Thermal chemistry of CF3I on Ag(111): a TPD and RAIRS study

surface science ELSEVIER

Surface Science 364 (1996) 345-366

Thermal chemistry of CF3I

on

Ag(111):

a

TPD and RAIRS study

Andrfis Szab6, Stacy E. Converse, Sandra R. Whaley, J.M. White * Centerfor Materials Chemistry, Department of Chemistry and Biochemistry, Universityof Texas, Austin, TX 78712, USA Received 2 November 1995; accepted for publication 19 February 1996

Abstract

Temperature orogrammed desorption and reflection absorption infrared spectroscopy have been used to study the thermal chemistry of CF3| on Ag(111). At coverages <0.03 CF3I/Ag, adsorption occurs with I CF 3 bond dissociation at T= 86 K, while at higher coverages CF3I adsorbs molecularly. The C-I bond direction of the adsorbed molecules depends on the coverage. When molecular adsorption occurs at coverages _<0.15 CF3I/Ag, the C-I bond direction is parallel to the surface. Above this coverage the adsorbed molecules reorder to a form in which the molecular axis is tilted away from the surface plane. The first layer saturates at 0.3 CF3I/Ag coverage. Multilayers form readily at 86 K. When the CF3I/Ag (111) system is heated, the multilayers desorb at ~ 100 K, and some of the CF3I thermally dissociates into CF 3 and I at ~ 110 K. The fate of the products depends on the molecular orientation prior to the fragmentation. When the coverage is high and the C-I axis is tilted from the surface, the CF 3 radicals desorb during dissociation at ~ 110 K, leaving only iodine atoms on the surface. At low coverage, when the C I axis is parallel to the surface, both the CF 3 and the iodine remain bound. The non-dissociated CF3I desorbs molecularly at T_< 150 K. The CF 3 that remains after C-I bond scission desorbs as radicals at ~ 300 K, and the adsorbed iodine desorbs atomically at ~ 800 K. The relationship of molecular orientation to previous results on CF3I/Ag (111) thermal and photochemistry is discussed.

Keywords: Chemisorption; Halogens; Reflection spectroscopy; Silver; Thermal desorption spectroscopy

1. Introduction

To better understand the behavior of the C-F and C-I bonds, CF3I adsorption has been studied on various metal single crystal surfaces, including Ni(100) [1,2], N i ( l l l ) [3], Ru(001) [4,5], P t ( l l l ) [6] and A g ( l l l ) [7-9]. Thermal chemistry of the C-I and C-F bonds has been studied because of interest in the chemical stability of fluorocarbon ether lubricants at high temperature [4]; adsorption and thermal decomposition of CF3I on silicon and silicon oxide surfaces has been studied to elucidate the role of the CF 3 radicals in * Corresponding author. Fax: + 1 512 4718696.

etching of Si and silicon oxide in C F 4 based plasmas [ 10,11 ]. The relatively low energy needed to cleave the carbon-iodine bond makes it easy to study surface-alkyl radical chemistry [ 12-14], and fluorine substitution in hydrocarbons has been used to obtain information about hydrocarbon surface chemistry [12]. The use of CF3I for these purposes is supported by the enormous literature on its behavior in the gas phase, where it has large cross sections for photon and electron stimulated dissociation of the C-I bond; its vibrational, rotational and electronic structures are well understood [15-34]. In the present work we studied adsorption of CF3I on the close packed (111) surface of the relatively unreactive metal, silver. While both C-I

0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S0039-6028 (96)0061 7-6

346

A. Szab6 et al./Surface Science 364 (1996) 345-366

and C - F bond cleavage occurs readily on many metal surfaces [1-5], on the silver surface the C - F bond does not undergo thermal dissociation [8]. The thermal chemistry of CF3I adsorbed on Ag(111) involves molecular desorption as well as carbon-iodine bond cleavage at temperatures <150 K. The dissociation products, CF 3 and iodine, desorb as radicals at temperatures ,-~300 and ,-~800 K, respectively. There are no desorption products other than CF3I, CF3 and atomic iodine. Iodine preadsorbed on Ag( 111 ) inhibits C-I bond cleavage. By cycling iodine adsorption and thermal desorption, a saturation coverage of 0.33 ML of iodine is obtained, i.e. 0.33 1 per surface Ag Atom. Adsorption of CF3I on the iodine saturated surface results only in molecular thermal desorption [8]. Recently, desorption of CF3 radicals from the CF3I/Ag(lll ) system at T ~ 110 K was observed [35]. Since CF3I is thermally stable in the gas phase (C-I bond energy 224kj mo1-1 [36]), and the C F 3 - substrate binding energy has been estimated to lie between 16 and 66kJ tool -1 [8], desorption of CF3 radicals at this low temperature must occur concomitantly with the C - I bond cleavage before thermal equilibrium is reached. This is supported by a study showing that desorption of the CF3 radicals at ,-~ 110 K is maximized at ,-~20° from the surface normal [35]. Non-thermal chemistry of CF3I in the gas phase has been studied extensively. Halofluoromethanes exhibit large cross sections for dissociation by low energy photons and electrons [37]. When CF3I collides with K the K+CF31 , K + + I - + C F 3 process occurs with a large cross section; this was interpreted as harpooning of the CF3I molecule by the "s" electron of the potassium, forming CF3I- that spontaneously dissociates [19]. Using the hexapole technique, CF3I can be spatially oriented in molecular beams. Crossed beam experiments show that potassium is similarly reactive when it approaches the CF3I at either the CF3 or I of CF3I [20]. Other studies exhibit radical desorption of CC12 from CClzF 2 [38]. Detailed knowledge of the electronically induced gas phase chemistry of CF3I spurred interest in its electron and photon induced chemistry when adsorbed [1,5,7]. Orientation by and increased coordination of weakly physisorbed particles on a surface may modify the photochemical properties

from that in the gas phase. In studying the photochemistry of the CF3I/Ag(lll) system we found interesting dependence of the yield and kinetic energy distribution of the desorption products on the coverage and the wavelength of the incident light [7,9]. The main purpose of this study is to elucidate the CF3I-Ag(lll) interaction and the reaction mechanism(s) leading to dissociation of the adsorbed CF3I and low temperature desorption of the CF3 radical. We used reflection absorption infrared spectroscopy (RAIRS) to gain information about the identity and the molecular orientation of the adsorbate, and temperature programmed desorption (TPD) to identify the desorption states and quantify the desorption products after exposing the Ag(111) to CF3I. The present results on clean A g ( l l l ) show that CF3I adsorbs with various orientations, depending on the coverage. We were able to establish a relationship between the low temperature desorption of CF3 radicals and the adsorbate orientation. The correlation between adsorbate orientation (present result) and other results (Refs. [7,9,35]) is also discussed.

2. Experimental methods The work was carried out in a two level UHV system with a base pressure in the low 10 -1° Torr range. The system is equipped with a differentially pumped Extrel C-50 quadrupole mass spectrometer, a Physical Electronics single pass CMA Auger spectrometer (both in the lower chamber), an upgraded Mattson Cygnus 100 Fourier transform infrared spectrometer (upper chamber), and also a sputter gun that was used to clean the sample. CF3I (Aldrich Chemical Co., >99% pure) was used as received. We could identify no impurities by mass analysis. Dosing of CF3I was accomplished through a 3 mm ID stainless steel tube positioned ~2.5 cm from the sample surface and oriented towards the sample. Exposures were measured by the pressure increase of an ionization gauge located about 10 cm from the doser, behind the sample. The exposures are reported in units of Torr.s, calculated by multiplying the pressure gauge reading with the exposure in seconds. Although the exposures are reported in 10 -6

A. Szab6 et al./Surface Science364 (1996) 345-366

Torr's units, we do not use the conventional "Langmuir" units, since the pressure gauge reading was not corrected for the CF3I sensitivity, nor was the proximity focusing of the doser tube taken into account. The silver single crystal, oriented and cut within 1° of the (111) plane, was polished with 1.0/t diamond paste. The crystal was held between a U-shaped tungsten wire (diameter 0.75 mm) by the spring force of the wire. The sample temperature was measured using a chromel-alumel type thermocouple sandwiched and spotwelded between a 0.05 mm thick tantalum foil, further sandwiched and spotwelded between a bent silver wire that was finally spotwelded to the sample. Although during the course of the measurements the sample had to be reconnected to the sample holder in which case the above procedure was repeated to re-attach the thermocouple to the sample, after identical sample preparation the temperature of the TPD peaks was the same within + 2 K. Because of the differing temperature scales in our experimental system and in other laboratories, we have compared TPD data collected in our system to that in another laboratory. For this purpose we used TPD data collected during multilayer CC14 desorption from Ag(111) in our laboratory and from Fe(ll0) from Ref.[38]. Since desorption is from multilayers, the desorption kinetic parameters do not depend on the substrate material. For zero order desorption the position of the desorption peak increases with the surface coverage. In Ref. [38], for 4.4 ML CC14 initial coverage and 0.61 K - v s heating rate, the desorption peak position is at 146-150 K. In our system the desorption peak from multilayers was at 142 K, for ~5 layers of CC14 coverage at a heating rate of 0.5 K-1S, comparing nicely to that in Ref. [38]. Assuming that the desorption rate is proportional to exp ( - 3 H / k T ) , the kinetic parameter AH in Ref. [38] was found to be 10.7+0.9 kcal tool-1 in our case the same fit resulted in A H = 9.8 kcal mol- 1. The value of this parameter is fairly sensitive to the temperature; a + 10 K shift in the temperature scale would change this value by ___1.4 kcal mol- 1. Based on these comparisons, we estimate that our temperature calibration is, at most, 10 K different from that in Ref. [38].

347

The sample holder XYZ manipulator is fixed on a rotating stage. During measurements the sample temperature could be varied between 83 and 960 K. The lowest temperature was achieved using the He bubbling technique described in Ref. [39]. The sample was prepared for measurements by Ar + bombardment (4 kV, 10 #A, 5 min) and subsequent annealing at 700 K for 5 min. After this treatment no contaminants were observed with AES. TPD measurements were done with the differentially pumped quadrupole mass spectrometer. We assume that the integrated TPD peak areas are proportional to the adsorbate coverage before desorption. This is an approximation, since the velocity and angular distribution of the desorption products, which influence the mass spectrometer signal intensity, might depend on the surface temperature and the desorption mechanism [40]. Infrared measurements were done with resolution ranging from 4 to 16 cm 1, as indicated in the figure captions. From 500 to 2000 scans were averaged, so that a typical measurement took about 8 min. Although we routinely achieved a noise level in the absorbance of less than 5 x 10 -5 when collecting a spectrum immediately after the background collection, the noise level rose with the increasing delay between measuring the background and the sample. This appears in some of the RAIR spectra that were collected using a single background while the cumulative effect of exposure or heating was studied.

3. Results 3.1. TPD data

3.1.1. CF3I adsorption on clean Ag(11 I)

Fig. 1 shows CF3I + and CF£ TPD traces collected after exposing clean Ag(111) to CF3I. While the CF3I + traces indicate desorption of molecular CF3I only, CF~ may indicate desorption of either molecular CF3I or CF 3 radicals E8]. At the lowest exposure shown in Fig. 1 (0.5× 10 -6 Torr's), no molecular desorption is observed, indicating that CF3I desorption requires a minimum initial coverage. The low temperature desorption features

348

A. Szab6 et aL /Surfaee Science 364 (1996) 345-366

C F31' A N D CF 2 ' TPD AFTER CF31 EXPOSURE OF THE Ag(1 1 1 )

dT/dt-1 K/sec I

I

I

I

wA,

~" E

._J < Z (.9

EXPOSURE [10- 6 tort sec]

Lo C~ +_

2.0 •


,

100

.

1.5

1.0

-=.I

"

200

--

"

'

I

300

'

'

. . . .

I

400

"

---

0.5

500

TEMPERATURE [K]

li

I

I

l

& EXPOSURE

j)R

,:',O"torrsec

Ig I I

2.0

u

1.0 0.5

i

100

200

:300

400

500

TEMPERATURE [K]

Fig. 1. CF3I ÷ and CF~ TPD traces after exposing Ag(lll) to CF3I at T=86 K. The indicated exposures correspond to initial coverages of 0.08, 0.15, 0.23 and 0.3 ML (see Section 3.2). (T<200 K) grow in sequentially as the exposure is increased, finishing with the peak at ,-~100 K. Molecular CFaI desorption is complete below 150 K at all exposures, in agreement with previous reports [8]. The CF3I + peaks at 110 and 128 K saturate above ,-~ 1 x 10 -6 Torr" s exposure. The lowest temperature CF3I + peak is the characteristic of desorption from multilayers, that is, it does not saturate and the peak temperature increases with exposure. Unlike previous reports on A g ( l l l ) [8] and nickel single crystal surfaces [ 1-3], in which multilayer desorption was attributed to T P D peaks at T _ 110 K, we assign the desorption peak at 100 K to multilayer desorption, in agreement with a

Pt(111) study [6] where multilayer CFaI desorption was observed at --~100 K. We would like to point out that none of Refs. [ 1-3,8] have provided a graph showing the growth of multilayer T P D peaks with increasing exposure. Significant desorption from the multilayer occurs above 90 K: when we exposed the surface to CF31 at 100 K or above, like it was reported in references [ 1-3], no multilayer adsorption was detected with either T P D or RAIRS. The CF~" signal at ~300 K, but no CF3I desorption, indicates desorption of CF3 radicals [8]. Also, the difference in the fragmentation pattern of the low temperature features, in particular the extra CF~- signal in the feature at 110 K,

349

A. Szab6 et aL/Surface Science 364 (1996) 345-366

has been attributed to desorption of CF 3 radicals [35]. Desorption of iodine, present on the surface due to dissociation, occurs above 700 K (see below). Fig. 2 shows CF~ and I ÷ TPD spectra, collected after exposing clean Ag(111) to CF3I. The sample was sputtered (Ar ÷) and annealed between each exposure. The CF~- spectra in the temperature range 250-450 K indicate CF 3 radical desorption, while the I ÷ spectra from 650 to 970 K show desorption of atomic iodine, which remained on the surface after dissociation of the adsorbed CF3I. No molecular iodine desorption was observed, in agreement with a previous report [8]. The complex coverage dependence of the TPD peak shapes and positions can be explained assuming repulsive interactions within the adsorbate layer. The shift to lower temperature of the high temperature tail of the CF 3 traces with increasing exposure is due to repulsive interaction between the desorbing CF3 radicals and the iodine that remains on the surface. The shift of the iodine

desorption feature to lower temperature most likely indicates repulsive interactions within the iodine adlayer [8]. The leading edge of the CF3 desorption initially shifts to lower temperature, then moves to slightly higher temperature again above 1 x 10 -6 Torr.s exposure. This complex behavior is probably due to repulsion of the adsorbed CF3 radicals by both iodine and CF 3. As the initial exposure increases and the iodine coverage increases, the CF 3 coverage decreases, reducing the CF3-CF 3 repulsive interaction and suggesting that the CF3-CF 3 repulsion is stronger than the CF3-I repulsion. The amounts of I and CF 3 desorbing, calculated by integrating the corresponding TPD traces of Fig. 2, are shown in Fig. 3. Above ~ 200 K both are initially proportional to the CF3I exposure; however, above 0.5x 10-6Torr.s exposure, the CF 3 radical desorption levels off and eventually decreases, while the I signal rises to 1.8 × 10 -6 Torr.s before leveling off. Since only CF3I , I and

C F2 + AND IODINE + TPD TRACES AFTER CF31 EXPOSURE OF THE Ag(1 1 q ) TAos=86 K I

I

B ~"

~

r,JV'--,,~

r"

I

dT/dt= 1K/sec

I

EXPOSURE [10"s torr sec] 2.1

~

x...,,

0.90.

s'x..,0.3_3L0 250

300

350

TEMPERATURE [K]

400

450

700

800

900

1000

TEMPERATURE [K]

Fig. 2. TPD spectra monitoring(a) CF3 radical and (b) atomic iodine desorption after exposing Ag(111) to CF3I at T= 86 K. The indicated exposures correspond to initial coverages of 0.02, 0.05, 0.08, 0.10, 0.12, 0.15, 0.18, 0.20, 0.22, 0.25, 0.28, 0.30 and 0.35 ML (see Section 3.2).

A. Szab6 et al./Surface Science 364 (1996) 345-366

350

C F2 ÷ AND IODINE + INTEGRATED TPD AREAS 6O

I

I

I

I

I

I

-

i+ 5o

8 40

"~

30

C F2 +

z~ lo

0 0.0

i 0.5

I 1.0

I 1.5

I 2.0

I 2.5

I 3.0

I_ 3.5

I

EXPOSURE[10" s torr sec]

Fig. 3. Integratedareas of the CF~ and I * TPD traces presentedin Fig.2.

CF 3 desorb, this change in the stoichiometry indicates that there must be an additional channel for CF3 desorption. The fragmentation pattern of the desorption indicates that CF3 radical desorption contributes at 110 K for exposures above 0.5x 10 -6 Torr.s. Fig. 4 shows CF3I + and CF~- TPD traces, collected at a heating rate of 0.03 K ' s - 1 . At this low heating rate, which enhances temperature resolution as compared to the other spectra collected at a heating rate of 1 K" s -1 (Fig. 1), the desorption features shift to lower temperature, as expected from the kinetic model developed to explain curve shapes in temperature programmed desorption [41]. The multilayer peak, split into a doublet, appears between 85 and 95 K. While the high temperature part of this doublet saturates with increasing exposure, the low temperature part does not. The desorption feature observed at 128 K at the 1 K . s -1 heating rate (Fig. 1) shifts to ,-~119 K for both in the CF~ and CF3I + desorption traces at the lower heating rate (Fig. 4). The 110 K desorption feature (1 K. s-l) drops to between

100 and 103 K, and, importantly, the position of the CF + peak is ~3 K lower than the CF3I + peak. The CF~ signal is attributed to both CF3I and CF3 radical desorption, while the CF3I + signal indicates only molecular CF3I desorption. Thus, CF 3 radical desorption occurs slightly before molecular CFaI desorption. Within our accuracy, the ratio of the integrated CF~- and CF + signals did not change when the heating rate was increased two orders of magnitude, from 0.03 to 3 K ' s - 1 (not shown).

3.1.2. CF3I adsorption on the iodine saturated Ag(111) Fig. 5a shows CF~ TPD traces collected after CF3I exposure of iodine-saturated Ag(ll 1). The surface was prepared by repeated adsorption of CF3I and desorption of CF3I and CF 3 radicals until molecular CFaI was the only desorption product at T< 500 K. The CF3I desorption feature at ~ 118 K saturates, and we attribute this feature to first layer adsorption; because the feature at 100 K has the characteristic features of desorp-

A. Szab6 et al./Surface Science 364 (1996) 345-366

351

LOW HEATING RATE TPD AFTER CF31EXPOSURE OF THE Ag(1 ] 1) I

I

I

I

EXPOSURE = 4 • 1 0 -6 torr s e c dT/dt=

0.03

K/sec

_J < Z

+

:£

C f 31

I 90

...... 100

I 110

120

130

TEMPERATURE [K]

Fig. 4. CF31 + and CF~ T P D traces collected after exposing Ag(111) to CF31 at T = 86 dT/dt = 0.03 K . s - 1. The exposure corresponds to 0.66 ML (see Section 3.2).

tion from a multilayer (does not saturate, overlapping leading edge, peak temperature increases with exposure), we assign it accordingly. Fig. 5b shows peak areas, calculated from the TPD traces, for monolayer and multilayer features. The sum is also shown and, within measurement uncertainty, the CF3I uptake rate (slope of the dashed line) does not change as the first layer saturates and multilayer adsorption starts.

3.2. Coverage calibration Knowing the adsorbate coverage in terms of adsorbed molecule/substrate atom is important in establishing a model for adsorbate dynamics. For this calibration we assume that the integrated TPD areas are proportional to the desorbed coverage. The proportionality constant can be calculated from a TPD trace collected from a sample with a known adsorbate coverage. With repeated CF3I exposure and heating to 500 K of the A g ( l l l ) surface, a well defined

iodine coverage of 0.33 ML is obtained [42,43], allowing calibration of the iodine TPD traces. The integrated peak area of the iodine TPD trace collected from our iodine saturated surface was 82 000 counts, which gives a proportionality constant of 250,000 per ML of iodine (1 M L = I adsorbate particle/1 surface silver atom). At exposures _<0.5 )< 10 6 Torr.s, all the adsorbed CF3I dissociates and desorbs in the form of CF 3 radicals and iodine atoms. The above iodine calibration then allows us to calibrate the uptake in monolayers vs. exposure in system units (Torr. s) at exposures <0.5 x 10 - 6 Torr. s. From Fig. 3 we calculate that 6 x 10 - 6 Torr. s exposure results in 1 M L = 1 CF3I molecule/1 surface Ag atom. Also, when all the adsorbed CF3I dissociates and the fragmentation products do not directly desorb during dissociation (at coverages < 0 . 5 X 1 0 - 6 Torr. s), the CF3 to iodine ratio is fixed. Based on the iodine calibration, we can calibrate the integrated TPD area for CF3 using Fig. 3. In the following, we also assume that the sticking

352

A. Szab6 et aL/Surface Science 364 (1996) 345-366

C F3+ TPD TRACES AFTER CF31 EXPOSURE OF THE IODINE SATURATED Ag(1 1 1 ) I

I

I

I

[

dT/dt=l K/sec

+.D

E ~3 .-.I

EXPOSURE [lOGtorrsec]

< Z

',.9 U3

O'

16.2

+ m 11

10.8 5.4

U

2.7 1,3s __._.T.~..,~_ 100

(a)

0.66

I

I

I

120

140

160

I

180

200

TEMPERATURE [KI

INTEGRATED TPD AREAS I

,<,, r~

<

• -II-

---0-

I

I

I

I

I

monolayer + multilayer multilayer monolayer / / /

I

/ / //e

/

/

/

coefficient of CF3I is the same, and independent of the CF3I coverage, on both the clean and iodine saturated Ag(111 ). This is supported by our results (Fig. 5b) showing that the uptake on the iodine covered surface increases linearly with exposure. This assumption is also sensible considering that physisorbed CF31 is bound to the surface by weak electrostatic forces that are predominantly dependent on the permanent dipole moment and the polarizability of the adparticle. Except for the multilayer case, molecular CF3I desorbs from both the iodine covered and the initially clean Ag(111) surfaces above 105 K, that is, higher than the exposure temperature. Recently the use of a sticking coefficient of unity has been suggested, under conditions similar to ours, to calibrate mass spectrometer sensitivity in terms of adsorbed monolayers [44]. Here we assume that if C-I bond cleavage occurs during adsorption, both fragments are retained on the surface. Adsorption through extraction of atoms from gas phase molecules has been observed [45], but this mechanism appears to be rare, and we assume it does not happen in the case of CF3I adsorption on Ag(111). Consequently, we proceed with the postulate that 6 x l 0 - 6 T o r r - s CF3I exposure at T_<90 K leads to 1 ML CF3I coverage both on the clean and iodine saturated Ag(111). In the following the exposure ML (monolayer) will be shown in parenthesis after the value of the exposure in system units (Torr. s).

3.3. Infrared results t--

g

1_/5/. e~He~ 0

(b)

2

I 4

I 6

I 8

I 10

I 12

I 14

I 1

EXPOSURE [10" 6 torr sec]

Fig. 5. (a) CF~- TPD traces collected after exposing the iodine saturated A g ( l l l ) to CF3I at T=86 K. (b) The integrated areas of the monolayer (high temperature peak) and multilayer (low temperature peak) desorption features. The dashed line is a least squares fit to the sum of the integrated peak areas in the monoand multilayer desorption features. The exposures in (a) correspond to 0.11, 0.22, 0.45, 0.9, 1.8, and 2.2 ML (see Section 3.2).

3.3.1. Clean Ag(111) Fig. 6 shows a typical RAIR spectrum collected after 4 x 10 -6 Torr. s (0.66 ML) CF3I exposure of the clean A g ( l l l ) at 85 K. Although the peak positions and shapes depend on the CF3I exposure, the annealing temperature and the surface iodine coverage, we can use gas phase and surface literature data to distinguish three regions in the IR spectrum. The regions ~1150-1200 and 1000-1100 cm -1 correspond to the asymmetric and the symmetric CF3 stretch modes, and 740-745 cm-1 corresponds to the symmetric CF 3 bending mode [2-4,6,16-18]. A peak observed at

A. Szab6 et aL /Surface Science 364 (1996) 345-366

353

TYPICAL RAIR SPECTRUM FROM THE CF31/Ag(1 1 1) SYSTEM EXPOSURE =

4-10

-6 torr sec

T~)s=86 K I

T

I

I

I

I

I

Isymmetric [ asymmetric C F3 stretch I

l.IJ Z < r~ r~ 0 t~O O0 <

/

700

I

I

I

I

I

I

800

900

1000

11O0

1200

1300

WAVENUMBER [cm" 1] Fig. 6. Typical RAIRS spectrum collected after exposing Ag(111) to 4 x 10-6 Torr. s CF3I (0.66 ML). Resolution = 8 c m - l .

high exposures at ~ 1020 cm -1 is a combination mode, from the C-I stretch (284 cm -1 in gas phase) and the symmetric CF 3 bending mode (741 cm-1 in gas phase) [ 18]. Since the intensity of the symmetric CF 3 bending mode is close to the noise level and is difficult to observe, we did not follow its development. For further measurements we used a narrow band Mercury-Cadmium-Telluride (MCT) detector which has a low energy cut-off limit at ~ 800 cm- 1 but enhanced sensitivity above 1000 cm -~ as compared to the wide band MCT detector. Fig. 7 shows the development of the asymmetric and symmetric CF 3 stretching vibrations with increasing CF3I exposure. At the lowest exposure only the feature corresponding to the symmetric CF 3 stretch mode appears. At higher exposures, we see the two features corresponding to the symmetric and asymmetric stretching modes developing simultaneously at ~ 1050 cm-1 and ~ 1200 cm- 1. As the exposure increases, the peak positions gradually shift to ~ 1080 and 1190 cm -1. Above ~ 6 x 10-6Torr.s (1 ML) new peaks appear at 1097 and at 1182 cm-1; these correspond to multilayer CF3I adsorption, and their position does not change with further exposure.

In Fig. 8 we show the relative integrated absorbance corresponding to the symmetric and asymmetric stretch features, calculated from the data in Fig. 7 (squares), as well as data points (circles) calculated from additional RAIRS measurements not shown in Fig. 7. The integrated absorbance was calculated for the asymmetric and symmetric regions (approximately 1000-1100 and 1140-1210 cm -~, respectively). No attempt was made to deconvolute overlapping peaks in these regions. The exposure dependence of the ratio shows interesting, reproducible trends. At the lowest exposures, the symmetric mode dominates. As the exposure increases to ~0.5-1.0 × 10 - 6 Torr's (0.08-0.16 ML), the ratio of the integrated absorbances becomes approximately one. The symmetric mode dominates again between 1 . 5 - 2 × 1 0 -6 Torr.s (0.25-0.33 ML) exposure, and the two absorbances become approximately equal between 4-8 × 10 -6 Torr.s (0.66-1.33 ML) exposure. At the highest exposure (multilayer) the ratio is ~ 0.62. In the gas phase, the ratio is 0.84, measured by backfilling the chamber with CF3I. We know that heating the CF3I/Ag (111) system above 150 K leaves only iodine and CF 3 radicals on the surface [8]. After an exposure of 0.4.10 -6

354

A. Szab6 et aL /Surface Science 364 (1996) 345-366

RAIRS SPECTRA FROM CF31/Ag(1 1 1) vs. EXPOSURE T~s=86 K I

l

0.001 EXPOSURE

!

I

I

(~ I I

I

I O.O1 f~

LU L)

z<

Z

<

m

0

0.9

nn n,,

EXPOSURE

0

[10"6 torr sec]

rn

<

<

0.4

15.0 - lo.o - 6 . o j

-4.z _j :.3 .o 2.4__..~__j

0.1 900

4

:---.)

lOOO

11oo

1zoo

13oo

WAVENUMBER[cm- 1]

900

1000

1100

1200

1300

WAVENUMBER [cm- 1]

Fig. 7. RAIRS spectra collected after exposing A g ( l l l ) to various amounts of CF3I at T = 8 6 K. Resolution is 8 cm -1 at exposures 0.1-0.3 x 10 -6 Torr" s and 4 cm-1 at higher exposures. The exposures correspond to 0.015, 0.03, 0.05, 0.07, 0.14, 0.15, 0.2, 0.3, 0.4, 0.5, 0.7, 1.0, 1.6 and 2.5 ML.

Torr's (0.07 ML), the asymmetric stretch decays and the symmetric stretch increases with heating (Fig. 9). After heating to 150 K, we see only the symmetric CF3 stretch mode. Fig. 10 and Fig. 11 show RAIR spectra collected after exposing clean A g ( l l l ) to 0.9 and 1.5 x 10 -6 Torr. s CF3I (0.15 and 0.25 ML), respectively, and heating to the indicated temperatures at a rate of 0.5 K ' s followed by cooling below 90 K. For the 0.9 x 10 -6 Torr. s (0.15 ML) exposure, as the sample temperature is increased the RAIR feature corresponding to the asymmetric CF3 stretching mode gradually disappears, and a new broad feature develops in the region 950-1020 cm-1. Although this feature is barely above the noise level and is difficult to distinguish from a modest shift in the baseline, it is reproducible. No baseline shift was observed in control experiments involving identical sample treatment without CF3I exposure. Note that the intensity and the

position of the symmetric CFa vibration at ~ 1060 cm-1 does not change significantly, while the feature at ~1200 cm -1 completely disappears and the 1020 cm-1 wavenumber develops. When the sample temperature is increased to 105 K after the 1.5 x 10 - 6 Torr.s exposure (0.25 ML) the initially sharp symmetric CF 3 stretching peak disappears, and a narrow asymmetric CF 3 stretching band develops (Fig. 11). With further heating to 126 K, the asymmetric mode disappears, and the symmetric CF 3 stretch mode becomes similar to that observed for 0.9 x 10 -6 Torr. s (0.15 ML). The symmetric stretch modes are present after heating to 250 K but disappear after brief annealing above 400 K (not shown). The ratio of the integrated absorbance of the asymmetric and symmetric stretches, calculated from the spectra in Fig. 10 and Fig. 11, appears in Fig. 12. The initial large difference in the absorption ratios agrees with the result in Fig. 8. As the

A. Szab6 et al./Surface Science 364 (1996) 345-366

355

RATIO OF THE INTEGRATED ABSORBANCE OF THE ASYMMETRIC AND SYMMETRIC CF3 STRETCH MODES FOR CF31/Ag(1 1 1 ) I

.

.

.

.

.

.

.

.

I

.

.

.

.

.

.

.

.

I

1.5

o p.< o~ "tC.~ t--

1.0 I

I-el) eo

I

b

±

C_) puJ

~E >U3 C~

0.5

LU

~E ~E >c/3

.<

0.0

2

3

4

567

0.1

2

3

1

4

567

10

EXPOSURE [10" ~ torr see]

Fig. 8. The ratio of the integrated absorbance of the asymmetric and the symmetric CF a stretch IR features. The solid line is included to guide the eye.

samples are heated to above 120 K the difference in the ratios disappears and the two curves converge. In Fig. 13 we show RAIR spectra collected after the Ar + sputtered, but not annealed, A g ( l l l ) surface was exposed to 0.9x 10 - 6 T o r r ' s (0.15 ML) CF3I at 86 K. We expect that Ar + sputtering without annealing results in a rough surface with defect sites. In the spectrum collected immediately after the exposure, a new feature at ~ 1040 cm -x is visible. After annealing to 120 K, this feature decreases and a broad shoulder between 950 and 1050 c m - 1 appears.

3.3.2. lodine saturated Ag(111) Fig. 14 shows RAIR spectra collected after saturating the A g ( l l l ) surface with iodine. For exposures up to 1.4 × 10 - 6 Torr" s (0.25 ML), the position and the shape of the vibrational features do not change significantly, but a qualitative change is observed at 1.9 x 10 - 6 Torr. s (0.32 ML). Above this exposure only the intensity changes, that is, the peak shapes and positions are unaffected by exposure. Fig. 15 shows that the ratio of the integrated absorbance corresponding to the asymmetric and symmetric CF 3 stretch modes stabilizes at 0.62, characteristic of multi-

A. Szab6 et al./Surface Science 364 (1996) 345-366

356

RAIR SPECTRA FROM CF31/Ag(1 1 1)vs. HEATINGI]

RAIRSPECTRAFROMCF31/Ag(1 11) vs. ANNEALINGTEMPERATURE EXPOSURE=0.9.10 s torr sec T~as=86K

T~s=86 K I

T

I

I

i

0.4"10-6 torr sec 0.0005

i

i

exposure

t

iO,O01 ANNEALING TEMPERATURE

A

/

<

0.4"10 6 torr sec exposure and short I

800 900

1000 1100 1200 WAVENUMBER[cm 1]

1300

Fig. 9. RAIR spectra collected after exposing Ag(111) to 0.4 x 10 -6 T o r r . s C F f l (0.07 M L ) and heating the system to the indicated temperatures. Resolution = 8 c m - 1.

layers, after about 3 x l 0 - 6 T o r r ' s (0.5 ML) exposure. Heating the CF3I/I/Ag(lll) system results in partial desorption of CF3I; no decomposition is expected. As the CF31 desorbs, the RAIR spectra (not shown) behave similarly to those in Fig. 14, telling us that the adsorption-desorption process on the iodine covered surface is reversible.

4. Discussion

Heating the CFaI/Ag (111) system results in rich thermal chemistry. In agreement with previous results, we find that the adsorbed CFaI desorbs molecularly or dissociates and desorbs as atomic iodine and CF 3 radicals [8,9]. CFa radical desorption occurs at ~ 110 K and also at -,~300 K 1-35]. The simplest process, molecular CFaI desorption, occurs in several steps. In the following, we first discuss how the adsorbate orientation can be deduced from RAIRS data and the extent of ther-

I

900

I

I

I

1000 1100 1200 WAVENUMBER[cm"1]

1300

Fig. 10. RAIR spectra collected after exposing A g ( l l l ) to 0.9x 1 0 - 6 T o r r . s CF3I (0.15 ML) and heating the system to the indicated temperatures. R e s o l u t i o n = 4 cm -1.

mal decomposition of CF3I on the Ag(111) during adsorption. From this, we propose a model of the surface dynamics of CF31 on the Ag( 111 ).

4.1. Infrared spectroscopy and CF3I orientation Surface infrared selection rules determine whether a given vibrational mode of the adsorbate can be excited by IR irradiation of the surface. These rules limit the observable modes to those with the proper symmetry and with non-zero projection of the dynamic dipole moment on the surface normal 1-46]. Assuming the optical response does not change with varying orientation and binding state of the weakly bound adsorbate, the infrared absorption is proportional to cos2(0), where 0 is the angle between the direction of the dynamic dipole moment and the surface normal [47-49]. While this is true for individual molecules in a fixed orientation, the RAIR spectra are collected on ensembles of molecules vibrating about their equilibrium orientation. For example, a vibrational amplitude of 21 ° about the rotational axis

357

A. Szab6 et aLISurface Science 364 (1996) 345-366

RAIR SPECTRAFROMCF31/Ag(1 1 1) vs. ANNEALINGTEMPERATURE EXPOSURE=1.5-10" 6 torr

sec

T~s=87 K l

I

RATIO OF THE INTEGRATED PEAK AREA OF THE ASYMMETRIC AND SYMMETRIC STRETCH MODES vs. ANNEALINGTEMPERATURE

FROM CF31/Ag(1 1 1 ) I

I I

ANNEALING TEMPERATURE

1 .J~/

i I O IZ I

I .002

I

I

I

I

EXPOSURE: X ---0- 0.9 "10"6 torr sec ~ X ~ - I - l " 5 ° l O ' 6 t ° r r sec

1,4

i 1,0

~7 0.8 k)

i

~ 0.6

7.\T.L.

~

0.4

U

~ 0.2 ~ 0.0

800

900

1000 11O0 1200 WAVENUMBER[cm- 1]

90

100 110 120 130 140 150 ANNEALINGTEMPERATURE[K]

1300

Fig. 11. RAIR spectra collected after exposing A g ( l l l ) to 1.5 x 10 6 Tort. s CF3I (0.25 ML) and heating to the indicated temperatures. Resolution =4 cm- 1.

of the molecule was reported for solid CFaI [50]. Vibration of adsorbed molecules about the equilibrium position, especially in the case of weakly physisorbed particles, may lead to the appearance of otherwise non-observable transitions. From the six fundamental vibrational transitions of CF3I (three of the eigenmodes are degenerate) we can observe two between 800 and 4000 c m - 1 The symmetric CF3 stretching (~1000-1100 cm- 1) mode has a dynamic dipole moment parallel to the C-I axis, while the (degenerate) asymmetric C - F stretching modes (~ 1150-1200 cm -1) have dynamic dipole moments normal to the C-I axis. For the gas phase CF 3 radical, the symmetric and asymmetric stretch were observed at 1090 and 1260 cm -1, respectively [51]. Similar to the CF3I, the dynamic dipole moment of these transitions for CF3 are parallel to the rotational axis for the symmetric mode and normal to the rotational axis for the asymmetric stretching modes. For adsorbed species, observation of the symmetric modes indicates that the rotational axis of the adsorbate (CF3I or CF3) is not parallel to the

Fig. 12. Ratio of the integrated absorbance corresponding to the asymmetric and the symmetric CF 3 stretch modes after exposing A g ( l l l ) to 0.9 and 1.5 x 10 6 T o r r s CF3I (0.25 ML) at 86 K and annealing to the indicated temperatures.

RAIR SPECTRA AFTER ADSORPTION OF CF31 ON Ar + SPUTTERED Ag(1 1 1 ) EXPOSURE=0.9"10"6 torr sec T~=96 K lextr a feature , , i

~:

i

on the Ar+ sputtered surface 0.001 ~/~

after

short a n n e a l j ~ _ _ at 120K

800

900 1000 11O0 1200 1300 WAVENUMBER [cm" i]

Fig. 13. RAIR spectra collected after exposing Ar + sputtered A g ( l l l ) to 0.9× 10 -6 Torr-s CF3I (0.15 ML) and heating to the indicated temperatures. Resolution = 16 cm- '.

358

A. Szab6 et aL/Surface Science 364 (1996) 345-366

RAIR SPECTRA AFTER CF31 EXPOSURE OF THE IODINE SATURATED Ag(1 1 1 )

RATIO OF THE INTEGRATED ABSORBANCE OF THE ASYMMETRIC AND SYMMETRIC CF3 STRETCH MODES

TAPS=86 K

~ 1.2

I

I

I

I

i

2

I 3

I 4

I 5

~ 1.0 ~ 0.8 I'--

0.6

~t~ 0.4

~ o.z i >~ 0.0~

I 1

,I

EXPOSURE [1 O"s torr sec] Fig. 15. Ratio of the integrated absorbance corresponding to the asymmetric and the symmetric CF 3 stretch modes after exposing iodine-saturated A g ( l l l ) to various amounts of CF3I at 86 K.

900

1O00

11O0

1200

1300

WAVENUMBER [crn" 1] Fig. 14. RAIR spectra collected after exposing iodine-saturated A g ( l l l ) to various amounts of CF3I at 86 K. Resolution=4 cm -1. The exposures correspond to 0.08, 0.18, 0.23, 0.32, 0.38, 0.46, 0.53, 0.68 and 0.98 ML.

surface; to observe the asymmetric modes, the rotational axis must be tilted from the surface normal. Consequently, when the asymmetric-tosymmetric stretch ratio (either for molecular CF3I or CF 3 radical) is large, the rotational axis of the adsorbates is strongly tilted from the surface, and when it is small the adsorbates are bound to the surface with their rotational axis close to the surface normal. While the purpose of the present work is qualitative description of the molecular orientation, we briefly overview how the intensity of IR absorbance is related to the molecular orientation in order to estimate the orientation of the CF 3 and CF3I. Intensity analyses of IR absorbance of orthogonal vibrational modes have been used previously to quantify the orientation of adsorbates (e.g. Refs. [49]). The integrated IR absorbance is propor-

tional to the dipole density on the surface (n), the square of the transition dipole moment component in the direction of the surface normal (Mz)2, the peak frequency and the weakly frequency dependent surface reflectivity (G(fl)), where fl is the beam angle of incidence from the surface normal. The integrated absorbance ratio of the CF3 asymmetric and symmetric modes (A/S) can be expressed then as,

A / S = M 2 x sin(0) 2 x va × G(fl(va))/M 2 x cos(0) 2 x

Vs x 6(/~(Vs)),

where 0 is the angle between the molecular axis and the surface normal and v~ and vs are the vibrational frequencies of the asymmetric and symmetric CF 3 stretch modes 1-49]. Collecting the constants, we get A/S = C x tan (0)2. If for the present case we assume that, for multilayer CF3I, the average molecular orientation is 45 ° from the surface normal, then from the A/S = 0.62 measured for multilayers we obtain C---0.62. In the following we use this value to estimate the average orientation of the I - C F 3 bond for the ensemble of adsorbed molecules.

4.2. Molecular versus dissociative adsorption Interpretation of our results requires knowing whether C - I bond scission occurs during adsorp-

A. Szab6 et aL /Surface Science 364 (1996) 345-366

tion at T< 90 K, or if further thermal activation is needed for dissociation. Previous XPS results show that at low exposures CF3I dissociatively adsorbs (I-CF 3 bond scission) on the A g ( l l l ) when the sample temperature is 105 K [8]. In the present work we use RAIRS data to decide whether CF3I adsorbs molecularly or dissociatively at T< 90 K. After an exposure of 0.1 × 10 - 6 Torr" s (0.02 ML) at 86 K, only the symmetric CF 3 stretch mode is observable. This absorption may be attributed to either CF3I molecules or CF 3 radicals bound to the surface with their rotational axis along the surface normal. Fig. 16 compares the RAIRS spectra collected after 0.1 and 0.4× 10 - 6 Torr.s CF3I exposure (0.02 and 0.07 ML) with spectra of C F 3 + I and CF3I+I. Looking only at the symmetric stretch, note the similarity (peak posiCOMPARISON OF RAIR SPECTRA COLLECTED AFTER LOW CF31 EXPOSURE OF THE CLEAN Ag(1 1 1 ) TO THOSE COLLECTED FROM MOLECULARLY ADSORBED CF31 AND CF3 RADICALS I

I

I

i

i

i

molecular CF31: 0.51 • 10" 6 torr sec CF31 exposure of the I/Ag(1 1 1 )

0 . 4 , 1 0 - 6 t o r r sec CF31

exposure of the Ag(111) z

<= ',

0.1,10" G torr sec CF31 exposure of the Ag(111 )

t

C F3 radical:0.4,10"6 torr sec C F31 exposure of the clean Ag(1 1 1), followed by short annealing at 1 50 K 950

I

I

I

I

I

I

1000

1050

1100

1150

1200

1250

WAVENUMBER [cm" 1]

1300 _

Fig. 16. RAIR spectra collected after 0.1 and 0.4 x 10 -6 Torr. s CF3I exposure (0.02 and 0.07 ML) of the clean A g ( l l l ) compared with spectra collected from molecular CF3I on I/Ag(111) and CF 3 on A g ( l l l ) . The data are from Fig. 7, Fig. 9 and Fig. 14.

359

tion, width and intensity) of the spectra collected after low CF3I exposure of the clean A g ( l l l ) at low temperature to that collected from the CF3 radicals, and the dissimilarity to the spectrum for CF3I. This suggests that at low exposures CF3I adsorbs with C--I bond cleavage at T = 86 K. The CF a radical may adsorb with its rotational axis along the surface normal if it binds to the surface through an sp 3 hybridized carbon atom. At the lowest CF3I exposure (0.1 x 10-6 Torr.s, 0.02 ML), we see only the symmetric CF a stretch. Likewise, in the case of the PF3/Ni(lll) system, when PF3 adsorbs with its rotational axis normal to the surface [52], only the symmetric PF3 stretch mode was observed with RAIRS [53]. As the exposure is increased to 0.2 x 10 .-6 Tort. s (0.03 ML) and above, an absorption feature appears in the asymmetric CF3 stretch region. Increasing the exposure from 0.2x 10-6 to 0.9x 10-6Torr.s (0.03 to 0.15 ML) does not noticeably change the symmetric CF3 stretch absorbance, while the asymmetric CFa stretch absorbance increases significantly. Also, as noted in Section 3.3.1, annealing the sample to 140 K after 0.9 x 10 - 6 Torr.s CF3I exposure (0.15 ML), which eliminates all molecular CF3I, results in no noticeable change in the peak position and integrated absorbance of the symmetric CF3 stretch, while the asymmetric CF3 stretch mode disappears. Their independent behavior suggests that the symmetric and asymmetric CF3 stretch modes belong to separate adsorbed species. We assign, as noted earlier, the symmetric stretch to CF3 adsorbed with its rotational axis normal to the surface. The asymmetric mode is most likely due to CF31 molecules adsorbed with their C-I axis parallel to the surface; assignment to tilted CF 3 radicals would be inconsistent with the absence of intensity in this region when, after a high initial exposure, the CF3I/Ag(lll) system is annealed so that only I and CF3 are left on the surface. After this treatment, only a broad absorption feature around ~ 1020 cm-1 is seen in addition to the symmetric CFa stretch mode. Although we have not yet made an assignment of this broad absorption feature, it is most likely due to inhomogenous adsorption sites; some CF3 adsorbs close to another CF 3 or I.

360

A. Szab6 et aL/Surface Science 364 (1996) 345-366

The integrated absorbance of the symmetric CF 3 stretch mode, which is approximately proportional to the CF 3 radical coverage, does not change substantially as the exposure is increased from 0.2 to 0.9 × 10 -6 Torr" s (0.03-0.15 ML). From this we estimate that dissociative CFaI adsorption occurs only at exposures less than 0.2 × 10 -6 Torr- s (0.03 ML), resulting in 0.06 ML combined CF 3 and I coverage. As the iodine coverage builds up, it hinders and eventually prevents further C-I dissociation. Inhibition of I - C F 3 bond cleavage by preadsorbed iodine on A g ( l l l ) is well documented

[8]. Some of the 0.03 ML CF3I dissociation during adsorption may be related to a small number of defect sites present on the surface. However, based on Fig. 13, a predominant role of defect sites in CF3I dissociation can be ruled out. CF3I adsorbed on intentionally roughened surfaces results in an absorption band at ~1020 cm -1, which is not observed when well annealed Ag(111) is exposed to CF3I. This indicates that the defect induced adsorption state of CF 3 differs from that we observed on annealed surfaces. 4.3. Adsorbate orientation - CF31/Ag(111)

As discussed in Section 4.1, the ratio of the integrated absorbance of the asymmetric and symmetric CF 3 stretch (A/S) is related to the orientation of the C-I bond. From the data in Fig. 8, we conclude that at low coverage (~<0.15 ML) the non-dissociated CF3I adsorbs on Ag(111) with its molecular axis oriented parallel to the surface. This is similar to the theoretical prediction for adsorption of a single methyl halide molecule on a nonconductive ionic surface, for which the preferred orientation is with the molecular axis parallel to the surface [54]. In both cases weakly physisorbed particles are bound to the surface through dispersive, van der Waals and electrostatic forces between the adparticle and the surface. A conspicuous decrease in the A/S ratio appears as the CF3I exposure is increased from 0.9 to 1.2 x 10 - 6 Torr. s (0.15 to 0.2 ML). This decrease indicates reorientation of the adsorbate, bringing the rotational axis of the CF3I closer to the direction of the surface normal. Adsorbate reorientation

with increasing coverage has been reported for alkyl halides on P t ( l l l ) [48,49], MgO(100) [55] and NaCI(001) [56], and the problem has also been treated theoretically [54]. From the discussion in Section 4.1 we can make a rough estimate of the C-I bond tilting from the surface normal after reorientation. Assuming that the contribution of the CF 3 radicals to the symmetric CF 3 stretch absorbance after 1.2 x 10 -6 Torr. s exposure (0.2 ML) is the same as after 0.9 × 10 -6 Torr.s exposure (0.15 ML), we can subtract the CF 3 contribution from the symmetric stretch mode and arrive at an A/S ratio of ~0.84. This corresponds to 49 ° tilting of the C-I bond from the surface normal. The A/S ratio starts increasing again, meaning that molecules with the C-I axis tilted from the surface normal appear, as the CF3I exposure is increased above 2 x 10 -6 Torr's (0.3 ML). This change likely signals the completion of the first layer, and in the following we will assume that 0.3 CF3I per substrate atom corresponds to saturation of the first layer. Based on the density of solid CF3I a full layer of CF3I, on the A g ( l l l ) was estimated to be 0.25 ML [9], close to the above value. 4.4. Low temperature CF 3 radical desorption

Considering that the C-I bond energy in CF3I is ~224 kJ.mo1-1 [36], the desorption of CF 3 radicals from fragmentation of adsorbed CF3I during sample surface heating from 85 to 110 K is really surprising. Summarizing our results, we propose the following model to explain this interesting phenomenon. On clean A g ( l l l ) in the low coverage limit (<0.03 ML), CF3I adsorbs with C-I bond dissociation. As the I + CF 3 coverage builds up, a potential barrier develops that hinders further C-I bond breaking. An additional ~0.12 ML CF3I adsorbs molecularly (a combined dissociated and non dissociated coverage of ~0.15 ML), with the C-I axis parallel to the surface, but in the coverage range of 0.15-0.30 ML the adsorbed molecules reorient to bring their C-I axes closer to the surface normal. Further adsorption occurs in the second and additional layers.

A. Szab6 et al./Surface Science 364 (1996) 345-366

If the CF3I/Ag(lll) system is heated, some of the CF3I molecules dissociate. The fate of the dissociation products - adsorption versus desorption - is dictated by the adsorbate orientation. Both of the dissociation fragments are retained on the surface when the admolecules are bound with their C-I axis close to the surface. However, when the C-I axis is tilted away from the surface the CF 3 radical is ejected into the gas phase during dissociation (Fig. 17). This model, in which CF 3 desorption is dictated by the C-I bond orientation, is supported by the conspicuous coincidence of the CF 3 radical desorption with the reorientation of the CF3I rotational axis. Results from our research group also show directionally focused desorption of the CF 3 radicals [35]. Within the framework of the van Willigen model ['40], this implies non-thermal kinetic energy of the desorbate, most likely due to immediate CF 3 radical desorption as the C-I bond breaks, without reaching thermal equilibrium with the surface. Another model, related to the limited number of adsorption sites available for the dissociation pro-

A. Low covera_oe : after thermal dissociation both the iodine and the CF3 are bound to the surface

B. High coverage : during thermal dissociation the CF 3 radical desorbs

t

Fig, 17. Illustration showing the proposed models. At exposures up to 0.15 ML, molecular CF3I adsorbs with its C-I bond axis parallel to the surface (a) and at exposures between 0.15 and 0.3 ML, CF3I adsorbs with its C-I bond axis tilted away from the surface (b).

361

ducts after C-I bond cleavage, may also explain the coverage dependence of the CF3 radical desorption, As the dissociation proceeds, the surface iodine and CF 3 coverage builds up, limiting the number of adsorption sites. There may not be two neighboring sites available for the iodine and the CF 3 that derive from the dissociation of one CF3I molecule, which would allow only one dissociation fragment to adsorb. Assuming that these fragments compete for the same adsorption site and the driving force for adsorption is monotonically related to the binding energy to the surface, we would predict adsorption of iodine and desorption of CF 3. Although this model is perfectly plausible, we believe that the former model, which not only explains the coverage dependence of the low temperate CF 3 radical yield but correlates the CF 3 desorption with the reorientation of the adsorbate, is more complete. Our results, showing that the ratio of the amount of CF 3 desorbed in the low temperature and high temperature TPD peaks is invariant as the heating rate increases by two orders of magnitude (0.03 K s-1 versus 3 K s-1) argue strongly against a model in which the dissociation-desorption process and the ratio of these processes are kinetically controlled. Desorption of CF3I molecules following the CF 3 radical desorption at ~ 110 K (Fig. 4) is most likely due to fluctuations of the local coverage during C-I bond cleavage. For example, when both fragments are retained on the surface (dissociation with CF 3 adsorption), the local surface coverage increases, which could destabilize, and lead to desorption of some CF3I molecules. As the sample temperature is increased above 105 K and the surface CF3I coverage decreases, a narrow asymmetric CF 3 stretch peak appears and the A/S absorbance ratio increases (Fig. 11). This indicates that the remaining CF3I are adsorbed with their C-I bond tilted from the surface normal. As the I + C F 3 coverage is increased, the C-I dissociation is hindered and eventually inhibited, and CF3I desorption is favored over dissociation during thermal agitation of the system. The nondissociated CF3I desorbs from the surface at ~ 128 K. On clean Ag(111), CF3I in excess of 0.3 ML adsorbs in the second and further layers. These

362

A. Szab6 et al./Surface Science 364 (1996) 345-366

layers contribute nothing to the CFaI/Ag(lll) interface chemistry and desorb without dissociation in the multilayer peak at ~ 100 K. The second layer is slightly different from a true multilayer. In the TPD spectra at low heating rate the second layer peak can be resolved (Fig. 4) and also the RAIR spectrum from a double layer is substantially different from a genuine multilayer structure (Fig. 7).

4.5. On the strength of the CFaI/silver interaction In the previous section we implicitly assumed a weak interaction between the adsorbed CF3I and the substrate, an interaction dominated by London and electrostatic forces, without formation of (covalent or ionic) chemical bonds. However, the interaction destabilizes the C-I bond sufficiently so that it breaks during a modest annealing to 110-120 K. The conflict is resolved by a model, in which the carbon-iodine bond cleavage is initiated by electron transfer from the substrate to the adsorbate [35]. This results in formation of CF3I-, in which the potential is repulsive with respect to the carbon-iodine bond that, assuming the excited state is not quenched in the meantime, leads to C-I bond cleavage. This process is initiated by a sudden electron transfer, before the formation of a strong chemical bond. The desorption activation energy of CF3I, calculated from the analysis of the desorption rate, is ~ 20 kJ mol- 1. Although calculated for I and CF 3 covered Ag(111), and not for clean A g ( l l l ) (molecular CF3I desorption occurs only after substantial amount of dissociation), it qualitatively supports our assumption of weak CFaI-substrate interaction. The full interaction potential can be described only on a multi-dimensional surface that must include the C-I bond distance and at least the CF3I-surface distance for a given molecular orientation. The dissociation process is not very sensitive to the C-I bond orientation; our RAIRS data show dissociation at ~ 100-120 K both when the C-I bond direction is close to the surface plane and when the C-I axis is tilted towards the surface normal. This is similar to the results in the oriented CF31 + K crossed beam experiments, in which the

reaction rate is not sensitive to whether the potassium atom approaches the CFaI from the iodine end or from the CFa end [20]. The C-I bond cleavage must be exothermic on the clean Ag(lll), since it spontaneously and irreversibly occurs at T~90 K. The CF3-Ag(lll) and I-Ag(111) bond energies estimated from analysis of TPD spectra are 16-66 and 208-226 kJ mol-1, respectively, and the I-CF3 bond energy is ~224 kJ mol. Summing the extremes of these values for CF3I dissociation in which both fragmentation products adsorb on the surface, yields 0-68 kJ mo1-1 exothermicity, in agreement with our observation of reaction at ~ 100 K.

4.6. Multilayer structures The RAIRS data in Fig. 7 show that in the CF3I/Ag(lll) system, the surface independent multilayer appears after adsorption of > 1.5 ML CF3I. Since one full layer of CF3I on the Ag(111) corresponds to ~0.3 ML, about five layers are adsorbed before the surface independent multilayer appears. The TPD spectrum collected at 0.03 K s-1 also reveals a structured multilayer desorption peak. The small peak corresponding to about 0.3 ML CF3I desorption at the high temperature tail of the multilayer TPD peak is assigned to desorption of the second layer CF3I. At the highest exposure in Fig. 7 and Fig. 15 × 10 -6 Torr.s (2.5 ML), the two intense peaks observed at 1097 and 1182 cm -1 belong to the symmetric and asymmetric stretch modes of CF3I in the multilayer. Observation of singlet peaks, as opposed to doublets, suggests that the CFa moieties are in chemically equivalent sites in the solid (multilayer) phase. This is consistent with the results of Clark et al. [50], who, based on neutron diffraction results, found that solid CF3 I above 115 K can be best described by one crystallographically distinct molecule. They found a different phase of solid CF3I under 115 K, but unfortunately they were not able to provide a detailed model for its crystal structure. Singlet but somewhat broadened RAIRS features were observed in the case of CH3I multilayers [57]. At a coverage of ~0.8 ML we observe doublet peaks similar to those reported by Myli and Grassian

A. Szab6 et al./Surface Science 364 (1996) 345-366

which they assigned to the multilayer [3]. This coverage, although it corresponds to several layers of CF3I, is apparently insufficient to form a substrate independent multilayer.

4.7. CF3I adsorption on iodine saturated surfaces CF3I adsorbs on the iodine saturated surface without dissociation. TPD spectra of CF3I from the iodine covered surface show one feature that saturates at ~0.3 ML coverage, and one for the multilayer desorption. The RAIR spectra also show a qualitative change as the exposure (coverage) increases from 0.22 to 0.32 ML, which we attribute to saturation of the first layer. Within the accuracy of the measurement the single layer coverage on the clean and the iodine saturated Ag(111) are the same, --~0.3 ML. The ratio of the integrated absorbance of the asymmetric and symmetric CF 3 stretches indicates that, as the coverage increases above ,,~0.15 ML, the average C-I axis orientation changes, tilting towards the surface normal. This was also observed for CF3I adsorption on clean Ag(lll); with increasing coverage the adsorbate in the first layer changes orientation to relieve stress due to crowding. On the iodine-saturated surface, RAIR spectra characteristic of multilayers develop at ,~0.6 ML CF3I coverage, less than that needed for clean A g ( l l l ) to reach the surface independent multilayer state. Thus, compared to clean Ag, I/Ag has less long-range influence on the CF3I orientation. 4.8. Molecular orientation and photochemistry of

the CF3I/Ag(111) system In previous work in this research group, I - , I and C F 3 desorbed when the CF3I/Ag(111) system was irradiated by 248 and 193 nm photons [9]. The onset of CFa radical desorption at 0.05 ML coverage I was correlated with the appearance of physisorbed, non-dissociated CF3I on the surface. At coverages <0.15 ML the kinetic energy of the photodesorbed CF3 radicals was ~0.3 eV; above 1 Coverage calibrations here and in Ref. [9] differ: 1 ML in Ref. [9] corresponds to ~0.15 ML in the present work. We quote coverages based on the present reference calibration.

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~0.15 ML, along with this slow channel, a new, fast desorption channel appeared with a kinetic energy >0.6 eV. At both 193 and 248 nm, the fast channel grew with coverage and saturated between 0.45 and 0.6 ML, i.e. 3 to 4 layers of CF3I. At 248 nm, the slow channel decayed with increasing coverage and became negligible when the coverage reached 0.45 ML. The situation at 193 nm was qualitatively different. The slow channel intensity rose monotonically with coverage and dominated the fast channel. At both wavelengths, the intensity of the slow CF 3 channel tracked the I - signal. The model proposed involved a combination of substrate-mediated and direct photoexcitation of CF3I. The former involved electron attachment and lead to slow CF 3 and I - . The latter lead to fast CF 3. Based on the present results, we propose the following refinement of this model, particularly at low coverages. At coverages less than 0.15 ML, where the C-I axis of the adsorbed CF3I is parallel to the surface, we propose that photon induced dissociation of the C-I bond, by either excitation path, leads to desorption only after the momentum of the CF3 radicals is redirected towards the vacuum by a collision with the Ag surface. As the coverage is increased above 0.15 ML the C-I bond tilts away from the surface, allowing some of the CF3 radicals to leave the surface directly, without collision. In the absence of collisions with the surface, we expect the kinetic energy to be higher. The slow desorption channel contributes even above 0.15 ML CF3! coverage and the yield tracks the yield of I - [9]. This has been interpreted in terms of contributions from charge transfer processes to form CF31 -. Another plausible contribution may come from CF3I molecules bound to the surface by their CF 3 end. A similar model was suggested for methyl halides adsorbing on dielectric surface with their dipole moments in an antiferroelectric configuration [55]. The above model does not contradict our RAIRS results, which tell us only along which direction the dynamic dipole moment is oriented, and not whether CF3I binds to the A g ( l l l ) through I or CF 3. CF3 radical desorption at ~ 110 K, suggests that some CF31 binds through I. CF31 molecules bound to the surface through

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CF3 close to the surface, would energetically inhibit iodine desorption, since the I-CF3 bond energy (224 kJ tool-1) is much larger than the energy that is gained when only CF 3 adsorbs on the surface (16-66kJ mol-1). Consequently, the absence of iodine radical desorption at low temperature does not exclude the possibility that some CF3I adsorb with their CF 3 end close to the surface. In Ref. [9] the onset of I - photodesorption occurred at ~0.15 ML CF3I coverage, i.e. less than saturation of the first overlayer at ca. 0.3 ML. The absence of I - desorption when the C-I axis is parallel to the surface indicates that I ions cannot desorb when the C-I axis is parallel to the surface. It also suggests, consistent with the above refinement, that some CF3I molecules are bound with their iodine atom pointing away from the surface. This differs from the crystal structure of solid CFaI where bilayers form, in which the neighboring CF3I molecules have parallel dipole orientation and the subsequent layers have opposing dipole orientation. The change of the RAIR spectrum (Fig. 7) after adsorption of several layers of CF3I on A g ( l l l ) may indicate a structural change to that found in bulk CF3I crystals. The similarity of the CF3I/Ag(lll ) and the CHaI/Mg(100) [-55] systems may represent a broader class of phenomena for physisorbed particles that have a large permanent dipole moment. At low coverages these particles are held on the surface by substrate-adsorbate interactions. On the other hand, Monte Carlo modeling of CH3I and CH3Br on the LiF(001) surface suggests that at high adsorbate coverages when a large number of molecules are accommodated near the surface, interactions within the adlayer dominate the adsorbate-substrate interactions [54]. The largest contribution to the adsorbate binding energy comes from dispersive forces between neighboring admolecules; electrostatic forces between the permanent dipole moments of the admolecules also make a significant contribution. The dipole-dipole (electrostatic) attraction is maximized when the admolecules are oriented in an antiparallel fashion. The antiparallel dipole orientation was favored by ,-~0.03eV (3kJ mo1-1) for CH3I or CH3Br adsorbed on the LiF(001 ), and in these two systems

the neighboring adsorbates are believed to bind in this way.

5. Summary We have studied the thermal chemistry of CF3I adsorbed on clean and iodine saturated (O~ = 0.33 ML) Ag(lll). On clean A g ( l l l ) we learned that: At low coverages (Ocr3~<0.03 ML), CF3I adsorbs, at 85 K, with dissociation of the C-I bond. Above this coverage CF3I adsorbs in molecular form. Molecular adsorption at OCF3~< 0.15 ML occurs with the C-I axis parallel to the surface and at OCF3~ >0.20 ML, with the C-I axis tilted about 49 ° from the surface normal. After adsorption of CF31 on the clean Ag(111) at 85 K, annealing of the system causes some I-CF3 bond dissociation, which is accompanied, at initial CF3I coverages >0.15 ML, by desorption of CF3 radicals at ~ 110 K. The saturation coverage of the first layer is 0.3 ML (0.3 CFaI molecule per surface silver atom). RAIRS data suggest that the surface independent multilayer appears only after ~1.5 ML CFaI adsorption (i.e. 5 layers). CF3I adsorbed in the second and further layers desorbs without dissociation. On the iodine covered Ag(111) we found that: The first layer is saturated after adsorption of 0.3 ML CF3I. RAIRS data show that the surface independent multilayer appears after about 0.6 ML CF3I adsorption. Similar to clean Ag(lll), molecular CF3I adsorbs with its C-I axis closer to the surface at coverages <0.15 ML than at higher coverage. A model has been proposed in which dissociation of CF3I and desorption of CF3 radicals at low temperature is controlled by the orientation of the I-CF3 axis. We have discussed the compatibility of the model proposed for the coverage dependence of the CFaI orientation with previous results from this group on the photochemistry of the CF3I/Ag(111) system. We have also compared the proposed model with other weakly physisorbed systems.

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Acknowledgement This work was supported in part by the National Science Foundation, Grant CHE9319640.

References [1] M.B. Jensen and P. A. Thiel, Journal of the American Chemical Society 117 (1995) 438. [2] K. B. Myli and V.H. Grassian, Journal of Physical Chemistry 99 (1995) 1498. [3] K.B. Myli and V.H. Grassian, Journal of Physical Chemistry 99 (1995) 5581. [4] M.B. Jensen, U. Myler, C.J. Jenks, P.A. Thiel, E.D. Pylant and J.M. White, Journal of Physical Chemistry 99 (1995) 8736. [5] M.B. Jensen, J.S. Dyer, W.-Y Leung and P.A. Thiel, Langmuir (1995), to be published. [6] Z.-M Liu, X.-L Zhou, J. Kiss and J.M. White, Surface Science 286 (1993) 233. [7] Z.-J. Sun, A.L. Schwaner and J.M. White, Chemical Physics Letters, 219 (1994) 118. [8] M.E. Castro, L.A. Pressley, J. Kiss, E.D. Pylant, S.K. Jo, X.L. Zhou and J.M. White, Journal of Physical Chemistry 97 (1993) 8476. I-9] Z.-J. Sun, A.L. Schwaner and J.M. White, Journal of Chemical Physics 103 (1995) 4279. [ 10] R.M. Robertson, D.M. Golden and M.J. Rossi, Journal of Vacuum Science and Technology B 6 (1988) 1632. [11] J.-L. Lin and J.T. Yates Jr, Journal of Vacuum Science and Technology 13 (1995) 178. [12] J.G. Forbes and A.J. Gellman, Journal of the American Chemical Society 115 (1993) 6277. [ 13] A.J. Gellman and Q. Dai, Journal of the American Chemical Society 115 (1993) 714. [14] Q. Dai and A.J. Gellman, Journal of Physical Chemistry 97 (1993) 10783. [ 15] W.F. Edgell and C.E. May, Journal of Chemical Physics 20 (1952) 1822. [16] P.R. McGee, F.F. Cleveland and S.I. Miller, Journal of Chemical Physics 20 (1952) 1044. [ 173 S.R. Polo and M.K. Wilson, Journal of Chemical Physics 20 (1952) 1183. [18] W.F. Edgell and C.E. May, Journal of Chemical Physics 22 (1954) 1808. [19] P.R. Brook, Journal of Chemical Physics 50 (1969) 5031. [20] H.S. Carman, P.W. Harland and P.R. Brooks, Journal of Physical Chemistry 90 (1986) 944. [21] L.C. Hoskins and C.J. Lee, Journal of Chemical Physics 59 (1973) 4932. [22] M. Suto and L.C. Lee, Journal of Chemical Physics 79 (1983) 1127. [23] M. Heni and E. Illenberger, Chemical Physics Letters 131 (1986) 314.

365

124] S.R. Gandhi and R.B. Bernstein, Journal of Chemical Physics 88 (1988) 1472. [25] D. Raybone, T.M. Watkinson and J.C. Whitehead, Chemical Physics Letters 139 (1987) 442. [-26] C.W. Walter, B.G. Lindsay, K.A. Smith and F.B. Dunning, Chemical Physics Letters 154 (1989) 409. [27] P.W. Harland, H.S. Carman Jr, L.F. Phillips and P.R. Brooks, Journal of Chemical Physics 93 ( 19901 1089. [28] B. Abel, H. Hippler and J. Troe, Journal of Chemical Physics 96 (1992) 8872. [29] L.D. Waits, R.J. Horwitz, R.G. Daniel, J.A. Guest and J.R. Appling, Journal of Chemical Physics 97 (1992) 7263. [-30] B. Abel, H. Hippler and J. Troe, Journal of Chemical Physics 96 (1992) 8863. [31] T. Oster, O. Ingolfsson, M. Meinke, T. Jaffke and E. Illenberger, Journal of Chemical Physics 99 (1993) 5141. [32] C. Angelie, Journal of Chemical Physics 98 (1993) 9284. [33] C.A. Taatjes, W.H. Mastenbroek and S. Stolte, Chemical Physics Letters 216 (1993) 100. [34] C.A. Taatjes, J.W.G. Mastenbroek and S. Stolte, Chemical Physics Letters 216 (1993) 100. [35] K. Junker, Z.-J. Sun, T.B. Scoggins and J.M. White, Journal of Physical Chemistry (1995), to be published. [36] S.I. Ahonkhai and E. Whittle, International Journal of Chemical Kinetics 16 (1984) 543. [37] T. Underwood-Lemons, T.J. Gergel and J.H. Moore, Journal of Chemical Physics 102 (1995) 119. [38] V.S. Smentkowski, C.C. Cheng and J.T. Yates Jr., Langmuir 6 (1990) 147. [39] J. Xu, H.J. Jansch and J.T. Yates, Journal of Vacuum Science and Technology A 11 (1993) 726. [40] W. Van Willigen, Physics Letters 28A (1968) 80. [41] P.A. Redhead, Vacuum 12 (1962) 203. [42] F. Forstmann, W. Berndt and P. Buttner, Physical Review Letters 30 (1973) 17. [43] W.M. Kang, C.H. Li and S.Y. Tong, Solid State Communications 36 (1980) 149. [44] V.P. Holbert, S.J. Garrett, J.C. Bruns, P.C. Stair and E. Weitz, Surface Science 314 (1994) 107. [45] Y.L. Li, D. P. Pullman, J.J. Yang, A.A. Tsekouras, D.B. Gosalvez, K.B. Laughlin, Z. Zhang, M.T. Schulberg, D.J. Gladstone, M. McGonigal and S.T. Ceyer, Physical Review Letters 74 (1995) 2603. [46] F.A. Cotton, Chemical Applications of Group Theory (New York, John Wiley and Sons, 1964) [47] Y.J. Chabal, Surface Science Reports 8 (1988)211. [48] F. Zaera, H. Hoffmann and P.R. Griffiths, Journal of Electron Spectroscopy and Related Phenomena, 54/55 (1990) 705. [49] J. Fan and M. Trenary, Langmuir 10 (1994) 3649. [50] S.J. Clarke, J.K. Cockcroft and A.N. Fitch, Zeitschrifl fur Kristallographie 206 (1993) 87. [51] G.A. Carlson and G.C. Pimentel, Journal of Physical Chemistry 44 (1966) 4053. [52] M.D. Alvey and J.T. Yates Jr., Journal of the American Chemical Society 110 (1988) 1782.

366

.4. Szab6 et al./Surface Science 364 (1996) 345-366

1-53] S. Liang and M. Trenary, Journal of Chemical Physics 89 (1988) 3323. [54] Z.-H. Huang and H. Guo, Journal of Chemical Physics 98 (1993) 7412. 1-55] D.H. Fairbrother, K.A. Briggman, P.C. Stair and E. Weitz, Journal of Physical Chemistry 98 (1994) 13042.

[56] G.N. Robinson, N. Camillione, III; P.A. Rowntree, G.-Y. Liu, J. Wang and G. Scoles Journal of Chemical Physics 96 (1992) 9212. 1-57] M.X. Yang, S.K. Jo, A. Paul, L. Avila, B.E. Bent and K. Nishikida, Surface Science 325 (1995) 102.