Electron induced decomposition of Ni(CO)4 adsorbed on Ag(111)

9

Surface Science 257 (199119-21 North-Holland

Electron induced decomposition of Ni(CO), adsorbed on As( 111) R.D. Ramsier,

M.A. Henderson

and J.T. Yates, Jr.

Surface Science Center, Department ofCfiemisfry, University of Pittsburgh, Pittsburgh, PA 15260, USA Received 18 January 1991; accepted for publication 14 May 1991

Nickel te?ra~ar~nyl ~Ni~CO~~~adsorption on Ag(lll) was studied with tem~~t~re-pronged desorption @?D), digital el~t~n-st~u~ated desorption ion angular distribution fESDL%D~ and Auger electron spectroscopy fAES>. NifCO& physisorbs molecularly at 90 K on Agflll), with thermal desorption maxima observed near 170 and 150 K. Analysis of AEZSspectra indicates no Ni, C or 0 is present at the surface following TPD, showing that Ni(CO), does not thermally decompose on A&11). Bombardment by 100-300 eV electrons induces positive ion and excited neutral particle desorption, and conversion of molecularly adsorbed Ni(CO), into uniden~~ed Ni,(CO& surface species. CO thermally desorbs from these Sudan-Lund species near 240, 300 and 400 K, leaving a Ni deposit on the Ag surface. This Ni diffuses below the surface above 450 K. ESDIAD patterns from molecularly adsorbed Ni(CO), are composed of ESD fragments (CO+, O+, and neutrals) ejected normal to the surface, suggesting that Ni(CQ), on A&11) orients one CO ligand nearly perpendicular to the substrate. In contrast, Ni,(COjy surface species do not produce directed ESDIAD beams, possibly reflecting a non-normal or random orientation of CO ligands in these adspecies. The total cross section for electron induced Ni(CO)4 fragmentation is about 2 X 10-‘6 cm’, and is not strongly dependent on incident electron energies between 100 and 300 eV.

1. htiu&an Metal carbonyl complexes have traditionally been the object of exhaustive study in orga~~metallic chemistry as these molecules are often used as precursors for the production of heterogeneous and homogeneous catalysts. In general, metal carbonyls may be easily activated both photol~i~a~y and thermally. Such complexes can be handled quite well theoretically, providing a good testing ground for calculations of electronic structure and vibrations force CORstar&s. More specifically, metal carbonyls have become important as reactants during metal film deposition 111.CO ligands can be liberated, either thermally or by particle ~mbardme~t, producing a metallic film. Microelectronics fabrication involving layered film growth relies heavily on such processes, as does the production af surface alloys [2-S]. An understanding of the surface chemistry of carbonyl complexes is required if one is to gain insight into the processes governing this type of metal film formation. ~39-~~Zg/9~/$03.50

This study focusses on the adsorption of nickel-te~a~arbonyl (Ni~C~~~~ on the A&11) surface and the effects of subsequent low energy electron bombardment. NXCO), Cas opposed to other carbonyls) was chosen because of its telrahedral IT,) symmetry [6-91. As Ni(CO), is highly sensitive to electron-induced fragmentation via Ni-C bond scission {l&16], it was critical to choose a substrate
2. Experimental All measurements were made in a stainless steel ultrahigh vacuum (UHV) chamber with a base pressure less than 2 x 1O-'o Torr. Ag(ll1) surfaces were prepared by thermal evaporation of Ag (Johnson Matthey 99.999%) onto a clean, well-characterized Pt(ll1) single crystal, followed by annealing at 750 K for several minutes. Auger electron spectroscopy &ES) was used to monitor

Q 1991 - Elsevier Science Publishers B.V. All rights reserved

10

RD. ~amsier et al. / Electron induced deco~~s~tio~ of Ni(COj, adsorbed cmAg(llI)

the thickness and cleanliness of the Ag films, with Ag deposition maintained until at least 20 monolayers had accumulated on the Pt substrate. Low-energy electron diffraction (LEED) verified the epitaxial ordering of silver following annealing to form a Ag(l11) surface. Heating the crystal above 900 K desorbed the Ag, expediting the preparation of fresh, clean Ag(ll1) surfaces. This procedure is discussed in greater detail by Davies et al. [17]. Liquid Ni(CO), was purchased from Pressure Chemical Company and transferred into a stainless steel cylinder. This was then attached to a stainless steel gas handling system adjacent to the UHV chamber. Freeze-pump-thaw cycles were employed to remove possible contaminants before dosing the crystal. At room temperature, the vapor pressure of Ni(CO), was sufficient to allow for dosing of the Ag(ll1) surface (cooled to 90 K) through an effusive pinhole source positioned N 1 cm in front of the crystal and containing an internal pinhole of known conductance (C = 2.01 x 1013 N,/(Torrs)) to limit molecular flux. It was found that NifCO), decomposed rapidly in the stainless steel gas handling system and doser line (section toward the UHV-side of the pinhole). This decomposition resulted in a mixture of Ni(CO), and CO effusing from the dosing aperture, and is a phenomenon observed by others [2,4,18-201. Reproducible amounts of Ni(CO), (within 5%, as measured by TPD) could be adsorbed on the Ag surface by maintaining stringent dosing parameters, but quantification of the absolute Nip exposure was impossible due to the unknown Ni(CO),/CO ratio effusing from the doser. Therefore, reported exposures for the gas mixture are considered as upper limits for the actual Ni(CO), exposure. Temperature-programmed desorption (TPD) experiments were performed with the crystal positioned - 1 cm in front of a quadrupole mass spectrometer (QMS) with the crystal biased negatively to repel electrons escaping from the ionizer region. A linear temperature ramp of 3.3 K/s was employed, with the substrate temperature being monitored by a chromel-alumel thermocouple spot-welded to the crystal edge. The multiplexed QMS simultaneously monitored signais

at m/e = 28, 58, 86 and 114, corresponding to CO+, Ni+, NiCO+ and Ni(CO)l, respectively. Relative ratios of these signals were in good agreement with the cracking pattern of Ni(CO),, ionized by 70 eV electrons, as determined from the literature [lo-131, and also as measured by dosing Ni(CO), directly into the QMS. The 60Ni/58Ni ratio was also consistent with the natural relative isotopic abundance ratio, 0.38/1.0 [7,10]. The CO TPD signal from chemisorbed CO was calibrated by pe~o~ing TPD from saturation coverages (0.67 ML = 1.03 X 1015 CO/cm*) of CO on the clean Pt(ll1) crystal used to support the Ag(111) film [211. Electron bombardment of the Ni(CO),/ Ag (111) system was performed by positioning the sample in front of the QMS (as for TPD), and then positively biasing the crystal to draw electrons from the QMS ionizer onto the substrate, with an incident energy (Ei) of 115 eV. Current densities near 1.5 X 10M4 A/cm* caused no significant heating of the crystal, which was at 90 K during electron bombardment. Digital electron-stimulated desorption ion angular distribution (ESDIAD) measurements were performed with an electron gun which delivered a well-focussed beam of electrons (Ei = 100-300 eV) onto the crystal at approximately 54” from the surface normal with current densities near 0.8 x 10m6A/cm’. ESDIAD data have been digitally smoothed and corrected for a background signal [22]. Details of the micro-channel plate detectors and 4-grid optics used in ESDIAD imaging have been described elsewhere 1223. Low-energy electron diffraction (LEED) was accomplished by changing potentials on the ESDIAD optics, and the substrate was held at 90 K during all ESDIAD and LEED measurements.

3. Results 3.1. Thermal behavior of Ni(CO),

on Ag(lll)

Figs. 1 and 2 show the TPD signal (nz/e = 28) from Ni(CO)~/A~lll) as a function of exposure to Ni(CO),. These curves are quantitatively similar in appearance to QMS signals at m/e = 58,

R.D. Ramsier et al. / Electron induced decomposition of Ni(CO),

86 and 114, and represent the thermal desorption of molecular Ni(CO), from the surface. The ratios of these signals at different m/e values were in good agreement with published data on the cracking pattern of Ni(CO), ionized by 70 eV electrons [lo-131. The first desorption maximum to appear occurs at 173 K in fig. 1 (bottom trace). With increasing exposure this peak broadens, and both the onset and centroid shift to lower temperature. A second TPD feature develops with further exposure. This TPD state does not initially exhibit a constant leading edge shape at low exposures, but superimposable leading edges are observed at higher exposures. The development of this desorption feature is seen in fig. 2. By monitoring QMS signals at m/e = 28, 58, 86, and 114, it was observed that no additional desorption maxima occurred between 190 and 750 K for any exposure (up to 32 X 10i4/cm2) on clean Ag(ll1). In addition, AES showed no evidence for Ni, C or 0 at the surface following these TPD experiments. 3.2. Electron bombardment of Ni(CO), on Ag(ll1) 3.2.1. ESD desorption

studied by temperature-programmed

Fig. 3 shows TPD spectra from Ni(CO),/Ag (111) (gas exposure = 13 X 1014/cm2) subjected to 115 eV electron bombardment for various time intervals. The TPD signal intensity corresponding to molecular Ni(CO), desorption (region A: 140190 K> decreases as the electron fluence increases, while new features appear above 190 K. Desorption maxima near 240, 300 and 400 K are observed in areas labelled region B: 190-260 K, region C: 260-330 K, and region D: 330-440 K. No Ni or Ni-containing species desorb in regions B, C or D for any initial gas exposure or electron fluence. Thus, these TPD features represent CO desorption from surface-bound Ni,(CO), species produced by electron impact on molecularly adsorbed Ni(CO),. Fig. 4 is the integrated area under the TPD curves of fig. 3 for each desorption region (after linear background subtraction). The ordinates of figs. 4B-4D were quantified by comparison to TPD data from 0.67 ML CO on Pt(ll1) (not

adsorbed on Ag(ll1)

11

:: 1.1

:

.90

: :

.74

‘1 0

.56 .53

; ; :,

.27

_ 60

160

320

240

TEMPERATURE

.lO

(K)

Fig. 1. Temperature-programmed desorption of Ni(CO)4 from Ag(ll1) for m/e = 28 (low gas exposure regime). A linear temperature ramp of 3.3 K/s was employed.

shown) 1211. Fig. 4A shows the depletion of molecular Ni(CO), (region A> due to electronstimulated fragmentation and species conversion and possibly also to NKCO), desorption. Figs. 4B-4D show the filling of states B, C, and D. State B is populated most rapidly during the initial stage of electron bombardment, with a

I =

L 60

6.0

nA

_-_/y--_/1.1 160 TEMPERATURE

320

240 (K)

Fig. 2. Temperature-programmed desorption of Ni(CO), from Ag(lll) for m/e = 28 (high gas exposure regime). A linear temperature ramp of 3.3 K/s was employed.

R.D. Ramsier et al. / Electron induced decomposition of Ni(CO),

12

80

160

240

TEMPERATURE

320

400 (K)

Fig. 3. Temperature-programmed desorption (m/e = 28) from Ni(CO), on A&l 11) (gas exposure = 13 x 10’4/cm2) following ~mbardment by 115 eV electrons. Region A represents molecular Ni(CO), desorption, whereas regions B, C and D result from CO desorption from unidentified Ni,(CO), species, A linear temperature ramp of 3.3 K/s was employed. slower filling of states C and D. It appears that B and D saturate after an electron fluence of - 20 X lOI electrons/cm 2, decreasing slightly with additional electron bombardment, whereas peak C continues to increase slowly. The sum of features B, C and D represents about 5.5 X lOi CO molecules/cm2 desorbing from the surface after bombardment by - 39 x 1Or5electrons/cm’. This electron fluence will be defined as a “total conversion” value for subsequent discussion. Fig. 5 shows the TPD signal (m/e = 28) from Ni(CO),/AgU 11) after total electron conversion for different initial gas exposures. Note that the low-temperature background features (seen near region A and below) increase with dosing time, becoming most evident in the upper curves of fig. 5. This arises from CO thermal desorption from the crystal holder and heating leads (from Ni(CO), decomposition and/or inadvertantly dosed CO), as confirmed by examining TPD spectra after dosing with pure CO (not shown). These background features are therefore neglected in the analysis. The lower curves of fig. 5 illustrate that for low Ni(CO), exposures, very Iittle molecular CO remains at the surface foIlowing electron bombard-

adsorbed on Ag(llI)

ment. As the initial Ni(CO), coverage increases, eIectron bombardment predominantly populates states B and C, with smaller amounts of CO desorption detected in region D. It is of interest to note that surface species C and D seem to increase in coverage (relative to species B) as the initia1 gas exposure increases in these ESD experiments involving the total conversion of adsorbed NXCO), . Several families of curves like those of fig. 5 have been integrated to produce fig. 6, which shows the total amount of CO desorbed from the electron-converted Ni(CO),/Ag(ll 1) system versus initial exposure. Two regions exist in fig. 6, distinguished by an inflection point near an exposure of 2.5 x 1014/cm2. This shows that the amount of CO bound to Ni at the surface, per Ni(CO), molecule, following extensive electron irradiation is about five times smaller for initial gas exposures less than - 2.5 x 10’4/cm2 than for exposures above this value. Auger electron spectroscopy was used to monitor the presence of C, 0 and Ni on the Ag surface following total electron conversion (producing states B, C and D). Since the incident AES beam (Ei = 2 keV, 1.5 x 10m4 A/cm’, spot size = 1 mm2) causes significant damage to molecularly adsorbed NKCO), during acquisition of an AES spectrum, reliable data from the adsorbed Ni(CO), species (state A) could not be measured. After total conversion of Ni(CO), by 115 eV electrons (from the QMS), analysis of AES spectra (90 K) showed that the Ni(61)/Ag (351) ratio increased roughly linearly with the initial gas exposure, and carbon and oxygen Auger signals rose accordingly. Comparison with similar spectra acquired after identical electron bombardment foilowed by heating above 450 K (to desorb CO from states B, C and D) showed that the Ni(61)/Ag(351) ratios significantly decreased as a result of heating. Heating to 750 K resulted in nearly complete Ioss of the Ni AES signal, presumably due to Ni diffusion into the substrate. 3.2.2. ESD products and ESDIAD from Ni(CO), / Ag(lll) The depletion of Ni(CO),(a) (state A) by electron irradiation occurs by conversion of the par-

R.D. Ramsier et al. / Electron induced decomposition of Ni (CO),adsorbed on Ag(lll)

been exposed to about 0.9 x 1Ol5 electrons/cm2 (Ei = 115 eV) from the ESDIAD gun. Using the measured cross section for Ni(CO), depletion @ = 2 x lo-l6 cm’, shown below), less than 20% of the molecules are decomposed during these ESDIAD measurements. The ion patterns contain a single central feature which grows in intensity with initial gas exposure. The ESDIAD patterns exhibit a half-width at half-maximum (HWHM) that remains roughly constant at 7.6 ~fr 1.0 *, with the crystal at ground potential to eliminate any effects of compression fields on the ion trajectories. The micro-channel plate detectors used for ESDIAD are capable of being excited by the decay of metastable neutral particles [23]. In addition, positive ions can be repelled by applica-

ent molecules to surface-bound Ni,(CO), species. This was verified by bombarding NXCO), layers with electrons from an auxiliary electron gun, positioned so that desorbing particles would enter the QMS (ionizer off). The only ESD products detected were CO’ and O+, with no Ni or N&containing ions observed. Thus, the depletion of state A results from electron-induced decomposition and possibly desorption of molecularly adsorbed Ni(CO),, with CO+ and 0’ ionic species being ejected in the process. The carbonyl fragments remaining at the surface yield CO by thermal desorption from the states B, C and D. In fig. 7 a series of digital ESDIAD patterns of positive ions desorbed from the Ni(CO),/Ag(lll) system are shown versus gas exposure. Upon completion of data acquisition, the surface had

STATE

Ni(CO), DEPLETION

^o 1.0 z

A

z

0.8

“:

z I i+

0.6

z

PRODUCTION

2.5

E o 2.0 z

0.4

1.5

x 1.0 8 = 0.5

,‘ 0” 0.2 .0.0

3 -

3.0

-

=

13

0

15

ELECTRON

30

45

FLUENCE

STATE

C -

0.0

60

X 1Ol5

0 cm-*

PRODUCTION

3.0

15

ELECTRON

STATE Cl

N

30

45

FLUENCE

D -

60

X 10”

cm-*

PRODUCTION

3.0

Ol

‘E 2.5 f0

2.0

0 ;z

0.07

0.0

15 ELECTRON

1.5

30

FLUENCE

45

60

X 10lg

wn-2

0 15 30 45 60 ELECTRON FLUENCE X 1015

cm-*

Fig. 4. Integrated TPD areas of regions A-D from fig. 3 showing (A) depletion of molecular NiCCO), and (B-D) filling of high-temperature CO states induced by 115 eV electron bombardment. Lines have been hand-drawn through the data to guide the eye.

R. D. Ramsier et al. / Electron induced decomposition of Ni(CO),

14 ITA

B

C

adsorbed on Ag(ll1)

EXPOSURE

D/

x 1o14

V, = cm-*

N,

= 0.9

115

eV

x ,015c~-2

.25

.75

160

00

240

320

TEMPERATURE

400 (K)

Fig. 5. Temperature-programmed desorption (m/e = 28) from Ni(CO), /Ag(lll) following conversion by 39 X lOI electrons/cm’. The incident electron energy was 115 eV and a linear temperature ramp of 3.3 K/s was employed.

tion of a retarding potential between the crystal and the ESDIAD grid optics, allowing only neutral species to pass through the analyzer. With these two points in mind, fig. 8 is presented, which shows a series of digital ESDIAD patterns for excited-state neutral species produced by ESD from adsorbed NKCO),. The data acquisition exposed the surface to about 1.8 x lOI electrons/ cm2 (Ei = 115 eV), and only a single central feature similar to the ion patterns was observed. 14 12

N

‘E 0 z 0

6 6

x z

10

8

4 2 0

0

0 EXPOSURE

16

24

X 1014

32 cm-’

Fig. 6. Data from fig. 5 (and similar sets) showing the amount of CO that thermally desorbs from high-temperature states following 115 eV electron-induced conversion of adsorbed IWCO),. Note the inflection point, better seen in the inset. Lines have been hand-drawn through the data to guide the eye.

2.0

Fig. 7. Positive ion ESDIAD patterns from Ni(CO), /Ad1111 at 90 K. Data acquisition exposed the surface to 0.9~ 10” electrons/cm’. The incident electron energy was 115 eV and the crystal was at ground potential, resulting in no artificial compression of the ion angular distribution toward the normal.

It must be emphasized that the intensity of the ion signals in fig. 7 cannot be compared directly to that from neutrals of fig. 8, since different electron fluences and detector efficiencies were involved. Removal of molecularly adsorbed Ni(CO),, either by bombardment with more than 20 X 101’ or by partial electron-induced electrons/cm2, conversion followed by heating to 190 K, resulted in the disappearance of the normally directed ESDIAD beam. Thus this central ESDIAD feature results exclusively from molecularly adsorbed Ni(CO),. The conversion states (B, C and D) do not produce directed ESDIAD patterns but simply contribute to an increase in a broad background signal. Since only state A (Ni(CO),(a)) produces a normally directed ESDIAD beam, the depletion of molecularly adsorbed Ni(CO), can be monitored by ESDIAD. Fig. 9 shows ESDIAD patterns from all ESD products (ions and neutrals)

R.D. Rmnsier et al. / Electron induced decom~s~t~ V,

EXPOSURE x 1o14

cm-*

Ne

= 115 eV

= 1.8 X 1015 cm

of Ni(CO),

15

adsorbed on Ag(lll)

Ve = 115 eV

EXPOSURE

= 11 X 1014 cm-*

-2

.25

.75

Ne

x

= 1.2

1015cm-*

No = 1.8 X IO

15

cm

-2

2.0

Ne Fig. g. Electronically excited neutral ESDIAD patterns from N~(CO)~/~l~l) at 90 K. Data acquisition exposed the surface to 1.8 x iOr electrons/cm2. The incident electron energy was 11.5eV and the crystal was at ground potential.

following various electron fluences. The peak height and volume of the central ESDIAD feature decreases for increasing electron fluence from the ESDIAD gun. The measured total cross-section (Q> for Ni(CO), depletion by electron bombardment versus incident electron energy is shown in fig. 10, and is derived from ESDIAD data recorded sequentially (as in fig. 9). For Ei in the range 100-300 eV, it is evident that Q is roughly constant at about 2 x lo-r6 cm’, The “filled” data point in fig. 10 is derived from data for the depletion of Ni(CO), using TPD, as shown in figs. 3 and 4A. It should be noted that no evidence for photolysis of adsorbed Ni(CO), by stray light (chamber filaments, viewports, etc.> was observed during this study. In addition, digital LEED gave no information on lateral ordering of adsorbed NXCO),, but an increase in background signal

= 2.7 X 10’5cm-2

N

= 5 9 x ~o’~cm-*

’

e

Fig. 9. ESDIAD images (positive ions and neutrals) from NitCOI, on Agflll) at 90 K (gas exposure = 11 x 10’4/cm2). Signal depletion with increasing electron fluence indicates the conversion of NifCO), to unidentified Ni,(CO),, surfacebound species. The incident electron energy was 115 eV and the crystal was at ground potential.

with increased gas exposure was observed. ever, the LEED electron beam causes ESD age to condensed Ni(CO),, preventing one drawing reliable conclusions from LEED surements.

Howdamfrom mea-

3.0 :

F 2.5 0

0

to 2.0 ;

1.5

0

8”

0

O= ESDIAD l = TPD

1 .o 100

150

INCIDENT

200

250

ELECTRON

300 ENERGY

(eV)

Fig. 10. Cross sections for Ni(CO), conversion by electron irradiation on Agflll) at 90 K. Values derived from TPD (filled point) and ESDIAD (open points) agree well.

16

R.D. Ramsier et al. / Electron induced decomposition of Ni(CO),

4. Discussion 4.1. Previous studies of adsorbed Ni(CO),

Ni(CO), has received little attention in surface science studies to date. Most studies involved characterization of Ni films or surface alloys formed by pyrolysis of Ni(CO), on various surfaces. This Ni deposition was usually accomplished by raising the substrate to high temperature [2-51 or by local heating of the surface with intense laser pulses [24-281 in the presence of Ni(CO), vapor. A few studies have included reports of the behavior of Ni(CO), at 90 K on metal surfaces [19,20], and complement the present findings. However, previous studies of low temperature Ni(CO),/surface interactions have not emphasized effects of incident electrons or photons, even though Ni(CO), is known to be highly sensitive to fragmentation by such particles [lo-16,29-351. Using X-ray photoelectron spectroscopy (XPS), Kishi and co-workers observed that Ni(CO), weakly adsorbs molecularly at 90 K on polycrystalline Fe and Pd surfaces [20]. They noted a significant decrease in molecular Ni(CO), XPS signals when the substrate was warmed above 175 K, presumably a result of Ni(CO), thermal desorption. Following heating to 290 K, XPS showed some Ni, C and 0 remaining on these Fe and Pd surfaces. This was attributed to thermal decomposition of Ni(CO), followed by adsorption of the fragments. Gland and colleagues used TPD and highresolution electron energy loss spectroscopy (HREELS) to study the Ni(CO),/Ni(lll) system [19]. They concluded that Ni(CO), molecularly adsorbs on Ni(ll1) at 100 K, as observed by characteristic Ni(CO), vibrational transitions. Their TPD results show that molecular Ni(CO), thermally desorbs from Ni(ll1) near 150 K. An important point to realize when comparing the studies of Kishi et al. [20] and Gland et al. [193 with the present report is that Ag(ll1) does not bind CO at 90 K, whereas the previous studies involved Fe, Pd and Ni substrates, to which CO readily adsorbs at low temperature. Thus experiments on the Ni(CO),/Ag(lll) system do

adsorbed on Ag(lll)

not involve features from CO species adsorbed on the substrate, as do these previous investigations. This permits a clear view of CO bonding to Ni sites produced by ESD of Ni(CO),(a). 4.2. Thermal behavior of Ni(CO),

on Ag(ll1)

TPD results of figs. 1 and 2 suggest that Ni(CO), molecularly physisorbs at 90 K on Ag(ll1). The first desorption maximum to be observed with increasing exposure appears at 173 K, broadening and shifting to 167 K, and reflecting first-order desorption kinetics [36]. A second TPD peak in figs. 1 and 2 grows with additional exposure, desorbing near 150 K and displaying zero-order desorption kinetics in its later stages of development [36]. Throughout, QMS ratios of Ni-containing fragments are consistent with gasphase Ni(CO), mass spectra [lo-131, and AES shows no evidence for dissociative fragmentation on the surface due to thermal processes. The temperatures of the Ni(CO), desorption peaks imply that Ni(CO), is weakly bound in both states. The gas exposure required to saturate the high-temperature TPD feature (= 0.5 X 1014/ cm*) is considerably smaller than that likely to be necessary for the filling of one complete monolayer of Ni(CO), on Ag(lll), where geometrical considerations predict a value of N 4 X 1014/cm2 [20]. The conductance of the molecular doser was calibrated with standard gases. The reported exposures for the Ni(CO>,/CO gas mixture were corrected for the fraction of the flux actually intercepted by the crystal [371 and involve an unknown ratio of Ni(CO), to CO. Since our gas exposures represent an upper limit of Ni(CO), flux (due to the presence of CO in the gas mixture), it is likely that even less than 0.5 x 1014 Ni(CO),/cm* is required to saturate the hightemperature TPD feature. Thus we assign this high temperature state to Ni(CO), adsorbed on defect sites on the Ag(ll1) surface. The low temperature state of Ni(CO), is due to overlapping desorption from the monolayer and multi layer which are unresolved from each other. This behavior is quite different from that observed for Fe(CO), adsorption on Ag(ll1) in the same apparatus [38]. For Fe(CO),, a high-tem-

RD. Rmker et al. / Electron induced dec~~sit~~

perature signed to Fe(CO), lowed by

(181 K) thermal desorption state (asthe monolayer) is seen to saturate at an exposure of 7.5 X 10i4/cm2. This is folthe formation of Fe(CO), multilayers.

4.3. Electron bombardment Ag(ll1) system

of the Ni(CO),/

4.3.1. ESD studied by tem~rat~re-programed d~ur~t~~ Figs. 3 and 4 show that molecularly adsorbed Ni(CO), (state A) is readily converted to other surface-bound carbonyl species (states B, C and D) by electron bombardment. Recall that no Nicontaining fragments were observed in TPD spectra above 190 K, so that regions B, C and D represent CO desorption from unidentified types of Ni sites containing CO species. It is clear that the kinetics of CO desorption from the NiCCO)4/AgW) system after electron ~mbardment are complicated. In fact, the specific surface species responsible for CO desorption in regions B, C and D may not even be present on the 90 K surface after electron bombardment, but may result from condensation reactions involving coordinately unsaturated subcarbonyl species during crystal heating. Note that fig. 4 shows the low-temperature state (B) initially filling most rapidly upon electron exposure, with states C and D following. The present results suggest that surface species produced from adsorbed Ni(CO), by ESD do not form large metallic agglomerates of Ni-~ntain~g chemisorbed CO. This hypothesis comes by comparison of TPD spectra from CO on various Ni surfaces [39], which always exhibit major desorption maxima above 300 K. In the present study, CO thermal desorption occurs at 240, 300 and 400 K. One would anticipate TPD spectra similar to those from CO on Ni substrates if ESD-produced species from adsorbed Ni(CO), coalesce to form large metallic-like Ni islands. Additionally, calculations have shown that Ni clusters with as few as 10 Ni atoms behave similarly to a semi-infinite Ni surface [40]. Thus, we believe that species B and C (and possibly D) represent subcarbonyl condensation products stabilized by interaction

of Ni(CO), a&orbed on Ag(lll)

17

with the Ag(ll1) substrate; these species probably contain only a few Ni atoms. It is of interest to note that cluster species such as Ni ,(CO), Ni,(CO), NXCO), and Ni 2(CO)2 have been produced and studied by matrix isolation spectroscopy [41]. Fig. 5 shows that surface species C and D seem to increase in coverage, relative to B, as the initial gas exposure increases in these experiments concerning total ESD conversion of adsorbed NXCO),. This may suggest that the Ni,(CO), surface species (B, C and D) involve successively larger numbers of Ni atoms (x = 2,3,4, etc.). One may assume that destabilization of surface species caused by CO thermal desorption permits Ni diffusion into the Ag surface before extensive lateral motion and Ni ag~omeration occurs, This is substantiated by a decrease of the Ni AES signal when the crystal is heated above 450 K, and by the inability to readsorb pure CO on these ESD converted surfaces (not shown). The slope change near an exposure of 2.5 x 1014/cm2 seen in fig. 6 is difficult to explain. This inflection point implies that at low gas exposures, conditions for CO retention following ESD are unfavorable compared to those at high exposures. This effect may be related to defects in the Ag(ll1) surface, which apparently saturate near 0.5 x 1014/cm2 gas exposure. This may also be indicative of critical sized Ni clusters required for CO retention, which could be produced more efficiently by ESD of the more dense Ni(CO), layers. 4.3.2, E,SLIand ESLX4L3 of Ni(Co), on Ag(lll) The observation of directed ejection of ESD fragments from a physisorbed molecule is of great interest. These fragments consist of positive ions (CO+ and O+) as well as electronically excited neutrals. Analogous to photo&tic processes [30351, one might envision surface processes such as Ni(CO),(a)

+ e-+

Ni(CO):(a)

+ Ni(CO)T(a) + (CO)*(g), (I) where liberated CO ligands carry electronic (* > as well as translational energy. Free CO has an a3r excited state containing about 6 eV of electronic energy, and metastable ESD ejection from

18

R.D. Ramsier et al. / Electron induced decomposition of Ni(CO),

CO on Ni substrates has been postulated to involve such a final state [23]. While a37 CO has not been identified as the metastable species ejected in the present ESD measurements, it is possible that the neutral excited species produced from ESD on CO ligands of Ni(CO),(a) is the same as that observed from CO chemisorbed on Ni surfaces. The ESDIAD results of figs. 7-9 show a single central feature corresponding to stimulated desorption of positive ions and metastable neutrals from adsorbed molecular NKCO),. Retarding potentials of + 10 V were sufficient to prevent all positive ions from reaching the detector array. HWHM values were roughly constant at 7.6 f 1.0 o for the patterns from both ion and excitedneutral species. A general observation was that the ESDIAD patterns became slightly narrower with increasing exposure. A large central ESDIAD peak is interpreted as being due to the desorption of energetic ESD fragments near the surface normal. This implies that Ni(CO), preferentially orients with one CO ligand nearly perpendicular to the substrate. Indeed it is difficult to visualize a stable mode of tetrahedral Ni(CO), adsorption on Ag(lll) which does not include three CO ligands near the surface and the fourth oriented normal to the substrate. The absence of directed ESDIAD beams from states B, C and D may be explained in the following ways: (1) The ESD cross section for CO present on Ni clusters may be very small compared to that for CO from adsorbed Ni(CO),. (2) CO species on Ni clusters may be oriented randomly or at large angles from the surface normal. The cross-section data of fig. 10 shows no strong dependence on incident electron energy. These values
It is known that NKCO), undergoes dissociative attachment (DA) reactions with low-energy

adsorbed on Ag(lll)

electrons in the gas phase [14-161, producing anionic species via processes such as Ni(C0)4 + e-+

Ni(CO),

+ CO.

(2)

These processes are thermo-neutral and occur with extremely large cross sections [16]. The cross section for tri-carbonyl anion production exhibits maxima near 0.0, 1.4 and 6.8 eV, whereas other sub-carbonyl anions (having lost 2 or more CO ligands) also result from DA mechanisms in the energy range 0.0-8.0 eV [161. Electrons with higher incident energies are capable of inducing valence and core level electronic excitations of Ni(CO), [lo-131, resulting in various fragmentation products. Many possibilities for electronic excitations leading to decomposition of NKCO), exist. Molecular orbital diagrams from gas-phase experiments and theoretical calculations are available [41-551. The lowest group of unoccupied orbitals (2t, lot,, 3e) are mainly CO(27r*) in character, and the highest filled orbitals (9t,, 2e and 8t,) are predominantly Ni(3d) and CO(% + 1~) in nature, respectively. Ni(CO), does not absorb visible light, but UV irradiation initiates many transitions leading to dissociation. In the gas phase, predominant photolytic channels involve the production of excited state species which dissociate by Ni-C bond rupture, forming excited tri-carbonyl and CO fragments. The excited subcarbonyl can radiatively decay in the visible region and can also dissociate further, yielding more free CO [43], depending on the partitioning of available energy. From the literature available on gas-phase NXCO),, one can draw several conclusions. These are: (1) Ni(CO), is highly prone to DA reactions in the presence of low-energy (O-10 eV) electrons, yielding anionic carbonyl fragments and free CO. (2) Ni(CO), dissociates efficiently when subjected to higher energy (70 eV) electron bombardment, forming a variety of positive ion and neutral fragmentation products. (3) Transient excited-state species with microsecond lifetimes have been observed following NKCO), electronic excitation.

RD. Ramsier et al. / Electron induced decompositionof Ni(CO), adsorbed on Ag(ll1)

4.4.2. Condensed phase The presence of the metal surface in the Ni(CO),/Ag(lll) system can cause differences between condensed and gas-phase Ni(CO), dissociation pathways. These differences may result from metal-induced excitation/de-excitation mechanisms such as secondary electron production, charge transfer, electron-hole pair excitation, image field damping, etc. In addition, interadsorbate interactions and charge transfer, as well as chemical reactions between condensed sub-carbonyls, may occur. Such processes are not possible in the gas phase. Secondary electron emission from Ag(l11) subjected to an incident beam of primary electrons of energy 5-300 eV has been studied [56]. Secondary emission intensity from Ag maximizes at secondary energies near 1.0 eV, but shows large secondary electron intensities .up to 10 eV [56]. In general, the emission energy and angular distribution of secondary electrons is independent of the primary beam energy and direction [57]. This is because secondaries undergo so many scattering events that any “memory” of the incident beam is lost [57]. Thus, electron bombardment from the QMS ionizer (115 eV, normal to surface) and from the ESDIAD gun (100-300 eV, 54” with respect to the normal) are expected to produce secondary electrons which are indistinguishable in terms of their effect on condensed molecules. Thus for Ni(CO), on AgUll), the role of secondary-electron-induced ESD may enhance the total cross section for dissociation. Primary electrons may contribute to direct dissociative processes, whereas low-energy secondary electrons from the Ag(lll) substrate can provide additional routes to molecular decomposition. Physisorption is not expected to appreciably shift the relative energies of adsorbate ground state molecular orbitals [58,59]. In addition, this weak adsorption may not significantly perturb the NXCO), Td symmetry 1601, as suggested by the HREELS results of Gland et al. [19] for Ni(CO),/Ni(lll). These two points suggest that dissociation pathways, similar to those observed in gas-phase studies, may be sampled in the present study of Ni(CO),/Ag(lll). However, the

19

metal substrate can drain energy from excited adsorbates by mechanisms which include electron-hole pair creation [61-631 and surface plasmon excitation [64-661. These complications make it difficult to use gas-phase excitation schemes solely as a basis for understanding the nature of excited state curves sampled during ESD processes from the Ni(CO),/Ag(lll) system.

5. Conclusions (1) Ni(CO), has been shown to molecularly physisorb on Ag011) at 90 K, with thermal desorption maxima observed near 150 and 170 K. Thermal desorption occurs without dissociation of Ni(CO),. (2) Electron irradiation induces CO+, O+, and excited-state neutral desorption and conversion of molecular Ni(CO), to unidentified Ni,(CO), surface species. These surface-bound species thermally decompose, yielding CO desorption maxima near 240, 300 and 400 K. (3) Ni species, produced from Ni(CO), by ESD followed by TPD, begin to dissolve in the Ag(ll1) substrate near 450 K, after CO desorption has occurred. (4) Ejected ESD fragments from NXCO), on Ag(lll) predominantly desorb normal to the surface, indicating that Ni(CO), adsorbs with one CO ligand nearly perpendicular to the surface. (5) Ni,(CO), species trapped at the surface following electron bombardment do not yield directed ESDIAD beams. (6) Cross sections for Ni(CO), decomposition
Acknowledgements This study was supported by AFOSR, contract 82-0133. The authors express their sincere gratitude to And& Szabo for many helpful suggestions throughout the course of this work.

20

R.D. Ramsier et al. / Electron induced decomposition

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