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Interaction of NH3 with oxygen-predosed Ni(111)
Surface Science I 19 (1982) 422-432 North-~olIand Publishing Company
422
INTERACTION Falko P. NETZER
OF NH, WITH OXYGEN-PREDOSED * and Theodore E. MADEY
surface science Diuisian, National Bureau of Standards, Received
16 February
Ni(ll1)
1982; accepted
for publication
Washington, 30 March
DC 20234, USA
1982
We have used ESDIAD (electron stimulated desorption ion angular distributions), LEED and thermal desorption to study the structure and kinetics of NH, interacting with preadsorbed oxygen on a Ni( Ii I) surface. We find evidence for a striking effect: Traces of preadsorbed oxygen will induce a high degree of azimuthal ordering in a fractional monolayer of adsorbed NH, molecules; the H ligands are oriented azimuthally in the [ii21 directions. In contrast. adsorption of NH, on a clean Ni(1 I I) surface results in random azimuthal orientation of the NH, molecules, which are bound to the metal via the N atom with H pointing away from the surface. Even for very low oxygen coverages, 8, -0.05 (annealed layer), the majority of NH, is azimuthally oriented, so that one 0 atom influences more than one NH, molecule. LEED reveals that long range order is absent in the composite NH, +0 overlayer. Thermal desorption reveals that the presence of 0 leads to an increase in NH, desorption energy. We postulate that the NH, interacts with atomic 0 via a hydrogen bond, leading to focaf azj~~t~af ordering in the absence of Iong range orders
1. Introduction The structure and kinetics of the interaction of NH, with Ni( 111) have been the subject of several recent investigations ]1,2]. Angle resolved UPS {l] has been used to demonstrate that NH, adsorbs molecularly on Ni(lll), and is bonded to the surface via the N atom, with the H atoms pointed away from the surface. Evidence obtained using ESDIAD (electron stimulated desorption ion angular distributions) demonstrates that the H atoms in the array of adsorbed NH, on clean Ni( 111) have random azimuthal distribution, i.e., no preferential azimuthal ordering exists [2]. We have recently observed a new effect involving an interaction between adsorbed NH, and adsorbed impurity atoms [3]. Preadsorbed oxygen on a Ni(ll.1) surface can induce a high degree of local azimuthal ordering in adsorbed molecular NH, even though the molecules have random azimuthal orientations on the clean surface. Similar ordering effects have been seen for H,O + 0 on Ni(l11) [3,4]. In the present paper, we report details of the interactions of NH, with oxygen-predosed Ni(ll1) surfaces as studied using * NBS Guest Worker and Visiting Professor, Permanent address: University of Innsbruck,
0039-6028/82/~0-~~/$02.75
Department Innsbruck,
of Physics, Austria.
0 1982 North-Holland
University
of Maryland.
ESDIAD, LEED and thermal desorption. The structures, binding energy and surface chemistry of NH, on Ni( 111) are profoundly influenced by preadsorbed oxygen. This new observation of steric effects in coadsorption may have a direct bearing on the mechanisms by which catalyst promotors and poisons function. ESDIAD [S] is a useful method for studying the local geometry of adsorbed molecules, primarily because the directions of ion desorption are determined by the directions of the surface bonds ruptured by the electronic excitation (electron bombardment of adsorbed molecules). Thus, bond angles appear to be directly related to ion desorption angles.
2. Experimental procedures The ultrahigh vacuum apparatus and methods used for these studies have been described previously in detail [2,3,5]. ESDIAD and LEED patterns were viewed directly on a fluorescent screen following image intensification of the desorbing ion (or scattered electron) signal using a double microchannel plate detector in a hemispherical retarding grid analyzer. NH, was deposited onto the front surface of the Ni(ll1) sample using a calibrated molecular beam doser having a micro-capillary array as an effusion source. Mass analyses of ESD ions and of thermal desorption products were accomplished using a quadrupole mass spectrometer. The sample was cryogenically cooled to 80 K and could be heated continuously to 1100 K using resistive heating via two short Ta wires spotwelded to the crystal. The sample was cleaned using Ar’ ion bombardment and annealing in vacuum; sample cleanliness and oxygen coverages were determined using Auger electron spectroscopy (AES) and LEED (an oxygen coverage of So = 0.25 corresponds to a we&developed p(2 X 2) LEED pattern (61).
3. Results 3.1. Thermal desorption Figs. 1A and IB contain a series of thermal desorption spectra for NH, adsorbed on Nif 111) at - 80 K. The first layer in direct contact with the Ni surface gives rise to a broad desorption feature in the range 150-300 K. As the coverage increases, a second layer forms with a sharp desorption peak at - 120K (spectra Id-li); these data are in good agreement with previously published results [1,2]. A further increase in exposure at - 80 K leads to formation of multilayers of NH,, evidence for which can be seen in spectra Ij-Im. The formation of the second layer has been studied in detail by Fisher [7] for NH, on Pt(l1 I) (his /3 state), and seen by Grunze et al. [S] on Ni(1 IO)
424
E P. Netzer,
T. E. Madey / NH, on oxygen-pm-dosed Ni(I 1 I)
r
I
@
5s -
Sens. x
1
6ens. x 0.16
Fig. 1. Thermal desorption spectra for NH, on Ni(l11). The NH, coverage BNH, corresponding to each spectrum is: (a) ~0.05; (b) -0.12: (c) -0.2; (d) ~0.35; (e), (D aO.4; (g) =0.2; (h) -0.4; (i) ~0.5; (j)-(m) >0.5.
and Gland and Kollin on stepped Pt(ll1) [9]. Thermal desorption following multilayer NH, growth has been reported by Fisher [7]. The dominant desorption product from Ni( 111) is NH, [ 1,2], although evidence for some thermal decomposition has been seen [2]. Although we have not made absolute coverage measurements for NH, on Ni( 11 l), Fisher [7] has used XPS to determine the saturation coverage of the first layer of NH, on Pt(ll1) to be about l/4 monolayer, i.e., 3.8 X lOI molecules/cm2 (his (Ystate). Grunze et al. [8] also report a value of 3.8 X lOI molecules/cm for NH, saturation on Ni(ll0) at 120 K. Based on these measurements [7,8] we shall assume for the following discussion that saturation coverage for the first layer occurs between curves c and d of fig. 1, and corresponds roughly to l/4 monolayer (the coverage 8 = 0.25 corresponds to a surface density of 4.6 X lOI molecules/cm for the Ni(ll1) surface). (Note that in ref. [2], a different convention was adopted, in which saturation of the first NH, layer was defined as 0 = 1.) The influence of preadsorbed oxygen on the thermal desorption of NH, from Ni( 111) is illustrated in fig. 2. For the spectra of fig. 2A, the surface was monolayer of oxygen and annealed to 600K before predosed with -0.05 being cooled to 80 K and exposed to NH,. The major difference between these data and the spectra for NH, on clean Ni(ll1) is the appearance of NH,
425
F.P. Netter, T.E. Madey / NH, on oxygen-predosedNi(l II)
e
80150200250300350 400 450
,
,
,
(
-80150m2503003$0 TIKl
,
,
,
I
1
400
45u
Fig. 2. Thermal desorption of NH, from oxygen-precovered Ni( 111).Curves A: Constant oxygen coverage, 0, = 0.05, and values of BNn, are: (a) ~0.05; (b) -0.12; (c) ~0.2; (d) 10.25; (e) ~0.4. Curves B: Constant NH, coverage BNH,= 0.4; (a) Bo=O (clean); (b) 8, =0.05; (c) eo=O.ls; (d) 8, =0.25, p(2 X 2) LEED.
in the temperature range 320-360 K. This shift of the NH, desorption to higher temperatures has also been seen for H,O on Ni( 111) predosed with oxygen [4], and is indicative of an increase in the binding energy of a fraction of the adsorbed NH, due to an NH,-0 interaction [lo]. Auger electron spectroscopy following several adsorption-desorption cycles of NH, on oxygen-predosed Ni( 111) revealed no loss of oxygen (wit~n expe~mentai uncert~nty) so that the decomposition of NH, to form OH, and the subsequent disproportionation (2 OH -+ I-I,0 + O(ads)) was not an important process. There is evidence for some NH, decomposition however, and this will be discussed in section 3.3. As the NH, coverage is increased (fig. 2A, c-e) the desorption characteristics for Ts 300 K are similar to NH, on clean Ni( 11 I), including the appearance of the “second layer” peak. Thus the binding energies of most of the adsorbed NH, are relatively unaffected by the low coverage of oxygen. The influence of increasing the 600 K annealed oxygen coverage prior to dosing a constant coverage of NH, is shown in fig. 2B. As the oxygen coverage increases to the point where NH, molecules can interact with more than one adsorbed oxygen atom (6, N 0.25 for fig. 2B, d), the range of desorption temperatures for NH, extends to - 425 K. desorption
426
F. P. Netrer, T. E. Madey / NH, on oxygen-predosed Ni(l1 I)
3.2. Low energy electron diffraction The only LEED pattern seen for NH, on Ni(ll1) is a poor!v-ordered structure of hazy beams which was previously tentatively indexed as (n/2 X fi/2)R19”; this pattern appears at about half the saturation coverage for the first layer [2]. Predosing the surface with oxygen prevents the formation of an ordered LEED pattern. Note that the coverage of an ideal (J’r/2 X 0/2)R19” structure is 19= 0.43 [ 111, considerably greater than the coverage of 0.25 estimated for saturation coverage of the first layer of NH,. The LEED pattern may arise from islands of NH, in which some second-layer NH, is present. 3.3. Electron stimulated desorption Fig. 3 is a plot of the H+ ESD signal in the direction normal to the surface measured as a function of NH, dose from the microcapillary doser. After a short initial induction period, the yield rises monotonically to a plateau (the arrow marks the approximate dose at which the first layer is saturated, 050.25). The second step rise coincides with the growth of multilayers of NH,. The initial induction is due to the fact that the H+ ESDIAD pattern has
NH, Dose (Arbitrary Units) Fig. 3. Hf ESD signal versus NH, exposure for adsorption at 80 K on Ni( 111) (arbitrary units). The arrow corresponds roughly to the formation of the first layer ( ON,3 = 0.25). The exposure scale is not exactly linear, due to wall effects in the doser: the exposure necessary to form the first layer is -30% larger than that necessary to form subsequent layers.
F.P. Netzer,
T.E. Madey / NH, on oxygen-predosed
Ni(lIl)
427
a minimum in intensity normal to the surface for low coverages of NH, at TZ 80 K [2]; the plateau appears to coincide with filling of the second layer. For B 5 0.25, the only ESD ions detected were H+ . For ESD of multilayers, traces of high mass ions (NH,),H+ were seen, with intensities less than 1% of the Ht yield. This is similar to the ESD results for H,O multilayers [4,12]. Fig. 4 contains LEED and ESDIAD patterns for the adsorption of NH, on Ni( 111). A more complete description of NH, adsorption on clean Ni( 111) as characterized using ESDIAD is given in ref. [2]. Fig. 4a is a LEED pattern from the clean surface, and fig. 4b is an Hf ESDIAD pattern for NH, adsorbed on Ni( 111) (0 < 0.25, T- 150 K). The continuous “halo” of H+ desorption in directions away from the surface normal is consistent with an array of NH, molecules bonded via the N atoms, with no preferred azimuthal orientation of the H atoms which point away from the surface [2]. Another possibility is that of NH, rotating freely about an axis normal to the surface. If the Ni(ll1) surface is predosed with oxygen (0.1 to 0.2 L O,, 13, - 0.03 to 0.05, heated to 600 K to anneal, then cooled to 80 K) and then dosed with NH,, an ESDIAD pattern exhibiting three-fold symmetry is observed [3]. Heating to 2 140 K causes desorption of all NH, except the chemisorbed layer in contact with the substrate, and the well-ordered Hf ESDIAD pattern illustrated in fig. 4c results. The H+ ion emission appears as a “broken” halo, with maxima in intensity along [ 1121 azimuths. Even at the lowest oxygen coverages used in this work (8, N 0.03 monolayer), the oxygen-induced azimuthal ordering was seen. Upon heating to Tk 300 K, the ESDIAD pattern disappears as the NH, desorbs. Fig. 5 illustrates the effect of both NH, coverage and surface temperature on the azimuthal ordering effect. In each case, the annealed oxygen pre-coverage was &=O.OS. Each vertical pair of ESDIAD patterns (a, A; b. B; c, C) corresponds to adsorption of NH, at 80 K, followed by heating to 150 K. The initial NH, coverages increased from left to right, and were -0.08, 0.12, and 0.25 for patterns a, b, and c, respectively. In all cases evidence for azimuthal
Fig. 4. LEED and ESDIAD of NH, on clean and oxygen-dosed Ni( 111): (a) clean LEED, V, = 110 eV; (b) H+ ESDIAD of NH, on Ni( 11 I), V, = 300 eV; (c) H+ ESDIAD of NH, on oxygen-predosed Ni( 111). 8, = 0.05.
428
F.P. Netzer,
i? E. Madey / NH, on oxygen-predosed
Ni(ll I)
Fig. 5. Effect of NH, coverage and temperature on ESDIAD of NH, on oxygen-predosed Ni( 11l), 8,=0.05; (a, A) adsorb B.,,,=0.08 at 80 K, heat to 150 K; (b, B) adsorb 0,,,-0.12 at 80 K, heat to 150 K; (c, C) adsorb 0 NH, -0.25 at 80 K, heat to 150 K; (D) after repeated adsorptions of NH,, followed by heating to 610 K.
ordering was evident at 80 K, but heating to - 150 K resulted in an increase of azimuthal order at the higher coverages; this was accompanied by desorption of any second-layer NH, which may have formed at 80 K. Even when the first NH, layer is nearly filled (fig. SC), the ESDIAD pattern indicates a high degree of azimuthal order. In this case, the ratio of NH, to 0 is 2 5, so that one 0 atom influences the orientation of more than one NH, molecule. The intensity of the normal beam (central spot) seen in the ESDIAD patterns of fig. 5 varies with coverage as well as position of the electron beam on the surface. This suggests that this beam is due to a surface complex (e.g., NH or OH) whose concentration depends on defect and step density, as well as possible electron beam damage. If the oxygen-predosed surface is exposed repeatedly to NH, following by heating to. - 600 K, a dim hexagonal ESDIAD pattern (fig. 5D) appears in which the dominant ion normal to the surface is O+. The same pattern was seen following NO adsorption on Ni( 111) [ 131 when the surface was heated to - 600 K; it was attributed to a mixed layer of N + 0 on Ni( 111) following dissociation of the molecular NO. Thus, it appears that the presence of the oxygen promotes dissociation of a fraction of the adsorbed NH,; indeed, AES demonstrates that traces of N remain on the oxygen-predosed surface after this experiment. Finally, we note that the azimuthal ordering is less pronounced as the oxygen coverage increases. For 0, = 0.25, no evidence for azimuthal ordering in ESDIAD of adsorbed NH, was observed.
F.P. Netzer, T.E. Madqv / NH3 on oxygen-predosedNi(llI)
429
4. Discussion It is of interest to compare and contrast the adsorption of NH, on Ni(ll1) with the adsorption of H,O [4]. Both of these polar molecules are adsorbed molecularly on Ni(1 11), and are bonded to the surface via the lone pair electrons on the N(0) atom, with the H atoms pointing away from the surface. Desorption occurs with little dissociation. Each molecule is disordered azimuthally on the clean Ni(l11) surface, but undergoes a high degree of azimuthal ordering in the presence of adsorbed oxygen. However, there are quantitative differences in the thermal desorption behavior of the two molecules. Water desorbs from the first layer on Ni( 111) in a sharp desorption peak as the at - 170 K; the sharp peak (FWHM - 9 K) shifts to higher temperatures coverage increases, an effect which has been related to attractive lateral interactions in the adsorbed layer due to hydrogen-bonding between neighboring H,O molecules [4]. In contrast, the first NH, layer desorbs over a wide temperature range (- 150 to 300 K). The low temperature onset of desorption shifts to lower temperatures as NH, coverage is increased (cf. fig. l), consistent with repulsive lateral interactions between neighboring NH, molecules. Qualitatively similar results have been seen for H,O and NH, thermal desorption data from Ru(001) [ 14, IS] and Pt( 111) [7,16] surfaces. A possible explanation for these differences involves the relative strengths of the hydrogen bonding interactions in adsorbed H,O and NH,, and how these interaction energies compare with the NH,/Ni( 111) and H,O/Ni( 111) adsorption energies. In the low-coverage limit, the values of desorption peak temperature for NH, and H,O are - 290 and 165 K, respectively (implying desorption energies of - 74 and - 42 kJ/mole): NH, is bonded to Ni( 111) with an adsorption energy almost twice as great as H,O. From UPS [l], it is known that NH, is bonded via the 3a, orbital, which is composed mainly of the nitrogen lone pair electrons. The adsorbed NH, thus has no additional lone pair orbitals to participate in attractive lateral interactions via hydrogen bonding to neighboring NH,; the dominant lateral interactions between neighboring NH, molecules appear to be repulsive as the coverage increases. In contrast, H,O has two orbitals with some lone-pair, non-bonding character, lb, and 3a, ; when H,O is bonded to a planar metal surface via one of its lone pair orbitals, the other lone pair orbital is available for hydrogen bonding. Thus the more weakly-bound H,O can be “pulled around” to interact with its neighbors via hydrogen bonds (attractive lateral interactions) as the coverage increases. The ESDIAD patterns observed for NH, and H,O on clean Ni( 111) are consistent with the above pictures of bonding of these molecules. The “halo” pattern of fig. 4C is unchanged as the NH, coverage increases in the temperature range above - 150 K, indicating that the bonding geometry is unchanged (bonding via the N atom, with H atoms away from the surface). The ESDIAD pattern for H,O on Ni(ll1) (fig. 1 of ref. [3]) is consistent with a range of
430
F. P. Netzer, T. E. Madey / NH, on ox_vgen-~~edosed Ni(I Ii)
bonding orientations for the non-hydrogen-bonded H atoms in a two-dimensional layer of hydrogen-bonded, adsorbed H,O. When NH, is adsorbed on Ni( 111) predosed with oxygen, both ESDIAD and thermal desorption measurements provide evidence for a chemical interaction between NH,(ads) and O(ads). This interaction is sufficiently strong as to increase the NH, desorption temperature, and to introduce azimuthal ordering into the NH, layer which has no preferred azimuthal ordering on the clean Ni( 111) surface. The NH, may interact with 0 via a weak hydrogen bond, as suggested previously 131, or via an interaction through the metal substrate. The H atoms (ions) which contribute to the ESDIAD patterns (fig. 4C; fig. 5) are presumably those atoms which do not participate in the interaction with oxygen. The azimuthal ordering of NH, has been described previously [3]. Oxygen on Ni( 111) is bonded in a three-fold hollow site [6]; whether this site is above a second layer atom or a second layer vacancy is not known, although the vacancy site is favored for 0 on the close packed fee surfaces, Ir( 111) ] 171 and Al(lll) [18]. In fig. 6, a structural model is proposed which is consistent with ref. [3]. If the 0 atoms are located in three-fold hollows above second layer atoms, NH, is bonded in the other three-fold hollow (this is the site preferred for NH, on clean Pt(ll1) {7]). The surface O-N distance for O-NH, is 2.87 A, as compared with the N-O separation of 2.714 in hydrogen-bonded (H~NHOH~)~ [ 191 and N-O separations of 2.78 to 2.93 A reported for other hydrogen-bonded systems involving N-H * . .O 1201. The close agreement between the presumed N-O separation and the hydrogen bonding N-H-O distance [19,20] is the basis for suggesting that a hydrogen bonding interaction may be responsible for the azimuthal orientation. This interaction overcomes the random azimuthal configuration (or free rotation) of the NH, on clean Ni( 111). Studies presently
Fig. 6. Model of NH,
adsorption
on oxygen-predosed
Ni( 111).
F.P. Netzer, T.E. Madey / NH, on oxygen-predosedNi(Il I)
431
underway are designed to evaluate the role of substrate geometry and adsorbate chemistry (0. S, Na, . . .) on the azimuthal ordering of NH, and H,O. As discussed previously [3], if the 0 atoms are actually located above the second layer vacancies, the NH, sites in fig. 6 are shifted accordingly, with no change in N-O separation. In this model, the non-hydrogen-bonded H atoms are the ones that are seen in ESDIAD. Either the hydrogen-bonded ligands are more effectively neutralized following electronic excitation, or the H+ ions from the hydrogen-bonded ligands follow shallow trajectories under the influence of the image potential [5] and are recaptured by the surface. This latter possibility is consistent with adsorbed NH, molecules which are slightly “tilted”, i.e., inclined toward the 0 atom so that the N-H . . . 0 angle opens (note that the XH . . . 0 angle is not necessarily linear in hydrogen-bonded systems, such as certain forms of ice [21]). Such a tilt may be related to the vertical displacement of nickel atoms on Ni( 111) which is induced by adsorbed oxygen [22]. Note also that the central beams in figs. 4 and 5 may be due to a small fraction of dissociation products (NH, OH); the intensity of this beam varies at different points on the crystal and it is sensitive to bombardment time. In fig. 6, we show a single 0 atom influencing the orientation of more than a single NH, molecule, consistent with the observations of Bowker et al. [23] and Kretschmar et al. [24] for H,O + 0 on metals. Whether this direct interaction of one 0 with three NH, molecules occurs, or whether 0 atoms “nucleate” ordered layers by orienting single NH, species which interact laterally with their neighbors is not clear. It is clear, however, that 6, ~0.05 will cause substantial short-range order in a saturated NH, layer (a,,, - 0.25). cf. fig. 5C; The ratio of NH, to 0 is - 5 to 1. Since it is unlikely that one 0 will interact directly with more than 3 NH, molecules, the remaining adsorbed NH, is either ordered by lateral interactions, or is randomized azimuthally, thereby contributing to the H+ background signal in ESDIAD pattern 5C. At higher oxygen coverages (8, - 0.25), the azimuthal ordering effect is not observed, presumably because NH, molecules can interact with more than one 0 atom. Finally, we note that the azimuthal ordering reported for NH, on clean Ni(l11) [25] using angle resolved UPS may have been influenced by traces of impurities, as reported herein.
5. Summary
(1) Thermal desorption data for NH,/Ni( 111) indicate that repulsive lateral interactions are present in the adsorbed layer in contact with the Ni; evidence for a second layer and multilayer formation are also seen at high exposures. (2) Preadsorption of oxygen has a strong influence on the bonding of NH,
432
F.P. Netzer,
T. E. Madey / NH, on oxygen-predosed
Ni(ll1)
to Ni( 111). Thermal desorption data indicate an increase in the binding energy of NH, in the presence of O(ads). (3) Small coverages (19,s 0.05) of preadsorbed oxygen induce a high degree of azimuthal order in adsorbed NH, which is disordered azimuthally on the clean Ni( 111) surface.
Acknowledgements
This work was supported in part by the Office of Naval Research. acknowledges financial support from the Max Kade Foundation.
F.P.N.
References
[l] C.W. Seabury, T.N. Rhodin, R.J. Purtell and R.P. Merrill, Surface Sci. 93 (1980) 117. [2] T.E. Madey, J.E. Houston, C.W. Seabury and T.N. Rhodin. J. Vacuum Sci. Technol. 18 (1981) 476. [3] F.P. Netzer and T.E. Madey, Phys. Rev. Letters 47 (1981) 928. [4] T.E. Madey and F.P. Netzer, Surface Sci. 117 (1982) 549. [5] T.E. Madey, in: Inelastic Particle-Surface Collisions, Topics in Chemical Physics, Vol. 17, Eds. W. Heiland and E. Taglauer (Springer, Heidelberg, 1981) p. 80. [6] L.D. Roelofs, A.R. Kortan, T.L. Einstein and R.L. Park, J. Vacuum Sci. Technol. 18 (1981) 492; A.R. Kortan and R.L. Park, Phys. Rev. B23 (1981) 6340. [7] G.B. Fisher. Chem. Phys. Letters 79 (1981) 452. [8] M. Grunze, M. Golze, R.K. Driscoll and P.A. Dowben. J. Vacuum Sci. Technol. 18 (1981) 611. [9] J.L. Gland and E.B. Kollin, J. Vacuum Sci. Technol. 18 (1981) 604. [lo] T. Au and M.W. Roberts, Chem. Phys. Letters 74 (1980) 472. [ 1 l] K. Jacobi, E.S. Jensen, T.N. Rhodin and R.P. Merrill, Surface Sci. 108 (1981) 397. [12] R.H. Prince and G.R. Floyd, Chem. Phys. Letters 43 (1976) 326. [13] F.P. Netzer and T.E. Madey, Surface Sci. 110 (1981) 251. [14] T.E. Madey and J.T. Yates, Jr.. Chem. Phys. Letters 51 (1977) 77; also. in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Eds. R. Dobrozemsky et al. (Berger, Vienna, 1977) p. 1183. [ 151 P.A. Thiel, F.M. Hoffmann and W.H. Weinberg, J. Chem. Phys. 75 (1981) 5556. [16] G.B. Fisher and J.L. Gland, Surface Sci. 94 (1980) 446. [17] C.M. Chan and W.H. Weinberg, J. Chem. Phys. 71 (1979) 2788. [18] S.A. Flodstrom, C.W.B. Martinsson, R.Z. Bachrach, S.B.M. Hagstrom and R.S. Bauer, Phys. Rev. Letters 40 (1978) 907. [19] S. Scheiner and L.B. Handing, Chem. Phys. Letters 79 (1981) 39. [20] M.D. Joesten and L.J. Schaad, Hydrogen Bonding (Dekker, New York. 1974) table l-1 1. [21] B. Kamb, in: Water and Aqueous Solutions, Ed. R.A. Horne (Wiley-Interscience, New York, 1972) p. 9. [22] T. Narusawa, W.M. Gibson and E. Tornqvist, Phys. Rev. Letters 47 (1981) 417. [23] M. Bowker, M.A. Barteau and R.J. Madix, Surface Sci. 92 (1980) 528. [24] K. Kretzschmar, J.K. Sass, P. Hoffman, A. Ortega, A.M. Bradshaw and S. Holloway, Chem. Phys. Letters 78 (198 1) 410. [25] W.M. Kang, C.H. Li, S.Y. Tong, C.W. Seabury, K. Jacobi, T.N. Rhodin, R.J. Purtell and R.P. Merrill, Phys. Rev. Letters 47 (1981) 931.
422
INTERACTION Falko P. NETZER
OF NH, WITH OXYGEN-PREDOSED * and Theodore E. MADEY
surface science Diuisian, National Bureau of Standards, Received
16 February
Ni(ll1)
1982; accepted
for publication
Washington, 30 March
DC 20234, USA
1982
We have used ESDIAD (electron stimulated desorption ion angular distributions), LEED and thermal desorption to study the structure and kinetics of NH, interacting with preadsorbed oxygen on a Ni( Ii I) surface. We find evidence for a striking effect: Traces of preadsorbed oxygen will induce a high degree of azimuthal ordering in a fractional monolayer of adsorbed NH, molecules; the H ligands are oriented azimuthally in the [ii21 directions. In contrast. adsorption of NH, on a clean Ni(1 I I) surface results in random azimuthal orientation of the NH, molecules, which are bound to the metal via the N atom with H pointing away from the surface. Even for very low oxygen coverages, 8, -0.05 (annealed layer), the majority of NH, is azimuthally oriented, so that one 0 atom influences more than one NH, molecule. LEED reveals that long range order is absent in the composite NH, +0 overlayer. Thermal desorption reveals that the presence of 0 leads to an increase in NH, desorption energy. We postulate that the NH, interacts with atomic 0 via a hydrogen bond, leading to focaf azj~~t~af ordering in the absence of Iong range orders
1. Introduction The structure and kinetics of the interaction of NH, with Ni( 111) have been the subject of several recent investigations ]1,2]. Angle resolved UPS {l] has been used to demonstrate that NH, adsorbs molecularly on Ni(lll), and is bonded to the surface via the N atom, with the H atoms pointed away from the surface. Evidence obtained using ESDIAD (electron stimulated desorption ion angular distributions) demonstrates that the H atoms in the array of adsorbed NH, on clean Ni( 111) have random azimuthal distribution, i.e., no preferential azimuthal ordering exists [2]. We have recently observed a new effect involving an interaction between adsorbed NH, and adsorbed impurity atoms [3]. Preadsorbed oxygen on a Ni(ll.1) surface can induce a high degree of local azimuthal ordering in adsorbed molecular NH, even though the molecules have random azimuthal orientations on the clean surface. Similar ordering effects have been seen for H,O + 0 on Ni(l11) [3,4]. In the present paper, we report details of the interactions of NH, with oxygen-predosed Ni(ll1) surfaces as studied using * NBS Guest Worker and Visiting Professor, Permanent address: University of Innsbruck,
0039-6028/82/~0-~~/$02.75
Department Innsbruck,
of Physics, Austria.
0 1982 North-Holland
University
of Maryland.
ESDIAD, LEED and thermal desorption. The structures, binding energy and surface chemistry of NH, on Ni( 111) are profoundly influenced by preadsorbed oxygen. This new observation of steric effects in coadsorption may have a direct bearing on the mechanisms by which catalyst promotors and poisons function. ESDIAD [S] is a useful method for studying the local geometry of adsorbed molecules, primarily because the directions of ion desorption are determined by the directions of the surface bonds ruptured by the electronic excitation (electron bombardment of adsorbed molecules). Thus, bond angles appear to be directly related to ion desorption angles.
2. Experimental procedures The ultrahigh vacuum apparatus and methods used for these studies have been described previously in detail [2,3,5]. ESDIAD and LEED patterns were viewed directly on a fluorescent screen following image intensification of the desorbing ion (or scattered electron) signal using a double microchannel plate detector in a hemispherical retarding grid analyzer. NH, was deposited onto the front surface of the Ni(ll1) sample using a calibrated molecular beam doser having a micro-capillary array as an effusion source. Mass analyses of ESD ions and of thermal desorption products were accomplished using a quadrupole mass spectrometer. The sample was cryogenically cooled to 80 K and could be heated continuously to 1100 K using resistive heating via two short Ta wires spotwelded to the crystal. The sample was cleaned using Ar’ ion bombardment and annealing in vacuum; sample cleanliness and oxygen coverages were determined using Auger electron spectroscopy (AES) and LEED (an oxygen coverage of So = 0.25 corresponds to a we&developed p(2 X 2) LEED pattern (61).
3. Results 3.1. Thermal desorption Figs. 1A and IB contain a series of thermal desorption spectra for NH, adsorbed on Nif 111) at - 80 K. The first layer in direct contact with the Ni surface gives rise to a broad desorption feature in the range 150-300 K. As the coverage increases, a second layer forms with a sharp desorption peak at - 120K (spectra Id-li); these data are in good agreement with previously published results [1,2]. A further increase in exposure at - 80 K leads to formation of multilayers of NH,, evidence for which can be seen in spectra Ij-Im. The formation of the second layer has been studied in detail by Fisher [7] for NH, on Pt(l1 I) (his /3 state), and seen by Grunze et al. [S] on Ni(1 IO)
424
E P. Netzer,
T. E. Madey / NH, on oxygen-pm-dosed Ni(I 1 I)
r
I
@
5s -
Sens. x
1
6ens. x 0.16
Fig. 1. Thermal desorption spectra for NH, on Ni(l11). The NH, coverage BNH, corresponding to each spectrum is: (a) ~0.05; (b) -0.12: (c) -0.2; (d) ~0.35; (e), (D aO.4; (g) =0.2; (h) -0.4; (i) ~0.5; (j)-(m) >0.5.
and Gland and Kollin on stepped Pt(ll1) [9]. Thermal desorption following multilayer NH, growth has been reported by Fisher [7]. The dominant desorption product from Ni( 111) is NH, [ 1,2], although evidence for some thermal decomposition has been seen [2]. Although we have not made absolute coverage measurements for NH, on Ni( 11 l), Fisher [7] has used XPS to determine the saturation coverage of the first layer of NH, on Pt(ll1) to be about l/4 monolayer, i.e., 3.8 X lOI molecules/cm2 (his (Ystate). Grunze et al. [8] also report a value of 3.8 X lOI molecules/cm for NH, saturation on Ni(ll0) at 120 K. Based on these measurements [7,8] we shall assume for the following discussion that saturation coverage for the first layer occurs between curves c and d of fig. 1, and corresponds roughly to l/4 monolayer (the coverage 8 = 0.25 corresponds to a surface density of 4.6 X lOI molecules/cm for the Ni(ll1) surface). (Note that in ref. [2], a different convention was adopted, in which saturation of the first NH, layer was defined as 0 = 1.) The influence of preadsorbed oxygen on the thermal desorption of NH, from Ni( 111) is illustrated in fig. 2. For the spectra of fig. 2A, the surface was monolayer of oxygen and annealed to 600K before predosed with -0.05 being cooled to 80 K and exposed to NH,. The major difference between these data and the spectra for NH, on clean Ni(ll1) is the appearance of NH,
425
F.P. Netter, T.E. Madey / NH, on oxygen-predosedNi(l II)
e
80150200250300350 400 450
,
,
,
(
-80150m2503003$0 TIKl
,
,
,
I
1
400
45u
Fig. 2. Thermal desorption of NH, from oxygen-precovered Ni( 111).Curves A: Constant oxygen coverage, 0, = 0.05, and values of BNn, are: (a) ~0.05; (b) -0.12; (c) ~0.2; (d) 10.25; (e) ~0.4. Curves B: Constant NH, coverage BNH,= 0.4; (a) Bo=O (clean); (b) 8, =0.05; (c) eo=O.ls; (d) 8, =0.25, p(2 X 2) LEED.
in the temperature range 320-360 K. This shift of the NH, desorption to higher temperatures has also been seen for H,O on Ni( 111) predosed with oxygen [4], and is indicative of an increase in the binding energy of a fraction of the adsorbed NH, due to an NH,-0 interaction [lo]. Auger electron spectroscopy following several adsorption-desorption cycles of NH, on oxygen-predosed Ni( 111) revealed no loss of oxygen (wit~n expe~mentai uncert~nty) so that the decomposition of NH, to form OH, and the subsequent disproportionation (2 OH -+ I-I,0 + O(ads)) was not an important process. There is evidence for some NH, decomposition however, and this will be discussed in section 3.3. As the NH, coverage is increased (fig. 2A, c-e) the desorption characteristics for Ts 300 K are similar to NH, on clean Ni( 11 I), including the appearance of the “second layer” peak. Thus the binding energies of most of the adsorbed NH, are relatively unaffected by the low coverage of oxygen. The influence of increasing the 600 K annealed oxygen coverage prior to dosing a constant coverage of NH, is shown in fig. 2B. As the oxygen coverage increases to the point where NH, molecules can interact with more than one adsorbed oxygen atom (6, N 0.25 for fig. 2B, d), the range of desorption temperatures for NH, extends to - 425 K. desorption
426
F. P. Netrer, T. E. Madey / NH, on oxygen-predosed Ni(l1 I)
3.2. Low energy electron diffraction The only LEED pattern seen for NH, on Ni(ll1) is a poor!v-ordered structure of hazy beams which was previously tentatively indexed as (n/2 X fi/2)R19”; this pattern appears at about half the saturation coverage for the first layer [2]. Predosing the surface with oxygen prevents the formation of an ordered LEED pattern. Note that the coverage of an ideal (J’r/2 X 0/2)R19” structure is 19= 0.43 [ 111, considerably greater than the coverage of 0.25 estimated for saturation coverage of the first layer of NH,. The LEED pattern may arise from islands of NH, in which some second-layer NH, is present. 3.3. Electron stimulated desorption Fig. 3 is a plot of the H+ ESD signal in the direction normal to the surface measured as a function of NH, dose from the microcapillary doser. After a short initial induction period, the yield rises monotonically to a plateau (the arrow marks the approximate dose at which the first layer is saturated, 050.25). The second step rise coincides with the growth of multilayers of NH,. The initial induction is due to the fact that the H+ ESDIAD pattern has
NH, Dose (Arbitrary Units) Fig. 3. Hf ESD signal versus NH, exposure for adsorption at 80 K on Ni( 111) (arbitrary units). The arrow corresponds roughly to the formation of the first layer ( ON,3 = 0.25). The exposure scale is not exactly linear, due to wall effects in the doser: the exposure necessary to form the first layer is -30% larger than that necessary to form subsequent layers.
F.P. Netzer,
T.E. Madey / NH, on oxygen-predosed
Ni(lIl)
427
a minimum in intensity normal to the surface for low coverages of NH, at TZ 80 K [2]; the plateau appears to coincide with filling of the second layer. For B 5 0.25, the only ESD ions detected were H+ . For ESD of multilayers, traces of high mass ions (NH,),H+ were seen, with intensities less than 1% of the Ht yield. This is similar to the ESD results for H,O multilayers [4,12]. Fig. 4 contains LEED and ESDIAD patterns for the adsorption of NH, on Ni( 111). A more complete description of NH, adsorption on clean Ni( 111) as characterized using ESDIAD is given in ref. [2]. Fig. 4a is a LEED pattern from the clean surface, and fig. 4b is an Hf ESDIAD pattern for NH, adsorbed on Ni( 111) (0 < 0.25, T- 150 K). The continuous “halo” of H+ desorption in directions away from the surface normal is consistent with an array of NH, molecules bonded via the N atoms, with no preferred azimuthal orientation of the H atoms which point away from the surface [2]. Another possibility is that of NH, rotating freely about an axis normal to the surface. If the Ni(ll1) surface is predosed with oxygen (0.1 to 0.2 L O,, 13, - 0.03 to 0.05, heated to 600 K to anneal, then cooled to 80 K) and then dosed with NH,, an ESDIAD pattern exhibiting three-fold symmetry is observed [3]. Heating to 2 140 K causes desorption of all NH, except the chemisorbed layer in contact with the substrate, and the well-ordered Hf ESDIAD pattern illustrated in fig. 4c results. The H+ ion emission appears as a “broken” halo, with maxima in intensity along [ 1121 azimuths. Even at the lowest oxygen coverages used in this work (8, N 0.03 monolayer), the oxygen-induced azimuthal ordering was seen. Upon heating to Tk 300 K, the ESDIAD pattern disappears as the NH, desorbs. Fig. 5 illustrates the effect of both NH, coverage and surface temperature on the azimuthal ordering effect. In each case, the annealed oxygen pre-coverage was &=O.OS. Each vertical pair of ESDIAD patterns (a, A; b. B; c, C) corresponds to adsorption of NH, at 80 K, followed by heating to 150 K. The initial NH, coverages increased from left to right, and were -0.08, 0.12, and 0.25 for patterns a, b, and c, respectively. In all cases evidence for azimuthal
Fig. 4. LEED and ESDIAD of NH, on clean and oxygen-dosed Ni( 111): (a) clean LEED, V, = 110 eV; (b) H+ ESDIAD of NH, on Ni( 11 I), V, = 300 eV; (c) H+ ESDIAD of NH, on oxygen-predosed Ni( 111). 8, = 0.05.
428
F.P. Netzer,
i? E. Madey / NH, on oxygen-predosed
Ni(ll I)
Fig. 5. Effect of NH, coverage and temperature on ESDIAD of NH, on oxygen-predosed Ni( 11l), 8,=0.05; (a, A) adsorb B.,,,=0.08 at 80 K, heat to 150 K; (b, B) adsorb 0,,,-0.12 at 80 K, heat to 150 K; (c, C) adsorb 0 NH, -0.25 at 80 K, heat to 150 K; (D) after repeated adsorptions of NH,, followed by heating to 610 K.
ordering was evident at 80 K, but heating to - 150 K resulted in an increase of azimuthal order at the higher coverages; this was accompanied by desorption of any second-layer NH, which may have formed at 80 K. Even when the first NH, layer is nearly filled (fig. SC), the ESDIAD pattern indicates a high degree of azimuthal order. In this case, the ratio of NH, to 0 is 2 5, so that one 0 atom influences the orientation of more than one NH, molecule. The intensity of the normal beam (central spot) seen in the ESDIAD patterns of fig. 5 varies with coverage as well as position of the electron beam on the surface. This suggests that this beam is due to a surface complex (e.g., NH or OH) whose concentration depends on defect and step density, as well as possible electron beam damage. If the oxygen-predosed surface is exposed repeatedly to NH, following by heating to. - 600 K, a dim hexagonal ESDIAD pattern (fig. 5D) appears in which the dominant ion normal to the surface is O+. The same pattern was seen following NO adsorption on Ni( 111) [ 131 when the surface was heated to - 600 K; it was attributed to a mixed layer of N + 0 on Ni( 111) following dissociation of the molecular NO. Thus, it appears that the presence of the oxygen promotes dissociation of a fraction of the adsorbed NH,; indeed, AES demonstrates that traces of N remain on the oxygen-predosed surface after this experiment. Finally, we note that the azimuthal ordering is less pronounced as the oxygen coverage increases. For 0, = 0.25, no evidence for azimuthal ordering in ESDIAD of adsorbed NH, was observed.
F.P. Netzer, T.E. Madqv / NH3 on oxygen-predosedNi(llI)
429
4. Discussion It is of interest to compare and contrast the adsorption of NH, on Ni(ll1) with the adsorption of H,O [4]. Both of these polar molecules are adsorbed molecularly on Ni(1 11), and are bonded to the surface via the lone pair electrons on the N(0) atom, with the H atoms pointing away from the surface. Desorption occurs with little dissociation. Each molecule is disordered azimuthally on the clean Ni(l11) surface, but undergoes a high degree of azimuthal ordering in the presence of adsorbed oxygen. However, there are quantitative differences in the thermal desorption behavior of the two molecules. Water desorbs from the first layer on Ni( 111) in a sharp desorption peak as the at - 170 K; the sharp peak (FWHM - 9 K) shifts to higher temperatures coverage increases, an effect which has been related to attractive lateral interactions in the adsorbed layer due to hydrogen-bonding between neighboring H,O molecules [4]. In contrast, the first NH, layer desorbs over a wide temperature range (- 150 to 300 K). The low temperature onset of desorption shifts to lower temperatures as NH, coverage is increased (cf. fig. l), consistent with repulsive lateral interactions between neighboring NH, molecules. Qualitatively similar results have been seen for H,O and NH, thermal desorption data from Ru(001) [ 14, IS] and Pt( 111) [7,16] surfaces. A possible explanation for these differences involves the relative strengths of the hydrogen bonding interactions in adsorbed H,O and NH,, and how these interaction energies compare with the NH,/Ni( 111) and H,O/Ni( 111) adsorption energies. In the low-coverage limit, the values of desorption peak temperature for NH, and H,O are - 290 and 165 K, respectively (implying desorption energies of - 74 and - 42 kJ/mole): NH, is bonded to Ni( 111) with an adsorption energy almost twice as great as H,O. From UPS [l], it is known that NH, is bonded via the 3a, orbital, which is composed mainly of the nitrogen lone pair electrons. The adsorbed NH, thus has no additional lone pair orbitals to participate in attractive lateral interactions via hydrogen bonding to neighboring NH,; the dominant lateral interactions between neighboring NH, molecules appear to be repulsive as the coverage increases. In contrast, H,O has two orbitals with some lone-pair, non-bonding character, lb, and 3a, ; when H,O is bonded to a planar metal surface via one of its lone pair orbitals, the other lone pair orbital is available for hydrogen bonding. Thus the more weakly-bound H,O can be “pulled around” to interact with its neighbors via hydrogen bonds (attractive lateral interactions) as the coverage increases. The ESDIAD patterns observed for NH, and H,O on clean Ni( 111) are consistent with the above pictures of bonding of these molecules. The “halo” pattern of fig. 4C is unchanged as the NH, coverage increases in the temperature range above - 150 K, indicating that the bonding geometry is unchanged (bonding via the N atom, with H atoms away from the surface). The ESDIAD pattern for H,O on Ni(ll1) (fig. 1 of ref. [3]) is consistent with a range of
430
F. P. Netzer, T. E. Madey / NH, on ox_vgen-~~edosed Ni(I Ii)
bonding orientations for the non-hydrogen-bonded H atoms in a two-dimensional layer of hydrogen-bonded, adsorbed H,O. When NH, is adsorbed on Ni( 111) predosed with oxygen, both ESDIAD and thermal desorption measurements provide evidence for a chemical interaction between NH,(ads) and O(ads). This interaction is sufficiently strong as to increase the NH, desorption temperature, and to introduce azimuthal ordering into the NH, layer which has no preferred azimuthal ordering on the clean Ni( 111) surface. The NH, may interact with 0 via a weak hydrogen bond, as suggested previously 131, or via an interaction through the metal substrate. The H atoms (ions) which contribute to the ESDIAD patterns (fig. 4C; fig. 5) are presumably those atoms which do not participate in the interaction with oxygen. The azimuthal ordering of NH, has been described previously [3]. Oxygen on Ni( 111) is bonded in a three-fold hollow site [6]; whether this site is above a second layer atom or a second layer vacancy is not known, although the vacancy site is favored for 0 on the close packed fee surfaces, Ir( 111) ] 171 and Al(lll) [18]. In fig. 6, a structural model is proposed which is consistent with ref. [3]. If the 0 atoms are located in three-fold hollows above second layer atoms, NH, is bonded in the other three-fold hollow (this is the site preferred for NH, on clean Pt(ll1) {7]). The surface O-N distance for O-NH, is 2.87 A, as compared with the N-O separation of 2.714 in hydrogen-bonded (H~NHOH~)~ [ 191 and N-O separations of 2.78 to 2.93 A reported for other hydrogen-bonded systems involving N-H * . .O 1201. The close agreement between the presumed N-O separation and the hydrogen bonding N-H-O distance [19,20] is the basis for suggesting that a hydrogen bonding interaction may be responsible for the azimuthal orientation. This interaction overcomes the random azimuthal configuration (or free rotation) of the NH, on clean Ni( 111). Studies presently
Fig. 6. Model of NH,
adsorption
on oxygen-predosed
Ni( 111).
F.P. Netzer, T.E. Madey / NH, on oxygen-predosedNi(Il I)
431
underway are designed to evaluate the role of substrate geometry and adsorbate chemistry (0. S, Na, . . .) on the azimuthal ordering of NH, and H,O. As discussed previously [3], if the 0 atoms are actually located above the second layer vacancies, the NH, sites in fig. 6 are shifted accordingly, with no change in N-O separation. In this model, the non-hydrogen-bonded H atoms are the ones that are seen in ESDIAD. Either the hydrogen-bonded ligands are more effectively neutralized following electronic excitation, or the H+ ions from the hydrogen-bonded ligands follow shallow trajectories under the influence of the image potential [5] and are recaptured by the surface. This latter possibility is consistent with adsorbed NH, molecules which are slightly “tilted”, i.e., inclined toward the 0 atom so that the N-H . . . 0 angle opens (note that the XH . . . 0 angle is not necessarily linear in hydrogen-bonded systems, such as certain forms of ice [21]). Such a tilt may be related to the vertical displacement of nickel atoms on Ni( 111) which is induced by adsorbed oxygen [22]. Note also that the central beams in figs. 4 and 5 may be due to a small fraction of dissociation products (NH, OH); the intensity of this beam varies at different points on the crystal and it is sensitive to bombardment time. In fig. 6, we show a single 0 atom influencing the orientation of more than a single NH, molecule, consistent with the observations of Bowker et al. [23] and Kretschmar et al. [24] for H,O + 0 on metals. Whether this direct interaction of one 0 with three NH, molecules occurs, or whether 0 atoms “nucleate” ordered layers by orienting single NH, species which interact laterally with their neighbors is not clear. It is clear, however, that 6, ~0.05 will cause substantial short-range order in a saturated NH, layer (a,,, - 0.25). cf. fig. 5C; The ratio of NH, to 0 is - 5 to 1. Since it is unlikely that one 0 will interact directly with more than 3 NH, molecules, the remaining adsorbed NH, is either ordered by lateral interactions, or is randomized azimuthally, thereby contributing to the H+ background signal in ESDIAD pattern 5C. At higher oxygen coverages (8, - 0.25), the azimuthal ordering effect is not observed, presumably because NH, molecules can interact with more than one 0 atom. Finally, we note that the azimuthal ordering reported for NH, on clean Ni(l11) [25] using angle resolved UPS may have been influenced by traces of impurities, as reported herein.
5. Summary
(1) Thermal desorption data for NH,/Ni( 111) indicate that repulsive lateral interactions are present in the adsorbed layer in contact with the Ni; evidence for a second layer and multilayer formation are also seen at high exposures. (2) Preadsorption of oxygen has a strong influence on the bonding of NH,
432
F.P. Netzer,
T. E. Madey / NH, on oxygen-predosed
Ni(ll1)
to Ni( 111). Thermal desorption data indicate an increase in the binding energy of NH, in the presence of O(ads). (3) Small coverages (19,s 0.05) of preadsorbed oxygen induce a high degree of azimuthal order in adsorbed NH, which is disordered azimuthally on the clean Ni( 111) surface.
Acknowledgements
This work was supported in part by the Office of Naval Research. acknowledges financial support from the Max Kade Foundation.
F.P.N.
References
[l] C.W. Seabury, T.N. Rhodin, R.J. Purtell and R.P. Merrill, Surface Sci. 93 (1980) 117. [2] T.E. Madey, J.E. Houston, C.W. Seabury and T.N. Rhodin. J. Vacuum Sci. Technol. 18 (1981) 476. [3] F.P. Netzer and T.E. Madey, Phys. Rev. Letters 47 (1981) 928. [4] T.E. Madey and F.P. Netzer, Surface Sci. 117 (1982) 549. [5] T.E. Madey, in: Inelastic Particle-Surface Collisions, Topics in Chemical Physics, Vol. 17, Eds. W. Heiland and E. Taglauer (Springer, Heidelberg, 1981) p. 80. [6] L.D. Roelofs, A.R. Kortan, T.L. Einstein and R.L. Park, J. Vacuum Sci. Technol. 18 (1981) 492; A.R. Kortan and R.L. Park, Phys. Rev. B23 (1981) 6340. [7] G.B. Fisher. Chem. Phys. Letters 79 (1981) 452. [8] M. Grunze, M. Golze, R.K. Driscoll and P.A. Dowben. J. Vacuum Sci. Technol. 18 (1981) 611. [9] J.L. Gland and E.B. Kollin, J. Vacuum Sci. Technol. 18 (1981) 604. [lo] T. Au and M.W. Roberts, Chem. Phys. Letters 74 (1980) 472. [ 1 l] K. Jacobi, E.S. Jensen, T.N. Rhodin and R.P. Merrill, Surface Sci. 108 (1981) 397. [12] R.H. Prince and G.R. Floyd, Chem. Phys. Letters 43 (1976) 326. [13] F.P. Netzer and T.E. Madey, Surface Sci. 110 (1981) 251. [14] T.E. Madey and J.T. Yates, Jr.. Chem. Phys. Letters 51 (1977) 77; also. in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces, Eds. R. Dobrozemsky et al. (Berger, Vienna, 1977) p. 1183. [ 151 P.A. Thiel, F.M. Hoffmann and W.H. Weinberg, J. Chem. Phys. 75 (1981) 5556. [16] G.B. Fisher and J.L. Gland, Surface Sci. 94 (1980) 446. [17] C.M. Chan and W.H. Weinberg, J. Chem. Phys. 71 (1979) 2788. [18] S.A. Flodstrom, C.W.B. Martinsson, R.Z. Bachrach, S.B.M. Hagstrom and R.S. Bauer, Phys. Rev. Letters 40 (1978) 907. [19] S. Scheiner and L.B. Handing, Chem. Phys. Letters 79 (1981) 39. [20] M.D. Joesten and L.J. Schaad, Hydrogen Bonding (Dekker, New York. 1974) table l-1 1. [21] B. Kamb, in: Water and Aqueous Solutions, Ed. R.A. Horne (Wiley-Interscience, New York, 1972) p. 9. [22] T. Narusawa, W.M. Gibson and E. Tornqvist, Phys. Rev. Letters 47 (1981) 417. [23] M. Bowker, M.A. Barteau and R.J. Madix, Surface Sci. 92 (1980) 528. [24] K. Kretzschmar, J.K. Sass, P. Hoffman, A. Ortega, A.M. Bradshaw and S. Holloway, Chem. Phys. Letters 78 (198 1) 410. [25] W.M. Kang, C.H. Li, S.Y. Tong, C.W. Seabury, K. Jacobi, T.N. Rhodin, R.J. Purtell and R.P. Merrill, Phys. Rev. Letters 47 (1981) 931.