Conformational changes in chemisorbed CO on Ni(110) due to molecular interactions: An ESDIAD study

Surface Science 165 (1986)447-465 North-Holland, Amsterdam

447

CONFORMATIONAL CHANGES IN CHEMISORBED DUE TO MOLECULAR INTERACTIONS: AN ESDIAD M a r k D. A L V E Y , M i l e s J. D R E S S E R

CO ON STUDY

Ni(ll0)

* a n d J o h n T. Y A T E S , Jr.

Surface Science Center, Department of Chemistry, Universi(v of Pittsburgh, Pittsburgh, Pennsvh~ania 1526(], USA Received 16 June 1985; accepted for publication 23 July 1985

The surface bonding geometry of CO chemisorbed at 83 K on the Ni(110) surface was studied as a function of CO coverage using the ESDIAD technique. A study of the O + angular distribution from CO indicated that at CO coverages below about 0.75 CO/Ni. a normal orientation of the C - O bond exists. Above this coverage, CO species begin to be tilted approximately 19 ° away from the normal in a plane perpendicular to the Ni atom rows. The O + ion angular distribution evolves from a single normal beam to a split beam during this transformation. The yield of positive ions from chemisorbed CO was studied as a function of electron energy, Since a sharp threshold for O + emission occurs at the O(ls) core level energy, measurements above this threshold are particularly effective in observing the conformational transformation. In addition, smaller yields of O 2+ and CO + have been characterized as functions of the electron energy. The conformational behavior of CO has been correlated with thermal desorption spectra and LEED behavior for the overlayer. The work was carried out in a new ESD1AD analyzer which acquires digital ion angular distribution data. The new apparatus is able to eliminate a serious soft X-ray background present in all photographic ESDIAD measurements. Coherent accumulation of ESDIAD data and subsequent background subtraction yields excellent signal-to-noise ratios for ion angular distributions~ and facilitates quantitative analysis of ion beam orientations.

1. Introduction J

The study of the structure and the electronic properties of chemisorbed CO m o l e c u l e s o n t r a n s i t i o n m e t a l s u r f a c e s f o r m s a s i g n i f i c a n t t h e m e in t h e d e v e l o p m e n t o f t h e field o f s u r f a c e s c i e n c e [1]. M a n y s t u d i e s h a v e f o c u s s e d o n the properties of chemisorbed CO species on smooth, close packed transition m e t a l s u r f a c e s . T h e s e s t u d i e s a r e b e g i n n i n g to i n d i c a t e t h a t at h i g h s u r f a c e c o v e r a g e s , r e p u l s i v e i n t e r a c t i o n a l e f f e c t s b e g i n to d o m i n a t e in d e t e r m i n i n g t h e structural character of the chemisorbed CO. On smooth, close packed surfaces, these interactions may cause deviations of the CO bond orientation from the n o r m a l d i r e c t i o n , as f o r e x a m p l e i n t h e c a s e o f R u ( 0 0 0 1 ) [2]. * Present address: Department of Physics, Washington State University, Pullman, Washington 99164-2814, USA.

0039-6028/86/$03.50 © E l s e v i e r S c i e n c e P u b l i s h e r s B.V. (North-Holland Physics Publishing Division)

448

M.D. Alvev et al. / Conformation changes #7 CO on Ni(l lO)

The adsorption of CO on more open surfaces may be expected to exhibit similar interactional phenomena. Thus, in the case of a corrugated metal surface, such as the N i ( l l 0 ) surface studied here. it is possible that the interactional effects observed at high CO coverage may reflect the underlying s y m m e t r y of the substrate. Several recent investigations have been directed at determining the structure of CO chemisorbed on the N i ( l l 0 ) surface, using a variety of surface sensitive techniques [2-7]. For CO coverages less than 0 = 0.75 C O / N L it has been shown through angular resolved photoelectron spectroscopy [8] and E S D I A D [3,5a] that the CO bond axis is normal to the macroscopic plane of the Ni surface. For coverages which are higher, up to the limit of 0 = 1.0 C O / N i = 1.14 × 10 Ls cm 2 there has been strong circumstantial evidence that the CO axis tilts away from the surface normal [5]. Only very recently has this postulate been directly confirmed by Riedl and Menzel by E S D I A D [2,3]. This paper is in agreement with these measurements and extends the E S D I A D observations reported previously [2,3,5]. In particular, our results represent a refinement of the use of E S D I A D display methods in which the removal of serious photon background effects has been achieved [9]. The results reported here represent an extension of E S D I A D to a new level of measurement capability, permitting the observation of subtle conformational changes due to intermolecular interactional effects between CO adsorbate molecules at high coverages.

2. Experimental These experiments were conducted in an ultrahigh vacuum system described in a previous publication [10]. The base pressure was 3 × 10 ~J Torr, and the residual gases consisted primarily of At. A clean, well-ordered (by LEED) N i ( l l 0 ) surface could be obtained by Ar ion bombardment ( E = 1000V: 1 = 0.5/~A; time = 30 rain), followed by heating to 970 K at 8 K/s, and then cooling to 83 K. C, O, and S surface concentrations were below minimum detectable levels using a CMA Auger analyzer. Thermal desorption of CO achieved by temperature programming to 970 K was found to result in a clean surface free of C or O adsorbate species. The CO used for these studies was of reagent grade quality, stored in a glass bulb. All CO adsorption was carried out at a crystal temperature of 83 K, using a calibrated molecular beam doser, and all E S D I A D data were acquired at the same temperature. An important modification was made to the ESD1AD apparatus used for these experiments. The micro channel p l a t e / p h o s p h o r screen detector assembly was replaced by a micro channel plate/resistive anode detector system [11]. This detector system allows the positive ion angular distribution to be obtained electronically and to be stored digitally. Digital E S D I A D data collection has a

M.D. Alvey et al. / Conformation changes in CO on Ni(llO)

449

i

~z

"rod s : 8 3 K

dT/dt

=

2.1 K/s

/,~ cf!

(~1

E

I

I x Io-IOA

o

f

e d '~~"

I

200

I

I

300 400 Temperature / K

b

'

o I

500

Fig. 1. Thermal desorption spectra for CO on N i ( l l 0 ) for increasing exposure. Mass 28 was monitored as a function of thermocouple EMF. The crystal temperature linearly varies with time, with d T / d t = 2.1 K s -1. The coverages assigned to the spectra, in units of C O / N i , are: (a) 0.14: (b) 0.30; (c) 0.64; (d) 0.87; (e) 0.89; (f) 1.00.

clear advantage over the use of a phosphor screen detector which required that the screen be photographed for subsequent analysis. In particular, with the digitized system it is possible to remove a photon background effect present in all photographic E S D I A D measurements, resulting in excellent signal-to-noise ratios and in high contrast patterns. The photon background subtraction method is discussed in greater depth in another paper [9].

3. Results

3.1. Temperature programmed desorption for CO chemisorbed on Ni(11 O) The temperature programmed desorption spectra presented in fig. 1 were obtained with a quadrupole mass spectrometer (QMS) oriented such that there was line-of-sight from the crystal front surface into the QMS. In order to insure that stray electrons emitted from the QMS ionizer did not cause damage to the CO layer on the crystal, a bias of - 170 V was placed on the crystal [12]. It can be seen (fig. 1) that a desorption state, a2 forms first, and develops initially at a constant temperature of 422 K. As the CO coverage rises, the desorption state broadens on its low temperature edge. At an initial CO

450

M.D. Ah'ev et al. / Conformation changes in CO on Ni(l lO)

coverage above 0 = 0.64 C O / N i , and below 0 = 0.87 C O / N i , a second desorption state, designated ct1 is first observed. The cq state exhibits a constant peak temperature of 343 K. The desorption spectra in fig. 1 differ somewhat from previous work [4,7], but are in reasonable agreement with other studies [3,5a,13]. The difference is in the coverage where the high coverage cq - CO state first appears. In refs. [5a,6] this initial coverage for observation of ch - CO is slightly below 0 = 0.72 in agreement with the result reported here. For other studies, the critical coverage for cq initiation lies as high as 0 = 0.9 C O / N i [4,7]. The peak temperature for the two desorption states are in reasonable agreement with all of the previous work. Applying the Redhead equation [14] for first order desorption for each of these states, heats of adsorption of 27 and 22 k c a l / m o l are calculated assuming a first order pre-exponential factor of v o = 10 ]2 s J. Analysis by Falconer and Madix [13] yielded a value of 33 k c a l / m o l for c~2-CO while Behm et al. [4] calculate 2 0 - 2 3 k c a l / m o l for the al - CO state for ~0 = 1013-1015 s~ i Fig. 2 shows the coverage-to-exposure relationship for CO chemisorption on Ni(110) for two sets of data taken one year apart in our laboratory. The good agreement demonstrates our ability to reproduce surface coverages using an effusion source capillary array doser [10]. The exposure scale is derived from the measured absolute total flux of CO emitted from the doser and a calculated fraction of this flux which is intercepted by the crystal at its position 2 m m in front of the collimated array doser [15]. Because of the uncertainty in the position of the crystal in front of the doser and in the exact behavior of the

1.0

0.8 o

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0.6 •

<3

Experiment

I

0 Experiment

2

6

I

0.4

0.2

O0

4

8

12

Exposure / IOIScm -e Fig. 2. CO coverage versus exposure on Ni(110), Integrated thermal desorption spectra (fig. 1) are plotted as a function of exposure.

M.D. Alvey et al. / Conformation changes in CO on Ni(llO)

451

collimation of the doser, the exposure scale in fig. 2 may contain accuracy errors larger than its rather high precision of 4%. All of the CO coverages quoted in the remainder of this paper are interpolated from fig. 2 using exposures of known magnitude. The results shown in fig. 2, in which a linear function of CO coverage versus exposure is obtained over a wide range, are consistent with CO adsorption via a mobile precursor mechanism as reported by others [4,16].

3.2. Electron stimulated desorption for CO chemisorbed on Ni(l lO) 3.2.1. Data acquisition E S D I A D patterns of the positive ion angular distributions from chemisorbed CO layers were acquired as total counts received as a function of position on the resistive anode detector. Digitization of the x and y coordinates of the signal imposed a 128 × 128 channel matrix on an active detector area of 35 m m × 35 mm. Thus the digital resolution was 13.4 c h a n n e l s / r a m 2. This is ample resolution for E S D I A D patterns whose linear spatial extent was typically 10-20 m m on the detector. It should be noted that an ion compression field was necessary to limit the solid angle of the trajectories to match the acceptance angle of the detector. The compression was achieved by applying a bias of +100 V to the crystal during E S D I A D measurements. The term collection time used here (figs. 5 - 7 and 9) was an estimate of the total integrated time the electronics were able to respond to incoming ions, and not to the clock time required to do an experiment. It is important to reproduce collection times for proper data comparisons. All of the E S D I A D patterns shown here have been corrected for a reproducible, well behaved photon background which is present in E S D I A D [9,17]. 3.2.2. The dependence of E S D I A D patterns on CO coverage Fig. 3 shows the different methods which are used to display E S D I A D results in this work. Also, the orientation of the N i ( l l 0 ) surface with respect to the square detector edges is shown, as determined by L E E D measurements in the apparatus [9]. Fig. 3A shows a hard sphere model of the fcc N i ( l l 0 ) surface. The [001] direction is inclined 50 ° with respect to the horizontal x-axis. At high CO coverages, a two beam E S D I A D pattern is observed, and fig. 3B shows a contour diagram for Oco = 1.00 on the N i ( l l 0 ) surface. These measurements were made with an electron beam energy of 1000 V. The closed paths are contours of constant counts per channel as indicated. As shown here, a line through the two maxima is parallel to the [001] direction, inclined 50 ° from the horizontal. Fig. 3C is an average profile of the same data shown in fig. 3B. Ten channels extending perpendicular to, and centered about the dashed line ab in fig. 3B are averaged together to produce one averaged profile channel. The

452

M.D. Ah~'er et al. / Conformation changes in CO on Ni(llO)

ESDIAD PATTERN CONTOUR

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/

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Fig. 3, Various methods of displaying ESDIAD data. Comparison of (A) and (B) shows that the two beam ESDIAD pattern is oriented parallel to the [001] axis of the Ni(ll0) surface,

process is repeated along line segment ab for the 128 channels necessary to produce the profile in fig. 3C. This averaging and subsequent 5 point smoothing gives a profile that is representative of the symmetry of the pattern along the [001] direction. The vertical axis in fig. 3C is the average count rate, while the horizontal axis is distance along the line segment, ab. Fig. 3D is a three-dimensional projection of the data. The diamond shaped base is at the zero count level after correction for the photon background, and represents the square configuration of the detector plate. The vertical scale represents the total ion counts received in each channel. Each of the profile lines shown is 128 channels in length, and is an average of 5 profiles parallel to and centered on the profile line drawn. E S D I A D patterns 3B and 3D were both smoothed using a two-dimensional, 25 point quadratic least-squares smoothing function described previously [9]. The dramatic dependence of E S D I A D behavior on electron energy for C O / N i ( l l 0 ) is shown in summary form in fig, 4. Here, E S D I A D patterns at two CO coverages on N i ( l l 0 ) are shown for two electron energies, E e = 1000

M.D. Alvey et al. / Conformation changes in CO on Ni(llO) 1800

453

Counts

O o-- o. o

O o:,.oo

I Ee:,OOOeVi

Ee:,OOO~V]

Fig. 4, E S D I A D patterns obtained at two conditions of coverage of CO and for two electron beam energies on N i ( l l 0 ) . This summarizes E S D | A D patterns from CO on N i ( l l 0 ) . (A) and (B) are for 0c,o = 0.50 and 1.00 respectively for electrons of 1000 eV energy; (C) and (D) are for the same CO coverages for electrons of 300 eV energy.

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Fig. 5. ESDIAD profile development for CO on N i ( l l 0 ) . The upper set of profiles was taken at 1000 eV. The lower set was taken at 300 eV.

454

M.D. Alvey et al. / Conformation changes in CO on Ni(llO) ,

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Fig. 6. Integrated intensity of pattern profiles for CO on Ni(110). The total number of counts in the profiles in fig. 5. are plotted as a function of the coverage of CO.

eV and E~ = 300 eV. At 0c.o = 0.50 C O / N i , a single normal E S D I A D beam is seen at both electron excitation energies. As the coverage of CO increases to 0co = 1.00 C O / N i , the normal beam is converted into the two beam pattern shown in fig. 4B. This two beam pattern is evident for 1000 eV excitation, but is almost invisible for E~ = 300 eV (fig. 4D). In fig. 4B, the two lobes of positive ion emission are centered on the [001] azimuth of the N i ( l l 0 ) surface. This result verifies work done recently by Riedl and Menzel [2,3]. The dependence of the E S D I A D patterns on CO coverage are shown in more detail in fig. 5 for the two electron energies, 300 and 1000 eV. The profiles are obtained in the [001] azimuth as described in fig. 3C. CO coverage increases from left to right in the figure. For E~ = 1000 eV, it can be seen that the shape of the E S D I A D pattern changes from the single normal beam structure to the two beam structure in the coverage range, Oco = 0.66 to 0.82 C O / N i . In the same coverage range for E~ = 300 eV, a rather sharp decrease in total ion yield is observed, and the splitting of the beams can barely be seen. Fig. 6 shows the area measured under the profiles plotted in fig. 5 and is therefore a relative measure of the total positive ion yield as a function of CO coverage. For 1000 eV excitation, the ion yield increases to Oc.o = 0.6, then decreases to 0co = 0.85, then increases to Oc:o = 1.0. For 300 eV excitation, a similar pattern is observed except that the increase in ion yield above 0c,o = 0.85 is not observed. It is well k n o w n that in addition to the production of positive ions, ESD processes with higher cross sections can destroy adsorbed species during electron b o m b a r d m e n t . Since we are measuring E S D I A D patterns for various surface coverages, it is important to determine that the process of E S D I A D measurement does not involve appreciable destruction of the CO overlayer during the period of electron b o m b a r d m e n t used to acquire the ion angular

M.D. Aluey et aL / Conformation changes in CO on Ni(l lO)

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Distance Fig. 7. ESDIAD pattern average profiles versus extent of electron bombardment for CO on Ni(ll0). The values of electron fluence, Ne, are indicated to the right of each figure.

distribution data. This was tested by bombarding a full-coverage CO layer for long periods while measuring the change in the E S D I A D pattern. It can be seen in the profile patterns of fig. 7 that ESD causes the production of the normal ion emission pattern at the expense of the two beam pattern. This effect is followed by a general loss in E S D I A D intensity as extensive electron b o m b a r d m e n t takes place. The total fluence of electrons responsible for the change observed in going from the initial pattern to the second pattern (1.12 x 1017electrons/cm 2) is about 100 times greater than the electron fluence used to acquire E S D I A D patterns shown in this paper. This indicates that electron beam damage effects are unimportant during the acquisition of E S D I A D data for this system. An evaluation of the cross section for ESD destruction of the CO layer can be made by monitoring the positive ion yield as a function of the electron fluence to the adlayer [18]. In this analysis, the positive ion yield is considered to be linearly dependent upon the CO coverage. By plotting the natural logarithm of the ion yield (total area, fI d x ) observed under the profiles of fig. 7 for the last six profiles versus electron fluence, a total cross section for the loss of ion-producing species can be calculated. The derived cross section is -- 4.8 x 10 18 cm 2. This value is dependent upon our estimate of the electron beam cross sectional area of 1 m m 2. The total cross section derived above may be used to calculate the fractional

M.D. Alu£v et a L / Conformation changes in CO on Ni(l lO)

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x

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Fig. 8. Determination of ESD total cross section for CO on Ni(110). This is a semi-log plot of the data in fig. 7, used to extract the total cross section of 4.8×10 is cm z for CO on Ni(ll0). electron stimulated damage done to a monolayer of CO on Ni(110) during the acquisition of the E S D I A D data shown in this paper. The da.mage is approximately 0.005 monolayer during measurement, and is therefore considered to be negligible.

3.2.3. The dependence of E S D 1 A D patterns on electron excitation energy F r o m our observations of the E S D I A D profile development at two different electron energies as shown in figs. 4 and 5, it is clear that significant effects occur in pattern intensification as the electron energy is increased. This has been investigated in detail as shown in fig. 9, and indicates that the pattern profile for 0~.o = 1.00 varies with E e. It can be seen that as the electron energy decreases from 1000 eV, the ion yield decreases but the symmetry of the pattern remains invariant. We observe at all electron energies the two beam ion pattern characteristic of full coverage of CO on N i ( l l 0 ) . The ratio of intensity in the center of the ion pattern to the average intensity of the two off-normal beams remains at about 0.25-0.20 throughout the range of measurement at different electron energies, An inspection of the two beam ion angular distribution pattern indicates that the majority of the intensity in the normal direction, midway between the two beams, is due to overlap of the two beams in the field-compressed E S D I A D display. In fig. 10, we show the results of a study to analyze the ion composition responsible for the E S D I A D patterns reported here. Positive ions desorbed from the C O layer were mass analyzed using the quadrupole mass spectrometer (QMS) which in this case is operated with its ionizer source turned off. Three positive ions were detected: O + ( r n / e = 16); 0 2+ ( r n / e = 8); and CO + ( r o l e = 28). For O + and O 2~ no focusing into the Q M S was necessary to obtain a m a x i m u m signal. However, for CO +, a - 3 V potential difference between the

M.D. AIvey et aL / Conformation changes in CO on Ni(llO)

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Distance Fig. 9. ESDIAD pattern average profile versus electron beam energy for CO on Ni(ll0). The values of the electron beam energy are indicated to the right of each curve. ecoll.oo

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Fig. 10. Threshold measurements for ESD for CO on Ni(]10), 0co = 1.00.

458

M.D. Ah~cv et aL / Conformation changes in CO on Ni(l lO)

crystal and the aperture was necessary to achieve a m a x i m u m in signal. A saturated CO layer (Oc() = 1.00) was prepared for each point in fig. 10. For all three ions, the observation with the Q M S indicated that the positive ion signals increased to a m a x i m u m in the initial stages of measurement and the ion intensities shown in fig. 10 represent the values measured at the maximum. Each of the three positive ions is associated with different threshold values. For O +, two thresholds are observed in these measurements at 340 and at 540 eV. For O 2+, a threshold at about 650 eV is observed. For CO +, a threshold in the vicinity of 300 eV is seen. These threshold estimates are c o m p a r e d with the appropriate core level excitation energies ( O ( l s ) = 538 eV; C ( l s ) = 288 eV for the free atoms) [19]. We see that the second threshold for O + is closely associated with O(ls) excitation. The threshold for CO + may be correlated with C(ls) excitation. It is also clear that the 0 2+ threshold is about 100 eV above the O ÷ threshold. 3.3. Quantitative ion angular distribution measurements using E S D I A D ; measurement of surface orientation

the

All of the E S D I A D data shown to this point were obtained with a compression field produced by the use of a + 100 eV potential on the Ni(110) crystal relative to the first analyzer grid. This is a c o m m o n procedure used with

O C= I.OO0

t

~craystal

crystal

@ ESDIAD PATTERN Ton a =AX/R

I

ESDIAD PATTERN Ton J3 =AR/R

Tan /9 = Z~R Tan a/Z~X Horizontal Angular Calibration (No ion compression field)

Measurement of Tilt Angle (for ions from crystal normal)

/9=19.0+_0.5 ° Fig. 11. Measurement of CO tilt angle on Ni(110). The tilt angle is determined using the indicated functions and measurements.

M.D. Alvey et al. / Conformation changes in CO on Ni(llO)

459

ESD1AD display analyzers [20], to cause all of the available pattern to be observed by compressing ion trajectories toward the normal axis of the hemispherical grid system. In order to make true angular distribution measurements of the ion trajectories from the surface, it is necessary to operate the analyzer without the compression field. Fig. 11 shows the method by which the angular displacement of the two lobes of ion intensity from the normal to the N i ( l l 0 ) crystal was determined. Two ion patterns for 0co = 1.00 were determined at two crystal rotation angles which differ by a. The midpoint between the two lobes was observed to move by a distance Ax, allowing a calibration of the displacement on the detector with respect to the angle for the ion optical system used here. Geometrical measurement of the distance between the midpoint of the two lobes of ion intensity and the center of each lobe yielded a value for AR as shown in the right hand frame of fig. 11. The formula, fl = t a n - l ( A R tan a/Ax), permits the calculation of/3, the angle of inclination of the positive ion lobe from the normal. From these measurements, /3 = 19.0 _+ 0.5 °. The plane of the tilt angle, /3, is accurately aligned parallel to the [001] crystallographic direction. A similar quantitative measure of/3 = 19 ° has been made by Riedl and Menzel [2,3] using a mass spectrometer for detection of the two ion beams.

4. Discussion

4.1. Adsorption behavior," CO/Ni(llO) The temperature programmed desorption data of figs. 1 and 2 clearly indicate that CO adsorbs on N i ( l l 0 ) by means of a mobile precursor mechanism, in agreement with the work of others [4,16]. This adsorption mechanism results in a series of incommensurate L E E D structures which have been seen by Behm et al. [4] and are reproduced in fig. 12. During the production of CO layers of higher and higher coverages, three stages of desorption state development are observed in our work as well as in the work of others. Initially an ch is observed with a constant desorption peak temperature of 422 K. As is often seen for CO desorption from transition metals, higher CO coverages produce a broadening of the desorption state on its low temperature edge, possibly due to variations in the pre-exponential factor for first order desorption [21], or to a decrease in the desorption energy, or to both factors combined [21]. The broadening stage is followed by the development of an a~-CO desorption state exhibiting a constant peak maximum of 343 K. In our work, the initial development of a~-CO corresponds closely to major changes in the E S D I A D pattern, and we believe that the correspondence

460

M.D. Alvey et al. / Conformation changes in CO on Ni(llO)

c(8x2)

c(4x2)

( 2 x l ) plgl

I

[ooi] 0CO = 0 . 6 2 5

Oco= 0 . 7 5

Oco = 1 . 0 0

Fig. 12. Proposed structures of CO on N i(110) interpreted from LEED measurements. This figure is taken from ref. [4].

between CO configurational change and T P D behavior is quantitatively correlated. The LEED measurements of Behm et al. [4] show that the c(4 x 2) overlayer structure is complete at Oco = 0.75 C O / N i . Above this coverage coverage the p(2 x 1) p l g l LEED pattern begins to form and the E S D I A D behavior begins to convert from the single beam pattern to the double beam pattern. In ref. [4] the critical coverage for observation of the cq desorption state is at about Oco= 0.9 C O / N i , somewhat in disagreement with this and other work, EELS measurements on this system have been carried out by Gurney and Ho [7]. These measurements indicate that terminally-bound CO species exist at CO coverages when only the ~2 state is observed in thermal desorption. The CO stretching frequency for these terminally-bound CO species ranges from 2015 to 2065 cm ~ depending on coverage. The species convert to predominantly bridge-bonded CO when the ~j-CO state is evident, and these bridge bonded species exhibit a coverage-dependent CO stretching frequency that ranges from 1910-2000 cm 1. It can be seen from the overlayer structures drawn in fig. 12 that increasing coverage is consistent with a conversion from terminal-CO to bridged-CO molecules, but that incommensurate structures are involved below the final close-packed structure. The rehybridization from terminal-CO to bridged-CO with increasing CO coverage is just the opposite of that seen on the smoother N i ( l l l ) surface, where bridged-CO species form first and are then partially converted at high coverages to terminal CO species [22]. This opposite behavior for two crystal faces of the same metal illustrates the subtle structural and electronic effects which are at work as a mobile precursor CO species forms a chemisorbed entity.

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4.2. Dependence o f positive ion yields on electron energy

An important paper by Netzer and Madey [23] has suggested that the decrease in yield of positive ions in ESD as adsorbate coverage rises above a certain intermediate coverage may be due to intermolecular quenching effects. This effect has been seen on N i ( l l l ) for CO adsorbate species and has been ascribed to a "through-space charge exchange" between neighboring CO molecules. In the work described in this paper for N i ( l l 0 ) plus CO, a similar effect has been seen as coverage increases past Oco = 0.55 (figs. 5 and 6) for an electron excitation energy E e = 300 eV. This effect has also been reported by others [5a,2,3]. This effect is seen to produce a larger fractional decrease in ion yield for E~ = 300 eV than for E c = 1000 eV (fig. 6). The quenching effect would be expected to be more significant for valence level excitations than for core level excitations. Hence, as recommended by Houston and Madey [24], for monitoring E S D I A D behavior at high coverages, it is important to work above the critical core level excitation energies for the adsorbate. This has been achieved in this work, and provides the explanation for the lack of an observation of the two-beam ion pattern at high CO coverages in our earlier study [5a,6]. It is anticipated, in agreement with Riedl and Menzel [3], that the use of higher excitation energies will lead to an extension of E S D I A D to a number of adsorbate systems which yield low positive ion signals at high coverages. The O + yield data (fig. 10) also show a threshold at about 340 eV, some 50 eV above the C(ls) excitation energy. This could correspond to the energy necessary to produce an initial shake-up state on the C(ls) excitation, through which the events leading to desorption of O + proceed [25]. Fig. 10 also shows that 0 2+ ions are produced in ESD of CO on Ni(ll0). The threshold for this process differs from that for O + by about 100 eV, showing that the O + process is not connected to the, O 2+ process. We cannot think of the O + ion as originating from 0 2+ via some partial neutralization channel, at least not in the threshold region. Such processes could occur at excitation energies well above the threshold [26]. Our data are not sufficiently well behaved for CO + measurements to be sure that there is a threshold for CO + linked to C(ls) excitation. It can however be seen that the fractional yield of CO + is relatively small at 1000 eV excitation energy ( ~ 0.1 or less) compared to O +. If the ion angular distributions of CO + and O + differ significantly, the interference of the two in the E S D I A D display measurements will be of minor importance. The same conclusion can be applied to interference between 0 2 + and O +. 4.3. Conformational change in C O bond orientation as C O coverage increases

We assert in this work, consistent with previous interpretations, that the ion angular distribution observed in ESD is primarily a reflection of the orienta-

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tion of the chemical bond being ruptured by the electronic excitation process(es). This assertion is tempered by two factors which can have a minor influence on ionic trajectories in the vicinity of a surface: (1) the influence of the interaction of the positive ion with its image in the substrate; (2) the influence of ion neutralization processes which may be more effective for ions having higher angles away from the normal than for ions more normally directed. The two processes will have an opposite effect in modifying ion angular distributions. The image effect [27] will increase the observed angle of the trajectory from the normal. The neutralization effect will be most important for ions at high angles, causing the ion angular distribution to be more strongly peaked closer to the normal direction [18]. Riedl and Menzel [2] have concluded that these two opposing effects are both small in the case of the tilted CO patterns from Ni(ll0). We therefore conclude, in agreement with Riedl and Menzel [2,3], that the angle of tilt for CO from the normal direction is near the experimentally measured value of 19 ° as measured in both laboratories. The measurements of Riedl and Menzel were made on O ÷ ions using an apertured QMS. In both cases, the CO tilt angle occurs accurately along the [001] azimuth, in agreement with previous LEED measurements [28,4]. In our study the tilting of the C O bond axis begins near a coverage of 0.75 C O / N i (fig. 5) corresponding to the completion of the c(4 × 2) overlayer (fig. 12). We therefore associate the tilting configuration with the final stages of packing of CO into the adsorbed layer as the p(2 x 1) p l g l structure begins to form. It is interesting to note that the angle of tilt is coverage independent. This may be seen by inspection of the final 1000 eV patterns in fig. 5, and has also been verified by a more detailed inspection of the experimental data. The absence of an intermediate set of tilt angles argues for island formation of the p(2 x 1) structure rather than a continuum of surface structures with variable tilt angles [29]. Similar conclusions about tilting of diatomic species at high coverages have been reached by a number of previous investigations. In particular, a tilting NO species has been observed on Ni(111) using EELS methods [30]. A tilted CO species has been detected on the Pd(210) surface using ESDIAD [31]. A tilted CO species has been observed on Pt(ll0) using angular resolved photoemission [32,33]. A previous investigation of the C O / N i ( l l 0 ) system [5a] has given indirect evidence that tilting of the CO axis occurs above Oco = 0.75 C O / N i . This work involved the study of the surface Penning ionization spectrum of CO as a function of surface coverage. It was shown that above the critical CO coverage the relative and absolute intensity of electron emission from the 40 molecular

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orbital began to decrease as CO coverage increased. This effect was postulated to be due to the decrease in distance between the 40 orbital and the surface which influenced the competition between the Penning process and a surface ionization process involving the He metastable probe atom. Comparative studies of Ni(111) and Ni(ll0) have shown that this effect on the intensity of 4o orbital emission does not occur in the case of CO adsorption on Ni(111) [34,35]. Thus we have strong evidence that an inclination of the C - O axis occurs at CO coverages above Oco---0.75 C O / N i . This inclination occurs in a specific crystallographic direction in contrast to similar effects seen on smoother surfaces where no azimuthal preference of tilt angle is observed at high CO coverage. The results for C O / N i ( l l 0 ) are consistent for a number of physical measurement techniques, namely, ESDIAD, LEED, HREELS, SPIES, and TDS.

5. Summary This work has shown the following characteristics of the CO/Ni(110 chemisorption system: (1) Below a CO coverage of approximately 0.75 C O / N i , the C - O axis is perpendicular to the Ni(110) surface and exhibits a normal positive ion angular distribution in ESDIAD. (2) At CO coverages above 0.75 C O / N i , a tilting of approximately 19 ° begins to occur for CO molecules as repulsive intermolecular interactions build up. This tilting occurs in the plane perpendicular to the rows of Ni atoms on the Ni(ll0) surface. This result quantitatively verifies recent work done by Riedl and Menzel [2,3]. (3) During the transition between normally oriented CO molecules and tilted CO molecules, intermediate tilt angles are not observed, indicating that the tilting is not caused by multiple C O - C O interactions which are a result of an average over many neighbor interactions. (4) For excitation energies below the O(ls) core level, quenching effects have been observed at CO coverages above about 0.55 C O / N i which prevent the observation of the tilting conformational change with ease. By operating above the O(ls) threshold, strong O ÷ ESDIAD beams are observed over the entire CO coverage range. (5) This study has employed an enhanced ESDIAD technique for digital analysis of ion angular distributions. The enhanced digital method also permits the removal of a serious background effect due to soft X-rays which is inherent in all photographic ESDIAD measurements.

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Acknowledgements We gratefully thank AFOSR for support of this work under contract 82-0133. M.J. Dresser would like to thank the administration of Washington State University for the professional leave to pursue these investigations, and the University of Pittsburgh Surface Science Center for partial support. Helpful assistance from Mr. Kurt Kolasinski is gratefully acknowledged. In addition, we wish to thank Dr, T.E. Madey, NBS, for suggesting that a study of the effect of electron energy could be useful in this ESDIAD investigation.

References [1] J.T. Yates, Jr., T.E. Madey and J.C. Campuzano, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis. Vol. 3A, Eds. D.A. King and D.P. Woodruff (Elsevier. Amsterdam. 1985). [2] W. Riedl and D. Menzel, Surface Sci., submitted. [3] W. Riedl and D. Menzel, in: Desorption Induced by Electronic Transitions, Proc. DIET II Conf., Eds. W. Brenig and D. Menzel (Springer, Berlin, 1985) p. 136. [4] R.J. Behm, G. Ertl and V. Penka, Surface Sci. 160 (1985) 387 (see this paper for a complete listing of C O / N i ( I 1 0 ) references). The full coverage of CO on N i ( l l 0 ) involves 1 C O / N i surface atom = 1.14 × 1015 C O / N i according to the LEED results in this and earlier papers (for example, ref. [28]). [5] (a) J. Lee, J. Arias, C. Hanrahan, R. Martin, H. Metiu, C. Klauber, M.D. Alvey and J.T. Yates, Jr., Surface Sci. 159 (1985) L460. (b) B.J. Bandy, M.A. Chesters, P. Hollins, P. Pritchard and N. Sheppard, J. Mol. Struct. 80 (1982) 203: also see refs. [7,28]. [6] J.T. Yates, Jr., C. Klauber, M.D. Alvey, H. Metiu, J. Lee, R.M. Martin, J. Arias and C. Hanrahan, in: Desorption Induced by Electronic Transitions, Proc. DIET II Conf., Eds. W. Brenig and D. Menzel (Springer, Berlin, 1985) p. 123. [7] B.A. G urney and W. Ho, J. Vacuum Sci. Technol. A3 (1985) 1541. [8] D. Rieger, PhD Dissertation, Universiti~t, Mgnchen (1984). [9] M.J. Dresser, M.D. Alvey and J.T. Yates, Jr., Surface Sci. in press. [10] C. Klauber, M.D. Alvey and J.T. Yates, Jr., Surface Sei. 154 (1985) 139. [11] The resistive anode and position computer were manufactured by Surface Science Laboratories, Inc. [12] This necessity was reported by us in ref, [10] in which N H 3 (a very sensitive molecule to ESD beam damage) could be maintained in an undamaged condition using this procedure. Because of the lower damage cross section for C O / N i ( l l 0 ) (fig. 8), the bias procedure was less necessary. [13] J. Falconer and R. Madix, Surface Sci. 48 (1975) 393. [14] P.A, Redhead, Vacuum 12 (1962) 203. [15] C.T. Campbell and S.M. Valone, J. Vacuum Sci. Technol. A3 (1985) 408. [16] Other examples of CO adsorption via a mobile precursor on other transition metal surfaces are available; see: E.D. Williams, P.A. Thiel, W.H. Weinberg and J.T. Yates, Jr., J. Chem. Phys. 72 (1980) 3496; P.A. Thiel, E.D. Williams, J.T. Yates, Jr. and W.H. Weinberg, Surface Sci. 84 (1979) 54. [171 H. Niehus and B. Krahl-Urban, Rev. Sci. lnstr. 52 (1981) 56. [18] T.E. Madey and J.T. Yates, Jr., J. Vacuum Sci. Technol. 8 (1971) 525.

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[28] [29]

[30] [311 [32] [33] [34] [35]

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T.A, Carlson, Photoelectron and Auger Spectroscopy (Plenum, New York, 1978) p. 338. T.E. Madey and J,T. Yates, Jr., Chem. Phys. Letters 51 (1977) 77. P.A. Thiel, E.D. Williams, J.T. Yates. Jr. and W.H. Weinberg, Surface Sci. 84 (1979) 54. M. Trenary, K. Uram and J.T. Yates, Jr., Surface Sci. 157 (1985) 512 (see references to other work therein), F.P. Netzer and T,E. Madey, J. Chem. Phys. 76 (1982) 710. J.E. Houston and T.E. Madey, Phys. Rev. B26 (1982) 554. D.E. Ramaker, J. Chem. Phys. 78 (1983) 2998. P.J. Feibelman, Surface Sci. 102 (198l) L51. T.E. Madey, in: Inelastic Particle-Surface Collisions, Eds. E. Taglauer and W. Heiland (Springer, Berlin, 1981) p. 80; W.L. Clinton, Surface Sci. 112 (1981) L791. R.M. Lambert, Surface Sci. 49 (1975) 325. The two beam ESDIAD patterns we obtain at 0co > 0.82 C O / N i consistently exhibit a difference in intensity. Observations at various ion compression fields (including zero field) suggest that this difference is not due to distortions in our measurement system. The effect is most likely due to a physical property of the specific Ni crystal used, or to specific geometrical effects of the electron beam angle of incidence on the tilted CO species. S. Lehwald, J.T. Yates, Jr. and H. Ibach, in: Proc. 4th Intern. Conf. on Solid Surfaces [Suppl. Le Vide, Les Couches Minces 201 (1980) 221]. T.E. Madey, J.T. Yates, Jr., A.M. Bradshaw and F.M. Hoffman, Surface Sci. 89 (1979) 370. S.R. Bare, K. Griffiths, P. Hofmann, D.A. King, G.L. Nyberg and N.V. Richardson, Surface Sci. 120 (1982) 367. D. Rieger, R.D. Schnell and W. Steinman, Surface Sci. 143 (1984) 157. F. Bozso, J.T. Yates, Jr.. J. Arias, H. Metiu and R.M. Martin, J. Chem. Phys. 78 (1983) 4256. F. Bozso, J. Arias, J.T. Yates, Jr., R.M. Martin and H. Metiu, Chem. Phys. Letters 94 (1983) 243.