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Kinetics of H2 and D2 physisorption on Cu(100)
Surface Science 205 (1988) 387-396 North-Holl~d, Amsterdam
KINETICS OF H, AND D, PHYSISORPTION L. WILZkN,
387
ON Cu(100)
S. ANDERSSON
Chalmers University of Technology, S-412 96 Giiteborg Sweden
and J. HARRIS Institut fir Festkiirperforsehungder KFA Jiilich, D-51 70 Jiilich 1, Fed. Rep. of Germany Received 24 May 1988; accepted for publication 5 July 1988
The kinetics of molecular hydrogen physisorption on a cold ( - 10 K) Cu(100) surface have been investigated using molecular beams with incident energies in the range 30-70 meV. The adsorption probability, S, is found to increase linearly with the fractional coverage, 8. The data are consistent with a kinetic model that comprises sticking via impact on the bare surface and via collision with a pre-adsorbed molecule, with probabilities S, and S, respectively. St is found to be larger than St, by an order of magnitude which implies that collisions between incident and adsorbed molecules provide a very efficient means for absorbing the incident kinetic energy on initial impact.
1. Introduction The physisorption interaction of molecular hydrogen with noble metal single crystal surfaces has been thorougbly investigated in recent years, both experimentally [l-5] and theoretically [6,7]. Molecular beam techniques employing diffraction- and rotation-mediated selective adsorption allow a determination of the spectrum of bound state levels and give information about e.g. the degree of rotational hindering. Such experiments have been performed for H,, D2 and HD scattering from single crystal surfaces of Cu [l-3], Ag [4,5], and Au 121. Stable physisorbed species of H,, D, and HD on cold surfaces of Cu [8] and Ag [9] have been detected using high-resolution electron energy-loss spectroscopy (HREELS). The HREELS spectra display rotational excitations and internal H-H stretch vibrations that show only minute shifts compared with the gas-phase values. This shows that the chemical state of the physisorbed molecules is virtually identical to that of free molecules and that rotational motion within the physisorption well is essentially unhindered. In addition, the weak corrugations observed in beam experiments imply that 0039-6028~88/$03.50 0 Elsevier Science Publishers B.V. (North-Holl~d Physics ~blishing Division)
388
L. WilzJn et al. / Kinetics of H, and D, physisorption
on Cu(IO0)
centre of mass motion parallel to the surface in the physisorption well is essentially unhindered. These observations and their implications suggest that pre-adsorbed physisorbed molecules influence sticking behaviour in a way that is quite different from what is commonly found in studies on chemisorption systems, where pre-adsorbed species tend to block the chemisorption because of the strongly different interactions experienced by gas-phase particles incident on the bare surface and on a pre-adsorbed particle. In the simplest cases, the coverage dependence of the chemisorption probability can be described via “chessboard” models where the chemisorbed species are assumed merely to render a portion of the surface - 0 inactive [lO,ll]. Though allowance must often be made for parallel transport across the inactive part of the surface [12], the chemisorption probability usually falls as the coverage increases (with some notable exceptions, see, for example ref. [13]). Two factors are of importance in understanding the way prephysisorbed species influence the sticking behaviour of a system like H,-Cu. Firstly, the physisorption interaction is of the order of tens of meV and involves no essential change in chemical configuration. This means that electronic excitations are extremely inefficient in absorbing the centre of mass kinetic energy of incident particles and so give rise to extremely low levels of sticking [14]. Furthermore, since the phonon bandwidths of the noble metals are of the order of - 20-30 meV, the sticking of an adparticle with incident energy larger than this value in a single step process requires at least two phonons. For light particles such as H, and D, such processes occur with only a small probability [15]. In such cases the overall probability for sticking as a result of collision on the bare surface is expected to be of the order of a few percent which is in accordance with our previously reported experimental measurement [16]. When an open shell atom or molecule chemisorbs on the uncovered part of a metal surface, on the other hand, the interaction is of the order of eV rather than meV and the incident particle, whatever its incident gas-phase energy, smashes into the surface creating showers of phonons and electron-hole pairs. The energy carried away by these excitations will in general be on the scale of eV and quite sufficient to stick a thermal energy incident particle with unity probability. Secondly, physisorbed H, molecules are not tightly bound in localized sites, as chemisorbed atoms and molecules tend to be, and their freedom to translate parallel to the surface when struck by an incident particle provides an efficient means for the absorption of a substantial amount of the incident kinetic energy. In addition, rotational excitation of pre-adsorbed molecules can take up part or all of this energy. The energy imparted to the adsorbed molecule on impact can be carried away from the impact region and dissipated via further particle-particle collisions and, finally, through the phonon bath. Thus, the impact of a light gas-phase particle on a prephysisorbed particle offers channels for the conversion of the incident kinetic energy that are not
L. Wilzknet al. / Kineticsof HI and D2 physisorptionon Cu(lO0)
389
available on the uncovered part of the surface. For the chemisorption system, on the other hand, impact on a precovered part of the surface implies that the incoming particle experiences a much weaker acceleration than on the bare surface. If the chemisorbed atom is tightly bound and localized with respect to the substrate lattice, the incoming particle will not have sufficient energy to knock it to one side and so is quite likely to rebound and revert to the gas phase. Of course, allowance must be made for the “steering effect” of the interaction in the neighbourhood of the adatom, and also for the possibility of normal-to-parallel kinetic energy conversion and “‘skidding” along the surface to a free site. For a more complex chemisorption system (e.g. dissociative chemisorption) such processes may dominate the sticking behaviour. For simple chemisorption, however, we would expect pre-adsorbed species to inhibit further chemisorption, while prephysisorbed atoms and molecules should promote further physisorption. This role of prephysisorbed particles in promoting sticking has been demonstrated by Menzel and coworkers [17,18] who presented data taken using an effusive source for rare gases sticking on Ni(ll1) and Ru(001). The sticking coefficient was found to increase linearly with coverage over the first monolayer so that the monolayer grows exponentially. The effect of particle-particle collisions was most marked for the system Ne-Ru(OO1) where an increase of the average sticking coefficient from a value of the order at 10m3 at zero coverage to about 0.5 at monolayer coverage was found. The aim of the present paper is to demonstrate that the sticking of H, and D, molecular beams on a Cu(100) target displays similar behaviour. We noted in an earlier publication [16] that the sticking coefficient for this system showed a marked increase with coverage and attributed this to the influence of particle-particle collisions. We now consider this dependence in detail and show that energy and angle resolved uptake curves can be reproduced quantitatively by a simple kinetic law obtained on the basis of different sticking probabilities for impact on the bare surface and on a pre-adsorbed molecule over .a coverage range 0 < (9 < 0.8. On this basis we believe to demonstrate that, at all but the lowest coverages, the sticking of H, and D, on Cu(100) is dominated by particle-particle collisions.
2. Experimental The scattering apparatus used in these experiments consisted of hydrogen molecular beams shaped by skimmers in three differentially turbopumped chambers operating at typical pressures 10-3, 10P6, and 2 x 1O-9 Torr, respectively. The gas was expanded from a 10 pm nozzle source at temperatures between 100 and 300 K combining liquid-nitrogen cooling and resistive heating. The source temperature was electronically controlled via a thermocou-
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ple signal and could be kept steady or ramped linearly in time. Gas pressures in the range l-2 bar produced adequate beams with an optimum energy spread of - 15%. The beam angular divergence was 0.25” and the beam was allowed to expand to a diameter of 5 mm and covered essentially the entire sample surface. The adsorption experiments were performed in a cryopumped (1000 8/s) UHV system, with the Cu(100) sample mounted so that the scattering plane comprised the surface normal and the (010) direction in the surface. The sample could be cooled to - 10 K using liquid helium as cooling agent and was heated resistively. Incident and scattered beam intensities were measured with a rotatable stagnation arrangement (1.5” angular resolution) equipped with a calibrated ionization gauge. Diffraction measurements were carried out using this detection system and provided calibration of the molecular beam energy versus nozzle source temperature. The base pressure in the UHV chamber was 3 X 1O-‘1 Torr, which increased to - 5 X 10-l’ Torr with the beam on. The sticking behaviour was determined by measuring continuously the adsorbate coverage. For this purpose we measured the adsorbate induced work function change A+, with a tracking electron-beam retardation method. This technique, which caused no obstruction of the incident molecular beam, turned out to be quite sensitive and fast; 0.5 meV rms noise level at 0.3 s time constant. The electron beam sampled the adsorbate coverage at the centre of the crystal. It was found important that the area illuminated by the molecular beam should be substantially larger than the area sampled by the electron beam. A full monolayer of H, (4) resulted in work function changes of A+ = -120 (- 145) meV. The relation between A+ and adsorbate coverage was measured via partial monolayer desorption. The monolayer saturation densities, n,, required for normalization were also determined in the desorption experiments and found to be 0.65 X 1015 and 0.75 X 1015 mol/cm2 for H, and D2 respectively. (These figures are uncertain by up to 20%. The saturation density, n S, was taken to correspond to the break point in the adsorption curves shown below.) The Cu(100) crystal was kept at - 10 K during the adsorption measurements and was cleaned by flash heating to 900 K between each measurement.
3. Results and discussion Figs. 1 and 2 show measured adsorption data (open circles) obtained for D, at three different beam energies, 35, 49, and 63 meV, and two angles of incidence, 40 o and 60 O. The fractional surface coverage 19= n Jn s is plotted versus the D, dose in equivalent monolayers ni/nS. The solid curves represent calculated adsorption curves and will be discussed below. The characteristic
L Wiirhn et al. / Kinetics
ofH2andD2physisotpion on Cu(l#)
391
0.6 0 4 c? 0.4
0 0
4
2
6
6
D2 dome
(ni/nc)
Fig. 1. MeasuredDz adsorption, “*/ns versus D, dose, ni/n, (open circles) for three Dz beam energies,35,49, and 63 meV at 40 o angle of incidence.The solid curves representtwo parameter fits to the data of eq. (3). Parameter vaks: S,,, S, = 0.048, 0.60; 0.022, 0.52; 0.015, 0.45, for the three energies.
features of the experimental kinetics data are (i} a low level of uptake at low coverages, (ii) a monotonic increase in the rate of uptake as coverage increases, and (iii) an abrupt end to the uptake as the saturated monolayer is formed. The coverage dependence of the sticking coefficient, S, to which these data correspond is illustrated in fig. 3. S was determined from the slope of the uptake curve obtained for a D2 beam of 44 meV incident energy and 40° angle of incidence. S is a linear function of 8 over the range of coverage
Q 5 ro
0.6
0. 4
0
2
4 D2 dose
6
a
10
(n,/rQ
Fig. 2. Analogousadsorptioncurves for angle of incidence60 O. Parameter values: S,,, S, = 0.10, 0.64, 0.055,0.56; 0.033,0,48.
392
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al. / Kinefics of Hz and B2 ph~s~or~~ionon Cufloo)
I. 0
O2fie
i
-
cuaoo> 44
mev
4o”
i
Fig. 3. Measured sticking probability, S versus fractional coverage 8 = n,/n, for D, at 44 meV beam energy and 40 ’ angle of incidence. The solid line represents a least-squares straight line fit to the data.
0 < 0 < 0.8 and falls off at higher coverage as saturation of the monolayer is approached. We discuss this fall off later and focus first on the linear region. If S, and S, are the intercepts of the straight line characterizing this region at 8 = 0 and 1, respectively, then we can write s = S,(i -
8) + s,e,
0)
and, in accordance with the discussion in the introduction, identify S, and S., as representing average sticking coefficients for molecules whose initial impact is on the bare, and covered parts of the surface. The values of S, and S, as determined by a straight line fit over the linear region of fig. 3 were 0.03 and 0.62. These figures are typical and reflect the inefficiency of the bare surface in promoting sticking at these incident energies and the extraordinary efficiency of molecule-molecule collisions in doing so. This gross difference in efficiency is readily understood in terms of lattice stiffness and a mass ratio unfavourable for phonon creation, contrasted with the ease of recoil and the favourable mass ratio for energy transfer due to p~ticle-panicle collisions. The sticking coefficient versus coverage relation (1) implies a kinetic equation of the form de,‘dp = S,(l - 0) + Si8, where p = ni/ns uptake is then
e=
(2) is the exposure in monolayers. The equation governing the
-$j-g{exPKSl- S&4 - l>
393
Fig. 4. Mmxmed X2 adsorption, n,ln, (opencircles> for two Ii, beam tmrgit?s, 33, asxi 40 tneV at 6tlQ angle of irdience, The solid curves ate fits using eq. (3). Parameter values: SO, St = 0.025, 0.80; 0.017,0.57 at the two energies, r~~~~ely.
is the exposure at sat~at~o~. The
solid curves in figs. 1 and 2 show the result of fitting eq. (3) to the data via adjustment of S, and 2&. Fits of similar quality were found for all data throughout the range of energies 30-70 meV and angles Bi = 30-70 * and for Hz as ~~41 a~ I?,. Fig. 4 shows the corr~p~nden~ of the ET, uptake curves for beam energies of 33 and 40 meV at 60” angle of incidence with eq. (3) (full lines). The values of S’e and S, that result from fitting the measured uptake curves to eq. (3) are shown in figs. 5 and 6 as a function of incident energy and angle, respectively, S, is larger than Se by at least an order of ma~tude, is very weakly d~endent on ai and falls off with incident energy. This behaviour is what one would expect of sticking processes involving moi~ule-rnoi~~e collisions. As reported previously 1161,the bare-surface sticking ~~ff~~ent, S,, increases with increasing angle of incidence and f&s off rather strongly towards higher incident energy. As is clear from figs. l-4, the kinetic law (3) fails for coverages larger than 8 *%0.8. At these higher coverages the sticking ooeffieient departs from the linear behaviour observed over most of the coverage range, ~0~~ it is not easy to qu~tify, this behaviour is qu~tative~y consistent with our interpretation. At high coverage the majo~ty of impacts involve eohision of the iu~~ng particle with one or more adsorbed particles. Ih order to adsorb,
394
L. Wilzbn et al. / Kinetics of Hz and D2 physisorption on Cu(100)
so
=1 02 8. 1
0. 10
Cu(100) 4o”
1.0
0. 5
0.05
0
0 0
10
20
30
40 Ei
50
60
70
Fig. 5. Measured values of the “bare-surface” (open circles) and “impact on adparticle” (filled circles) sticking coefficient S,,, S, as a function of the incident D2 beam energy at 40 o angle of incidence.
however, the incident molecule must not only lose its incident kinetic energy but also be able to push the pre-adsorbed molecules in the impact region to one side. This becomes more and more difficult as the coverage approaches saturation and the preadsorbed monolayer stiffens. Energy transfer can still be reasonably efficient and there could be substantial trapping of particles at the surface even after the monolayer has completed. However, the trapped molecules experience only a very weak attractive interaction and would immediately desorb from the surface. At extremely low temperatures, of course, they would not and further layers of condensed H, would form on the surface.
0. 10
!_! D2
-
Ei -
s1*
.
l
0. 05 0 so
CuClOO)
49
1.0
moV
. 0
z
60°
7o”
0. 5
0
o
0
0 3o”
4o”
5o” 8:
Fig. 6. Dependence of the sticking coefficients SO and S1 as a function of the angle of incidence of a 49 meV Dz beam.
L WiMn et al. / Kinetics of Ii2 and D, physisorption
on Cu(loO)
395
4. Conclusion Measured uptake curves when H, and D, molecular beams with incident energies and angles in the ranges 30-70 meV and 30-70” impinge on a cold Cu(100) surface show that the adsorption probability increases monotonically with exposure. The sticking coefficient is found to increase linearly with the coverage over a wide range. This behaviour is interpreted in terms of sticking processes involving impact of the incoming molecule with the bare surface and with pre-adsorbed species. A simple kinetic model constructed on this basis was found to give an excellent description of the data with reasonable values of the sticking parameters. The average sticking coefficient for processes involving molecule-molecule collisions was found to be about 0.6 in the energy range considered, at least one order of magnitude larger than the sticking coefficient on the bare surface. This difference reflects the importance of the mass ratio in energy transfer processes (cf. in an impulsive collision, the fraction of the incident particle energy transferred is given in classical mechanics by 4~/(1 + P)~, where p is the mass ratio). For physisorption systems where the adsorbate and substrate masses are similar one would expect SCI- S, and a much weaker dependence of S on the coverage. This is the behaviour observed by Wang and Gomer [19] for Xe on clean and oxygencovered tungsten surfaces, where the sticking coefficient was found to be close to unity with a weak linear increase with coverage. Similarly, Menzel and co-workers [17] found that the zero coverage sticking coefficient of rare gases on Ni(ll1) increased monotonically with adsorbate mass and that the influence of particle-particle collisions was greatest for the lighter particles. These data and the data presented above indicate that the role of prephysisorbed particles in promoting sticking is a rather general and, at least in qualitative terms, readily understandable phenomenon.
Acknowledgement Financial support from the Swedish Natural Science Research gratefully acknowledged.
Council is
References [l] [2] [3] [4] [5]
J. Perreau and J. Lapujoulade, Surface Sci. 121 (1982) 341. U. Harten, J.P. Toennies and Ch. Wall, J. Chem. Phys. 85 (1986) 2249. S. Andersson, L. Wil& and M. Persson, Phys. Rev. B, in press. M. Chiesa, L. Mattera, R Musenich and C. Salvo, Surface Sci. 151 (1985) L145. C.-F. Yu, K.B. Whaley, C.S. Hog and S.J. Sibener, Phys. Rev. Letters 51 (1983) 2210; J. Chem. Phys. 83 (1985) 4217.
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L.. Wil..& et al. / Kinetics of H2 and D2 physisorption on Cu(lO0)
[6] P. Nordlander, C. Holmberg and J. Harris, Surface Sci. 175 (1986) L753.
[7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17]
P. Nordhmder, C. Holmberg and J. Harris, Surface Sci. 152 (1985) 702. S. Andersson and J. Harris, Phys. Rev. Letters 48 (1982) 545. Ph. Avouris, D. Schmeisser and J.E. Demuth, Phys. Rev. Letters 48 (1982) 199. I. Langmuir, Trans. Faraday. Sot. 17 (1921) 607,621. P. Alnot, A. Cassuto and D.A. King, to be published. P. KisIiuk J. Phys. Chem. Solids 3 (1957) 95; 5 (1958) 78. P. Feulner and D. Menzel, Phys. Rev. B 25 (1982) 4295. K. Sch&rhammer and 0. Gunnarsson, Phys. Rev. B 22 (1980) 1629. M. Persson and J. Harris, Surface Sci. 187 (1987) 67. S. Andersson, L. Wilzkn and J. Harris, Phys. Rev. Letters 55 (1985) 2591. D. Menzel, in: Kinetics of Interface Reactions, Eds. M. Grunze and H. J. Kreuzer (Springer, Berlin, 1987). [18] H. Schhcting, D. Menzel, T. Brunner, W. Brenig and J.C. Tully, Phys. Rev. Letters 60 (1988) 2515. [19] C. Wang and R. Gomer, Surface Sci. 84 (1979) 329.
KINETICS OF H, AND D, PHYSISORPTION L. WILZkN,
387
ON Cu(100)
S. ANDERSSON
Chalmers University of Technology, S-412 96 Giiteborg Sweden
and J. HARRIS Institut fir Festkiirperforsehungder KFA Jiilich, D-51 70 Jiilich 1, Fed. Rep. of Germany Received 24 May 1988; accepted for publication 5 July 1988
The kinetics of molecular hydrogen physisorption on a cold ( - 10 K) Cu(100) surface have been investigated using molecular beams with incident energies in the range 30-70 meV. The adsorption probability, S, is found to increase linearly with the fractional coverage, 8. The data are consistent with a kinetic model that comprises sticking via impact on the bare surface and via collision with a pre-adsorbed molecule, with probabilities S, and S, respectively. St is found to be larger than St, by an order of magnitude which implies that collisions between incident and adsorbed molecules provide a very efficient means for absorbing the incident kinetic energy on initial impact.
1. Introduction The physisorption interaction of molecular hydrogen with noble metal single crystal surfaces has been thorougbly investigated in recent years, both experimentally [l-5] and theoretically [6,7]. Molecular beam techniques employing diffraction- and rotation-mediated selective adsorption allow a determination of the spectrum of bound state levels and give information about e.g. the degree of rotational hindering. Such experiments have been performed for H,, D2 and HD scattering from single crystal surfaces of Cu [l-3], Ag [4,5], and Au 121. Stable physisorbed species of H,, D, and HD on cold surfaces of Cu [8] and Ag [9] have been detected using high-resolution electron energy-loss spectroscopy (HREELS). The HREELS spectra display rotational excitations and internal H-H stretch vibrations that show only minute shifts compared with the gas-phase values. This shows that the chemical state of the physisorbed molecules is virtually identical to that of free molecules and that rotational motion within the physisorption well is essentially unhindered. In addition, the weak corrugations observed in beam experiments imply that 0039-6028~88/$03.50 0 Elsevier Science Publishers B.V. (North-Holl~d Physics ~blishing Division)
388
L. WilzJn et al. / Kinetics of H, and D, physisorption
on Cu(IO0)
centre of mass motion parallel to the surface in the physisorption well is essentially unhindered. These observations and their implications suggest that pre-adsorbed physisorbed molecules influence sticking behaviour in a way that is quite different from what is commonly found in studies on chemisorption systems, where pre-adsorbed species tend to block the chemisorption because of the strongly different interactions experienced by gas-phase particles incident on the bare surface and on a pre-adsorbed particle. In the simplest cases, the coverage dependence of the chemisorption probability can be described via “chessboard” models where the chemisorbed species are assumed merely to render a portion of the surface - 0 inactive [lO,ll]. Though allowance must often be made for parallel transport across the inactive part of the surface [12], the chemisorption probability usually falls as the coverage increases (with some notable exceptions, see, for example ref. [13]). Two factors are of importance in understanding the way prephysisorbed species influence the sticking behaviour of a system like H,-Cu. Firstly, the physisorption interaction is of the order of tens of meV and involves no essential change in chemical configuration. This means that electronic excitations are extremely inefficient in absorbing the centre of mass kinetic energy of incident particles and so give rise to extremely low levels of sticking [14]. Furthermore, since the phonon bandwidths of the noble metals are of the order of - 20-30 meV, the sticking of an adparticle with incident energy larger than this value in a single step process requires at least two phonons. For light particles such as H, and D, such processes occur with only a small probability [15]. In such cases the overall probability for sticking as a result of collision on the bare surface is expected to be of the order of a few percent which is in accordance with our previously reported experimental measurement [16]. When an open shell atom or molecule chemisorbs on the uncovered part of a metal surface, on the other hand, the interaction is of the order of eV rather than meV and the incident particle, whatever its incident gas-phase energy, smashes into the surface creating showers of phonons and electron-hole pairs. The energy carried away by these excitations will in general be on the scale of eV and quite sufficient to stick a thermal energy incident particle with unity probability. Secondly, physisorbed H, molecules are not tightly bound in localized sites, as chemisorbed atoms and molecules tend to be, and their freedom to translate parallel to the surface when struck by an incident particle provides an efficient means for the absorption of a substantial amount of the incident kinetic energy. In addition, rotational excitation of pre-adsorbed molecules can take up part or all of this energy. The energy imparted to the adsorbed molecule on impact can be carried away from the impact region and dissipated via further particle-particle collisions and, finally, through the phonon bath. Thus, the impact of a light gas-phase particle on a prephysisorbed particle offers channels for the conversion of the incident kinetic energy that are not
L. Wilzknet al. / Kineticsof HI and D2 physisorptionon Cu(lO0)
389
available on the uncovered part of the surface. For the chemisorption system, on the other hand, impact on a precovered part of the surface implies that the incoming particle experiences a much weaker acceleration than on the bare surface. If the chemisorbed atom is tightly bound and localized with respect to the substrate lattice, the incoming particle will not have sufficient energy to knock it to one side and so is quite likely to rebound and revert to the gas phase. Of course, allowance must be made for the “steering effect” of the interaction in the neighbourhood of the adatom, and also for the possibility of normal-to-parallel kinetic energy conversion and “‘skidding” along the surface to a free site. For a more complex chemisorption system (e.g. dissociative chemisorption) such processes may dominate the sticking behaviour. For simple chemisorption, however, we would expect pre-adsorbed species to inhibit further chemisorption, while prephysisorbed atoms and molecules should promote further physisorption. This role of prephysisorbed particles in promoting sticking has been demonstrated by Menzel and coworkers [17,18] who presented data taken using an effusive source for rare gases sticking on Ni(ll1) and Ru(001). The sticking coefficient was found to increase linearly with coverage over the first monolayer so that the monolayer grows exponentially. The effect of particle-particle collisions was most marked for the system Ne-Ru(OO1) where an increase of the average sticking coefficient from a value of the order at 10m3 at zero coverage to about 0.5 at monolayer coverage was found. The aim of the present paper is to demonstrate that the sticking of H, and D, molecular beams on a Cu(100) target displays similar behaviour. We noted in an earlier publication [16] that the sticking coefficient for this system showed a marked increase with coverage and attributed this to the influence of particle-particle collisions. We now consider this dependence in detail and show that energy and angle resolved uptake curves can be reproduced quantitatively by a simple kinetic law obtained on the basis of different sticking probabilities for impact on the bare surface and on a pre-adsorbed molecule over .a coverage range 0 < (9 < 0.8. On this basis we believe to demonstrate that, at all but the lowest coverages, the sticking of H, and D, on Cu(100) is dominated by particle-particle collisions.
2. Experimental The scattering apparatus used in these experiments consisted of hydrogen molecular beams shaped by skimmers in three differentially turbopumped chambers operating at typical pressures 10-3, 10P6, and 2 x 1O-9 Torr, respectively. The gas was expanded from a 10 pm nozzle source at temperatures between 100 and 300 K combining liquid-nitrogen cooling and resistive heating. The source temperature was electronically controlled via a thermocou-
390
L.. Wilzbn et al. / Kinetics of Hz and D, physisorption on Cu(lO0)
ple signal and could be kept steady or ramped linearly in time. Gas pressures in the range l-2 bar produced adequate beams with an optimum energy spread of - 15%. The beam angular divergence was 0.25” and the beam was allowed to expand to a diameter of 5 mm and covered essentially the entire sample surface. The adsorption experiments were performed in a cryopumped (1000 8/s) UHV system, with the Cu(100) sample mounted so that the scattering plane comprised the surface normal and the (010) direction in the surface. The sample could be cooled to - 10 K using liquid helium as cooling agent and was heated resistively. Incident and scattered beam intensities were measured with a rotatable stagnation arrangement (1.5” angular resolution) equipped with a calibrated ionization gauge. Diffraction measurements were carried out using this detection system and provided calibration of the molecular beam energy versus nozzle source temperature. The base pressure in the UHV chamber was 3 X 1O-‘1 Torr, which increased to - 5 X 10-l’ Torr with the beam on. The sticking behaviour was determined by measuring continuously the adsorbate coverage. For this purpose we measured the adsorbate induced work function change A+, with a tracking electron-beam retardation method. This technique, which caused no obstruction of the incident molecular beam, turned out to be quite sensitive and fast; 0.5 meV rms noise level at 0.3 s time constant. The electron beam sampled the adsorbate coverage at the centre of the crystal. It was found important that the area illuminated by the molecular beam should be substantially larger than the area sampled by the electron beam. A full monolayer of H, (4) resulted in work function changes of A+ = -120 (- 145) meV. The relation between A+ and adsorbate coverage was measured via partial monolayer desorption. The monolayer saturation densities, n,, required for normalization were also determined in the desorption experiments and found to be 0.65 X 1015 and 0.75 X 1015 mol/cm2 for H, and D2 respectively. (These figures are uncertain by up to 20%. The saturation density, n S, was taken to correspond to the break point in the adsorption curves shown below.) The Cu(100) crystal was kept at - 10 K during the adsorption measurements and was cleaned by flash heating to 900 K between each measurement.
3. Results and discussion Figs. 1 and 2 show measured adsorption data (open circles) obtained for D, at three different beam energies, 35, 49, and 63 meV, and two angles of incidence, 40 o and 60 O. The fractional surface coverage 19= n Jn s is plotted versus the D, dose in equivalent monolayers ni/nS. The solid curves represent calculated adsorption curves and will be discussed below. The characteristic
L Wiirhn et al. / Kinetics
ofH2andD2physisotpion on Cu(l#)
391
0.6 0 4 c? 0.4
0 0
4
2
6
6
D2 dome
(ni/nc)
Fig. 1. MeasuredDz adsorption, “*/ns versus D, dose, ni/n, (open circles) for three Dz beam energies,35,49, and 63 meV at 40 o angle of incidence.The solid curves representtwo parameter fits to the data of eq. (3). Parameter vaks: S,,, S, = 0.048, 0.60; 0.022, 0.52; 0.015, 0.45, for the three energies.
features of the experimental kinetics data are (i} a low level of uptake at low coverages, (ii) a monotonic increase in the rate of uptake as coverage increases, and (iii) an abrupt end to the uptake as the saturated monolayer is formed. The coverage dependence of the sticking coefficient, S, to which these data correspond is illustrated in fig. 3. S was determined from the slope of the uptake curve obtained for a D2 beam of 44 meV incident energy and 40° angle of incidence. S is a linear function of 8 over the range of coverage
Q 5 ro
0.6
0. 4
0
2
4 D2 dose
6
a
10
(n,/rQ
Fig. 2. Analogousadsorptioncurves for angle of incidence60 O. Parameter values: S,,, S, = 0.10, 0.64, 0.055,0.56; 0.033,0,48.
392
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Wi1.h
et
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I. 0
O2fie
i
-
cuaoo> 44
mev
4o”
i
Fig. 3. Measured sticking probability, S versus fractional coverage 8 = n,/n, for D, at 44 meV beam energy and 40 ’ angle of incidence. The solid line represents a least-squares straight line fit to the data.
0 < 0 < 0.8 and falls off at higher coverage as saturation of the monolayer is approached. We discuss this fall off later and focus first on the linear region. If S, and S, are the intercepts of the straight line characterizing this region at 8 = 0 and 1, respectively, then we can write s = S,(i -
8) + s,e,
0)
and, in accordance with the discussion in the introduction, identify S, and S., as representing average sticking coefficients for molecules whose initial impact is on the bare, and covered parts of the surface. The values of S, and S, as determined by a straight line fit over the linear region of fig. 3 were 0.03 and 0.62. These figures are typical and reflect the inefficiency of the bare surface in promoting sticking at these incident energies and the extraordinary efficiency of molecule-molecule collisions in doing so. This gross difference in efficiency is readily understood in terms of lattice stiffness and a mass ratio unfavourable for phonon creation, contrasted with the ease of recoil and the favourable mass ratio for energy transfer due to p~ticle-panicle collisions. The sticking coefficient versus coverage relation (1) implies a kinetic equation of the form de,‘dp = S,(l - 0) + Si8, where p = ni/ns uptake is then
e=
(2) is the exposure in monolayers. The equation governing the
-$j-g{exPKSl- S&4 - l>
393
Fig. 4. Mmxmed X2 adsorption, n,ln, (opencircles> for two Ii, beam tmrgit?s, 33, asxi 40 tneV at 6tlQ angle of irdience, The solid curves ate fits using eq. (3). Parameter values: SO, St = 0.025, 0.80; 0.017,0.57 at the two energies, r~~~~ely.
is the exposure at sat~at~o~. The
solid curves in figs. 1 and 2 show the result of fitting eq. (3) to the data via adjustment of S, and 2&. Fits of similar quality were found for all data throughout the range of energies 30-70 meV and angles Bi = 30-70 * and for Hz as ~~41 a~ I?,. Fig. 4 shows the corr~p~nden~ of the ET, uptake curves for beam energies of 33 and 40 meV at 60” angle of incidence with eq. (3) (full lines). The values of S’e and S, that result from fitting the measured uptake curves to eq. (3) are shown in figs. 5 and 6 as a function of incident energy and angle, respectively, S, is larger than Se by at least an order of ma~tude, is very weakly d~endent on ai and falls off with incident energy. This behaviour is what one would expect of sticking processes involving moi~ule-rnoi~~e collisions. As reported previously 1161,the bare-surface sticking ~~ff~~ent, S,, increases with increasing angle of incidence and f&s off rather strongly towards higher incident energy. As is clear from figs. l-4, the kinetic law (3) fails for coverages larger than 8 *%0.8. At these higher coverages the sticking ooeffieient departs from the linear behaviour observed over most of the coverage range, ~0~~ it is not easy to qu~tify, this behaviour is qu~tative~y consistent with our interpretation. At high coverage the majo~ty of impacts involve eohision of the iu~~ng particle with one or more adsorbed particles. Ih order to adsorb,
394
L. Wilzbn et al. / Kinetics of Hz and D2 physisorption on Cu(100)
so
=1 02 8. 1
0. 10
Cu(100) 4o”
1.0
0. 5
0.05
0
0 0
10
20
30
40 Ei
50
60
70
Fig. 5. Measured values of the “bare-surface” (open circles) and “impact on adparticle” (filled circles) sticking coefficient S,,, S, as a function of the incident D2 beam energy at 40 o angle of incidence.
however, the incident molecule must not only lose its incident kinetic energy but also be able to push the pre-adsorbed molecules in the impact region to one side. This becomes more and more difficult as the coverage approaches saturation and the preadsorbed monolayer stiffens. Energy transfer can still be reasonably efficient and there could be substantial trapping of particles at the surface even after the monolayer has completed. However, the trapped molecules experience only a very weak attractive interaction and would immediately desorb from the surface. At extremely low temperatures, of course, they would not and further layers of condensed H, would form on the surface.
0. 10
!_! D2
-
Ei -
s1*
.
l
0. 05 0 so
CuClOO)
49
1.0
moV
. 0
z
60°
7o”
0. 5
0
o
0
0 3o”
4o”
5o” 8:
Fig. 6. Dependence of the sticking coefficients SO and S1 as a function of the angle of incidence of a 49 meV Dz beam.
L WiMn et al. / Kinetics of Ii2 and D, physisorption
on Cu(loO)
395
4. Conclusion Measured uptake curves when H, and D, molecular beams with incident energies and angles in the ranges 30-70 meV and 30-70” impinge on a cold Cu(100) surface show that the adsorption probability increases monotonically with exposure. The sticking coefficient is found to increase linearly with the coverage over a wide range. This behaviour is interpreted in terms of sticking processes involving impact of the incoming molecule with the bare surface and with pre-adsorbed species. A simple kinetic model constructed on this basis was found to give an excellent description of the data with reasonable values of the sticking parameters. The average sticking coefficient for processes involving molecule-molecule collisions was found to be about 0.6 in the energy range considered, at least one order of magnitude larger than the sticking coefficient on the bare surface. This difference reflects the importance of the mass ratio in energy transfer processes (cf. in an impulsive collision, the fraction of the incident particle energy transferred is given in classical mechanics by 4~/(1 + P)~, where p is the mass ratio). For physisorption systems where the adsorbate and substrate masses are similar one would expect SCI- S, and a much weaker dependence of S on the coverage. This is the behaviour observed by Wang and Gomer [19] for Xe on clean and oxygencovered tungsten surfaces, where the sticking coefficient was found to be close to unity with a weak linear increase with coverage. Similarly, Menzel and co-workers [17] found that the zero coverage sticking coefficient of rare gases on Ni(ll1) increased monotonically with adsorbate mass and that the influence of particle-particle collisions was greatest for the lighter particles. These data and the data presented above indicate that the role of prephysisorbed particles in promoting sticking is a rather general and, at least in qualitative terms, readily understandable phenomenon.
Acknowledgement Financial support from the Swedish Natural Science Research gratefully acknowledged.
Council is
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396
L.. Wil..& et al. / Kinetics of H2 and D2 physisorption on Cu(lO0)
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