The molecular adsorption of ethane on sulfur- and ethylidyne-covered surfaces of Pt(111)

surface s c i e n c e ELSEVIER

Surface Science 364 (1996) 325 334

The molecular adsorption of ethane on sulfur- and ethylidynecovered surfaces of Pt(111) J a m e s A . S t i n n e t t a, M a r k

C. M c M a s t e r

b, R o b e r t J. M a d i x

a'*

a Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA b IBM Corporation, 625/0282 San Jose, CA 95193, USA

Received 12 December 1995; accepted for publication 29 January 1996

Abstract Initial adsorption probabilities, c%, were determined for ethane incident on sulfur- and ethylidyne-covered surfaces of Pt( 111) at 95 K for incident translational energies from 10 to 44 kJ mo1-1 and incident angles from 0 to 60°. At normal incidence and for all translational energies studied, So on these surfaces was found to be enhanced relative to clean Pt(111). The initial adsorption probabilities on both adsorbate-covered surfaces were found to be independent of the angle of incidence, suggesting a highly corrugated static gas-surface potential. The interconversion of normal and parallel momenta due to this corrugation appears to be the mechanism which best explains these results. The notion of increased energy transfer from a match in mass of the collision partners (ethane/sulfur)is clearly not consistent with the data, since the adsorption probability of ethane on the sulfur-covered surface at glancing incidence (60°) is less than that on the clean surface at this same angle. Ethylidyne was found to increase ct0 more than sulfur for all incident conditions, apparently due to energy transfer to the internal degrees of freedom of ethylidyne, a mechanism which is not available to the sulfur adsorbate. Keywords: Adsorption kinetics; Alkanes; Atom-solid interactions; Catalysis; Low index single crystal structures; Molecular dynamics;

Molecule-solid scattering and diffraction - inelastic; Platinum; Single crystal structures; Sulphides

1. Introduction M u c h a t t e n t i o n has been focused on atomic a n d m o l e c u l a r a d s o r p t i o n o n clean metal surfaces [ 1 - 4 ] . A c o m m o n m o t i v a t i o n for some of these studies is to elucidate the i m p o r t a n t energy transfer processes which govern t r a p p i n g a n d adsorption. M o s t i n d u s t r i a l processes, however, occur u n d e r c o n d i t i o n s in which the surface of a catalyst m a y be partially covered with one or m o r e adsorbates

* Corresponding author. Fax: + 1 415 723 9780; e-mail: [email protected].

such as carbon, sulfur or hydrogen, and the presence of these adsorbates m a y drastically change the a d s o r p t i o n process. C o n s e q u e n t l y , a detailed u n d e r s t a n d i n g of the d y n a m i c s of molecular a d s o r p t i o n o n a d s o r b a t e - c o v e r e d surfaces seems essential. I n this paper we report the effects of a d s o r b e d sulfur a n d ethylidyne o n the molecular a d s o r p t i o n of ethane o n P t ( l l l ) . The choice of sulfur a n d ethylidyne as adsorbates was m o t i v a t e d by simple energy transfer concepts such as the Baule f o r m u l a [ 5 ] a n d cube models [ 6 - 9 ] which suggest that a d s o r p t i o n will be maximized when the mass of the gas particle is n e a r that of the effective surface mass. To test the validity of this

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J.A. Stinnett et al./Surface Science 364 (1996) 325-334

idea we chose adsorbates, sulfur (32 amu) and ethylidyne (27 amu), with masses near the mass of ethane (30 ainu). The formation of sulfur- [10-16] and ethylidyne- [17-35] covered surfaces of P t ( l l l ) has been extensively studied and is well understood. Sulfur adsorbs in the fcc hollow site 1.62 A above the P t ( l l l ) surface to produce a well ordered (x/~ × v/3)R30° structure with an absolute sulfur coverage of 1/3 ML [12,15]. The sulfur overlayer can be formed by HES exposure and subsequent heating, which liberates molecular hydrogen, beginning around 250 K [13,14,16]. High resolution electron energy loss spectroscopy (HREELS) of Pt(111)-S shows a single sulfurplatinum stretching vibrational mode at 375 cm -i [13,14]. Though the barrier for diffusion of atomic sulfur on P t ( l l l ) is unknown, on an analogous closed packed surface, Re(0001), it has been estimated from scanning tunneling microscopy (STM) to be near 80 kJ tool -1 [36]. Ethylidyne (------CCHa) formation results via hydrogen liberation beginning at approximately 300 K from the adsorption and subsequent heating of ethylene on P t ( l l l ) [18-20,23,27,29,31-33]. Low energy electron diffraction (LEED) of ethylidyne on P t ( l l l ) exhibits a well ordered p(2 × 2) pattern [ 17, 21,25, 35 ]. Until recently, much debate has centered over the absolute coverage of ethylidyne on Pt(111). Several studies report an absolute coverage of 1/4 ML [ 19,24,29,33-35], while others suggest a 1/2 ML coverage from a p(2 x 1) rotated honeycomb structure with two molecules per unit cell [21,22,30]. Using tensor low energy electron diffraction (TLEED), Somorjai and co-workers have found strong evidence for an absolute ethylidyne coverage of 1/4 ML [35]. These investigators also report that the surface-bound carbon of ethylidyne resides in the fcc hollow site 1.21 A above the Pt(111) surface, producing a buckling within the top two surface layers. Prior studies indicate adsorbed species can significantly affect adsorption [37-50]. Arumainayagam et al. examined the molecular adsorption of ethane on clean [51] and ethanesaturated Pt(111) [41] as a function of incident translational energy, ET, and angle of incidence, 0i. Whereas the angular dependence of the initial

adsorption probability of ethane on the clean surface was found to scale according to the function Ea-cos°'60i, ethane adsorbing on the ethanesaturated surface was independent of angle, demonstrating total energy scaling. The independence of angle was interpreted as resulting from a corrugated gas-surface potential which efficiently couples perpendicular and parallel momenta. In addition, adsorbed ethane was found to enhance the initial adsorption probability relative to the clean surface. This result was attributed to increased energy transfer due to a better match in mass of the collision partners [37-39,48,49]. In a similar fashion Soulen et al. have recently determined that a monolayer of hydrogen on P t ( l l l ) reduces the adsorption probability of ethane by as much as 25% relative to the clean surface [42]. In this study the initial adsorption probability, So, of ethane was examined as a function of incident energy, E~, and angle, 0i, on P t ( l l l ) - S and Pt(111)-CCH3 and compared to the results from ethane adsorbing on clean P t ( l l l ) [51], ethanesaturated P t ( l l l ) [41], and P t ( l l l ) - H [42]. Gas-surface corrugation, indicated by the total energy scaling of the initial adsorption probabilities of ethane on P t ( l l l ) - S and P t ( l l l ) - C C H 3, and not the match in mass of incident ethane to the adsorbate is shown to be the primary mechanism which best explains both total energy scaling and enhanced adsorption. A forthcoming publication which will present experimental scattered angular distributions from these systems and stochastic trajectory calculations of ethane trapping on P t ( l l l ) - S supports the significance of corrugation [50].

2. Experimental Since the molecular beam scattering apparatus employed in this study has been described in detail elsewhere [52], only a brief description is provided here. The experimental apparatus consists of an ultrahigh-vacuum (UHV) scattering chamber coupled to a triply differentially pumped supersonic molecular beam source. An ethane beam (Matheson, 99.99% purity) directed toward the Pt(111) crystal was passed through a modulation

J.A. Stinnett et al./Surface Science364 (1996) 325-334

chamber which contains a 50% duty chopper for beam modulation and a solenoid-activated shutter to block or allow passage of the beam into the scattering chamber. The UHV chamber was pumped by turbo-molecular, ion and titanium sublimation pumps. These pumps are capable of producing a base pressure of 5 x 10 -11 Torr and a pumping speed for ethane of 800 d s-1 The UHV chamber is equipped with LEED and Auger electron spectroscopy (AES) which were used to examine surface structure and surface composition, respectively. The scattering chamber also contains two quadrupole mass spectrometers; one is rotatable about the crystal at a fixed distance of 12.1 cm and is used for direct trapping probability experiments (DTPE) and time of flight measurements (TOF). The other mass spectrometer can be positioned near the crystal with a bellows and used for temperature programmed desorption experiments (TPD). In addition, a goldplated flag positioned along the beam axis in the UHV chamber is used to block the passage of the beam to the crystal. The P t ( l l l ) crystal was cleaned by argon ion sputtering with a 10-15 #A ion current and by employing a 300-400 V bias on the crystal at a surface temperature of 300 K. The crystal temperature was measured by a chromel-alumel thermocouple spot-welded to the back of the crystal. After sputtering, surface order was restored by annealing the crystal at 1400 K for 8 min. In addition, oxygen titration cycles, consisting of a 3 L (1 L = 1 0 -6 Torr) background dose of oxygen and subsequent heating to 1000 K, were performed until AES indicated that no contaminants were present above the noise level (<0.02 ML). This procedure produced a hexagonal p(1 × 1) LEED pattern. The adsorbate-covered surfaces of sulfur and ethylidyne were prepared by the following well established procedures. The (xfi x x/3)R30 ° sulfur adlayer was prepared as follows. Initially, a clean P t ( l l l ) surface was exposed to a 9 L background dose of H2S at a surface temperature of 95 K. Upon heating the crystal to 550 K at approximately 5 K s- 1, desorption of both HzS and H 2 was detected. Molecular desorption of H2S (re~q= 34) exhibited a peak temperature of 130 K. H2 (rn/q=2)

327

desorption, which results from H2S dissociation, displayed a broad peak starting at approximately 200 K and ending at 250 K, which is in agreement with Koestner et al. [14]. The surface was then re-exposed to another 9 L background dose of HzS at 95 K with subsequent heating to 550 K, The surface was judged to be saturated with sulfur when no H 2 desorption from HzS dissociation (250 K) was detected. Further H2S exposure showed only HzS desorbing at 130 K and no H2 desorption. This procedure consistently produced a well ordered (xfi x xfi)R30 ° LEED pattern. The ethylidyne-covered surface was prepared by first exposing a clean P t ( l l l ) surface to a 2 L background dose of ethylene at a surface temperature of 95 K. The surface was then heated to 350 K with a heating rate of 3 K s-~; molecular desorption of C2H4 (m/q = 28) at 270 K and recombinative desorption of H2 (m/q =2) at 300 K were detected, which is consistent with the report of Steininger et al. [20]. This surface was then re-exposed to another 2 L background dose of ethylene at 95 K and heated to 350 K. Upon heating the surface after the second dose of C2H 4, no H2 desorption at 300 K was seen, indicating that the surface was saturated with ethylidyne, and no further C2H 4 exposure was necessary. This procedure gave a p(2 x 2) LEED pattern. Incident translational energies of ethane molecular beams were varied by seeding ethane in helium. Using a modulated beam, a lock-in amplifier and the rotatable mass spectrometer, incident translational energies were determined by measuring the phase difference at the front and back of the scattering chamber relative to a signal generated by the chopper used to modulate the beam [-53,54]. To eliminate variation of vibrational energy distributions of incident ethane, the nozzle temperature was held constant at 300 K. Direct trapping probability experiments (DTPEs) were performed using the rotatable mass spectrometer by a modified King and Wells method [-51]. Representative data are shown in Fig. 1 for ethane adsorbing on Pt(II1)-S. The trace represents the partial pressure of ethane in the UHV scattering chamber as a function of time measured by monitoring the intensity of the ethane parent

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J.A. Stinnett et al./Surface Science 364 (1996) 325-334

Ethane/Pt(111)-Sulfur k t ,,,/'1 ,|t ., ~ f!/qrlv",¢ ,w¢¢ " ' t

¢0

O4 II

"r

~

v,

v-~-

•

P,

O" v

,~.,~,~,~, I

E Er= 40 kJ/mol

¢03 ° i

R

Oi = O°

U) 6

Ts = 9 5 K

CL

(/) u)

¢0

= ~/R 0

I 15

I 30

I 45

I 6o

75

Time (seconds) Fig. 1. Typical direct trapping probability experiment for the molecular adsorption of ethane on Pt(111)-S at a surface temperature of 95 K for ethane incident with 40 kJ mol- ~ of initial normal translational energy. The initial adsorption probability is given by the ratio of PA to P~.

ion at m/q = 28. Briefly, the initial adsorption probability is determined by the ratio of the immediate drop in partial pressure of ethane as the flag is dropped so as to expose the crystal to the beam (denoted as PA) to the total partial pressure rise of ethane as it enters the chamber but is blocked from the crystal by the flag (denoted as PB). For this set of experiments the experimental error in c
be in excellent agreement with the previously observed values [41,51 ]. After the adsorbate surfaces of sulfur and ethylidyne were first prepared (as described above) several DTPEs were performed. Following each exposure of ethane from the molecular beam, the crystal was heated to 350 or 550 K for the ethylidyne- and sulfur-covered surfaces, respectively, to regenerate the original surfaces. As a check for reproducibility, for each beam energy a set of measurements at different angles of incidence were first taken from a prepared surface, then several data points were remeasured at randomly chosen angles on a second identically prepared surface; the measured adsorption probabilities for the same incident energy and angle were found to agree to within __+0.02. The average values for ~o are reported in this paper. Following some ethane exposures to the sulfurand ethylidyne-covered P t ( l l l ) surfaces, TPD, AES and LEED experiments were performed. The peak temperatures of ethane desorbing from both

329

J.A. S t i n n e t t et a l . / S u r f a c e S c i e n c e 364 ( 1 9 9 6 ) 3 2 5 - 3 3 4

surfaces were found to be approximately 115 K, slightly lower than the desorption peak temperature of ethane from the clean surface, which is 132 K. This result suggests the binding energies for ethane on the adsorbate-covered surfaces are approximately 4 k J mol 1 less that on clean P t ( l l l ) , D = 3 2 k J mo1-1 [51]. Furthermore, no species other than ethane desorbing from the surface was detected by TPD, indicating that the covered surfaces remained unchanged. Auger spectra taken after a surface was heated to desorb ethane showed no carbon accumulation, indicating that direct collisional activation of ethane did not contribute to the measured adsorption probabilities. Finally, LEED performed after several DTPEs showed that the surfaces retained their (x/3 x x/3) R30 ° and p(2 x 2) patterns for sulfur- and ethylidyne-covered Pt(111) surfaces, respectively.

Adsorbate Covered Surfaces Ts=95 K

;~ 0.8

thane •9 0.6

0.4

!frill e

0.2

3. Results and discussion 3.1. Dependence on incident translational energy

At normal incidence the initial adsorption probabilities of ethane on sulfur- and ethylidyne-covered surfaces of Pt(111) decrease monotonically with increasing translational energy (Fig. 2). This trend is expected for non-activated molecular adsorption where faster incident molecules are less likely to lose sufficient translational energy in order to adsorb [41-45,51,55-59]. The magnitudes of the initial adsorption probabilities at normal incidence of ethane on Pt( 111 )-S and Pt( 111 )-CCH 3 are greater than those on clean Pt( 111 ), indicating that energy transfer at normal incidence is more efficient in the presence of the adsorbates. Experiments at higher incident translational energies were not performed, since under such conditions carbon accumulation on the surface was detected by AES, signaling the onset of dissociative adsorption. Additionally, for the range of incident translational energies examined, the magnitude of c~o for ethane on the ethylidynecovered surface is consistently higher than ethane adsorbing on Pt( 111)-S.

0

I

I

[

[

I

10

20

30

40

50

Translational Energy (kJ/mol) Fig. 2. The initial adsorption probability of ethane on ethanesaturated P t ( l l l ) [,41] (solid square), ethylidyne-covered Pt( 111 ) (open squares), sulfur-covered Pt( 111 ) (solid triangles) and clean Pt( 111 ) [-51 ] (open triangles) as a function of incident normal translational energy at a surface temperature of 95 K. The lines connecting the symbols are presented simply as to guide the eye.

3.2. Dependence on angle of incidence

The initial adsorption probabilities of ethane on the sulfur- and ethylidyne-covered P t ( l l l ) are independent of the angle of incidence; i.e., they both scale with total translational energy (Fig. 3 and Fig. 4). For each translational energy studied the initial adsorption probability of ethane on both adsorbate-covered surfaces was found to remain constant between 0 and 60 °, suggesting that parallel and perpendicular momenta dissipation are both important to the adsorption mechanism. Relative to clean P t ( l l l ) , on which c% scales as ET cos°'60i, [51] the presence of adsorbed sulfur or ethylidyne appears to change the effective corru-

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J.A. Stinnett et aL/Surface Science 364 (1996) 325-334

Ethane/Pt(111)-Sulfur

I .Q O~

Ts = 9 5 K

10 k J / m o l 0.8

0 n 130 rO_ 0.6 EL

20 kJ/mol 26 kJ/mol

. - -

30 kJ/mol

F-

.m_ 0.4

o

t40 kJ/mol 0.2 0

20

40

60

Incident Angle (degrees) Fig. 3. The initial adsorption probability of ethane on a sulfur-covered surface of Pt(111 ) as a function of incident polar angle (0i) at a surface temperature of 95 K for 1 0 < E x kJ mo1-1 >40.

Ethane/Pt(111)-Ethylidyne Ts = 9 5 K ° - -

.Q

10 k J / m o l

03

.Q 0I.- 0.8 Q. 03 ¢-

18 k J / m o l 28 kJ/mol

° _

CL

0

0

0

0------___(9

~'0.6

31 k J / m o l

/=--

1-44 kJ/mol

03

° - -

c: o.4

0.2

I 0

~

I 20

Incident

i

I

~

40

I 60

Angle (degrees)

Fig. 4. The initial adsorption probability of ethane on an ethylidyne-covered surface of Pt( 111 ) as a function of incident polar angle (0i) at a surface temperature of 95 K for 1044.

J.A. Stinnett et al./Surface Science 364 (1996) 325 334

gation of the gas-surface potential. The reduction in the energy scaling exponent from n = 0.6 to 0.0 for both sulfur- and ethylidyne-covered P t ( l l l ) implies an increase in corrugation. This behavior is analogous to that for ethane adsorbing on the ethane-saturated surface of P t ( l l l ) , for which n equals zero as well [41]. Our results for ethane adsorbing on the sulfur- and ethylidyne-covered surfaces combined with the results for the ethane-saturated surface suggest that adsorbates on a relatively fiat surface with a size comparable to the incident species should increase gas-surface corrugation. While the sulfur surface clearly enhances adsorption at normal incidence relative to the clean surface, its effect is reversed at 60°. At this angle of incidence and for all translational energies studied, the initial adsorption probabilities of ethane on clean P t ( l l l ) always exceeds that of ethane on the sulfur-covered platinum surface (Fig. 5). This reversal implies that at glancing angles of incidence, the clean surface is slightly more efficient at energy transfer out of normal translational energy of the incident molecules than the sulfur-covered platinum surface. Though the increase in So produced by the adsorbates is consistent with the effect expected from simple energy exchange models based on the decreased mass of the adsorbate compared to the mass of the platinum atom on the clean surface, the effects of the adsorbates cannot be understood, even qualitatively, in terms of their masses. Simple cube models would predict that the adsorption probabilities on the sulfur-, ethylidyne- and ethanecovered surfaces would be nearly equal since the adsorbate masses of sulfur, ethylidyne and ethane are all near 30 amu [5-9]. In addition, from this viewpoint the higher coverage of sulfur (1/3 versus 1.4 ML ethylidyne) would also contribute to a higher value of 7 on the sulfur-covered surface; neither of these predictions are supported by the experimental evidence. Lastly, the reversal of the relative magnitudes of So on the clean and sulfurcovered surface at incident angles of 0 and 60 ° is entirely inconsistent with this picture. The increase in the initial adsorption probability of ethane on P t ( l l l ) - S at normal incidence, the reduction in adsorption relative to clean P t ( l l 1)

331

Adsorbate Covered Surfaces Ts= 95 K

0i = 60° ~=~

;-~ 0.8 .,-s

o

~-~ 0.4

Sulfur 0.2

0

I 10

I 20

t 30

I 40

[ 50

Translational Energy (kJ/mol) Fig. 5. The initial adsorption probability of ethane on ethanesaturated P t ( l l l ) [41] (solid squares), ethylidyne-covered Pt( 111 ) (open squares), sulfur-covered Pt( 111 ) (solid triangles) and clean Pt ( 111 ) [ 51 ] (open triangles) as a function of incident translational energy for ethane incident at 60 ° from the surface normal. The surface temperature for each system is 95 K. The lines connecting the symbols are presented simply as to guide the eye.

at 60 ° and the total energy scaling of s0 for ethane adsorbing on P t ( l l l ) - S can all be understood solely in terms of a highly corrugated gas-surface potential. The total energy scaling suggests parallel and normal momenta are strongly coupled in the collision of ethane with the sulfur-covered surface. The enhancement in adsorption at normal incidence relative to the clean surface occurs because the highly corrugated ethane/Pt(111)-S gas-surface potential allows for facile conversion of incident normal momentum into parallel momentum which assists adsorption. Conversion of parallel to normal momentum, however, reduces the adsorption of ethane on P t ( l l l ) - S relative to clean

332

J.A. Stinnett et al./Surface Science 364 (1996) 325-334

Pt(111) at glancing angles of incidence. Extremely broad experimental scattered angular distributions of ethane from Pt( 111)-S and stochastic trajectory simulations of this system provide strong support for this mechanism [50]. The increase in effective gas-surface corrugation suggested by ethane adsorption on P t ( l l l ) - S most likely occurs from an increase in static, not dynamic, corrugation. Following the root mean square displacement analysis presented by Arumainayagam et al. [41], the vibrational amplitude of the first two excited vibrational states of sulfur on Pt(111) are 0.07 and 0.09 ,~, respectively. These displacements are less than the root mean square displacement of surface platinum atoms of clean P t ( l l l ) at 95 K (0.14 and 0.18 A), suggesting that vibrating adsorbed sulfur atoms do not substantially increase dynamic corrugation. The ethane-saturated and ethylidyne-covered surfaces probably also produce facile interconversion of parallel and perpendicular momenta; however, the existence of additional energy loss mechanisms for these adsorbates increases the initial adsorption probability of ethane at glancing incidence relative to the clean surface. Such effects are consistent with the enhanced trapping of argon on argon-covered Ru(001) relative to clean Ru(001) [48,49]. Thus, the differences between the magnitudes of the initial adsorption probabilities on the adsorbate-covered surfaces can be qualitatively understood in terms of the dynamical properties of the adsorbates. Although each adsorbate has approximately the same mass and surface coverage, the energy transfer mechanisms available to dissipate energy for each adsorbate system are different. While surface-bound ethylidyne and ethane have several internal degrees of freedom, including low frequency frustrated rotations and translations [60,61], adsorbed sulfur has only a relatively strong sulfur-metal vibration (375 cm-1) [ 14]. Due to the average collisional time of ethane with these surfaces, excitation of this vibrational mode, while possible, seems unlikely since frequencies above 200 cm -1 are not expected to be excited for the range of incident translational energies studied [62]. Consistent with this notion are the results from our stochastic trjactory simulations of ethane trapping on P t ( l l l ) - S which do

not exhibit much energy transfer into the platinumsulfur vibration [50]. Collision-induced displacement (barrier~80kJ mo1-1) of sulfur atoms on P t ( l l l ) is also unlikely [36]. Thus, the sulfur adsorbate, which has fewer channels for energy transfer relative to the other adsorbates (ethylidyne and ethane), exhibits values of So closest to clean Pt( 1! 1). Additionally, because of the weak molecular bonding of adsorbed ethane, adlayer compression, which is not available to rigidly bound ethylidyne, can also provide energy accommodation [48,49]. Hence, because the number of adsorbate energy transfer mechanisms increases in going from sulfur to ethylidyne to ethane, it appears reasonable that the molecular adsorption of ethane on covered P t ( l l l ) is greatest for P t ( l l l ) C2H6, followed in order by P t ( l l l ) - C C H 3 and Pt(lll)-S. The observation of decreased energy transfer to the sulfur-covered surface at 60 ° is consistent with our suggestion that excitation of the sulfur-metal vibration is unlikely and that corrugation dominates the adsorption process. While excitation of this vibration would be easiest at normal incidence, ethane beams incident at 60 ° collide with the surface with approximately 30-40 kJ mo1-1 of normal energy for the range of incident conditions studied (with well depth considered). This quantity of normal energy is comparable to the amount of normal energy for a 10 kJ mo1-1 ethane beam at normal incidence. Because molecules incident on the sulfur-covered surface along the surface normal with 10kJ mo1-1 translational energy show enhanced adsorption relative to the clean surface (Fig. 2) while beams with equal normal energy incident at 60 ° show the opposite effect (Fig. 5), adsorption must be increased by a mechanism other than excitation of the sulfur-metal bond.

4. Summary and conclusions We have employed supersonic molecular beam experiments to examine the effect of adsorbates on the molecular adsorption of ethane. The results for ethane adsorption on P t ( l l l ) - S and Pt( 111 )-CCH3 suggest the following conclusions: The magnitudes of the initial adsorption proba-

J.A. Stinnett et al./Surface Science 364 (1996) 325 334

bilities at normal incidence for ethane on sulfurand ethylidyne-covered Pt(111) are enhanced relative to ethane adsorption on clean P t ( l l l ) . This increase in magnitude appears to be the result of increased static corrugation of the gas-surface potential which enhances adsorption at normal incidence via the conversion of translational momentum perpendicular to the surface into momentum parallel to the surface. Comparatively, the magnitude of ethane adsorption on P t ( l l l ) - C C H 3 was found to be enhanced relative to ethane adsorbing on P t ( l l l ) - S for all incident conditions. The origin for this increase is attributed to enhanced energy transfer to internal degrees of freedom in ethylidyne which are absent for the adsorbed sulfur. The initial adsorption probabilities on both adsorbate-covered surfaces remain constant for increasing angles of incidence. This observation is also consistent with a highly corrugated surface which scrambles incident translational parallel and perpendicular momenta. The concept of increased energy transfer from a match in mass of the collision partners of a gassurface system is inconsistent with the experimental results.

Acknowledgements We gratefully acknowledge the Department of Energy, Chemical Sciences Division, Office of Basic Energy Sciences (grant DE-FG03-86ER13468) for financial support of this work.

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