Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

101

Surface Science 155 (1985) 101-120 North-Holland, Amsterdam

DYNAMICS OF INTERACTION Ni(ll1) SURFACES

OF H,

H.J. ROBOTA

M.C. LIN **, J. SEGNER

*, W. VIELHABER,

AND D, WITH

Ni(ll0)

AND

and G. ERTL

Institut ftir Physikalische Chemie, Universitiit Miinchen, D - 8000 Miinchen 2, Fed. Rep. of Germany

Received

16 October

1984; accepted

for publication

14 January

1985

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H, and D, on Ni(ll0) and Ni(ll1) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(ll0). dissociative adsorption occurs with nearly unity sticking probability s,,, irrespective of surface temperature T, and mean kinetic energy normal to the surface (E I ). The desorbing molecules exhibit a cos 0, angular distribution indicating full thermal accommodation of their translation energy. With Ni(lll), on the other hand, se is only about 0.05 if both the gas and the surface are at room temperature. su is again independent of T,, but increases continuously with (E I ) up to a value of - 0.4 for (E, ) = 0.12 eV. The cos5@, angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(llO), it is about 0.1 eV for Ni(ll1) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.

1. Introduction Dissociative adsorption of a diatomic molecule on a clean surface - as well as associative desorption, which represents the reverse process - is frequently rationalized in terms of a simple one-dimensional potential diagram as proposed by Lennard-Jones [l] and reproduced in fig. 1. While this diagram is able to describe the overall energetics and also the possible existence of a molecular “precursor” state, its application to the kinetics may be rather misleading. This is the consequence of a series of dynamic processes, which have been studied in the present work to some extent by means of the molecular beam technique. The systems under investigation were H, and D, interacting with Ni(ll1) and Ni(ll0) surfaces for which a large series of results * Present address: UOP Research Center, ** Permanent address: US Naval Research

Desplaines, Laboratory,

Illinois 60016-6187, USA. Washington, DC 20375-5000,

0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

USA.

%d Fig. 1. One-dimensional Lennard-Jones [ 11.

schematic

potential

diagram

for

dissociative

chemisorption

after

on the energetic, kinetic and structural properties already exists in the hterature [2]. The main emphasis will be put on the phenomena in the limit of zero coverage, since at finite coverages additional effects come into play.

2. Experimental The experiments were performed in a combination molecular beam source-UHV scattering chamber described previously [3,4]. The hydrogen was expanded through a 0.070 mm orifice at pressures between 200-500 Torr. The beam diameter and angular divergence were determined by an aperture in the final pumping stage of the source. At the surface, diameters of 1.5-2 mm and angular divergence of 0.06” were used. Under the very mild expansion conditions of our nozzle, the rotational distribution remains virtually unaltered [S]. Thus, the translational kinetic energy of the pure H, or D, beams is determined only by the nozzle temperature. Kinetic energies of 26 and 64 meV beams were achieved by nozzle temperatures of 120 and 300 K, respectively, based upon the simple relation Ekin = G/CT f6]. D, could be raised up to 120 meV kinetic energy by seeding in H, [6]. These energies were confirmed by the angular positions of diffracted beams on both surfaces. The acceptance angle of the mass spectrometer was estimated at 5” which agrees well with the observed width of incident and specularly scattered He beams. Separation of molecules undergoing direct scattering from those undergoing chernisorption-desorption could be achieved by modulating the incident beam at 63 Hz and by recording the resulting modulated signal by means of a lock-in

H.J. Robota et al. / Dynamics

of interaction of H2 and D, with Ni

103

amplifier. At the surface temperatures used, the amplitude demodulation [7] of the desorbing chemisorbed hydrogen results in a modulated contribution less than 10d5 of the incident beam intensity. Such a small signal can be neglected in comparison to the directly scattered component. The nickel (110) and (111) surfaces were oriented within 0.5” of the respective principal faces. The azimuthal orientation was chosen to present the most corrugated direction on the surfaces to the incident beam: the (001) direction for Ni(ll0) and the (112) direction for Ni(ll1). These orientations were determined to be within 5” by LEED. The presence of trace quantities of adsorbed contaminants is known to sharply attenuate specular scattering of thermal particle beams [8]. Thus, great care was taken to ensure scrupulously clean surfaces throughout these experiments. Carbon is the major residual contaminant and its absence was monitored chemically as described in ref. [9]. Since both surfaces chemisorb hydrogen, the experimental conditions were chosen to assure scattering from surfaces covered with less than 0.01 monolayers of hydrogen at all times. This is easily accomplished on the (110) surface, where hydrogen is bound less strongly, by maintaining a surface temperature of 370 K [lo]. On the (111) surface, however, at surface temperatures where the hydrogen coverage remains low, the thermal attenuation of the already very weak higher order diffracted beams becomes unacceptable. Therefore, a compromise solution was used with the surface at 420 K and beam exposures of less than lo-15 s duration. The time interval from point to point was long enough to allow full desorption of any chemisorbed hydrogen. A check of the attenuation due to scattering by chemisorbed hydrogen was made by monitoring the specular intensity for times longer than normally used in measurement. An attenuation of about 10% was evident after about 45 s and was reversible by allowing time for desorption. This method is naturally less exact and each scan was repeated numerous times to ensure accurate results. Details of the sample preparation have been described previously [9]. The degree of atomic smoothness of the surfaces was probed by measuring the relative intensities of specular reflection of a thermal He beam. At T, = 300 K and incidence angle Bi = 60” values of Z/I, = 0.31 and 0.33 for clean Ni(ll1) and Ni(llO), respectively, were recorded which indicate a very high degree of surface perfection. This compares well with I/I, = 0.45 recorded by Wilsch and Rieder [ll] with a Ni(lOO) surface at T, = 100 K. The LEED pattern showed exceptionally sharp diffraction spots.

3. Results 3. I. Elastic scattering Elastic scattering

of a diatomic

molecule

(i.e. without

energy exchange

with

H.J. Rohoto et al. / Dytrumics

104

the solid) is governed

of interactionof

by the conservation

N, and D, wth NI

rules for energy

and momentum

k2=kF2+2m/fi2[E(J~)-E(Ji)].

(1)

k;’ = k;’ + g,

(2)

ki and k, are the initial and final momenta with components ki’ and k; parallel to the surface. g is a reciprocal lattice vector parallel to the surface. Eq. (1) contains the possibility for translation-rotation transformation from an initial rotational state J, to a final state J,. Effects of this kind have been observed for example by Cowin et al. 1121 for scattering of hydrogen molecules at a Pt(ll1) surface. The most prominent effects were detected with the excentric HD molecule, while the effects with H, and D, were hardly discernible. The same conclusion will be reached in the present work which only employed H, and D,. Translation-vibration transformation can be discarded since the energy of the u = 0 + u = 1 excitation (515 meV for H, and 370 meV for D2) is larger than the applied translation energies. For fixed k, these equations predict sharp angular distributions of the scattered molecules whose angular widths are determined by the spread in ki. Angular distributions for H, scattered from Ni(ll0) and Ni(ll1) are reproduced in figs. 2 and 3, respectively. The azimuthal orientation of the surfaces with respect to the scattering plane is indicated in the inserts. As outlined in section 2, the experimental conditions were chosen in a way that

_. i%ii

1

N11110) + H2

1

13, : 576O

IO

:

(001) _-...-

Fig. 2. Angular distribution of H, scattered from Ni(ll0) along the (001) directlti,l, Bi = 57.6’. T = 400 K, (E I ) = 43 meV (Hz : D2 j= 1: 1 beam), surface periodicity d = 3.52 A. The large fraction of molecules which underwent dissociative adsorption and subsequent desorption has been separated by using a modulated beam and lock-in techniques.

H.J. Robota et al. / Dynamics of interaction of HI and D, with Ni

-10

0

10

20

30

Fig. 3. Angular distribution of H, T,==420K,(EL)=85meV(H,:D,=1:1beam).

40 scattered

50

60

70

from Ni(ll1)

105

1

80 9( ef PI along

the (li2)

direction,

8, = 26”,

any signal originating from recombination of chemisorbed H atoms was completely demodulated and suppressed. These data therefore solely reflect scattering from the molecular potential well (dashed line in fig. 1). The angular distribution for H,/Ni(llO) shows - apart from a peak in specular direction - distinct diffraction peaks on a smoothly varying background which exhibits maximum intensity in the specular direction marked by the 00 beam. The angular half-widths of the diffracted beams are slightly larger than that of the specular beam which has to be ascribed to the finite velocity distribution and hence spread in ki [13]. The positions of the maxima agree with those calculated from the diffraction conditions with known lattice constant (4 g) and initial velocity ( + ki). NO additional maxima due to translational-rotational transitions are discernible which is in agreement with the findings for H,/Pt(lll) [12]. The relative intensities of the diffracted with respect to the specular beams are considerably larger for H, than for He. This agrees with previous reports on similar systems in the literature [14-161 and is explained by recent theoretical calculations [17]. Thereafter the repulsive part of the interaction potential (and hence the classical turning point) is closer to the surface for H, than for He. As a consequence an incoming H, molecule “feels” more of the corrugation of the potential parallel to the surface due to the periodic array of surface atoms. There is practically no intensity in diffracted beams with the data for H,/Ni(lll) (fig. 3) which is consistent with the much smoother nature of this densely-packed plane. The intensity wings around k 10” from the specular

106

H.J. Robota

et al. / Dynamics

of intrructwn

01 Hz and D, w’ith NI

direction suggest again the presence of a substantial background of direct-inelastic scattering, but separation is not as easily possible as with the H,/Ni(llO) system. With increasing surface temperature the total elastically scattered intensities decreased monotonically for all systems due to normal Debye-Waller behavior. There was no anomaly near the Curie temperature (631 K) as reported recently [ll], us long as the surf&es were rigorously kept clean. The quoted effects have to be attributed to carbon segregation [18] which also markedly influences the sticking coefficient for dissociative hydrogen chemisorption [9]. Angular distributions for D, scattering from the two planes are reproduced in figs. 4 and 5, respectively. The elastically scattered part with respect to the background intensities is substantially lower if compared with the data for H, scattering. Damping of the elastic intensity due to thermal diffuse and inelastic scattering depends on the mass of the gas particle [19]. while to a first approximation no such an effect would be expected if energy loss is dominated by electronhole excitations. Although no quantitative analysis was performed, all the data demonstrated a considerably reduced elasticity for D, scattering if compared with H,. 3.2. Direct-inelastic

scattering

The background in the angular distributions, which is more clearly discernible with the data for Ni(ll0) (due to the superimposed diffraction peaks), originates from energy exchange processes between the surface and the inci-

001

-20

-10

Fig. 4. Angular

0

10

distribution

20

30

LO

of D, scattered

50

60

from Ni(ll0).

70

80 90 ef 101

H.J. Robota et al. / Dynamics of interaction of H2 and D, with Ni

‘/IO ,

I

i%l

00

3-

-%O

107

Nail111+ D2

-10

Fig. 5. Angular

0

10

distribution

20

30

LO

of D, scattered

50

60

70

80 90 q ("1

from Ni(ll1)

dent particles during single collision events. Multiple collisions are of negligible importance because of the small ratio m/M, where m is the mass of the incident particle and A4 that of the surface atom. This energy exchange can, in principle, occur through electron-hole pair excitation or through phonon interaction. To a first approximation, electron-hole pair excitation is expected not to depend on the mass of chemically identical particles, but only on the time they spend in the region of the interaction potential. In order to check this possibility the data reproduced for H, and D, correspond to equal velocities of the incoming molecules (achieved by using a mixed H, + D, beam), i.e. equal times of interaction. Nevertheless a substantially lower fraction of D, than H, undergoes elastic scattering as visual comparison of figs. 2 and 4 as well as of figs. 3 and 5 demonstrates. Phonon interaction, on the other hand, depends strongly on the mass m of the scattered particles. If a particle with mass m and initial energy Ei strikes a stationary surface atom with mass A4 offering an infinitely steep repulsive potential (“hard sphere”) then the relative energy transfer to the surface is simply given by [20]:

and should therefore by More specifically, the the angular distribution maximum in a direction

about two times larger for D, than for H,. “hard-cubes” model [21,22] predicts a broadening of of the observed order of magnitude with its intensity deviating by an angle 9 from the specular direction.

10X

H.J. Robota et ul. / Dynumics of interuction OJ H, and D, with Ni

Since the latter is essentially determined by m/M [21], this deviation is predicted to be less than a few degrees for the present systems. Since the inelastic scattering is superimposed by the elastic contributions. accurate experimental determination of n was not feasible. The qualitative features are not affected by the more refined “soft-cubes” models [23]. Although an accurate determination of the relative inelastic intensities centered around the specular direction was not possibIe, qualitative inspection of figs. 2-4 demonstrates that these are considerably higher for D, than for H,. These findings are considered as strong indication that excitation or annihilation of phonons plays the dominant role in the energy exchange between hydrogen molecules and the repulsive part of the molecular interaction potential. This is also in agreement with the conclusions of recent theoretical treatments [24] of electron-hole pair excitation which predict this process to occur only with very small probability. 3.3. Dissociative chemisorption: Ni(ll0) The fraction of incident hydrogen molecules undergoing dissociation is so tightly held on the surface, that the rate of recombination and desorption is negligible for surface temperatures 7; ,< 300 K. The reIative probability for this chemisorption process (= sticking coefficient) was determined in a way that the surface was exposed to a defined H, or D, flux for a certain time, and subsequently thermal desorption spectra were recorded. The area under such a spectrum is proportional to the adsorbed amount. The thermal desorption spectra at various coverages were found to be practically identical to those published in earlier work 125,263 and are therefore not reproduced here. The absolute value of the initial sticking coefficient (i.e. at zero coverage) for 7; = T, = 300 K was taken as s,, = 0.96 from the literature [25]. Direct determination of s,, in the present work was only possible with low accuracy, but it was confirmed that its value has to be close to unity. The results presented in the preceding sections demonstrate, however, that a finite fraction of impinging hydrogen molecules is scattered back into the gas phase. No differences-in s0 were found between H, and DZ within the (admittedly rather poor) accuracy of &20%. The variation of the relative sticking coefficient, S/S,, with coverage is reproduced in fig. 6. These data are again very similar to those reported previously [25] and demonstrate that s remains constant up to about 40% of the saturation coverage. As a consequence, no corrections are necessary if s0 is extracted from data recorded at finite coverages of 8 6 0.1. If the angle of incidence fIi is varied at constant kinetic energy of the beam, the energy of the momentum normal to the surface (E I ) varies as (EL ) = Ei COS’~?~. Data for (EL > between 25 and 64 meV (which were derived by proper correction for the variation of the incident flux with ei) are plotted in fig. 7, curve a, and

H. J. Robota et al. / Dynamics of inieraction of H2 and D, with NI

0

109

N~illO)

0

0

o.oi. 0.5

00

Fig. 6. Variation r,=155 K.

‘.O %5at

of the relative

dissociative

sticking

coefficient

with coverage

for D, on Ni(llO),

demonstrate that sa is independent of (E I ) in this range. This result was in turn used to calibrate the variation of the flux if the nozzle was cooled down to about 120 K which offers an alternate possibility to change (E I ) if 8, is kept fixed. The dependence of sa on the surface temperature T, was determined in different ways, depending on the range of T,: (i) For T, < 300 K the same exposure-TDS technique as just described could be applied. (ii) For T, > 400 K desorption is so fast that the stationary coverage is

10

so

11

energy normal to Fig. 7. Variation of the initial sticking coefficient, so, with the mean translational the surface, (E I ). The change in (E i ) was achieved through variation of the angle of incidence (x, 0) or through variation of the nozzle temperature (0). (a) Ni(ll0); (b) Ni(ll1).

110

H.J. Robota et al. / Dynamics of interacrion of H, and Dz with Ni

Fig. 8. Varlatton Ni(ll1).

of .s,) with surface

Surface

Temperature

temperature

Ts at constant

(E,

) = 64 meV. (a) Ni(ll0);

(b)

negligibly small. If a mixed Hz/D, beam is used, the rate of HD formation r,,, is directly proportional to sg as outlined elsewhere [9]. (iii) The same technique can be applied for 300 < T, -C 400 K, but now ~nr) is reduced due to the decrease of s with coverage. The latter was estimated on the basis of published adsorption isotherms [IO]. However, the resulting sg values are rather inaccurate in this range. The results are shown in fig. 8: There is no detectable variation of sg with c from 150 to 800 K. Since the ratio of inelastic to elastic scattering is, on the other hand, markedly increasing with T,, these findings suggest that these two channels are essentially independent from each other and make dissociative adsorption through intermediate trapping in the molecular “precursor” state rather improbable. Additional support for this conclusion is obtained from the fact that the elasticity in D, scattering relative to H, is smaller by almost an order of magnitude, while the sticking coefficients for both isotopes are very similar.

The initial sticking coefficient of a thermal gas of hydrogen on this plane is with .F”= 0.96 for know to be much smaller, s0 = 0.05 [25]. By comparing Ni(ll0) we conclude s,=O.l for H,/Ni(lll) with (E,) =64 meV. The dependence of s/so on coverage under these conditions is plotted in fig. 9. In contrast to Ni(llO), it drops substantially already at fairly low coverages, so that proper correction for this effect was needed whenever conclusions on s,) were drawn from measurements at finite surface concentrations. s0 was no longer found to be independent of the angle of incidence. Instead the data could be described by a dependence sO( 0,) = ~(0’) cos??,, where n = 4. This result suggests that s,, depends in this case on (E i >. In order to check this

H.J. Robota et al. / Dynamics of interaction of H2 and D, with Ni

o.\+

005

Fig. 9. Variation

010

111

0.15 9,

of the relative

hat sticking coefficient

with coverage

for D, on Ni(ll1).

conclusion two types of experiments were performed: s0 was either measured for fixed total kinetic energy Ei at varying Bi, whereby (E I ) was changed, or 19~was kept constant and Ei (and accordingly (E I )) was varied by changing the temperature of the nozzle or by seeding a D, beam with H, (and measuring s0 for D2). All data were found to be on a common curve which is reproduced in fig. 7. This result demonstrates that (E I ), rather than E,, is the parameter governing dissociative sticking. There is a dramatic increase from very small values at the lowest energies up to s0 = 0.4 at (E I ) = 120 mV. The data may be fitted by a curve sc, a ((E *))*.*, and it may safely be concluded that even higher (E I ) would cause a further increase of s0 until it levels off at a value which is probably close to unity as for Ni(ll0). A behavior of this kind had been observed previously for dissociative hydrogen adsorption on different Cu single crystal surfaces [27]. In these measurements (E I ) could be varied over a much wider range, whereby limiting values of s0 = 0.1 were reached, however. More recently, similar experiments were performed with the system N,/W(llO) [28] where the beam energy could be varied between 0.1 and 1.4 eV while the dissociative sticking coefficient increased from 3 X 10m3 to a saturation value of about 0.3. The shape of the s0 versus Ei curve (at fixed ei, which means that actually (E I ) was varied) was quite similar to that for the present system. It was concluded that dissociative adsorption is translationally activated and cannot be adequately described by a one-dimensional potential diagram. The dependence of s0 on the surface temperature T, was determined in a similar manner as for Ni(ll0). The data for (E I ) = 64 meV are again shown in fig. 8 and are obviously independent of T,. (The highest degree of accuracy was achieved at high temperatures, and there is probably an increase by 5% from 600 to 770 K.) This result is in agreement with the findings by Rendulic and Winkler [29]. In view of the potential diagram of fig. 1 these results can be interpreted in

112

H.J. Robota et al. / Dynamics

of inferactionof

H, and D, with NI

terms of an activation barrier E* > 0 for Ni(lll), while it is < 0 for Ni(ll0). This activation barrier can obviously be surmounted by increased translational energy of the impinging molecule, while an increase of the thermal energy of the solid is inefficient. An influence of the surface temperature would necessarily be expected if dissociation would occur through a molecularly trapped intermediate state. Apart from the fact that E* could then be overcome by thermal energy of the solid, phonon assisted trapping in the molecular state should increase with T,, as was recently observed for HD/Au(lll) [30]. In this respect adsorption of H, on nickel is different from that on Pt(lll), where sg increases smoothly by about a factor of 5 between T, = 200 and 800 K [31,32]. This dependence could be fitted by a function s0 CCr exp( E/kT,) with E = 1.4 and E = 20 meV and was attributed to intermediate trapping in the molecular precursor state from where the particles diffuse to defect sites for dissociation [32]. The results presented here demonstrate, on the other hand, that direct collision from the gas phase presents the dominant channel for dissociation. The fact that for H,/Ni(lll) s0 increases so strongly with (E I ) rules also out any major effect of small concentrations of surface defects, which may be of importance in precursor-mediated processes. 3.5. Associative

desorption

At sufficiently high surface temperatures the adsorbed hydrogen atoms recombine and leave the surface after a mean surface residence time 7. If the sticking coefficient is high enough (as in the case of Ni(llO)), 7 can be directly determined by modulated beam experiments in which the phase lag between the incoming and outgoing molecules is recorded. This can most conveniently be done by applying a mixed H, - D, beam and by recording the signal of HD. By positioning the mass spectrometer behind the sample surface the total flux of desorbing molecules is recorded [31]. Results for the phase lag of the first Fourier component as a function of surface temperature are reproduced in fig. 10. Analysis of these data characteristic for second-order desorption kinetics follows a treatment by Olander and Ullmann [33] and yields an * = 17 + (at zero coverage). activation energy for desorption of Ede.\ _ 2 kcal/mol Since adsorption on Ni(ll0) is obviously non-activated this value may be set equal to the heat of chemisorption. It agrees fairly well with literature data derived from equilibrium measurements [lo] or TDS [34] if these are extrapolated to zero coverage. If the mass spectrometer is rotated into defined positions in front of the surface, the dependence of the flux of desorbing HD molecules on the exit angle 6’, can be determined. Resulting data are shown in fig. 11 and exhibit closely a cos 6, behavior. The same results were obtained if thermal desorption spectra after identical exposures were recorded at different 0, and the relative amount of molecules desorbing into this direction was derived by integrating

H.J. Robota et al. / Dynamics

MODULATED

8 -

7.

2 N k6 5

D, with Ni

113

RESULTS

*

vsl

0

Y=

A

ofinteructionofIf2 and

Hz

19Hz v=9 Hz

2” 43 2. 1.

i :3:5

La. 1.7

2.0

3.0

2.5

T-’ / l1O-3 K-l) Fig. 10. Modulated beam experiments: Arrhenius plot of the phase lag between impinging and desorbing H, molecules on Ni(ll0) at various modulation Frequencies. The slope of the straight line yields an activation energy of 17 kcal/mol.

the thermal desorption traces. A very similar result was obtained by Steinriick et al. [35] who found, by angular resolved TDS, a cos’%$ distribution for desorption of a saturated adlayer. The small deviation from the cos 8, behavior found here (for small coverages) might well be due to the different coverage

NiillO)

0

20

LO

60

-

Nijlll)

80*

0

8,

Fig. 11. Dependence of the intensity respect to the surface normal.

20

LO

-

6W Be

of associatively

desorbing

H,

molecules

on the angle 0, with

regimes probed in both studies. The H/Ni(I 10) system exhibits three different desorption states, which are partly associated with surface reconstruction [26]. A cos 8, distribution of desorbing molecules (Knudsen behavior) is equivalent to complete accommodation of their translation energy to the temperature of the solid [23]. With Ni(ll1) the modulated-beam technique was not applicable for determining the angular distribution of desorbing particles because the sticking coefficient is too low under ordinary conditions. Instead only angular resolved thermal desorption spectra were recorded for coverages in the range 0.05 to 0.35. The results are reproduced in fig. 11 and can be modelled approximately by a cos”0, distribution with d = 5. Quite similar results (even independent of the azimuthal orientation) had been reported previously on the basis of analogous experiments [35,36]. Horton and Masel [37] explained their findings in terms of a model, whereafter the desorbing molecules are directed toward the surface normal by the corrugation of the H, potential near the adsorption site. This model would predict an even more peaked angular distribution for Ni(ll0) where the potential is certainly more strongly corrugated than for the smooth (111) plane - which is not observed. Apart from this it is based on rather unrealistic assumptions and contains internal inconsistencies. For these reasons it is not further applied for the discussion. Sharper-than-cosine angular distributions of associatively desorbing molecules have been observed for a series of systems [3,35-471. In those cases where velocity distributions could be determined simultaneously by time-of-flight measurements, it turned out that (E I ) exceeded the value of 2kT,. So far the only exception was recently reported for HD desorbing from a stepped Pt(557) surface [47] where a cos28, angular distribution was found to be associated with molecules even slightly colder than the surface. A general theory developed by Doyen [48] established a simple relation between (E I ) and the exponent d of a cos”fI, distribution: (E,)

= ;(3 + d)kT,.

The validity of this equation was successfully checked with all existing experimental data (with the exception of ref. [47]), which justifies its application also in the present case. With d = 5 we obtain (E I ) = 4kT,, i.e. an excess Knudsen value of 2kT,. Since translational energy of 2k7” over the “normal” the thermal desorption spectra exhibit their maxima around T, = 500 K, this would correspond to an excess energy of about 0.1 eV. As a consequence of the principle of detailed balancing as well as of the Doyen model the sticking coefficient at fixed E, should vary with COS“‘ei, if desorption occurs like cosdO,, which is also confirmed by the present data. While this theory is free from any assumptions on the origin of excess translational energy, the latter effect can be traced back to the existence of an activation barrier for dissociative adsorption which has to be overcome by

H.J. Robota et al. / Dynamics of interaction of H, and D, with Ni

115

increased translational energy in an earlier model developed by Van Willigen [38]. This model is based on a one-dimensional potential of the type shown in fig. 1 and has its shortcomings which have been criticized [35,49]. In particular, it predicts a shift of the maximum in the thermal desorption spectra with 6’, by a few kelvin which has not been observed experimentally in previous or the present work [35,36]. Nevertheless it appears to reproduce the qualitative features rather well and will therefore be used for the further discussion.

4. Discussion The Lennard-Jones potential of fig. 1 can be quantified at least to some extent: Based on theoretical [17] and experimental [16] data for other H/metal systems the depth of the molecularly adsorbed state (H,,,) can be estimated to be of the order of 35 _t 15 meV, and it is located at a distance of about 2.7 + 0.2 A from the plane formed by the nuclei of the surface atoms. In the case of Ni(ll1) the location of the adsorbed H atoms follows from a LEED structural determination [50] to be at rM_n = 1.15 A and the depth of this minimum is 1 eV from the known chemisorption energy [lo]. (This means the energy E,, for the process H, --, 2 H,,. The energy of the Ni-H bond is accordingly ENi_ H = i( E,, + Ediss) = 2.8 eV.) The present results suggest that dissociative chemisorption is activated by about E* = 0.1 eV. The situation for Ni(ll0) will be quite similar, with the important exception that obviously no noticeable activation barrier for dissociation exists. This one-dimensional potential diagram leaves it open, if dissociative adsorption occurs “indirectly” through intermediate trapping in the molecular state or “directly” by overcoming the activation barrier through high enough energy of the incoming molecule [51]. The former mechanism was suggested for the system H,/Pt(lll) [32], and could be directly demonstrated for N,/Fe(lll) [52]. The data presented here demonstrate clearly, on the other hand, that the direct collision mechanism holds in the present case, whereby the activation energy can be overcome by the translational energy of the momentum perpendicular to the surface. Similar effects had been observed previously for H,/Cu [27], where the probability for dissociative chemisorption was found to increase by about an order of magnitude if the kinetic energy normal to the surface was increased from 35 to 370 meV. Whether vibrational excitation of the incoming molecule would assist this process is still an open question which would be interesting to investigate in detail. Translational-rotational coupling is obviously of negligible importance for the isonuclear H, and D, molecules but could be of some importance with HD [12]. Quite recently the internal states distributions of hydrogen molecules formed by recombination of atoms diffusing through Cu samples were investigated by laser spectroscopic techniques [53]. While the mean rotational

116

H.J. Robota et al. / Dynamics

of interactionof H2

and D2 wiih Ni

energy was found to be somewhat smaller than T,, the (v = l)/(v = 0) vibrational population was found to be up to a factor 100 larger than the equilibrium value given by T,. This would in turn mean that in this case vibrational excitation of the incoming molecules would strongly enhance their sticking probability. Even if (E I ) is high enough not all of the incoming molecules stick, as becomes particularly evident from the scattering data for Ni(ll0). If this effect would be due to scattering at the atomic potential, i.e. through intermediate dissociation and subsequent recombination (“hot precursor” [54]), it would be very hard to understand the conservation of initial momentum - which is necessary for explaining the observed elastic reflection, i.e. diffraction. The scattered molecules originate from interaction with the molecular potential, and energy exchange with the solid is in this case governed by phonons as demonstrated by the difference in elasticity between H, and D,. In the case of Ni(ll1) scattering will essentially take place (at least for low (E I )) before the crossing point of the molecular and atomic potential, while for Ni(ll0) this will mainly occur beyond this crossing point. If, however, scattering at Ni(ll0) is indeed due to non-adiabatic transitions will certainly be questionable as long as no more realistic multi-dimensional adiabatic potential hyperface is used for a quantum mechanical treatment of the scattering process. A potential of this kind has been calculated by Norskov et al. [55] for the system H,/Mg(OOOl) in terms of the variation of the total energy as a function of the distance x of the molecule from the surface (whose orientation was assumed to be parallel to the surface) and of internuclear separation y between the two H atoms. This potential exhibits in fact two activation barriers: one (A) for adsorption into the molecularly adsorbed state, and a second one (B) for dissociation. Since we have no evidence for the existence of the A barrier with the present system it is discarded for the subsequent discussion, and the modified Nsrskov potential would then be of the type as drawn schematically in fig. 12a. In this case an impinging molecule would not automatically transfer its kinetic energy normal to the surface, (E I ), into a H-H vibration parallel to the surface which is necessary for dissociation. This would rather predominantly take place through thermal activation from the solid of the intermediately trapped molecular state, and the sticking coefficient would be expected to increase with r, and to decrease with (E I ) - just opposite to the observations. This difficulty is removed if the potential is of the type as shown in fig. 12b, where the activation barrier is overcome by translational energy without trapping in the molecular state. A simple model potential of this type was in fact calculated by Tantardini and Simonetta [56] for the H,/Pt(lll) system which exhibits a saddle point in the entrance channel of about 0.1 eV and would probably offer a suitable starting point for trajectory calculations for the present system.‘Jt appears

H.J. Robota et at. / Dynamics of interaction of H2 and D, with Ni

Fig. 12. Qualitative two-dimensional potential diagrams for dissociative chemisorption: activation barrier in the exit channel; (b) with barrier in the entrance channel.

117

(a) with

rather plausible that with such a potential a steplike increase of the sticking coefficient at the threshold energy would not be expected. Rather with increasing E, , more and more trajectories would lead to dissociation and thus lead to a more continuous increase, as expe~mentally observed. The potential diagrams shown in fig. 12 are similar to those for atom-molecule reactions, A + BC -+ AB + C, which were investigated in detail by Polanyi and Schreiber [57]. Here also reaction is favored either by translational or vibrational excitation, depending on whether the activation barrier is located in the entrance (reactant) or exit (product) channel of the reaction coordinate. The molecular adsorption state is renascent of the potential well for a “collision complex” which is exhibited for example by the reaction H++ H, + H, + H+ and which may survive for many vibrational periods [58]. In contrast to the inelasticity of molecular scattering the dissociative sticking coefficient appears to be essentially independent of the mass of the impinging particles. This suggests that the energy release to the solid associated with dissociation and bonding to the surface after crossing the activation barrier is governed by electronic rather than by phonon excitations of the solid. There exist various theoretical treatments of such non-adiabatic processes during chemisorption [59] and experimental verification is for example found with the chemiluminescence observed with oxygen on Al or halogen adsorption on Na [60,61]. More specifically, the antibonding MO of a H, molecule

118

H.J. Rohota et al. / Dynamics of interaction of H2 and D2 wrth NI

coming close to the surface will be shifted below the Fermi level E,. The hole at the adsorbate thus created will have a certain lifetime during which this level is shifted further down below E, due to the movement of the particle. As a consequence, after tunneling a hole is left in the solid below E, which corresponds to an electron-hole excitation of the solid which transfers the chemisorption energy and enables effective sticking [62]. 5. Conclusions A hydrogen molecule impinging on a nickel surface may be either reflected (due to direct-inelastic or elastic scattering), or dissociatively chemisorbed. For the surface temperatures (T, = 120-800 K) and kinetic energies ((EL ) = 25-120 meV) employed there is no indication for effective trapping into a molecularly adsorbed state. Due to the corrugation of the potential parallel to the surface elastic scattering at Ni(ll0) leads to appreciable diffraction effects which, on the other hand, are only very weak with Ni(ll1). Elastic translational-rotational transitions are hardly detectable (if present at all) with Hz and D,. Direct-inelastic scattering manifests itself in a lobular angular distribution peaked at the specular angle and forms a background for the elastic contributions. Inelastic scattering is considerably stronger for D, than for H,, which indicates that energy exchange at the molecular scattering potential is governed by phonon rather than electron excitations of the solid. The sticking coefficient for dissociative chemisorption on Ni(ll0) is very high (sO = 0.96) and remains constant within the investigated limits of 7; and (E I). With Ni(lll), by contrast, it is only about s0 = 0.1 for q = 300 K and (E i ) = 64 meV. In this case s0 is again independent of T,, but increases steeply with (E, ) and reaches a value of 0.4 at (E I ) = 120 meV. These results suggest that no noticeable activation barrier (with respect to the translational energy of the incoming particle) for dissociative chemisorption exists on Ni(llO), while this is of the order of about 0.1 eV for Ni(ll1) and can be overcome by high enough translational energy in a direct collision process. These conclusions are supported by the observed angular distributions of the reverse process, i.e. associative desorption: With Ni(ll0) a cos 6, distribution indicates complete accommodation of the translational energy, while the cos’0, distribution with Ni(ll1) reflects enhanced mean velocity in the direction normal to the surface. These findings can be rationalized in terms of a two-dimensional potential diagram where the crest of the activation barrier is located in the entrance (i.e. reactant) channel. In contrast to the inelasticity of molecular scattering, there is no noticeable isotope effect observed for the probability of dissociative chemisorption. This suggests that the energy of the metal-hydrogen bond formation is effectively released by electron-hole excitations in the solid.

H.J. Robota et al. / Dynamics of interaction of H2 and D, with Nl

119

Acknowledgements Fruitful discussions with R.J. Behm, K. Christmann and G. Comsa are gratefully acknowledged. H.J.R. wants to thank the Alexander von Humboldt Foundation for a fellowship. M.C.L. would like to express his gratitude to the John Simon Guggenheim Foundation for a fellowship and to the Alexander von Humboldt Foundation for a Senior US Scientists Award. Financial support was obtained from the Deutsche Forschungsgemeinschaft (SFB 128).

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