Adsorption mechanisms of translationally-energetic O2 and N2: direct dissociation versus direct molecular chemisorption

surface ELSEVIER

science

Surface Science 380 (1997) L513 L520

Surface Science Letters

Adsorption mechanisms of translationally-energetic O2 and N2: direct dissociation versus direct molecular chemisorption J.E. Davis, C.B. Mullins * Department (?[Chemical Engineering. Unirersity o['Texas at Austin, Austin, TX 78712-I062, USA Received 24 October 1996: accepted for publication 27 December 1996

Abstract

A direct dissociation mechanism has been traditionally assigned to molecular beam data that exhibit an increase in the initial adsorption probability with increasing kinetic energy. Yet, recent experiments of nitrogen and oxygen adsorption provide support for an alternative high kinetic energy pathway in which incident energy assists in surmounting barriers to molecular chemisorption on a surface as the first step to dissociation. Moreover, systems for which the experimental evidence supports such a mechanism also demonstrate that molecularly chemisorbed intermediates can be spectroscopically observed at low temperatures and coverages from exposure to a gas in thermal equilibrium at room temperature. Likewise, such observations have not been measured for systems which are consistent with direct dissociation, A consideration of this trend regarding the existence of molecularly chemisorbed states and the implications for the dominant, dissociative chemisorption pathway at high kinetic energy is presented for a number of gas surface systems. :c~ 1997 Elsevier Science B.V.

Keywords: Chemisorption: Low index single crystal surfaces: Molecule solid reactions: Nitrogen: Oxygen

Studies of the dynamics of oxygen and nitrogen adsorption on single-crystalline metals have demonstrated that translational energy can promote the dissociative chemisorption of a molecule on a surface, and such a mechanism has been termed direct dissociation. Support for this type of mechanism is available from molecular beam experiments in which the reaction probability of a species as a function of incident gas and surface conditions has been investigated. Experimental data which demonstrate an increase in the initial adsorption probability So of a species with increasing kinetic energy Ei have been cited as evidence *Corresponding author. Fax + l 512 471 7060: e-mail: mullins(a:Iche.utexas.edu.

for a direct dissociation mechanism, with the rationalization that translational energy is effective in overcoming an activation barrier to the dissociated state. However, recent studies suggest that kinetic energy may assist in surmounting barriers to molecularly chemisorbed surface states as well (a direct molecular chemisorption mechanism) which then serve as precursors to dissociation. Much debate in the study of gas surface interactions currently centers on these high kinetic energy pathways and, in particular, the role of molecularly chemisorbed, intermediate states. A number of experimental features have been invoked to distinguish between direct dissociation and direct molecular chemisorption mechanisms, and these include surface temperature T~ effects,

0039-6l)28/97/$17.00 Copyright ~:31997 Elsevier Science B.V. All rights reserved PII S ( ) 0 3 9 - 6 0 2 8 ( 9 7 )00010-1

J.E. Davis, CB. Mullins / Sur/ace Science 380 (1997) L513-L520

..... ~ ~ll~,l~ scaling, and the dependence of saturation coverage on incident kinetic energy. An inverse surface temperature dependence in an energy regime characterized by an increase in the initial adsorption probability with increasing kinetic energy has been cited as evidence for a direct molecular chemisorption mechanism; the temperature dependence is believed to reflect a thermallydriven, kinetic competition for forward and reverse conversion from a molecularly chemisorbed, intermediate state. However, other origins for a surfacetemperature dependence in a high kinetic energy regime have been proposed, including thermal surface oscillator models [1,2] and electronic effects [3]. Energy-angle scaling refers to a method of data characterization in which values of the initial adsorption probability at various incident angles 0i are plotted as a function of Ei cos" 0i. The value of the exponent is adjusted between n = 0 (total energy scaling) and n = 2 (normal energy scaling) such that data for all angles fall along a curve defined by the data at 0i = 0 °. Experimental trends for a number of systems suggest that normal energy scaling may coincide with direct dissociation mechanisms, and deviations from normal energy scaling have been observed in a few systems believed to be governed by direct molecular chemisorption mechanisms at high kinetic energy. However, the origin of these effects is unclear, and the trends are not always consistent. Finally, an increase in the saturation coverage of a species with increasing kinetic energy has been reported for a number of systems believed to be governed by direct dissociation at high incident energies. For systems which have been described with direct molecular chemisorption mechanisms, including Nz/Fe(111) [4], O 2 / P t ( l l l ) [5], O 2 / I r ( l l 0 ) [6] and O 2 / I r ( l l l ) [7], no change in the saturation coverage is observed for incident energies up to a few electron volts. Such behavior is consistent with a mechanism in which the dissociation step occurs after a loss of "memory" of the incident molecule conditions. An additional indication that direct molecular chemisorption (rather than direct dissociation) may dominate the high kinetic energy adsorption of a particular gas-surface system concerns the existence of stable, molecularly chemisorbed intermedi-

ates. If a molecularly chemisorbed state is involved in dissociation, this state should correspond to a minimum in the gas surface interaction potential; consequently, the isolation and observation of this species may be possible at low surface temperatures. Indeed, molecularly chemisorbed states have been detected for a number of systems employing various spectroscopic techniques, including electron energy-loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), at low surface temperatures. Although the existence of molecularly chemisorbed states does not necessarily dictate a dynamical role for such species, a common characteristic of systems that exhibit behavior most consistent with a direct molecular chemisorption mechanism appears to be the presence of stable, molecularly chemisorbed species at low temperatures from low kinetic energy exposures. Conversely, the absence of evidence for these states in systems associated with direct dissociation complements this observation. A critical examination of molecular beam studies consistent with direct molecular chemisorption mechanisms and of studies supporting direct dissociation mechanisms is presented here. Also, for each system evidence concerning the existence of molecularly chemisorbed states is considered with emphasis on measurements at low adsorbate coverages; such data provide the most direct support for the involvement of such intermediates in initial (in the limit of zero coverage) dissociative chemisorption. Additionally, systems will be discussed for which the available data do not afford a clear indication of the nature of the high kinetic energy adsorption mechanism. The gas surface systems and experimental results presented in this article are summarized in Table 1. Evidence of molecularly chemisorbed states that are stable at low surface temperatures and low adsorbate coverages has been provided for a number of ga~surface systems. Of these systems, several have been the subject of molecular beam experiments conducted to characterize the dynamics and kinetics of initial adsorption. These studies indicate that direct population of molecularly chemisorbed states, rather than direct dissociation, is the dominant adsorption mechanism for incident kinetic energies up to a few electron volts.

.I.E. Davis, CB. Mullins / Su;7/bce Science 380 (1997) L513 L520 Table 1 Adsorption mechanisms of translationally-energetic O 2 and N 2 on selected transition metal surfaces Direct molecular chemisorption

Direct dissociation

Mechanism ambiguous

N 2/Fe( 111 )...... O2,'Pt( 111 ). . . . . O , :Agt 110) ..... 0 2/lr(110) a'c O2,"lrl 111 ).....

O2/W( 110}a'b'd

N2/W ( 110}a'd Na/W ( 100)..g O2/Pt( 100)-hex~'b'C'f O2/Ag( 111 y"~

O2/Ru(001

)a.b.c,d,f

O2/Cu(110) T M

" Initial adsorption probability increases with increasing kinetic energy. b Initial adsorption probability scales with normal kinetic energy. hfitial adsorption probability is surface temperature dependent. d Saturation coverage is kinetic energy dependent. Molecular intermediates detected after high kinetic energy exposures. r Initial adsorption probability increases with increasing surface temperature. g No surface temperature dependence observed between 300 and 1000 K.

Molecularly chemisorbed species are the intermediates for adsorption at these kinetic energies, rather than the physically adsorbed states which are often considered to be dissociation precursors (for low kinetic energy adsorption). The typical potentialwell depth for molecular chemisorption is several times greater than that for physical adsorption. The first experimental evidence for a direct molecular chemisorption mechanism was presented by Rettner and Stein to rationalize measurements of the adsorption of N 2 o n F e ( l l l j [4]. The authors observed that the initial adsorption probability increased with increasing kinetic energy but decreased with increasing surface temperature for all kinetic energies. They proposed that higher kinetic energies assisted in traversing a potential barrier to a molecularly chemisorbed state on the surface and that the surface temperature dependence reflected the kinetic competition between dissociation and desorption from this state. Indeed, Grunze et al. previously had identified molecularly chemisorbed N 2 species on F e ( l l l ) , including a 7rbonded, ~-N2 state stable below 170 K at low coverages, with XPS and EELS [8]. Temperatureprogrammed desorption (TPD) measurements

reported by Rettner and Stein indica ence of the :~-N2 state after high kinetic energy nitrogen exposures to the surface at 120 K; this observation supported a mechanism involving direct population of the ~-N, intermediate rather than direct dissociation [4]. Since the ground-breaking work of Rettner and Stein, other studies have suggested mechanisms involving direct molecular chemisorption with subsequent dissociation to explain experimental observations. Measurements of oxygen adsorption on Pt( 111 ) are consistent with such a mechanism The existence of two molecularly chemisorbed states of oxygen on Pt( 11 l j at low coverages (exposures of 0.2 L) and temperatures below - 1 5 0 K has been well-established by EELS, XPS, and near-edge X-ray absorption fine structure (NEXAFS) techniques [9,10]. Molecular beam studies of 0 2 on P t ( l l l ) showed a temperature dependence in the initial adsorption probability in a regime where So increased with increasing kinetic energy [11], as seen in Fig. 1. Evidence presented by Rettner and Mullins for the O 2 / P t ( I l l ) system suggested that the temperature dependence originated from the

02/Ru(001 ) ~0.8

0.6

• •

•

~ o

t

O2/lr(111)

-

o

©

0.4

o

,.~

O2/Pt(111) ©

"-~ 0.2

•

•

•aa

O~I,A A

+

T

A 0

•

. N2/W(100)

,~,l~,~l*,lllt,qlllLL,LIt

0

0.2

0.4

0.6

0.8

~l

1

1.2

1.4

Kinetic Energy, E i Fig. 1. Measurements of the initial adsorption probability versus kinetic energy for 0 2 on Ru(001) at 1~=77 ( i ) and 5 0 0 K (~} [3]: 02 on I r [ l l l ) at T~=77 (O) and 4 2 5 K i ) [7]; 02 on P t { l l l ) at 7.,=200 (A) and 3 5 0 K (/'..) [11]: and N 2 on W( 100} at T~=300 K (+j [26]. All measurements are for O i = O ,

J.E. Davis, C B. Mullins / Sur[ace Science 380 (1997) L513-L520

. . . . . . . . . . . . . . . of molecularly chemisorbed oxygen in high kinetic energy adsorption [5]. A technique involving the reaction of oxygen with carbon monoxide to distinguish molecular from atomic oxygen on the surface was employed to demonstrate that, primarily, molecular oxygen was present on P t ( l l l ) at 9 0 K after exposure to a 1.1eV oxygen beam. Another example of a system for which molecularly chemisorbed states have been identified and for which no evidence of direct dissociation has been presented is the O2/Ag(l10) system. EELS measurements of oxygen on A g ( l l 0 ) at surface temperatures below 150 K showed a feature at 645 cm -1 consistent with a molecularly chemisorbed oxygen state formed by charge transfer from the surface (the O-Ag stretch for oxygen adatoms on the surface appears at ~325 cm ~) [12]. For incident kinetic energies up to ~0.6 eV, the initial adsorption probability of O2 on Ag(110) increased with increasing kinetic energy, and over the entire energy range So decreased with increasing surface temperature. Moreover, EELS measurements indicated that molecular oxygen was present on the surface at low coverages and a surface temperature of 83 K after exposure to a 0.6eV oxygen beam [13]. On the (110) [6] and (111) [7] surfaces of iridium, the adsorption of oxygen has also been described with a direct molecular chemisorption mechanism for high kinetic energies (~0.1-1.3 eV). Measurements of the initial dissociative chemisorption of oxygen on both surfaces showed an increase in So with increasing kinetic energy and a decrease in So with increasing surface temperature in this energy regime (see Fig. 1 for representative O 2 / l r ( l l l ) data). The dependence on surface temperature as well as other observations (including no increase in the saturation coverage of oxygen with increasing kinetic energy) suggested a mechanism involving the direct access of an intermediate to dissociation [6,7]. Contact potential difference measurements of oxygen on I r ( l l 0 ) below 100 K suggested the presence of molecularly chemisorbed oxygen [14], but vibrational spectroscopy measurements confirming the existence of stable, molecularly chemisorbed states of 02 on Ir(110) remain to be provided. For intermediate oxygen coverages

(0>0.25 ML) on the (111) surface of iridium at --~68 K, EELS measurements show a loss feature at 550cm 1, indicative of oxygen adatoms, and features at 740 and 805 cm-~; the assignment of these latter features as molecularly chemisorbed states of oxygen is consistent with vibrational spectroscopic data of molecularly chemisorbed oxygen states on other transition metal surfaces. Furthermore, these molecularly chemisorbed states were populated for adsorption via a 1.36 eV oxygen beam, and this observation was invoked as further support for the direct molecular chemisorption mechanism proposed for this system [7]. Finally, the dissociative chemisorption of nitric oxide on It( 111 ) has been described with a mechanism involving direct population of molecularly chemisorbed states with subsequent dissociation. Molecular beam techniques and EELS were employed to demonstrate that nitric oxide chemisorbed directly as a molecule on It(111) at 77 K for kinetic energies up to 1.3 eV, evident by EELS loss features at 1860 and 1550cm 1 [15]. Additionally, molecularly chemisorbed nitric oxide dissociated thermally on the It(l 1 l) surface at temperatures above 350 K, suggesting its role as a precursor to dissociation. Unlike the other examples of direct molecular chemisorption discussed so far, no increase in So with increasing kinetic energy was observed for the N O / I t ( l 1 l) system. Indeed, the nature of the potential energy surface for this system appears to mask the transition from trapping-mediated to direct molecular chemisorption in measurements of So as a function of kinetic energy. The adsorption of NO on P t ( l l l ) was found to exhibit similar behavior [a decrease in the initial adsorption probability with increasing kinetic energy) and was also described with a mechanism involving direct population of a molecularly chemisorbed surface state. However, no dissociation of molecularly chemisorbed NO was detected on this surface [16]. Hence, the initial adsorption pathways of gas surface systems exhibiting molecularly chemisorbed states appear to be dominated by direct molecular chemisorption mechanisms with subsequent dissociation for kinetic energies up to a few electron volts. For systems which are consistent with a direct dissociation mechanism, no conclu-

J.E. Davis, C.B. Mullins / SurJace Science 380 (1997) L513 L520

sive evidence of molecularly chemisorbed states resulting from ambient adsorption has been presented. Systems that have been described with direct dissociation mechanisms include O2/W(110) [ 17], O2/Ru(001) [3], and O 2 / C u ( l l 0 ) [18]. For these systems, no conclusive evidence for stable, molecularly chemisorbed states at low coverages and low temperatures has been provided. The dissociative chemisorption of oxygen on W(110) was observed to increase with increasing kinetic energy above ~0.1 eV [17]. The high kinetic energy data scaled with normal kinetic energy (E i cos 2 0i), and the oxygen saturation coverage was reported to increase with increasing kinetic energy. These trends were applied in assigning a direct dissociation mechanism to high kinetic energy adsorption for this system. Employing UV and X-ray photoelectron spectroscopy (UPS, XPS), attempts to isolate a molecularly chemisorbed 02 species on this surface showed that atomic oxygen was formed up to saturation coverages at a surface temperature of 26 K [ 19]. (Recall that molecularly chemisorbed oxygen is stable on Pt( 111 ) below ~ 150 K [9,10]). For the Oz/Ru(001) system, measurements of the initial adsorption probability also exhibited an increase with increasing kinetic energy (between 0.1 and 1.4eV, as seen in Fig. 1) [3], Similar to the behavior reported in the Oz/W(110) study, the initial adsorption probability of 02 on Ru(001) scaled with normal kinetic energy, and the oxygen saturation coverage increased with increasing kinetic energy. Additionally, a surface temperature dependence that was inconsistent with the involvement of a molecularly chemisorbed intermediate was reported. Molecularly chemisorbed oxygen was not identified for this system at low coverages. EELS measurements of 02 on Ru(001) at 30 K showed molecularly chemisorbed species only after saturation of the atomic overlayer [20]. This obserwttion is consistent with the stabilization of molecular oxygen due to the large work function change associated with a saturated atomic layer and does not establish the existence of an intrinsic intermediate. The reader may recall that molecular oxygen on Ir( 111 ) at partial adatom coverages was invoked as evidence for a molecularly chemisorbed intermediate. The argument that molecular oxygen

on a saturated atomic layer on Ru(0( imply the existence of an intrinsic, molecularly chemisorbed precursor may seem contradictory. However, several key differences between the two studies should be noted. The temperature dependence of the high kinetic energy O2/Ru(001 ) data was inconsistent with a molecularly chemisorbed precursor, unlike the data for 02 on lrq 111 ). Also, EEL spectra of oxygen adsorption on Rul001) were measured at a lower sample temperature (where the probability of observing a molecularly chemisorbed state at low coverages would be greater) than those of O2 on Ir( 111 I. A direct dissociation mechanism is also consistent with results from a molecular beam study of the dissociative chemisorption of 02 on Cu(110) conducted by Hodgson et al. [18]. Many of the experimental observations of this study were similar to those of the O,/W(110) study, including an increase in the initial adsorption probability with increasing kinetic energy (above 0.05eV), no dependence of So on temperature [between ~ 200 and 700K at Ei=0.49eV) and normal energy scaling. Furthermore, EELS measurements of O, on the Cu(I10) surface for temperatures as low as 100 K indicated only oxygen adatoms on the surface [21]. Attesting to the complexity of gas surface kinetics and dynamics are experimental results for some systems that do not allow the assignment of direct dissociation or direct molecular chemisorption for high kinetic energy adsorption. For these systems, data concerning the presence of stable, molecularly chemisorbed intermediates to dissociation also have been inconclusive. A review of experimental results for such gas surface systems is presented below. Studies of the adsorption of N2 on W(I10) showed an increase in the initial adsorption probability f r o m S o < 3 x l 0 3 at 30 kJ/mol to So= 0.35 at 100 kJ/mol [22], characteristic of a direct-type mechanism. Consistent with direct dissociation was the observation of an increase in nitrogen coverage with increasing kinetic energy. Particularly interesting, however, were angular dependent data for the N z / W ( l l 0 ) system; near total energy scaling was reported at high kinetic energies instead of the normal kinetic energy scaling that has been associ-

J.E. Davis, C.B. Mullins / Surface Science 380 (1997) L513 L520

direct dissociative mechanisms. Moreover, the weak dependence of So on 0i at high kinetic energy was interpreted as due to the incident molecule sampling an intermediate state during the dissociation process. However, it was argued that such an intermediate was short-lived and therefore analogous to a transition-state (rather than to an equilibrated, kinetic intermediate state such as that involved in a direct molecular chemisorption process) [22]. That such a state (possibly a negative ion intermediate) could "scramble" parallel and perpendicular components of momenta and yield total energy scaling was discussed by Gadzuk and Holloway [23]. In summary, neither a direct dissociative nor direct molecular chemisorption mechanism was strongly suggested by the N z / W ( l l 0 ) data. However, molecularly chemisorbed states of N2 on W(I10) have been observed. Measurements of nitrogen on W ( l l 0 ) , employing T P D [24] and UPS [25], indicated the presence of molecularly chemisorbed nitrogen at low temperatures. Adsorption of nitrogen on W ( l l 0 ) at ~ 120 K resulted in the population of a ~ - N 2 state that desorbed at 150-190 K, but no dissociation of the molecule from this state was detected during heating of the crystal [24]. Additional experimental studies may be warranted for this system, including a thorough examination of the dependence of So on temperature in the high kinetic energy regime and attempts at spectroscopic identification of surface states at low temperatures adsorbed via translationally-energetic molecules. The reaction of nitrogen on another plane of tungsten, the (100) face, exhibits behavior similar to that on the (110) face, although the magnitude of the initial adsorption probabilities are greater for the former. An increase in the value of the initial adsorption probability of N 2 on this surface from So=0.14 at 0.45 eV to So>0.4 at 5 eV was observed [26] (see Fig. 1). Measurements of the temperature dependence of the initial adsorption probability in this energy regime suggested a direct dissociation mechanism because So varied minimally with temperature from 300 to 1000 K at energies of 1.2 eV and greater (not shown in Fig. 1). However, only a small change in the initial adsorption probability with angle of incidence was

detected in the high energy regime. Again, a mechanism similar to that proposed to explain the angular dependence of N 2 on W ( l l 0 ) was suggested for this system. An important difference in experimental phenomena between N 2 adsorption on W(100) and on W(I10) was reported; no increase in saturation coverage was observed for the N2/ W(100) system. (Recall that energy-independent saturation coverages have been reported for systems that exhibit direct molecular chemisorption.) The significance of this difference is unclear, but lends some support to a mechanism where the incident molecule conditions are "forgotten" prior to dissociation. In considering further the plausibility of a direct molecular chemisorption mechanism for this system, note that measurements of the temperature dependence of So between 300 and 1000K may fail to reveal the influence of a molecularly chemisorbed intermediate to dissociation, particularly if the difference in barrier heights between forward and reverse conversion from the intermediate were small; furthermore, the largest variation may be expected for temperatures lower than 300 K. Therefore, qualifying the high kinetic energy adsorption of this system as direct dissociation or direct molecular chemisorption may be premature with the available data. Spectroscopic measurements of N 2 on W(100) at low temperatures support the existence of molecularly chemisorbed states. Two forms of molecular nitrogen on W(100) for saturation exposures at 78 K have been detected by TPD, with desorption features peaked at 160 and 180 K [27]. These states were associated with loss features in EEL spectra of 1452 and 2137 cm 1 and identified as bridge and terminal bonded N2, respectively [28]. Low coverages of N 2 o n W ( 1 0 0 ) at 105 K showed features in EELS assigned to both dissociated and molecular nitrogen as reported by Sellidj and Erskine [29]. At the lowest coverage studied (a nitrogen exposure of 0.015 Langmuirs [-1 L a n g m u i r = l × 10 .-6 Torr.s]), a feature at 525 c m - 1 associated with the tungsten nitrogen stretch of nitrogen adatoms, was observed as well as broad features at 1050 and 1210 cm -1. The authors assigned the latter features to vNy for ~-bonded nitrogen states on the surface and suggested that these states served as precursors to dissociation. It is noted

J.E. Davis. C.B. Mullins / Surlace Science 380 (1997) L513 L520

that the energy losses of these states differ considerably from the 1492 cm ~ loss associated with rebonded nitrogen on F e ( l l l ) [8]. A lower temperature may be necessary to inhibit dissociation and observe solely molecularly chemisorbed N 2 on W(100) at low coverages. Finally, a discussion of experimental findings is presented for a few systems for which the magnitudes of the initial adsorption probability are very small (from 0.005 to less than 10 6) over a large range of incident conditions. Within the framework of the observations discussed in this article, a rationalization of the evidence concerning high energy adsorption pathways and the participation of molecularly chemisorbed intermediates in dissociation is unavailable, but results from these systems should be mentioned for completeness. Measurements of the dissociative chemisorption of oxygen on Pt(100)-hex-RO.7" have been reported by Guo et al. [30]. The Pt(100) surface readily reconstructs, and the Pt(100)-hex-RO.7 surface is the most stable structure. Oxygen adsorption on this surface has been studied by T P D and XPS. Norton et al. concluded that oxygen adsorbs molecularly on the hex surface at 123 K but desorbs with undetectable dissociation when the surface is heated [31]. This behavior is consistent with a one-dimensional potential energy surface involving a molecularly chemisorbed state and a large barrier to dissociation from this intermediate. The initial adsorption probability of oxygen on this surface was found to rise with increasing kinetic energy from So<5X l 0 - 4 at ~0.1 eV to So ~0.0055 at 0.9 eV and T~= 573 K. Additionally, the value of So increased with increasing surface temperature in this range, and limited energy-angle scaling data, indicated normal energy scaling. A direct dissociation mechanism was applied to this system with the temperature dependence rationalized via a surface oscillator model [30], similar to the one proposed by Hand and Harris in which surface vibrations increase the impact energy of particles colliding with the proper phase [1]. However, there is an alternative explanation that can explain these phenomena, and it is consistent with the measurements of Norton et al. [31 ]. Note that if a molecular intermediate were involved in dissociation on this surface and the barrier to

dissociation were greater than the desorption (as suggested by the measurements of Norton et al.I, an increase in So with increasing surface temperature would be expected. The adsorption of O2 on Ag(ll 1 )exhibits particularly interesting behavior. Separate measurements of molecular and dissociative adsorption for this system were reported by Raukema et al. [32]. The initial m o l e c u l a r adsorption probability was observed to increase with increasing E~ from t).l to l eV and to decrease with increasing kinetic energy above 1 eV. (Campbell had previously identified molecularly chemisorbed oxygen on Ag( 111 ) below 217 K using XPS [33]). However, the initial d i s s o c i a t i v e adsorption probability of O~ on A g ( l l 1) increased with increasing kinetic energy over the entire energy range studied (~0.1 to 1.7 eV). Also, below l eV, the initial dissociative adsorption probability decreased with increasing surface temperature while above 1 eV, S~ increased with increasing Ts. These measurements were provided as evidence for a direct molecular chemisorption mechanism for energies up to ~ I eV and a direct dissociation mechanism for higher kinetic energies (for dissociative adsorption). Indeed, the unusual behavior of this system may be a consequence of the low reactivity of the Ag( 111 ) surface (suggested by the very low values of the initial molecular and dissociative adsorption for this system) [33] compared with that of the other surfaces discussed for which adsorption is relatively facile. A consideration of all molecular beam data regarding diatomic dissociation is beyond the scope of this paper, but systems have been discussed for which thorough experimental studies are available, as summarized in Table 1. In this endeavor, an observed trend has been presented systems demonstrating molecularly chemisorbed states which are stable at low temperatures and coverages appear most consistent with dissociation mechanisms revolving direct molecular chemisorption for incident energies up to a few electron w~lts. More investigations are necessary to test this hypothesis and to discern the differences between direct molecular chemisorption and direct dissociation mechanisms. Of particular utility in exploring these differences are experiments aimed at probing

J.E. Davis, CB. Mullins / Sur/ace Science 380 (1997) L513-L520

surface states as a function of incident conditions. Indeed, the identification of phenomena common to various gas-surface systems may be important in providing a foundation for general theories of dissociative chemisorption and a molecular-level understanding of heterogeneous catalysis.

[13] [ 14] [15]

[16] [17]

Acknowledgements [18]

C.B.M. acknowledges support from a National Science Foundation Presidential Young Investigator Award. The authors also thank Koch Industries and Amoco for their assistance.

[19] [20] [21]

[22]

References [1] M. Hand and J. Harris, J. Chem. Phys. 92 (1990) 7610. [2] G. Katz and R. Kosloff, J. Chem. Phys. 103 (1995) 9475. [3] M.C. Wheeler, D.C. Seets and C.B. Mullins, J. Chem. Phys. 105 (1996) 1572. [4] C,T. Rettner and H. Stein, Phys. Rev. Lett. 59 (1987) 2768. [5] C.T. Rettner and C.B. Mullins, J. Chem. Phys. 94 (1991) 1626. [6] D. Kelly, R.W. Verhoef a~ld W.H. Weinberg, J. Chem. Phys. 102 (1995) 3440. [7] J.E. Davis, P.D. Nolan, S.G. Karseboom and C.B. Mullins, J. Chem. Phys., submitted. [8] M. Grunze, M. Golze, W. Hirschwald, H.-J. Freund, H. Pulm, U. Seip, M.C. Tsai, G. Ertl and J. Ktippers, Phys. Rev. Lett. 53 (1984) 850: M. Grunze, G. Strasser and M. Golze, Appl. Phys. A 44 (1987) 19. [9] N,R. Avery, Chem. Phys. Lett. 96 (1983) 371. [ 10] C. Puglia, A. Nilsson, B. Hernn~is, O. Karis, P. Bennich and N. Mgtrtensson, Surf. Sci. 342 (1995) 119. [11] A.C. Luntz, M.D. Williams and D.S. Bethune. J. Chem. Phys. 89 (1988) 4381. [12] B.A. Sexton and R.J. Madix, Chem. Phys. Lett. 76 (1980) 294:

[23]

[24]

[25] [26]

[27] [28] [29] [30] [31] [32] [33]

C. Backx, C.P.M. de Groot and P. Biloen, Surf. Sci. 104 ( 1981 ) 30O. L. Vattunone, M. Rocca, P. Restelli, M. Pupo, C. Boragno and U. Valbusa, Phys. Rev. B 49 (1994) 5113. J.L. Taylor, D.E. Ibbotson and W.H. Weinberg, Surf. Sci. 79 (1979) 349. J.E. Davis, S.G. Karseboom, P.D. Nolan and C.B. Mullins, J. Vac. Sci. Technol. A 14 (1996) 1598: J. Chem. Phys. 105 (1996) 8362. J.K. Brown and A.C. Luntz, Chem. Phys. Lett. 204 (1993) 451. C.T. Rettner, L.A. DeLouise and D.J. Auerbach. J. Chem. Phys. 85 (1986) 1131. A. Hodgson, A.K. Lewin and A. Nesbitt, Surf. Sci. 293 11993) 211. R. Opila and R. Gomer, Surf. Sci. 105 ( 1981 ) 41. W.J. Mitchell, J. Xie, K.J. Lyons and W.H. Weinberg, J. Vac. Sci. Technol. A. 12 (1994) 2250. J.M. Mundenar, A.P. Baddorf, E.W. Plummer, L.G. Sneddon, R.A. Didio and D.M. Zehner, Surf. Sci. 188 (1987) 15. H.E. Pfntir. C.T. Rettner, J. Lee, R.J. Madix and D.J. Auerbach, J. Chem. Phys. 85 (1986) 7452. J.W. Gadzuk and S. Holloway, Chem. Phys. Lett. 114 (1985) 314; S. Holloway and J.W. Gadzuk, J Chem. Phys. 82 (1985) 52O3. J.T. Yates, Jr., R. Klein and T.E. Madey, Surf. Sci. 58 (1976) 469; M. Bowker and D.A. King, J. Chem. Soc., Faraday Trans. I. 75 (1979) 2100. E. Umbach, A. Schichl and D. Menzek Solid State Commun. 36 (1980) 93. C.T. Rettner, E.K. Schweizer, H. Stein and D.J. Auerbach, J. Vac. Sci. Technol. A 7 (1989) 1863; C.T. Rettner, E.K. Schweizer and H. Stein, J. Chem. Phys. 93 (1990) 1442. L.R. Clavenna and L.D. Schmidt, Surf. Sci. 22 (1970) 365. W. Ho, R.F. Willis and E.W. Plummer, Surf. Sci. 95 (1980) 171. A. Sellidj and J.L. Erskine, Surf. Sci. 220 (1989) 253. X.-C. Guo. J.M. Bradley, A. Hopkinson and D.A. King, Surf. Sci. Lett. 292 (1993) L786; Surf. Sci. 310 (1994) 163. P.R. Norton, P.E. Binder and K. Griffiths, J. Vac. Sci. Technol. A 2 (19841 1028. A. Raukema, D.A. Butler, F.M.A. Box and A.W. Kleyn, Surf. Sci. 347 (19961 151. C.T. Campbell, Surf. Sci. 173 (1986) L641.