are shifted to lower values 134,39/. A coverage-de-

217

pendent property indicates the occurrence of coupled or collective behavior in the adsorbate layer. A number of explanations has been proposed for the coverage-dependent vibrational frequency shift: 1. Vibrating molecules interact "through space" via a long range dipole-dipole coupling /40-43/. 2. Vibrating molecules interact with their own images and with the images of other adsorbed molecules /41-43/. 3. Vibrating adjacent molecules interact via a short-range bonding interaction due to the overlap of the molecular wave functions of adjacent adsorbed molecules. The interaction is assumed to occur via a "through metal bond" coupling /44/. 4. Adsorbed rigid molecules interact with each other, the coverage dependent lateral interaction causing a coverage-dependent frequency shift. 5. Adsorbed rigid molecules interact with the adsorbent. A coverage-dependent change in the interaction, brought about by the metal, causes a coverage-dependent frequency /37/. The mechanisms 1 - 3 are dynamic effects, whereas mechanisms 4 - 5 are static. The combined mechanisms 1 and 2 are often called "dipolar coupling", mechanism 3 "vibrational coupling". The relative contributions of the dynamic and static mechanisms are determined by employing the isotope-mixture method.

4.3.1 Dipolar Coupling 4.3.1.1 Isotopically Pure Adsorbates Consider the stretching vibration of a large number of CO molecules adsorbed perpendicular to a metal surface. The adsorbed molecules form a regular two-dimensional array and are situated at cristallographically identical adsorption sites. Thus, there are many CO dipole moments vibrating within a wavelength of the external exciting infrared field. One may assume that all dipoles in the interaction area are vibrating in-phase parallel to each other and perpendicular to the surface. The electrostatic dipole field affects neighbouring CO molecules.

218 Fo~ gula~

an isotopically f~equency

all molecules

w

adsorbate the exact solution the only inf~a~ed active vib~ation, in-phase is 1411

pu~e

fo~

vib~ate

w2 = w~

(1

fo~

the anin which

(7)

+ rJ.v T) ,

the vib~ational o~ ionic pola~izability a v is introduced which is related to the derivative (d~/aQ) of the dipole moment with ~espect to the no~mal coo~dinate Q by 1331

whe~e

~

(8) is the angula~ frequency of an isolated or "singleton" molecule, c the velocity of light. The dipole sum T is given by

Wo

( 9)

T

Ri j is the distance between molecules i and j. The two-dimensional dipole-sum converges absolutely, unlike the case in th~ee dimensions. T can be w~itten in te~ms of the su~face-density no at full monolaye~, i.e. the numbe~ of adso~bed molecules pe~ unit a~ea ( 1 0)

T

The constant a is 9.0336 fo~ a square lattice, 8.8904 fo~ a t~ian gula~ lattice with no in cm- 2 . For CO on Pt(111), no = 5 x 10 1 4 cm- 2 , wo / 2 n c = v = 2000 cm- 1 and a y = 0.057 A3 taken to be equal to the gas phase value yield 6v = 1/2 v a v 9 n o 3 / 2 = + 6 cm- 1. In equation (7) any influence of the metal su~face othe~ than orienting the dipoles is neglected. If the refe~ence dipole is allowed to inte~act not only with the othe~ dipoles but also with their images, including the elect~onic contribution, the final formula obtained implying a coverage dependent blue shift is given by 141,431 2 w

(1

+

eay~O ------)

1+ ea

e

~

0

( 11)

219

where Eo is the full monolayer (0 = 1) dipole sum, Eo = T + V, which contains the sum of direct dipole interaction T and the sum of dipole image interaction V. In the perfect metal model the image dipole and the reference dipole are of equal magnitude, in the Thomas-Fermi dielectric model the image dipole is reduced. V excludes the self-image term as Wo is taken to be the zero-coverage frequency w(0 = 0), which contains the self-image effect. As mentioned in eq. (6) the self-image, being critically dependent upon the distance z between the image plane and the dipole causes a redshift. Any coverage induced change of z would change wOo The quantities a v and a e are the effective vibrational and electronic polarizability, respectively. For an unscreened interaction between the dipole and its self-image, an appreciable enhancement of a v over av(gas) is predicted. In the experiments, a v appears to be larger than the gas phase values. Note, that the coverage dependence of the shift 6w is not proportional to (0n o ) 3 / 2 but given by eq. (11), assuming that each site has an equal probability o = nino of being occupied. One should be aware of the crude approximations used in obtaining the formulas for the frequency shifts, e.g. classical images and point dipoles. The predictive power of the formulas is very limited, but they are useful to rationalize observed data. As already indicated before, the collective vibrations of adsorbates are expected to exist on extended single crystal surfaces, where molecules adsorbed on steps, other defects, which interrupt the regularity of the array of oscillators, and different crystal faces do not significantly contribute to the overall spectrum. However, due to the fast convergence of the interaction potential (9) it can be estimated, that the dipole-dipole coupling effects occuring on small facelets (with 10 nm edge length and approximately 10 3 CO molecules) are practically equal to those of an infinite face (see also chapter 3.3.1 and fig. 6).

4.3.1.2 Isotopically Mixed Adsorbates An experimental technique to study the intermolecular vibrational coupling of adsorbed molecules employs isotopic mixtures. At constant total coverage 0 the spectra are measured as a function of

220

the composition of an isotopic mixtu~e. The f~equency of the vib~ational absorption band from the low partial coverage isotopic species then ext~apolates in the dilution limit to the singleton frequency wo ' if negligible coupling between the isotopic species can be assumed. Any variation of Wo with total coverage must be att~ibuted to the static effects, which are often called chemical effects. This notation is misleading since also part of the dynamic intermolecular coupling of the vibrations can arise f~om chemical bonding effects. Assuming dipole-dipole coupling between e.g. vibrating CO molecules, two dipole active absorption bands from an isotopic mixed adsorbate layer are predicted: a high frequency band due to the in-phase 12eo-12eo and 12 e o- 13eo coupled modes and a low frequency band due to the in-phase 13eo-13co and very weak out-of-phase 12 eo- 13CO coupled modes /40/. For isotopic mixtures of CO on Cu(100) and Cu(111) the infrared spectra as a function of isotopic composition at a given coverage (8 = 1/3, 8 = 1/2) could be ~epro duced by dipolar coupling theo~y, assuming U v = 0.25 ~3, as compared to uv(gas) = 0.05 ~3, u e = 3 ~3 (ue(gas) 2.7 ~3) and z = 0.7 ~ /45/. From the agreement it was inferred that the dominating dynamical interaction between the CO molecules on Cu(100) and Cu(111) is the dipole-dipole coupling. However, the measured frequency increase for pure 12eo on Cu(100) with increasing coverage of 12CO is not reproduced by the formula (11) for dipoledipole coupling. That formula predicts a frequency shift from the singleton frequency Wo to Wo [1 + {u vEo/(1 + U e Eo ) } J 1 / 2 as the coverage increases from 8 = 0 to 8 = 1. It can be concluded that W is dependent upon coverage. o

=

The isotopic mixture technique in a different form was applied to the study of the dynamic coupling in the adsorbate layer of CO on the surfaces of Ag, Au and eu films /34/. The inf~ared spectra of 0.02 Langmuir 12 CO coadsorbed with increasing amounts of 13CO were measured (see fig. 7). It may be assumed that in a dilute isotopic mixture, where a molecule of one isotope is effectively surrounded by an environment of the other, the coupling of the first isotope into its surroundings is negligible. Thus the static part of the total shift can be determined as the "13CO" frequency shift with increasing total coverage. The vibrational frequency of the pure isotope adsorbate 12CO as a function of coverage was also measured

221

yielding the total f~equency shift (see fig. 8). A good fit between the fo~mula (11) fo~ dipole coupling and expe~iment can be obtained with physically ~easonable pa~amete~s: u y = 0.27 A3,

= 3 P. The inc~ease in the Yib~ational pola~izability u y upon chemisorption is thought to rep~esent the enhancement due to dynamic cha~ge t~ansfer from the metal's d-o~bital to the 2n* orbital, which is the lowest unoccupied molecula~ o~bital of CO. When the chemisorbed CO molecule is Yib~ationally excited, a pe~iodic cha~ge oscillation between the 2n* o~bital of CO and the metal occu~s. This causes an enhanced dynamic dipole moment and in turn an enhanced Yib~ational pola~izability /43/.

ue

(I)

1.0L 0.75 L

oc:

'" u; ~

.E .;; '0 c: .2

0.50L

1----

0.25L

l----~

0.15L

O.rOL

1------

-.;ol-----~

0.05L

.::

0.025L

'0 c:

.~

'"...

(e)

-:"

(d)

c:

(c)

::l

'" '0 .!:

t

0

L;:

(b)

(0)

0.0125L 0.005L

O.OOIL

2100

2150

Wavenumber (cm- I)

Fig. 7: Left: Inf~a~ed function of 12CO coadso~bed

2050

2100

2150

Wavenumber (cm")

2200

of 12CO adso~bed on Ag film as a Right: Inf~a~ed spect~a of 0.02 L with Ya~ying amounts of 13CO (f~om /34/)

spect~a exposu~e.

It is not immediately plausible, that the dipole-dipole coupling model is applicable to adso~bates on metal films, whe~e the su~face st~ucture is complex. In fact, one finds, that the model accounts well fo~ the measu~ed f~equency shifts, suggesting that othe~ than dipola~ couplings, such as inte~actions mediated by the metal, a~e not significant in these systems.

222 2ISO.-----.----r---,----,-------, •

\

0.

~1l00

E

'::2140

., .0 .~ ,

chemicol shill shift

o tore:

~8 o~

o

'0

I

0 00

Resol ution

~2130 L:

~

!r.n

o

u

2120

21100~--+--_='::_--:;;_':;--_='-:;---;:::'

0.1

02

03

Exposure (Ll

Fig. 8: Chemical (static) and total f~equency so~bed on Ag film as a function of

04

05

shift of 12CO ad(f~om 134/)

exposu~e

5.Intensities At p~esent it is difficult to use intensities in su~face vib~atio nal spect~oscopy fo~ cha~acte~izing adso~bates on metal su~faces. This is even mo~e se~ious in case of metal films. The intensity must be a ~ep~oducible function of the su~face cove~age of a given species and some well cont~ollable pa~amete~s ~efe~~ing to the sample and scatte~ing geomet~y. In the enhanced Raman scatte~ing and the inf~a~ed spect~oscopy on highly dispe~sed metal films the actual local field, acting upon the molecule, can ha~dly be specified, since it st~ongly depends on the scatte~ing geomet~y. F~e quently the su~face a~ea and the c~ysta110g~aphic faces exposed a~e not known. Thus sample and measu~ing cha~acte~ization is difficult. Sample p~epa~ation may lack ~ep~oducibi1ity. Secondly, quantitative measu~ements a~e needed. In this case special ca~e must be taken to obtain a p~ope~ ~efe~ence. Fo~ instance, in ~eflection inf~a~ed spect~oscopy of W(100)p(1 x l)H, the inf~a~ed abso~ption of the adso~bate is obtained by sUbt~acting the ~eflectivity of the hyd~ogen satu~ated su~face f~om that of the clean ~econst~ucted tungsten su~face (W(100) (12 x 12) R 45°)

223

and taking the ~atio of this diffe~ence to the clean su~face ~e flectivity /46/. Howeve~, variations in the b~oadband ~eflectivity over the ~ange 700 - 3000 cm- 1 a~e observed as the hyd~ogen coverage increases. This variations are attributed to the changes in the tungsten surface. The frequency dependence of these variations is established by measuring the reflectivity of the surface saturated with deuterium in the spectral region of the W(100)p(1 x l)H infrared absorption (1000 - 1300 cm- 1) and vice versa. It results that a linear reference can be taken for tungsten-hydrogen. Thirdly, a relation between the measured intensity and molecular properties is desired. As was pointed out already interactions between the adsorbed molecules can affect the vibrational frequencies. These lateral interactions also have influence on the relation between the intensity and coverage. In fact, for strong dipole scatterers there is no linear relation between the integrated absorbed intensity f~(V)dV and the coverage. The theory for pure dipole interactions between molecules adsorbed randomly on sites in a periodic lattice provides the relation

f

M v)

dv

o o.v

L

o -------(1

+ 0

o.e

L o)2

( 12)

o

is the fractional occupation of the adsorption sites, o.v the vibrational polarizability of the adsorbed molecule, o.e its electronic polarizability and L o the dipole sum as before /43/.

The values of the vibrational polarizability o.v for rough Ag, Au and Cu films, deduced from the dynamic frequency shift, account well for the measured intensities /34/. No evidence for strong infrared enhancement associated with the special structure of the rough metal films deposited at low temperature (4 K) is found. The o.v values observed for CO chemisorbed on the rough metal films are not greater than those observed for CO chemisorbed on single crystals (see also chapter 4.3.1.2). It should be realized that chemisorption of CO on annealed films (and single crystals) of Ag and Au does not take place (annealing temperatures T(Ag)~ 150 K, T(Au) ~ 290 K).

224

We should consider that the dipole coupling theory can become inadequate when the interacting molecules are very close together and short range forces may have to be taken into account.

Table 4: Singleton frequency vo ' vibrational polarizability avo electronic polarizability a e and dipole sum Zo for adsorbed CO

v

--_.~_._._------,-_._-

4 K 4 K 4 K 300 K 300 K

2148 2125 2102 2143 2085

Cu(100) c(2 x 2)CO Cu( 111) (/3x/3)R30 0 CO

2077 2089

CO gas phase

2143

Ag Au Cu Ag Cu

deposited deposited deposited deposited deposited

at at at at at

a 1$.3

/ cm- 1

o_._- _._._._-

--

±

_._,-

4

± 4 ± ± ±

4 4 4

-~

v --- -- _._--

a IX 3

Z IX-3 ref.

_- _e_._---_.__ 0._.- -_._-_._. __ ..

..

..

3.0 3.0 3.0

0.04 0.04 0.10

34 34 34 34 34

0.27 0.22

3.0 3.0

0.3

45 45

0.05

3.0

0.27 0.40 0.27

34

6. Line Shapes and Dynamics One of the ultimate goals of the study of adsorbate vibrational modes is a better understanding of their role in energy transfer and dissipation processes as well as in chemical reactions on surfaces. The spectral line shape of vibrational bands can provide valuable information on the mechanisms responsible for energy transfer between an adsorbed molecule and the adsorbent as well as between the adsorbed molecules /47/. On metal surfaces the creation of electron-hole pairs and the excitation of phonons are the most important energy dissipation channels of excited internal adsorbate vibrations. For well-ordered overlayers with neglegible inhomogeneous broadening, theory predicts symmetrical line shapes for relaxation by phonons. dephasing and diffusion. while electron-hole pair creation produces an asymmetric line shape. Experimental support for damping of a vibrational mode of an atom on a metallic

225 su~face by elect~on-hole pai~ c~eation has been ~epo~ted. By use of inf~a~ed ~eflection spect~oscopy, the asymmet~ic line shape of the ove~tone 2v2 of the wagging mode of H atoms (and D atoms) on tungsten W(100) at satu~ation cove~age was measu~ed and found to have a de~ivative-like form, a FANO line shape as theo~y predicts /28,48/. In cont~ast, the line shape of the symmet~ic st~etching mode v1 of H atoms on tungsten W(100) at saturation cove~age W(100)p(1 x l)H does not exhibit such an asymmet~y. P~eliminia~y ~esults f~om isotopic mixtu~e expe~iments show that the v1 vib~a tional line shifts down in f~equency and na~rows as the ~elative concent~ation of deute~ium inc~eases at constant total coverage. The na~~owing of the line upon D dilution ~ules out pu~e inhomogeneous b~oadening and suggests that H-H dynamic inte~actions b~oaden the line. Dipole-dipole inte~action is too weak to account fo~ the f~equency shift. Othe~ effects such as interactions mediated by the metal appea~ to be ope~ative. Appa~ently relaxation by elect~on-hole pai~ c~eation may in gene~al be st~onge~ fo~ parallel vibrations of adsorbates than for perpendicular.

For a metal film that is ce~tainly rough, there is reason to believe that the spectral line shape is dominated by inhomogeneous broadening. The band due to chemisorbed CO on Ag film is much broader than that due to physisorbed CO and its line shape is asymmetric with a tail to lower frequencies. For Cu films deposited at 4 K, a width of the CO line of 17 cm- 1 was observed, much broader than on single crystal faces /34/. Recently it has been shown that surface processes, such as desorption, can be induced by resonant excitation of internal vibrations of adsorbed molecules in the electronic ground state. For the resonant vibrational excitation tunable infrared lasers and optical mixing techniques have been employed /14,49/. A strong frequency dependence of the photodesorption yield has been observed and compared with the corresponding linear infrared absorption line shape. Corresponding to non-linear intensity effects, vibrational anha~monicity and surface hete~ogenity, characteristic differences in the spectra are found. Theories of resonant photodesorption via vib~ational excitation of adso~bates have been presented including 1) a selective desorption mechanism whereby vibrational energy tunnels directly into the external translational mode of the adsorbed molecule, 2) the effects of lateral vibrational energy

226

in the adso~bate and 3) non-selective enhanced the~mal due to ~esonant su~face heating /14,50/. In ~eson~nt heating of the su~face, photon ene~gy ~esonantly abso~bed in an inte~nal vib~ation of the adso~bed molecule is ~apidly t~ansferred to the adsorbent the~eby heating it. Molecular velocities in deso~ption induced by ~esonant vib~ational excitation have been measu~ed and specific diffe~ences fo~ highly dispe~sed film and single c~ystal su~faces obse~ved /51/. t~ansfe~

deso~ption

7.

Aspects

Expe~imental

measurements of the vib~ational spect~a of of adso~bates on a su~face p~esent a high challenge. Small signals a~e to be measu~ed which a~e superimposed by background ~adiation that can be st~onge~ by orders of magnitude. Often multiple surfaces, as in highly dispe~sed metal films with the equivalent of 10 3 monolayers, cannot be applied. The~efore some important aspects of the measu~ement of inf~ared intensities of adso~bates on flat metal surfaces are to be discussed. The techniques used imply ~eflection absorption, di~ect absorption and emission measu~ements. Inf~a~ed

monolaye~s

spect~oscopic o~

sUb-monolaye~s

Due to the high conductivity of metals the elect~ic field vector at the position of the adso~bate is always very nea~ly pe~pendicu lar to the surface. To generate this perpendicula~ field the infrared beam must have a large angle 0 of incidence and must contain the parallel polarized component. Unlike the perpendicula~ component parallel pola~ized radiation gives a significant electric field at the metal su~face, if the angle of incidence is la~ge (see fig. 9). But for a la~ge angle of incidence the infra~ed abso~ption by the metal is also la~ge , amounting to 10 - 20 % of the incident beam. The significant quantity is the ~atio S of the st~ength of the adso~bate signal to the adso~bent signal, which is a function of the angle of incidence too. At 2000 cm- 1 the ratio has its maximum in case of CO chemiso~bed on Ni: for abso~ption o(Sm) = 78°, Sm = 0.004, coverage (CO) = 0.1; for emission 0 (Sm) = 85°, Sm = 0.0025, coverage (CO) = 0.1 /52/.

227

2.0

x

1

-

w

N

W

n:3-30i

. 1.0 w

W"

z 2~

4~

e

6~

---

8~

Fig. 9: Parallel polarized electric field as a function of the angle of incidence e for a highly reflecting metal (n = 3 - 30i). Ex: normal component; Ez: tangential component; E: incident field In reflection absorption spectroscopy the vibrational signal of the adsorbate is contained in the reflected beam. The signal appears superimposed on a strong background radiation that is 80 90 % of the incident infrared intensity. Fluctuations in the background caused by source fluctuations of the spectrometer appear as noise. In almost all experiments the background photon intensity is so high, that many detectors cooled with liquid nitrogen such as the photovoltaic detectors InSb, HgCdTe and PbSnTe reach the background photon noise limit /52/. This is true also for bolometers (Ge) and photoconductive detectors (Ge:Ga, SilAs, etc.) combined with special amplifiers, when operated at liquid helium temperature. Direct absorption and emission measurements could be advantageous having a background smaller by a factor 0.1 than reflection absorption.

228

The large background in any infrared signal of adsorbates on metals requires techniques to distinguish these. In general the adsorbate signal is obtained by rationing the signal of the covered surface to that of the clean surface. As already indicated, there may be adsorbate induced variations in the background due to nonvibrational absorption, which can be detected by comparison with the spectra of corresponding isotopes. Fast modulation techniques can be effective in avoiding low drifts and in separating adsorbate signals from metal signals. The techniques used are polarization modulation /53/ and wavelength modulation /54/. Polarization modulaticn employs the fact that only the parallelpolarized component of the infrared beam contains the adsorbate signal. A rotating polarizer or a photoelastic modulator switches the polarization of the infrared beam. After demodulation, the detector signal is proportional to the difference in intensity between the two polarizations. However, even if a second polarizer is applied, the usefulness of this technique is limited, because of the polarization dependence of the window transparency and of the efficiency of the spectrometer, especially of a grating, which cannot be perfectly compensated. In wavelength modulation, the wavelength of the incident beam is swept over a small spectral range. After demodulation the detector signal is proportional to the derivation of the intensity with respect of the wavelength. This procedure strengthens sharp spectral features over braod ones. If the adsorbate signal is sharper than the adsorbent background, the adsorbate signal will be enhanced. The absorptivity and emissivity of metals does not contain sharp resonances. Therefore wavelength modulation can be efficient. In practice the main limits to accurate infrared intensity measurements are source brightness and detector noise. But when the above mentioned photovoltaic or photoconductive detectors are properly used, infrared spectroscopy of adsorbates on metals with thermal sources need not to be source brightness or detector noise limited. One essential point is to minimize the effect of ambient photons by cooling apertures and filters and if possible the whole spectrometer.

229

Advantages in the linea~ spect~oscopy of adso~bates on metals can also be expected from using mo~e powe~ful sou~ces, such as lasers, since la~ge ~atios of signal to noise can be obtained at a given backg~ound of ambient photons.

~ecific

Film Effects

- especially inf~a~ed spect~oscopy - is a useful tool in su~face investigations due to its high specifity fo~ diffe~ent adsorbates and adsorption sites. Measurable are f~equencies, intensities and - if pressure and sample temperature are known - site-specific adsorption isotherms as well as adsorption energies /61/.

Vib~ational

spect~oscopy

As already mentioned in preceding chapters care must be taken in transfe~ing experimental results as well as theories obtained for single crystals to thin film problems and vice versa. This transfer is even more serious since film prepa~ation is not always a well-controllable process. Furthermore, film characterisation is poor in many cases. Therefore we will try in this chapter to point out possible diffe~ences between vibrational spectroscopy of adsorbates on films and on single crystals. Due to the preparation the amount of imperfections, such as kinks and edges, on films is larger than on single crystals. This means that films contain more "active sites" with a higher adsorption energy and a stronger influence on intramolecular properties which may - for a given coverage - result in vibrational frequencies different from those on single crystals. This is also a serious problem if only films and no single crystals are under study; vibrational frequencies of adsorbates measured on different films often differ (see chapter 10.). Furthermore it is difficult to estimate the coverage and the number of adsorption sites. Because of its high specificity infrared absorption spectroscopy of suitable adsorbates can be used to characterize these films. It was shown that Ag films deposited at temperatures below 150 K

230

contain 0.01 monolayer of active sites at which CO chemisorbs, whilst films deposited (or annealed) above 150 K only physisorb CO 134/. Few measurements for the comparison of the vibrational spectra of adsorbates on single crystal and thin film surfaces have been performed as yet. In case of CO adsorbed on nickel at small surface coverage approximately the same spectral lines for argon ion bombarded and evaporated nickel film surfaces were detected as for well ordered (111) and (100) single crystal nickel surfaces 158/. This indicates that the CO stretching frequency is mainly affected by the number of Ni atoms bonded to the molecule and not by the long range order of the crystal. Similar conclusions can be drawn with respect to the system COICu (see table 5). The vibrational frequency shifts due to the metal surface structure are generally smaller than those due to the chemical bonding characteristics, but can be clearly resolved by means of infrared spectroscopy. Infrared studies of adsorbates on metal microcrystals supported on silica, alumina and others show that the same physical interactions operate on the supported dispersed systems as on metal single crystals. Especially, minor metal-support interactions occur on silica 164/. As already indicated in chapters 3. and 4., the intermolecular adsorbate interactions (e.g. dipole-dipole-coupling) on films may differ from those on single crystals due to the more limited size of the facelets. The few experiments, where the molecular interactions of adsorbates on metal films could be split into their static and dynamic contributions, suggest that there is no significant difference between adsorbates on single crystals and highly dispersed films of Cu (see also table 5). This is in agreement with theory predicting a convergence of the interaction potential with Ri j 3, Ri j being the distance between molecules i and j. In surface enhanced Raman scattering a special enhancement of the adsorbate scattering cross section due to the film roughness is observed. It seems that small effects of this kind are present in infrared spectroscopy too. For CO on Ag film deposited at 4 K an enhancement of the infrared absorption by a factor 1.4 was obser-

231

Table 5:

Compa~ison diffe~ent

subst~ate

adso~ption

Ni-film

site on top b~idged th~eefold

Ni

(111)

of vib~ational nickel and

- 2040 - 2100 - 1920 - 1970 - 1815 2045 1900 - 1915 1817 - 1846

th~eefold

on top

- 2046 - 1910

b~idged

no

coppe~

su~faces

vlcm- 1 total su~face _____________ cove~age B

on top b r i c ge d

Ni (100)

of CO adsorbed on

f~equencies

0.3 - 0.9 0.2 - 0.9 0.05

58

0.57 0.25 - 0.57 0.05 - 0.2

59 0.171 60

:> 0.5 :> 0.5

th~eefold

site

Cu-film * TD=300 K T D=4 K

on top on top

2085 2102

** **

Cu(100)

on top

2085

0.5

0.27 0.22

0.33

0,27

Cu(111)

- 2075

__________.

*

TD = deposition all cove~ages

.

.

34 0.27 43,54 45 33,39

Ji.5_ _

tempe~atu~e

** fo~

ved on wa~ming to 40 K. It is assumed that CO molecules mig~ate sites of enhanced elect~ic field located in po~es, cavities steps 134/.

o~

to at

low tempe~ature kinetic expe~iments have been as yet only on metal film systems. Inf~a~ed studies of the adso~ption on films as a function of the tempe~ature allowed the investigation of diffusion on metal su~faces. Fo~ po~ous films the initial adso~ption at low tempe~atu~es is limited to those adso~ption sites, whe~e molecules hit the su~face. By increasing the tempe~atu~e diffusion to vi~gin surfaces becomes an Ce~tain

pe~fo~med

inte~esting

232 p~ocess ~esulting in the fo~mation of the stable adthe diffusion can be monito~ed by e.g. the abso~bed inf~a~ed intensity 1341 and the f~equency shift 156/. Analyzing the rate as a function of tempe~atu~e can yield the order of magnitude of diffusion bar~ier on the surface. On warming the impo~tant

so~batej

adsorbate CO-Ni(film) from 15 K to 35 K, the infrared spect~a change substantially and the spect~um of the more stable chemisorbate appea~s with bands at 2090 and 2060 cm- 1 ascribed to terminal CO bonded to one surface Ni atom and a broad weake~ absorption at lower f~equency assigned to sites of higher coo~dination 156,57/. Also the p~oblem of activation in the adso~ption step can be studied. By means of low tempe~ature adso~ption it has been shown that even at T = 2 K CO chemiso~bs on silve~, gold and coppe~ films deposited at T = 4 K; no solid CO is observed by vib~ational spect~oscopy 134/. was observed in the adso~ption of CO on nickel film /62/. The phenomenon is similar to infra~ed chemiluminescence which has been successfully employed to gain insight into the dynamics of gas phase chemical reactions, vibrational ene~gy transfer and relaxation 163/. Infrared-adso~boluminescence

As detected by time-resolved infra~ed absorption measu~ements in the 1800 to 2400 cm- 1 region, the oscillations in the reaction ~ate of the CO oxidation on Pt or Pd films run parallel with periodic changes of the CO coverage on the metal surface. The shift in the peak frequency of CO du~ing ignition and du~ing the inc~ease of the CO co1), verage is very small (- 6 cmindicating CO island formation. Du~ing the ignition, the chemiso~bed CO is reacted away at island bounda~ies 165,66/.

Acknowledgements We should like to thank Prof. P.L. Richards, Dr. R.G. Tobin, Dr. P. Dumas, Prof. R.G. Greenler and Dr. Y.J. Chabal for sending preprints and allowing the rep~oduction of figures. Partial support by the "Deutsche Forschungsgemeinschaft" is g~atefully acknowledged. Particular thanks go to H.-R. Eggers, L. Matalla and M. SchlUter for help in the preparation of the manuscript.

233

9. REFERENCES (sections 1-8) 1. A useful

to vib~ational spect~oscopy of is, fo~ example, given in ~ef. 2 - 7 Spect~oscopy of Adso~bates, 2. R.F. Willis (ed.), Vib~ational Sp~inge~ Ve~lag, Be~lin-Heidelbe~g-New Yo~k, 1980 3. H. Veba and H. Yamada (eds.), Spect~oscopic Studies of Adso~ bates on Solid Su~faces, Su~f. Sci. 158 (1985) 4. R.F. Willis, A.A. Lucas, G.D. Mahan, v t or-at.t ona.i Pr-ope r t i es of Adso~bed Molecules, in: The Chemical Physics of Solid Su~faces and Hete~ogeneous Catalysis, Vol 2: Adso~ption at Solid Su~ faces, D.A. King and D.P. Wood~uff (eds.) Elsevie~, Amste~dam, pp. 59-163 (1983) 5. R. Caudano, J.M. Gilles and A.A. Lucas (eds.), Proceedings of the Second Inte~national Confe~ence on Vib~ations at Su~faces 1980, Plenum P~ess, New Yo~k 1982 6. C.R. B~undle and H. Mo~awitz (eds.), P~oceedings of the Thi~d Inte~national Confe~ence on Vib~ations at Su~faces 1982, Elsevier Scientific PUblishing Company, Amsterdam 1983 (Part A: J. Elect~on Spect~. ReI. Phenom. 29 (1983); Part B: J. Electron Spectr. ReI. Phenom. ~ (1983) 7. D.A. King, N.V. Richardson and S. Holloway, Proceedings of the Fourth International Confe~ence on Vibrations at Surfaces 1985, Elsevie~ Scientific Publishing Company, Amsterdam 1986 (Part A: J. Electron Spectr. ReI. Phenom. 38 (1986); Part B: J. Elect~on Spectr. ReI. Phenom. ~ (1986) 8. N. Sheppard and J. Erkelens, Appl. Spectrosc. 38,471 (1984); H.A. Pearce and N. Sheppard, Surf. Sci. 59, 205 (1976) 9. M. Adachi, K. Kishi, T. Imanaka and S. Teranishi, Bull. Chern. Soc. Japan~, 1290 (1967); O. Koga, T. Onishi and K. Tamaru, Chern. Commun., 464 (1974); M. Primet, P. Fouilloux and B. Imelik, Surf. Sci. 85, 457 (1979); M. Primet, P. Fouilloux and B. Imelik, J. Catalysis~, 553 (1980) 10. D. Kembe~ and N. Sheppard, Appl. Spect~osc. 29, 496 (1975); L.M. Gratton, S. Paglia, F. Scattaglia and M. Cavallini, Appl. Spect~osc. 32, 310 (1978) 11. D.L. Allara, D. Teiche~ and J.F. Durana, Chern. Phys. Lett. ~, 20 (1981) 12. S. Chiang, R.G. Tobin and P.L. Richards, J. Vac. Sci. Technol. A2, 1069 (1 984 ) adso~bates

int~oduction

234

13. D.M. Riffe, L.M. Hanssen, A.J. Sievers, Y.J. Chabal and S.B. Christman, Surf. Sci. ~, L 559 (1985) 14. J. Heidberg, H. Stein, E. Riehl, Z. SZilagyi and H. Weiss, in: see 3., pp. 553 15. B. Bolger, Overview of vibrational spectroscopy of adsorbed atoms and molecules, in: Surface Studies with Lasers, F.R. Aussenegg, A. Leitner, M.E. Lippitsch (eds)., Springer Verlag, Berlin 1983 16. see, for example: H. Ibach and D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York 1982 17. D.G. Walmsley, in 2 .• pp , 67 18. K.W. Hipps, in: see 6., part B, pp , 275 19. A. Campion, in 3., pp. 263 20. R.K. Chang and T.E. Furtak (eds.), Surface Enhanced Raman Scattering, Plenum Press, New York 1983 21. see 3., pp , 126 22. E. ErtUrk, C. Pettenkofer and A. Otto, in: see 7., part A, pp , 113 23. C.J. Wright, in: see 2., pp. 111 24. W. Ho, R.F. Willis and E.W. Plummer, Phys. Rev. Lett. 40, 1463 (1978); R.F. Willis, W. Ho and E.W. Plummer, Surf. Sci. 80, 593 (1979) 25. R.F. Willis, in: see 2., pp. 23 26. see for example: R. Richter and J.W. Wilkins, Surf. Sci. 128, L190 (1983); C.L. Fu, A.J. Freeman, E. Wimmer and M. Weinert, Phys. Rev. Lett. 54,2261 (1985) 27. J.J. Arrecis, Y.J. Chabal and S.B. Christman, Phys. Rev. B33, 7906 (1986) 28. Y.J. Chabal, in: see 7., part A, pp . 159 29. J. Heidberg and S. Zehme, unpublished results 30. J. Heidberg, R.D. Singh and H. Stein, Bel'. Bunsenges. Phys. Chem. 82, 54 (1978) 31. H. Ibach, in: see 6., part B, pp. 237 32. R.G. Greenler, F.M. Leibsel and R.S. Sorbello, in: see 7., part B, pp . 195 33. P. Hollins and J. Pritchard, in: see 2., pp.125 34. P. Dumas, R.G. Tobin and P.L. Richards, Surf. Sci lll, 555 (1986); Surf.Sci. lll, 579 (1986) 35. T.B. Grimley, Proc. Phys. Soc. ~, 1203 (1962)

235

36. N. Sheppa~d and T.T. Nguyen, in: Advances in Inf~a~ed and Raman Spect~oscopy, Vol. 5, R.J.H. Cla~k and R.E. Heste~ (eds.) Heyden & Son Ltd., London-Philadelphia-Rheine 1978, pp . 67 37. G. Blyholde~, J. Phys. Chern. ~, 2772 (1964); D.P. Wood~uff, B.E. Hayden,K. P~ince and A.M. B~adshaw, Su~f. Sci. 123, 397 ( 1982) 38. N. Sheppa~d, in: see 7., pa~t A, pp. 175 39. P. Hollins and J. Pr-I t ch a r d , sc-r . Sci. §.2., 486 (1979) 40. R.M. Hammaker, S.A. F~ancis and R.P. Eischens, Spectrochim. Acta £1, 1295 (1965) 41. G.D. Mahan and A.A. Lucas, J. Chern. Ph y s , ~, 1344 (1978) 42. M. Scheffler, Surf. Sci. ~, 562 (1979) 43. B.N.J. Per s s on and R. Ryber g , Ph y s . Rev. B24, 6954 (1981) 44. M. Moskovits and J.E. Hulse, Surf. Sci. 78, 397 (1978) 45. B.N.J. Pe r s s on , in: see 6., part A, pp , 43 46. Y.J. Chabal, J. Vac. Sci. Technol. A4, 1324 (1986) 47. J.W. Gadzuk and A.L. Luntz, Surf. Sci. 144, 429 (1984) 48. D. Langreth, Phys. Rev. Lett. 54, 126 (1985) 49. T.J. Chuang, sur r . Sci. Rep. i, 1 (1983) 50. Z.W. Gortel, H.J. Kreuzer, P. Piercy and R. Teshirna, Phys. Rev. B27, 5066 (1983) 51. J. Heidbe~g and D. Hoge, to be published in J. Opt. Soc. Am. B 52. P.L. Richards and R.G. Tobin, Infrared Spectroscopy of Adsorbates on Metals: Direct Abso~ption and Emission, in: Vibrational Spectroscopy of Molecules on Surfaces, Vol. 4 of Methods of Surface Characterization, Y.T. Yates, J~. and T.E. Madey (eds.), Plenum Publ. Co~p. New York, to be pUblished 53. A.M. Br-a ds haw and F.M. Hoffmann, Surf. Sci. 52,449 (1975); F.M. Hoffmann and A.M. Bradshaw, J. Catal. ~, 328 (1976) 54. R. Ryberg, Surf. Sci. ~, 627 (1982) 55. R.G. Greenler, D.R. Snider, D. Witt and R.S. Sorbello, Surf. Sci. ill, 415 (1982) 56. J. Heidberg, H. Stein and 1. Hussla, sur-r . Sci. 162, 470 ( 1985) 57. J. Heidberg, H. Stein and E. Hoefs, Ber. Bunsenges. Physik. Chern. 85,300 (1981); J. Hel dbe r g and 1. Hussla, in: see 5., pp. 323; J. Heidberg and I. Hussla, to be published 58. R.B. Bailey, T. lri and P.L. Richards, Surf. Sci. 100, 626 (1980) 59. J.C. Campuzano and R.G. Greenler, Surf. Sci. 83, 301 (1979)

236

60. S. Anderson, Solid State Commun. 61. J. Heidberg, 1. Hussla and Z.

~,

75 (1977)

Szilagyi, to be published

62. J. Heidberg, I. Hussla and K.-H. Stammberger, Thin Solid Films 90, 209 (1982) 63. J.C. Polanyi, Appl. Opt.

lQ, 1717 (1971)

64. E. Guglielminotti, G. Spoto and A. Zecchina, Surf. Sci.

l£.l.,

202 (1985) 65. D. Boeker and E. Wicke, Ber. Bunsenges. Phys. Chern. 89, 629 ( 1985) 66. V.A. Burrows, S. Sundaresan, Y.J. Chabal and S.B. Christman, Surf. Sci. 160, 122 (1985)

237

10. Tabulation of Investigated Systems In this chapter investigations of adsorbates on metal films by means of vibrational spectroscopy are tabulated up to 1985; the reference list follows the tables. Used abbreviations are: ATR DB EELS FT lETS Int.R. IRAS IRES Lumin. PAS PEM SB

SERS SEW TRIR

attenuated total reflection infrared spectroscopy double beam spectrometer electron energy loss spectroscopy Fourier transform spectroscopy inelastic electron tunneling spectroscopy internal reflection infrared spectroscopy infrared reflection absorption spectroscopy infrared ellipsometric spectroscopy luminescence photoacoustic spectroscopy polarization modulation single beam spectrometer surface enhanced Raman scattering surface electromagnetic wave spectroscopy transmission infrared spectroscopy

N2

-

adsorbate

C6Hti

-

-- f - - - - - -

SERS

SERS

r-------'

SERS

method

-----

--

----

------------------

evap orated onto aluminum at 11 K

1982

1982

1982

year

r-----.-

2.

2.

1.

------

ref.

---

----------- - - - - - -

evaporated onto aluminum at 11 K; al so study of C6D6

--

remarks

-------------------_._-------

----- -----

wavenumber (cm-1)

CO

Cr € vaporated onto glass; ad 30rption from sc lution. PMMA: polymethyl nethacrylat

----- - -

1985

1968

_._-_.-

TRIR

evap orated in 10-

1968

8.

-163.

8.

------------------ ----_.- -,----_.-

mbar CO 0 1to CaF2,T=170 K

-----1--------r--- -------------------------------- - - 2 1970

-------

evap orated onto CaF 2 in 10- 2 mbar CO at 170 K

-_._-

-

L . . -_____________._________________ L - __________________

W

f------

lRAS/FT

PMMA

1930 (290 K)

1950 (170 K)

-- ~,-------

TRIR

CO

Cr

SERS

C6Hti

Na

f----.

-evaporated onto a copper blo ck at 15 K 1982 3. in UHV; also study of C6D6 --~---~------------ r - - - - 1-----evap orated onto a copper bl,o ~k at 15 K SERS 4. 1984 in UHV ----- ----------- ----- --------- ------------------- ------ -----Ti SERS evaporated onto glass at -10 -5 Torr 1982 5. C5HsN f - - - - - - - ._--------------_._---- ----- 1-----SERS evap orated onto glass at -10 -5 Torr 6. 1983 --_._------~------------ ---------evaporated onto glass at -10 -5 Torr SERS 1983 7.

Li

--

adsorbent 00

""w

CO

Fe

CO/(CH3)3 N

CO

adsorbate

-

--_._._------_._---

Hn

adsorbent

------

1856. 569, 519, 436

-

------

--

-----------

onto CaF 2 at T=170 K and ar

TRlR

IETS

lETS

r------

----

A12~;

--l

11.

10.

-

--

8.

8.

-;~;-l-in-P~;~-T-or-r-c-o-;

-+---I 1981

I

1981

______~~~

...L

~

_

---f-1-9-7-2+--1-0-.-Fe evapora ::t::: redshift of the C-O stretching frequency due to t e Lewis base (CH3)3N

1-

Fe evapora ed onto A1 203 in 7*1 )" moor CO: top Pb e

1--------

1972

I

I

CO; ----- -1980- ----1~

.---------::"c----

ed onto ectrode

Fe evapo top Pb

--'----------

1856, 569. 519, 436

1968

-+-197~--9-.

---+-

1968

ref' •

-----------------------------t-----t--------

ectrode

---_._---_._-- --_._--

evaporat top Pb

CO

evaporat onto NaCl at T=113 K and p;10-9 bar

evaporat p<10-7

I

year

------------------------------·...-----1-----------,

remarks

evaporated in 10-2 moor CO onto CaF2 at 170 K

-------_._-

------1------evaporat -------------------------------l----~------TRlR onto NaCl at p=O. 1 Torr 1915 1--------- ----------- -----~nt~~1203 -at p=~:4mba['

lETS

K)

K)

2040, 1960 (170 K) 1930 (293 K)

1950 (170 1900 (293

TRlR

-

2000, 2080 (170 K) 1980 (293 K)

TRlR

TRlR

wavenUlllber (cm-1)

method ~---

""c.>

""

Co

Fe

adsorbent

----,-

-_._---_._._----~

CO/(CH3)3 N

-

-

1972

1982

--

10.

15.

-11.

10.

--------------------

-------------

- - - - - - - - - - - - f------ - - - - -

Co evaporated 0 to NaCl at 0.1 mbar CO; redshift of t e C-O st retching mode due to the Lewis base (CH 3 )3 N

- - - - - --------- --------------

TRIR

399

Co evaporated on to A1 203 ; top Pb electrode

1936, 519,

1980

1972

9.

-------------- f - - - - - f - - - - - -

evaporated onto NaCl in 0.1 mbar CO

f--.

lETS

-Co evaporated on to A1 2 0 3

1980 1880

1970

_._------- t - - - - - f - - - - - - -

lETS

TRIR

1----

evaporated onto NaCl at T=l13 K and p=l *1 0-9 mbar

1'-------- _._---- -----------

TRlR 2040 (113 K) 1925 (293 K)

--------

TRlR

CO

14.

14. Fe evaporated onto gold f ilm/ quartz 1975 at p;;2*10- 6 To rr --------------- - - - f---------- f - - - - - - - - - - - 1990 (170 K) evaporated onto CaF2 at T=170 K and 8. 1968 7 p<10- Torr 1970 (293 K)

1820

1975

IRAS/DB

Fe evaporated a to gold film/quartz at p;;2*10- 6 T rr

isoamyl nitrite

-

11.

ref.

1825

1981

-_._._-- t--

--.--

year

lRAS/DB

-

emarks

Fe evaporated a to A1 203 ; study of hydrocarbon s ynthesis

---------

NO

-

wavenumber (cm-1)

lETS

method

CO/H2

adsorbate

...""o

~-

Rh

Co

adsorbent

'--------

~.

2075

1964, 1744,

lRAS/DB

lETS

597

1935, 1730, 580,454,408

lETS

--

2020, 1885

2078, 2060, 1988

lRAS/SB

TRlR

2000, 1820

TRlR

--

Rh evapot'ated onto CaF2 ot' Csl in 3-12 Tort' CO

TRlR

~--

p<10- 4 Torr

lRAS/DB

CO

p<10- 4 Tort'

IRAS/DB

-5 Torr

--------

Rh evaporated onto A1 203; top Pb electrode

1978

I 1977 I

1976

1972

1970 I

1967

1965 I

1959 I I 1959 I

1983

I

I

1980 I

year

-----t_

in 10- 5 Torr CO;

Rh evaporated onto a tantal foil p~3*10-8 mbaI'; annealing at 78o K

Rh evaporated onto ~1203 top Pb electrode

Rh evaporated onto NaCl in 0.1 Torr CO

~._-----------------

Rh evapot'ated at p=3*10- 9 mbaI'; (110)-orientation of the film

I

---------+

Rh evaporated onto NaCl at p=10-8 Torr

------

p~10'

H2

Co evaporated onto glass at

SEAS

C5HsN

Co evaporated onto A1 203; study of hydrocarbon formation

remarks

----_.- - ----- -"--_._------_._---_._--

lETS

2111, 2055, 1905, 1852, 1817

wavenumber (cm-1 )

---------

CO/H2

-

----------- c-------adsorbate method

22.

21.

20.

10.

19.

18.

17.

16.

16.

5.

11.

ref.

I

---j

N

""

I-'

L--..

Rh

adsorbent

.L.

7~-----t-IET~---t~924

co

adsorbate

_

11955.2,1736.5, 588.3,484.5 I 2020, 1870,

lETS EELS

lETS

I

lETS

581

remarks

Rh evaporated onto Al203

-~~;-;b

elec;;~--

Pb, Tl, Sn and

-----------

Rh evaporated onto A1 203, top Pb electrode

Rh evaporated onto MaCI in 0.1 Torr CO

Rh evaporated onto A1 203, top Pb electrode

Rh evaporated onto oxidized aluminum in UHV. Canparison to lETS and lR

Rh evaporated onto A1 203, top Pb electrode

Rh evaporated onto A1 203; top Pb electrode. 13eo and e 180 also studied

I Rh evaporated onto AI2~; In top electrodes

......------ ----year ref.

26.

25.

24.

23.

.-

22.

------- --------

1978

-

----30.

29.

---- -----

1983

-----

1983

---_._-----1980 27. -- ----- r-------28. 1981 - f - - - - - - ------1981 11. - f------

1980

1979

1979

----

1978

.-------- f - - - - - - - - -

Rh evaporated onto A1 203; top Pb electrode

~ 1706:-T;-~vaporate~-~~;-~~;03;

1942, 1721, 600,465,413

-------

I 1942, 1721, 600,465,413

------~----

2045

-------------

~~~--------

430

-----------

lETS

TRlR

------

"TS

I 1942, 1721, 600,465,413

-

I

-~---------------------------

lETS

.--

wavenumber (cm-1 )

I 1935, 1730, 580,454,408

i

lETS

method

""

...""

Rh

adsorbate

~~-

~1i-j-

lETS

lETS

~._-

CO/C1 2 CO/Br 2

CO/(CH3 )3N

--

CO/C 2H4 CO/H2O

CO/0 2

-

TRlR

- - I--

TRlR

TRlR

'---

Rh evaporated onto A1 203 ; study of the Fischer-Tropsch rea tion

--

--------------------

--18.

1978

23.

-- ------

1967

------

- - ----25. 1979

--_._-- -------year ref.

2130, 2090 2110, 2090

1972

-

1981

----------- f--

0.1 Torr CO; tching mode H3)3 N

Rh evaporated onto NaCl i 0.1 Torr CO; blue-shift of the C-O s retching mode due to the halides

Rh evaporated onto NaCl i redshift of the C-O str due to the Lewis base (

28.

---10.

Rh evaporated onto A1 203; study of 1980 27. catalytic hydrocarbon fo rmation -----_._----,--- - - - - - - f--------Rh evaporated onto A1 203; top Pb electrode 111981 ---------+-------------------- ---------,--_._--_.- - - - --_._---Rh evaporated onto A1 203 30. 1983 f - - - - ---_._---Rh evaporated onto NaCl, p ;;1 ~=8~~;;-----1967 18.

--

(cm-1 )

-- ----------T----------------------wavenumber remarks

- - - - - -lETS 1916.6,1698.9,1 Rh evaporated onto A1 203 ; top Pb electrode 572.3,461.1 -;;10-8 Torr TRlR

method

f--------------- - - - -

CO/H2

C

-- f--- 180

adsorbent

------ ------------------- ,--_._---

I

I

co

...""

[

I

Ir

I

lETS

TRIR

HCHO

H4

TRIR

I

1970

2010 - 2093

------------

TRIR

TRIR

lRAS/DB

I

..

18.

1967

18.

-_._--------- 1------ ---------7

mba r

----------- ------ _._._------

32.

16.

_._-------

18.

11' evaporated onto NaCl, P~10-8

-

---

1980 32. study of vibrational coupling in 12C160/13C160 mixtures - - - - - - - - ----- --------1980 11' evaporated onto glass in UHV 33. ----_._--- - - - - - - - - 28. 11' evaporated onto NaCl in 100 nbar CO 1981

11' evaporated onto glass in UHV

c, p<10- Torr 1968 8. T~170 ------------_._-------------_._._- --------_._- ----- f - - - - - -

11' evaporated onto NaCl, p~10-8

L11' evaporated onto CaF2,

I

1967

1959 ------------ -_._-,11' evaporated onto glass in UHV 1980

p<10- 4 Torr

-

Torr

--

Rh evaporated onto NaCl, p~10-8

1967

31,

-----_._------ _.__._-_.-

----_.-

mbar 18. 1967 ---------- f------ f-------11' evaporated onto glass in UHV 1980 33. ------------1 - - - -1--------------~------.---------,-------18. 1967 I 11' ovapor-at.ed onto NaCl, p~1 0- 8 mbar

2035

lRAS/DB I 2010 - 2093

lRAS/DB

TRIR

-------,----

I

Torr

1982

ref.

-- r---- +------

year

---------- 1 - - - - -----

Rh evaporated onto NaCl, P~10-8

Rh evaporated onto A1 203 ; top P ) e.Lect rode , Dissociative adsorption.

remarks

r--------------------

------- +--_.

___..l-.__________

~~-

--------

CO/H2, CO/H20, CO/C2H2

CO/02

:~- - - -+~:~;_;~3~~--1900--

I

wavenumber (cm-1 )

--+-----lRAS/DB

----

HCOOH

--

I

I method I

--- - -

adsorbate

C2 ----r------------

Rh

adsorbent

tV

,jo> ,jo>

Ni

II'

adsoJ:'bent

2255, 2200

TRIR

2050, 1900 (170 K) 2020, 1900 (293 K)

TRIR

TRIR

2083, 2058, 1925, 1880, 1620

TRIR

Ni evaporated onto NaCl in UHV and in 2 Torr' CO

Ni evaporated onto CaF2, T=170 K, p<10- 7 Torr

1969

1968

1965

1959

p-l0-5 - 10- 4 Torr

2058

I RAS IDB

CO

Ni evapor'ated onto CaF2 and CsI in 10- 3 - 44 Torr CO

1970

Ni evapor'ated onto NaCl in UHV, T=83 K

TRIR

1970

1970

1967

1981

year

N2/CO

Ni evaporated onto NaCl in UHV, T=83 K (Ni also evaporated in N2)

Ni evapor'ated onto NaCl in UHV, T=113 K

2255, 2230, 2180

TRIR

N2

mbar

Ir' evapor'ated onto NaCl in 100 mbar' CO; blue-shift of the c-o str'etching mode due to the halides

remarks

Ir evaporated onto NaCl, p~10-8

wavenumber (cm-1 )

TRIR

TRIR

method

HCHO HCOOH

CO/Br'2

CO/Cl 2

adsoJ:'bate

35.

8.

34.

16.

37.

37.

36.

18.

28.

rer,

"" >I>0'

~---_._-

Hi

f---.

adsorbent

~--------------

CO

adsorbate

2085 2125 1895 1915

2049, 1984, 1791,1670, 480, 367

-

Ni evaporated onto A1 203 ; top Pb electro de

-_._---------------

Ni evaporated onto A1 203 ; top Pb electro de

-

Torr

Ni evaporated onto A1 203 in UHV; thel'mal detection technique

p<6*10-

----------------------------9

mbar; (110) orientation of the films

1.----------- 1-.-_--------- 1.-..____________________________________________

lETS

lETS

lRASISB

--

lRASISB 2065 2120 1880 1910

f----

2010, 1890

P~3*10-9

----1

1981

1981

1980 I

11.

42.

41.

40.

39.

1976 I 1978 I

38.

10.

19.

1973 I

1972 I

1970

9. I

1970 I

----+

ref.

Ni evaporated onto NaCl in UHV, T=113 K

year

36.

1970 I

remarks

Ni evaporated onto NaCl in UHV, T=113 K

-

Ni evaporated onto NaCl in 0.1 Torr CO -------------------------lRASISB 2060, 1920 Ni evaporated onto glass, p<2*10-8 mbar -------- --------------_._------------lnt.R. Ni evaporated onto NaCl and KBr

TRlR

f------

~.

-2090 1650 - 1950

(cm- 1 )

wavenumber

lRASISB 2056

f----

TRlR

TRIR

method

....""en

Ni

adsorbent

TRlR

NO

CO/C 2HsNC

--

mbar; (110) orientation of the films

-

-----

Ni evaporated onto CaF2 and Csl in 10-3 - 44 Torr CO

--- 1---------------

p~3*10-9

1--

1750

1860, 1805,

--'----

lRAS/DB r-------lRAS/DB

Torr

-

Ni evaporated onto quartz, p<6*10- 9 Torr

Ni evaporated onto KBr, p~2*10-6

Ni evaporated onto NaCl in 0.1 Torr CO; redshift of the C-° stretching mode due to the Lewis bases

lETS Ni evaporated onto A12°3 ; top Pb electrode r------- 1-------- f---. lETS Ni evaporated onto A12°3 ; top Pb electrode

~-

lRAS/SB 1998

----

I

I

-

< 4 K;

2010 - 2090 1860 - 1950 1790 - 1810

lRAS/SB

Ni evaporated onto A12°3 in UHV, T thermal detection technique

Ni evaporated onto NaCl film in UHV, T=77 K

2094

TRlR

r emarks

Ni evaporated onto glass in UHV; adsorboIumtnescence

(cm- 1 )

wavenumber

Lumin.

method

r------------- 1------TRlR CO/(CH3)3 N

CO/H2

13co

CO

adsorbate

, - - - - - - - - - - - - - - - - - - - - r------.-- - - - - - - . - - - - - ---.---.-------.--

I

I

1979

1975

J

47.

14.

10.

42. 1981 1972

11.

34.

19.

45.

44.

43.

ref.

1981

1965

1970

1983

1982

1982

year

"" -.J

.

------

Ni

adsorbent

Ni evaporated ont o A1 20 3 in UHV; thermal detecti on technique

lRAS/SB

-

-----------------

benzoic acid p-nitrobenzoate

isoamyl nitrite

C5HsN

~---------

C2H4

~._--------

C2H2

lRAS/DB

lRAS/DB

1--------- r--------------

41.

40.

51.

--

p;5*10-5 TOl:'r

---- --_.- --_._--

-------------

1400, 1355 1415, 1350

----------------

Ni evaporated ont o glass; adsorption f'r'om aqueous solution

-

1983

7.

------- ------ ------------

---,-

1981 49. - - - -1-----1 - - - - - Ni evaporated ont o glass, p;2*10-5 TOl:'r 1982 50. ----~ ----- - - - - - - - - - ---Ni evaporated ont o glass, p;2*10-5 Tol:'l:' 3112, 3072, 1982 6. 1008 - - - ----1 - - - - - - -Ni evaporated ont o KBI:', p;2*10-6 TOl:'r 14. 1975

-

-------------1-------I Ni evaporated ont o glass,

----------

-

SERS

SERS

SERS

------lRAS/DB

lRAS/DB

--

--

ref.

.------f------- -------

1980

1978

1982

year

48. 1977 ----- - - - - --------Ni evaporated ont o qual:'tz, p<6*10- 9 Tol:'l:'; 46. 1979 also study of C2 D2 1--------- - - ~---46. Ni evaporated ont o quartz, p<6*10- 9 TOl:'r; 1979 also study of CA

Ni evaporated ont o qual:'tz, p<6*10- 9 TorI:'

f---------------

------1--------- f-------------

lRAS/DB

_._------- ------ f - - - - - - - - -

CH4

-

p<6*10- 9 Torr; st udy of decomposition

I RAS IDB

-

DCOOD

r emarka

Ni evaporated ont. o glass, p;2*10-5 Tol:'l:'; adsorption from aqueous KCN solution

wavenumber (cm-1 )

lRAS/DB

method

CN-

adsorbate

tv

00

...

H2

CO/H2, CO/02• CO/H2O, CO/C 2H2

f---

CO

TRIR

lRAS/SB PEM

f-----

Int.R.

lRAS/SB PEM

f--------

Int.R.

f-----

TRIR

T~345

K

-

2002, 1987

2112, 1974 (230 K) 1987 (420 K)

2085, 1970, 1910, 1840 ---------2010, 1902

Torr,

--

-

Pd evapoeated onto NaCI at 10- 8 moor

Pd evapoeated onto glass, p<10- 9 Torr

Pd evapoeated onto NaCI an d KBr

Pd evapoeated onto glass i n UHV at substrate tempeeatures of 230 K an d 420 K

---------------

Pd evapoeated onto NaCI in 0.1 Torr CO 1----------------------Pd evaporated onto NaCI an d KBr

r---------------------

Pd evapocated onto CaF2 an d CsI in 20 - 50 Torr CO

--- -------"--- r--------------------------

f-------

TRIR

----

Pd evaporated onto glass, p~6*10-9

Ni evapoeated onto glass; Langmuir-Blodgett adeorbate film

Ni evaporated onto glass, p;;2*10- 5 Tore; Langmuie-Blodgett adsoet ate film

remarks

--------------------------

-- f---

--

wavenumber (cm-1)

----------

IRAS/DB 880, 760

IRAS/FT

Cd-arachidate

method

IRAS/DB

adsorbate

stearic acid

f---

r - - - - - - ~Pd

Ni

adsorbent

----- ----------------- -------_._-

17.

55.

163.

51.

ref.

1967

1976

1976

1975

-

1975

~-----

18.

56.

39.

53.

---------

54.

--------

--- --------10. 1972

1965

1975

1985

1982

year I

""~

-.

Pd

adsorbent

IRAS/DB

C2H2

C5~N

TRIR

HCHO HCOOH

.--------------

1982 I 1982 I

Pd evaporated onto quart z , P;;2*10-5 moor

- - --------'-------------- ---------------------

SEAS

SEAS

Pd evaporated onto A1 203 top Pb electrode Pd evaporated onto glass , P;;2*10-5 moor

------+--

1984

198~~~~~

-

lETS

top Pb electrode;

Pd evaporated onto A1 203 also study of C2D2

50.

6.

30.

--

46.

----_.-

18.

39.

48. I---_._-_.1979 I

1977

1967

1976

lETS

---

-----

Pd evaporated onto quart z, p<6*10-9 Torr

Pd evaporated onto NaCl, p;;10- 8 mbar

---------------------

Pd evaporated onto NaCl and KBr

1976

Pd evaporated onto quart z, p<6*10-9 Torr; also study of C2D2

3109, 1004

-

I

39. 1-------47. 1979

Pd evaporated onto quart z, p<6*10-9 Torr

Pd evaporated onto NaCl and KBr

10.

ref.

1972

year

Pd evaporated onto NaCl in 0.1 Torr CO; redshift of the C-O st retching mode due to the Lewis bases

--_.

remarks

---- ---------------------------

1860, 1805

(CDC1 )

wavenumber

IRAS/DB

~---

Int.R.

S02

_.

Int.R.

NO

lRAS/DB

TRIR

method

CO/(CH3)3 N CO/C 2!isNC

adsorbate

tv 0>

o

L---

co

Pt

I

~

CO/(CH3)3 N CO/C 2HsNC

._--

CO/H2 CO/02 CO/H20 CO/C 2H2

r--.--------

C5HsN

adsorbate

Pd

adsorbent

I

~t

--

TRIR

TRIR

TRIR

_

Pt evapor-ated onto glass, p<5*10- 9 mbar-

evaporat ed onto NaCl in 0.1 Tor-r CO

-------------------_._-_._-------_.-

2045 i

mbar-

Pt evaporated onto NaCl in 0.1 Torr CO; redshift of the C-O str-etching mode due to the Lewis bases

Pt evapor-ated onto NaCl, P~10-8

Pt evapor-ated onto NaCl in 100 mbar CO

10. -------56.

18.

17.

58.

5.

ref.

10.

18.

28.

----_._._-

1972 I

1967

1981

._----

1976

----_.-

1972

1967

Pt evaporated onto NaCl, p~10-8 mbar

1965

1984

1983

year

Pt evaporated onto CaF2 and CsI in 10-3 - 12 Tor-r CO

----_._-- ---_._------_._._---------_._---------------

lRAS/SB 120.80 - 2086 PEM

--_._---_.-

I 2040, 1850

2030

TRIR

TRIR

2053, 1840, 570, 477

TRIR

study of SEAS quenching of C5HsN adsorbed on Ag due to Pd

Pd evapor-ated onto silica in UHV, T=120 K;

Tor-r;

SEAS

remarks Pd evapor-ated onto quar-tz, P~2*10-5 different excitation wavelenghts

wavenumber (cm-1 )

SEAS

method

"" 01 ......

Cu

Pt

adsorbent

1RAS

C2H2 C2H4

-

mbap

mbap

Pt evapopated onto glass, p~2*10-5

1981

49.

----- -,--- - - - - - -

2105 2105

1RAS

TR1R

CO

Torp

--

------------ ------ ----------'-----------------------------------

Gu evaporated onto glass, p~2*10-8

---

Torr

Gu evapopated onto NaGl in UHV, T=113 K

Cu evaporated onto glass, P~10-9

study of vibrational overtones

evapopated onto Cu at 20 K, p<10- 10 Topp;

Torr

EELS

Torr

O2 N2

-

1982

6.

62.

9.

61-

--

-72.

5.

50.

- - - ------

1970

1970

1968

1982

1983

1982

-------------------- - - - - - -

Pt evapopated onto quaptz, p~2*10-5

1RAS/SB 2105, 2092

18.

-1967

59.

28.

1977

--

--

ref.

1981

--

year

---------------------- ----- --------

Torp

Pt evapopated onto glass, p~5*10-5

Pt evapopated onto quartz, p<6*10- 9 Topp

Pt evapopated onto NaCl, p~10-8

SERS

3105, 3069, 1015

-

remarks

Pt evapopated onto NaCl in 100 mbap CO; blueshift of the C-O stretching mode due to the halides

-

Pt evaporated onto glass. p~2*10-5

f----

SERS

-

(cm-1 )

wavenumber

SERS

C5HsN

SERS

TR1R

HCHO HCOOH

C2% ---

TR1R

method

CO/Cl2 CO/Br2

adsorbate

tv

tv

en

Cu

~sorbe~

HCOOH/DCOOD

HCOOH DCOOD

NO

co

adSorbate----r;;;~Od

lRAS/FT

l

I lRAS/DB I

I

lRAS/DB 11355 + TRIR 1350

2115

To~;---------r1976 _

0-6 Torr

Cu evaporated onto KBr, p~2*10-6

p~2*1

I Cu evaporated onto KBr, P~2*;0-6

-

Torr

Torr

Cu evaporated onto sapphire; 2 K < T < 425 K, P < 10-9 mbar. Thermal detection technique.

Sli;ht; -~~idiZ~~~

P<1~-8

Cu evaporated onto glaS; and 8 -+-P<10- Torr

glass,

evaporated onto glass, A1203 film and

onto glass in UHV

MgO film in UHV

I Cu

•

~_~~aporated

2102 -------Vu evaporated -~~to

i 2160,

1

I lRAS/DB I

I

~S

~~

lRAS/SB i 2102

h

07

--69:-~

I 1975 I

I 1973 I

14.

65.

14.

~1~73~

38.

64.

63.

ref.

~ ::j I 1975 I

1986

1975

1975

1973

Cu evaporated onto glass, p<2*10-8 mbar

lRAS/SB 12105

~~+1

1972

1971

I year

Cu evaporated onto glass, p<5*10- 8 Torr

Cu evaporated onto a tantal foil, p~3*10-8 Torr; frequency shift after oxidation of the copper film

remarks

lRAS/SB 12090 - 2119

lRAS/DB I 2105

{cm- 1 }

-r~~~enum~~--

""en w

-----

wavenumber (cm-')

C5HsN

remarks

---

Cu eva

--porated onto

glass, p~10-6

Torr

SERS

SERS

---------_.~

P~2*10-8-To;r-1

-_._----

800 °c (island formation)

revie _._--- ----_._--------Cu ev porated onto quartz and annealed at

--------

SERS

f-------

evapo ated onto Cu, T=15-150 K,

--------- ----

1601, 1217, 1012, 636

f------

SERS

evapor ated onto Cu, p<5*10-10 Torr, T=120-290

-

SERS

-- --------------------

evapor ated onto Cu in UHV, T=130 K

mbar

SERS

--------- 1-----

Cu in UHV, T=120 K

evapor ated onto Cu, T=4 K, P~3*10-10

- f-----

(PEM)

K

---------------------

p<6*10 -9 Torr; study of decomposition

f----

r-----

Cu eva porated onto glass and careful oxid izat1on; p<10-8 Torr -- ---1-----------------Torr SEW Cu eva porated onto glass, p~10-6 - - -f---------- 1------SERS evapor ated onto Cu in UHV, T=4 K 3068, 3050, 1176, 992, 608

IRES

IRASISB

I RAS IDB

method

--------------- ---------------- ---SERS evapor ated onto

C6%

HCOOH/0 2

HCOOH/DCOOD

adsorbate

----------------------- --------

Cu

adsorbent

,--

ref.

i

1984

1983

79.

90.

76.

75.

I 1983 I 1983

70.

73.

52.

74.

71.

--------

69.

-----

78.

40.

1983

1982

1983

1976 ---1982

--

1978

--

1983

1978

- - ------

---t--------

I year

N CJ1

...

Ag

Cu

adsorbent

-----.

N2

H2

°2

imidazole

-

EELS

EELS

I

II

4492, 4154

--

1286, 1053 838,815,68 7

-----

1053, 697

-----

-----

! 1596, 324,

EELS

1 - - - - - f---- - - -

~.

EELS

SERS

~----

SERS

~----

~~RS

~~-- +~~~-- -

SERS

~~---+;---

I

I

wavenumbe r (cm-1 ) -

-+-----

ATR/FT

IRES

SEW

cellulose acetate +

lRAS/DB

method

isoamyl nitroite

adsorbate

----_._-------- -_. __ __ ._--- ------

I

I

-

1983

Ag evaporoated onto

72.

85.

------

84.

----

77.

70.

--

1981

1982

1981

Ag evaporoated onto Cu, p<8*10- 11 Torr, T=20 K

Ag evaporoated onto Cu, p<10- 10 Torr, T=20 K

1982

Ag evaporoated onto Cu, p<10- 10 Torr, T=4 K -

-84.

72.

84.

129.

- ---- f - - - - - -

--------

Ag evaporoated onto Cu, p<8*10-11 Torr, T=20 K

109. ---- 1-----------------------Ag evaporoated onto Cu, P<5*10-10 Torr, T=120 K 1983 144.

1983

----------------- f--

--------------------- f - - - - - f-------Cu In UHV, T=130 K

1982

T=122 K

-71.

----_._.__ ._-------- f - - - - c-------

1981

1981

1983

Ag evaporoated onto Cu, T=20 K, p<10-10 Toror

-------

Ag evaporoated onto Cu In UHV, T=120 K

-----_._------------

Ag evaporoated onto Cu, p<8*10- 11 Torr, T=20 K

------------

1976

Cu evaporoated onto gla 3S, p;;;10-6 TorrCu evaporoated onto KBro , p;;;10-6 Toror

-----

-1975

Cu evaporoated onto KBro , P;;;2*10-6 Torr-

14.

ref.

year

---------------------- --_._-- _._-------

remark s

0> 0>

""

CO

adsorbate

L -_ _ _ _ I- _ _ _ _ _ _ _

Ag

adsorbent

2208, 2120, 2028, 1862

I

83.

82.

64.

9.

ref.

1983

1983

1982

--

I

I

1983

70.

7.

5.

88.

----

198t-' 85.

---{------

----t-------1981 I 84.

1980

1980

----+---_4_

Ag evapora ted onto Cu in UHV, T=130 K

1--------

Torr

ted onto quartz and Ag island film in UHV ( 150 K)

;~apora

Ag evapora ted onto glass, p~2*10-5

Ag

Ag evapora ted onto Cu in UHV, T=120 K

ted onto Cu, p<8*10- 11 Torr, T=20 K

1------Ag evapora ted onto glass

I

UHV, T=120 K

Ag evapora ted at T=11 K and p<10- 7 Torr

_ _ _ _ _ _ _ _ _ l-,_ _ _ _ _ _ _

2114, 1940

lRAS/DB SERS

2128, 2045, 2025, 1977 (90 K)

2112, 1940

SERS

I

-_.

Ag evapora ted onto Cu in

;i~evapoca- ----

C-;;42 (10K) -

2135, 160

SERS IRAS/DB

SERS

1---------

SERS

EELS

1------ ---------.

SERS

SERS

1972

Ag evapora ted onto glass, p<5*10-8 Torr

-

lRAS/SB 2130 - 2155

year

1970

remarks Ag evapora ted onto NaCl in UHV, T=113 K

---- f - - - - - - .

wavenumber (cm-')

2160

TRIR

method

-4

"" en en

Ag

adsorbent

,..----

SERS

SERS

HCN

H2O

S042SCN-

CN-

3355, 3127, 1611, 803, 421, 230

1---

Ag evaporated onto Cu, T=120 K, p~2*10-10 Torr; also study of H2180, D20

----_._._-

Ag evaporated onto Si (300*300 nm 2 structure)

--,-_._--

Ag evaporated onto aluminum in UHV, T=ll K

IRAS/DB

SERS

lRAS/DB

-2150

Torr;

-

adsorption from solution

Ag evaporated onto glass, p~2*10-5

Ag evaporated onto silica, T = 30

Torr;

.-

°c - 150 °c (island formation). Additional Au film evaporated onto Ag

Ag evaporated onto glass, p~2*10-5 adsorption from solution

I

1982 I

1983 I

1982 I

1982 I

1981

1982 I

51.

152.

51.

131.

128.

2.

172.

1986 I

-

108.

1983 I

Ag evaporated onto quartz and sapphire in UHV island formation at 390 K, "cold" film at 120 K Ag evaporated onto sapphire; 2 K < T < 425 K, p<10- 9 Torr. Thermal detection technique.

90.

ref.

1983 I

year I

review

-- f------ _._------- ---------------------------------------

-----

SERS

f---

2144

-----

2145

-----r-

lRAS/FT

SERS

EELS _.

(cm- 1 )

-- -.----- - - - - - - - r------------------------------wavenumber remarks method

CO/N 2

CO

adsorbate

"" C1 ....,

-940

-940

PAS

PAS

PAS

-1078

----- ------

-940

-940

PAS

f---.

PAS

~-

C2H4

C2D2

C2H2

--------------

HCOOH

IRES 2967, 289o - - - - - -----IRES -_.__._SF.:RS

----

SERS

1-----

IMS

SERS

----_. __ ._.

-----

-

2655,238: 1725,577, 514

3317,324: 1934,789, 635

---------------

·1

Torr, T=90 K

---

--year

°c

Ag evaporated at T=ll K and p<10- 7 Torr

Ag evaporated onto quartz, p<6*10-9 mbar; on mica epitaxial film (111)

Ag evaporated onto Cu in UHV, T=120 K

133.

91-

--- -----

1983

- - --

--

ref.

-----

--------~

81-

80.

;

1982

_._._--

83.

-----

--_.59.

153.

_._---

153.

_._----_.-

2.

------- _.._._.- _.__.-

- - - ---1980

1977

--

1983

-_._-- ---_._1983

(

----- ----_.- ----------

1975 --_._- -_._-1975

92. 1983 - - - ---- - - 143. 1983 - - ---143. 1983 --_._- ------ _._-------

----

--------------------------- - -

Ag evaporated onto Cu in UHV, T=120 K

Ag evaporated onto aluminum in UHV, T=ll also study of C2DZ

I film annealed in vacuum at 200

Ag evaporated in UHV, T=90 K

Ag evaporated in UHV, T=90 K

Ag evaporated in UHV, T=90 K

--

Ag evaporated in UHV, T=90 K

_._-

1982

--------------_._--------------- - -

remarks Ag evaporated at p~2*10-10

--+

---- -----method wavenumlber (cm-

----------- ------ ---------

NH3

SF6

adsorbate

1 - - - - - - - . - - - - - - - - - ___._

I

Ag

adsorbent 00

on ""

~.

Ag

c----adsorbent

-

C6~

trans-2-butene cis-2-butene isobutene 1-butene

SERS

K

UHV,

T=ll

-~-;:1~:~~1~:s

TO~~~=

K

P;;10-S-To~~------1

149.

.

------1 2. i

142.

70.

~0~9'

1980

1980

----

1981

1982

_~~~O

1980

1982

124.

95.

127.

.

I

I

I

2.!

~- ~

83. _~

----_ .,

-~-1~5

___________________________. l - - - . L - -

T=ll K; also study of C6D6

Ag evaporated onto aluminum in UHV,

I

onto aluminum,

onto aluminum in UHV, T=11 K

~to

at T=11 K and p<10-7 Torr

onto aluminum in

Ag evaporated onto silicon grating in UHV

Ag evapor T=ll K

Ag evapor

I

125.

r'ef.

-.------------.---------- ------ ---.-----1 i

1983

1983

1983

1983

1980

year

-+-

------t

T=130

onto Cu, T=120 K, p<6*10-10 Torr

-------------

--------'------------

--

--_._--

-- - - - -

Ag evapor.

1---------- 1 - - - - - - -

-

----- I--

SERS

SERS

SERS

SERS

~-----

SERS

SERS

Ag evapor. 3000, 2970 -_._- 1-------Ag evapor - -1---_._----Ag evapor

f-------

UHV,

Ag evaporated onto Cu, P<5*10-10 Torr, T=120 K

review

SERS

-

Ag evaporated onto Cu in

-

remarks

Ag evaporated onto aluminum, T=ll K, p;;10- 8 Torr

1331

--

(cm- 1 )

wavenumber'

SERS

SERS

SERS

SERS

method

-- f--

trans-2-butene

C3~

C2D4

~-------------

C2H4

adsor'bate

------------ r----.----- ......._---_._._.__.- r---.--------.--

so

tv

(Tl

Ag

adsorbent

3CN

C5HsN

r-----

-- H C4Hg° --CH

C2Hs°H

f-----------

CH 30H

C6 H12

f---------

C6Hci

~-

SERS

SERS

1036, 1006

SERS

-

remarks

----

--------

-

1982

year

1980

Ag evaporated onto silicon gr ti ng in UHV

10-5 Torr

-----

-------------_.__._--_._-- ---_._,_.------

p~5*

1980

Ag evaporated onto Cu in UHV, T= 130 K

94.

93.

_._-

1981

L-._ _ _ _ _ _ _ _

49.

-- - - - - -

95.

--------- --- f - - - -

1980

95.

120.

-- - - - -

1980

1974

Ag evaporated onto quartz in UHV ; island formation

Ag evaporated onto glass,

121.

119.

- - -------

1979

1971

---- -----

109. 1983 ----- - - - - 1980 95.

102.

2.

1.

---

----

ref.

------------ - - - --------

Ag evaporated onto silicon gr ti ng in UHV

----

Ag evaporated onto A1 203

----------------

-_._------

Ag evaporated onto silicon gratti ng in UHV

T=ll K; review

--1---------------------

-------------

---

Ag evaporated onto aluminum in UHV , T=ll K 1982 ---------- f--Ag evaporated onto silicon gra ti in UHV, 1982 100 K < T < 293 K

-- f----

3050, 1028, 988, 648

I---

2960, 2924, 2860, 2840, 2818

991, 984

wavenumber (cm-1 )

SERS

SERS

IRES

lETS

IRES

SERS

SERS

SERS

SERS

SERS

method

--'-------

adsorbate

tv

o '"

Ag

adsorbent

1

CSHsN

-----

adsorbate

SERS

--

T"80_~

; ~-; ~;'2'~~~"

SERS

1034, 100S,

SERS 1007, 992

991

Ag evaporated onto quartz; 370 K: island formation, lS0 K: "cold" film

1034, 1003, 990

SERS

---

Ag evaporated onto glass and sapphire, p=2-3*10- 7 Torr; SERS at 4 kbar

island formation, lS0 K: "cold" film

Ag evaporation onto quartz in UHV; 293 K:

-

Ag evaporated onto quartz in UHV, T=lS0 K

-------

SERS

1---

Ag evaporated onto quartz, T=lS0 K

SERS

evaporated onto

Ag evaporated onto glass, p~2*10-S

Torr

Ag evaporated onto aluminum in UHV, T=ll K

Ag evapor'at.ed onto Cu. p<3"0-'0 To".

=G

1034, 100S, 991

year

~------

1982

1982

1982

1982

1982

1982 -1982

1982

1981 -1982

1981

__._--

Ag evaporated onto Cu, p<6*10-10 Torr, T=120 K study of silver annealing

._._-- ---_.

1981

-_.-

.

__ __ Torr, T=120 K

-

Ag evaporated onto Cu, p~6*10-10

remarks

Ag evaporated onto Cu in UHV, T=120 K

1009

3120, 3110,

-

3032, 2983

(cm- 1 )

wavenumber

-------------,._--_._----_.----- ------_.

SERS

I~

. SERS

-

~~

!---

SERS

SERS

1---

---_._method

100.

99.

88.

87.

86.

73.

::=2°. =

6.

2.

--

--

98.

97.

96.

ref.

----_._--

>-'

""m

I

I

I

I

!

Ag

adsorbent

SERS

1035, 1005

Ag evaporated in UHV, T=90 K

SERS

Torr

-

390 K: island formation, 150 K: "cold" film

Ag evaporated onto quartz or sapphire in UHV;

-

evaporated onto Ag at 58 K in UHV; annealing between 58 K and 330 K

SERS

13033' 1596, 1008, 623

Ag evaporated onto Cu, T=120 K, p~10-10

-

Torr,

SERS

T = 15 K - 150 K

Ag evaporated onto Cu, p~2*10-8 review

1595, 1218, 1006, 624

Torr

Ag evaporated onto Cu in UHV, T=130 K

_.

Ag evaporated onto glass, p~2*10-5

SERS

SERS

SERS

SERS

76.

-'--

1983

1983

1983

I

I I

i

,

I

,

-"------

108.

-107.

106.

._- -~

1983

5.

--------1 1983 7~

1983

104.

102.

101.

ref.

90. 1983 - - - - ------~ 105. 1983

~-

1982

Ag evaporated onto Cu in UHV, T=130 K

1035, 1005, 994

SERS

--

1982

Ag evaporated onto silicon grating in UHV, T = 100 K - 293 K

island formation, 150 K: "cold" film

1032, 991

1982

year

SERS

Ag evaporated onto quartz in UHV; 360 K:

remarks

1030, 1000

(cm- 1 )

wavenumber

SERS

method

_________ ----c--.

C5HsN

adsorbate 0>

""

""

r

Ag

adsorbent

C5HsN

adsorbate

C5HsN/CO I-Cd-arachidate

i

SERS

SERS

I

SERS

SERS

SERS

f---

I

I

' 1044,

reenhancement after evaporation of Ag

I

I

I

Langmuir-Blodgett adsorbate films

Ag evaporated onto quartz in UHV, T=150 K Ag evaporated onto holographic grating;

Evaporation of Pd overlayer onto C5HsN

Ag evaporated onto glass: "cold" film at T=120 K, island film at room temperature

--+1------------

Ag evaporated onto Ag islands/sapphire (70

-j-----------------------

I

island formation, 70 K: "cold" film; Xe underlayer

evapo;;ted -onto sapphire in UHV; 370 K:

T------------------

T=120 K; submonolayer Al evaporated onto silver film

Ag evaporated onto quartz, p<10- 9 Torr,

-------t,~g

992

l 033

f------__t_ SERS

I

t

625

1034, 1006,

1598, 1215,

remarks

K)

Ag evaporated onto silica in UHV, T=120 K; quenching of SERS after evaporation of Pd,

--.,-----.-----

SERS

wavenumber (00- 1 ) review

1

SERS

--

method

~-

I

I

I

1980

1982

1985

~-1985

I

1

~=-f~ 1985

1984

1984

1983

year

112.

87.

171.

169.

111.

11O.

58.

109.

ref.

I

""co w

Ag

adsorbent

'------

I

...1...--

I

benzoic acid

Cd-arachidate

adsorbate

SERS lRAS/FT

lRAS/FT

method

I

I

lRAS/FT

lRAS/FT

lRAS

lRAS/FT

-'-

I SERS

i

Ag

grating;

Langmuir-Blodgett adsorbate films

evapo;~~~;;~to-hOlOgraPhiC

I

adsorbate films

evaporated onto glass, island formation; also study of Ag evaporation onto the adsorbate

_

1

I Ag

Ag evaporated onto glass; Langmuir-Blodgett adsorbate films

Langm,i,-Blodgett a"o,bate films

-J-

------+ ----------------~ Langmuir-Blodgett adsorbate films

L_____

I

;

II'

-J

1982

1982

year

1983

1 I

I

I

Ag evaporated onto gratin g, islands or smooth 1983 films; Langmuir-Blodget t adsorbate films. I Also study of Ag evaporation onto Cd-arachidate.

Ag evaporated onto quartz (grating, island and film); Langmuir-Blo dgett adsorbate films

Ag evaporated onto glass; Langmuir-Blodgett adsorbate films

remarks

---------

-+ - - - - - - - -

I

116.

115.

114.

113.

ref.

...J.

- l__

I 1980 I

1985

~~.--j

122.

163~

~-------: 1983! 118.

I

---------..,---------------------------- ---+--I Ag evaporated onto glass; Langmuir-Blodgett 1983 I 11~

I

1------

I

I

(cm- 1 )

wavenumber

UffiJon.

Uj0

PEM

SERS lRAS/SB

b-+

I

I

_

I

I

I

'

'

I

I

I

I

"'"

"" cr>

I

t

I

~-

Ag

adsorbent

I I

t

SEAS

SEAS

IRAS/DB

SEAS

ATR

SEAS

~--+--------

~~------

SERS

p-nitrobenzoic acid

I

I

t

I

remarks

--

--

1980

1982

1982

Ag evaporated onto teflon spheres on glass, p;1O- 7 Torr

Ag evaporated onto silica, p';;10-6 Torr, T=150 °c (island formation)

Ag evaporated onto glass

----

Ag evaporated onto a Ge prism, island formation; p;5*10-5 Torr

---------

-

formation at T=150 °c

Ag evaporated onto graphite and silica; island

132.

130.

1983

1983

1983

1982

--

-146.

145.

7.

140.

-- -----

1982

------------------ f - - - - - --

1982

--

Ag evaporated onto silica

103.

123.

130.

7.

130.

ref.

-- -_._----

1983

1982

yeaI'

--------------------------- ------ -----

Ag evaporated onto a A1 203 prism; Kretschmann configuration

Ag evaporated onto teflon spheres on glass, P';;10-7 Torr

-

Ag evaporated onto glas

Ag evaporated onto teflon spheres on glass, p';;10-7 Torr

------- --

1355

1415, 1350

1540 1--1340

1400, 1355

IRAS/DB

SEAS

1580, 1340

(cm- 1 )

wavenumber

SEAS

method

-- ----- ----------- ------------_._---------_._-------------------

phthalic acid

benzoic acid

adsorbate

Ol

CJl

""

'---..

Ag

adsorbent

1380, 1355

ATR

SERS

p-aminobenzoic acid

isonicotinic acid

1530, 1380, 1355

-

-- -------_._-----

SERS

pyridineSERS carboxylic acids

SERS

--

ATR

aminobenzoic acids

--

m-nitrobenzoic acid

1597

SERS

SERS

1596

SERS

p-ni t.robenzoi e acid

(00- 1 )

wavenumber

method

adsorbate

I

Torr, 151.

ref.

1983

~1

152.

-- ~--

1983

year

evaporated ont o glas s

Torr;

-------~-----

-------------

Ag evaporated ont o glas s; island formation

Ag evaporated ont o glas s

Ag evaporated ont-o a Al203 prism. Kretschmann configura tion

Ag

Ag evaporated ont o a Ge prism, P~5*10-5 island structur e

~

123.

122.

--

89.

--

---

_ _ _ _ _ L_._ _ _ _ _

1980

1982

1980

!

1982+8~

1984

Torr; Ag evaporated ont o a Ge prism, p~5*10-5 1984 155. island structur e ------------ - r-----r - - - - Ag evaporated ont o glas s , p~l 0- 7 mbar; 1985 i 168. . I island film. Ev aporat ion of Sb overlayer.

-

Ag evaporated ont o sili ~a at 30 - 50 °c (island formati on). Au evaporated additional

Ag evaporated ont o sili ~a, p~10-6 T=150 °c (islan d form at Ion)

r emarks

""en en

Ag

adsorbent

SERS

SERS

1,3,5-trifluorobenzene

rhodamin 6G

SERS

SERS

pyrazine

IRAS/DB

SERS

Torr.

--

Ag evaporated onto glass; island formation

Ag evaporated onto aluminum in UHV, T=11 K

--

Ag evaporated onto aluminum in UHV, T=11 K

Ag evaporated onto aluminum in UHV, T=11 K

IITAB: hexadecylmethylammoniumbromide

Ag evaporated onto glass, p~2*10-5

Ag evaporated onto polymer (320*320 nm 2 structure)

Ag evaporated onto glass; island structure

SERS

Torr;

Torr;

Ag evaporated onto holographic grating

Langmuir-Blodgett adsorbate films

Ag evaporated onto glass, P~2*10-5

Langmuir-Blodgett adsorbate films

Ag evaporated onto glass, P~2*10-5

remarks

SERS

(00- 1 )

--~-------------------

Ag evaporated onto holographic grating.

SERS

_.

-

wavenlJlDber

SERS

lRAS/DB

SERS

method

triazine

~._---

IITAB

polystyrene

stearic acid

adsorbate

~~-

112.

51.

50.

--

-----ref.

1980

1982

1983

1982

1982

1982

1982

122.

2.

150.

2.

-

---2.

51.

141.

126. r------1982 139. f-~---

1980

1980

1982

1982

year

~.-----

I

i

-J

"" m

Ag

adsorbent

phthalocyanine

tetrathiafulvalene

C6HsCH 2-

-

1420, 750, 508,500,432

SERS

°c

(island formation)

-

Ag evaporated onto phthalocyanine

T=150

1982

1983

Ag evaporated onto silica, p~10-7

SERS

Torr,

1982

1982

1982

1983

island structure

Ag evaporated onto silica, T=150 °c (island formation); adsorption from solution

Ag evaporated onto silica, T=150 °c (island formation); adsorption from solution

Ag evaporated onto Cr/silica; adsorption from solution

1983

1983

1982

year

SERS

SERS

SERS

RSSR, RSR

Torr,

Ag evaporated onto silica, p~10-6

SERS T=150 °c (island formation). Resonant Raman scattering RRS

Torr,

T=150 °c (island formation). Resonant Raman scattering RRS

remarks

Ag evaporated onto silica, p~10-6

SERS

R = C6Hs- or

wavenumber (cm- 1 )

SERS

SERS

method

crystalviolett

rhodamin 6G

adsorbate

137.

148.

136.

135.

134.

--

147.

15"

145.

140.

ref.

a> "" 00

Au

Ag

adsorbent

2110 2110-2115

TRIR

IRAS/DB

Ag evaporated onto the porphines adsorbed on CaF2

SERS

CO

Ag evaporated onto CaF 2 and on top of the adsorbate, p<10- 5 Torr.

SERS

158.

Au evaporated onto a tantal foil, p<3*10- 8 Torr 1972

167.

165.

156.

138.

170.

166.

164.

157.

ref.

9.

1985

1985

1984

1982

1985

1985

1985

1984

year

-~--r-----

1970

Au evaporated onto NaCI in UHV, T=113 K

Ag evaporated onto glass; adsorption from solution

-

porphines

1395

SERS

tetracyanoethylene

Ag evaporated onto aluminum at T=12 K in UHV

SERS

Ag evaporated onto CaF2 or Al on glass

Cu-phthalocyanine

Ag film evaporated onto Ag/glass.

diphenylhexatriene diphenyloctatetraene

SERS

SERS

Ag island film, substrate glass. LangmuirBlodgett adsorbate films.

SERS 1520,1331, 804

Ag evaporated onto glass/Sn02' Cu- and Zn-phthalocyanines

SERS

remarks

phthalocyanine

(cm- 1 )

wavenumber

method

adsorbate

os "" ec

Au

adsorbent 1972

Au evaporated onto glass, p<5*10-8 Torr Au evaporated onto KBr and NaCl

IRAS/SB 2110 - 2130

Int.R.

1982 1982 1983

Au evaporated onto glass, P~2*10-5

Ag evaporated onto Cu in UHV, T=120 K Au evaporated onto quartz, P;2*10-5 Torr Au evaporated onto Cu in UHV, T=130 K

SERS

SERS

SERS

SERS

1983

1982

Au evaporated onto glass, p;2*10-5 Torr

SERS

Torr

1981

Torr

Au evaporated onto glass, P~5*10-5

SERS

C5HsN 3105, 1014

1978

Au evaporated onto e.g. NaCl

1983

SEW

Au evaporated onto Ag island film / silica

SF6 KRe06

-2200

1986

1981

1980

SERS

Au evaporated onto sapphire; 2 K < T < 425 K, P < 10-9 mbar. Thermal detection technique.

Au evaporated onto Cu in UHV, T=120 K

2118

SERS

IRAS/FT

Au evaporated onto Cu in UHV, T=120 K

2118

SERS

1976

year

wavenumber (cm-1 )

remarks

method

CW

CO

adsorbate

70.

5.

73.

50.

6.

49.

159.

152.

173.

85.

82.

39.

64.

ref.

""c5

Al

Au

adsorbent

p~2*1

0- 6 Torr

1973

1600, 1360

IRAS/DB

Torr;

1975

also study of DCOOD

Al evaporated onto KBr, p~2*10-6

1600, 1360

IRAS/DB

HCOOH

1983

Au evaporated onto silicon; Langmuir-Blodgett adsorbate films

Torr,

SERS IRAS/FT

T=150 °c (island formation)

1983

1418, 748, 506, 488

1980

Au evaporated onto glass (island structure)

Au evaporated onto Cr/silicon; adsorption from solution. Resonant Raman scattering RRS

organic disulfides

SERS

SERS

1983

Au evaporated onto silica, p~10-7

SERS

tetrathiafulvalene

crystalviolett

1982

island structure of evaporated Au films

SERS

1983

Au evaporated onto Ag island film / silica

1980

isonicotinic acid

1597

Au evaporated onto glass (island structure)

SERS

1983

p-nitrobenzoic acid

Torr,

year

SERS

Au evaporated onto Cu, p~2*10-8 T=15K-150K

remarks

benzoic acid

1594, 1211, 1010, 630

(cm- 1 )

wavenumber

SERS

method

C5HsN

adsorbate

65.

14.

160.

147.

148.

136.

122.

152.

122.

76.

ref.

>-'

'-:l -J

In

Al

adsOl'bent

1983

p;10-7 Torr; adsorption fl'om solution

SERS

p-nitrobenzoic acid

1984

In evaporated onto glass/Sn02; Cu- and Zn-phthalocyanines In evaporated onto glass; Langmuir-Blodgett adsorbate films.

SERS

SERS

phthalocyanine

1985

1982

SERS

C6%

1520,1331, 804

1982

Al evaporated onto 8i/Si02; Langmuir-Blodgett adsorbate f ilrns

IRAS

1973

year'

Cd-arachidate

l'EIIlal'ks

p;2*10-6 Torr

(CIIl- 1 )

wavem.unbel'

lRAS/DB 1590, 1355

method

DCOOD

adsol'bate

164.

157.

1.

162.

161.

65.

l'ef.

..., "" ""

273

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

D.P. DiLella, J.-S. Suh and M. Moskovits, Proc. VIII Intern. Conf. Raman Spectrosc. ~' John Wiley &Sons 1982, pp. 63-64 M. Moskovits and D.P. DiLella, in: "Surface Enhanced Raman Scattering", R.K. Chang and Th.E. Furtak (eds.), Plenum Press 1982, pp. 243-273 P.A. Lund, R.R. Srnardzewski and D.E. Tevault, Chern Phys. Lett. 89, 508-510 (1982) P.A. Lund, D.E. Tevault and R.R. Srnardzewski, J. Phys. Chern. 88, 1731-1735 (1984) H. Yamada and Y. Yamamoto, Surf. SCi. ~' 71-90 (1983) H. Yamada and Y. Yamamoto, Proc. VIII Intern. Conf. Raman Spectrosc. ~' John Wiley &Sons 1982, pp. 73-74 H. Yamada, N. Tani and Y. Yamamoto, J. Electron Spectrosc. Relat. Phenorn. 30, 13-18 (1983) F.S. Baker, A.M. Bradshaw, J. Pritchard and K.W. Sykes, Surf. Sci. 1£, 426-436 (1968) A.M. Bradshaw and J. Pritchard, Proc. Roy. Soc. Lond. A316, 169-183 (1970) R. Queau and R. Poilblanc, J. Catal. 27, 200-206 (1972) R.M. Kroeker, P.K. Hansrna and W.C. Kaska, ACS Symp. Ser. 12? 203-212 (1981) R.M. Kroeker, P.K. Hansrna and W.C. Kaska, J. Chern. Phys. 72, 4845-4852 (1980) A. Bayman, P.K. Hansrna and W.C. Kaska, Physica B+C 108B, 1171-1172 (1981) M. Ito and W. Suetaka, J. Phys. Chern. 79, 1190-1193 (1975) R.M. Kroeker and J. Pacansky, J. Chern. Phys. 76. 3291-3294 (1982) H.L. Pickering and H.C. Eckstrom, J. Phys. Chern. 63, 512-517 (1959) C.W. Garland, R.C. Lord and P.F. Troiano, J. Phys. Chern. 69, 1188-1195 (1965) J.F. Harrod, R.W. Roberts and E.F. Rissrnann, J. Phys. Chern. 11, 343-352 (1967) H.C. Eckstrom, G.G. Possley, S.E. Hannum and W.M. Smith, J. Chern. Phys. 52, 5435-5441 (1970) P.K. Hansma, W.C. Kaska and R.M. Laine, J. Am. Chern. Soc. 98, 6064-6065 (1976) M.G. Wells, N.W. Cant and R.G. Greenler, Surf. SCi. 67, 541-554 (1977) S. de Cheveigne, J. Klein and A. Leger, J. Phys, Colloq. C6, 998-999 (1978) P.K. Hansrna, in: Springer Series in Solid-State Sciences Vol. 4 "Inelastic Electron Tunneling Spectroscopy", Springer-Verlag 1978, pp. 186-192 R.M. Kroeker, W.C. Kaska and P.K. Hansrna, J. Catal. 57, 72-79 (1979)

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