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Orientation and perpendicular energy dependence of dissociative scattering of hydrogen from Cu(111)
Surface Science 272 (1992) 240-246 North-Holland
surface s c i e n c e
Orientation and perpendicular energy dependence of dissociative scattering of hydrogen from Cu(111) J.-H. Rechtien, R. H a r d e r , G. H e r r m a n n and K.J. S n o w d o n Fachbereich Physik, Unicersith't Osnabriick, D-4500 Osnabriick, Germany Received 29 October 1991; accepted for publication 28 November 1991
We report measurements of the final molecular axis orientation and kinetic energy released in the dissociative neutralization of 2.98 keV H + incident at glancing angles to a Cu(111) surface. We observe two distinct struclures which we correlate with scattering on two distinct regions of the potential energy hypersurface (PES) describing the interaction. The relative contribution of each of these structures to the total spectrum exhibits a threshold-like behaviour as a function o~" the perpendicular energy of the beam in the (nominal) range 1-2 eV. The perpendicular energy at which this threshold appears is dependent on the molecular axis orientation. We correlate this threshold with a barrier on the PES, and its dependence on orientation with an orientation dependent barrier height. The height variation is of order 0.9 eV, and the barrier is lowest for the surface-parallel axis orientation.
1. Introduction A prerequisite for a detailed understanding of the interaction of a molecule with a surface is a knowledge of the topography of the corresponding potential energy surface (PES), or surfaces, and of the coupling between such svrfaces. The PES of lowest energy corresponding to dissociative adsorption may contain regions corresponding to physisorbed a n d / o r molecularly chemisorbed species, in addition to a region corresponding to the dissociatively chemisorbed state. These regions may be separated by barriers which may lie either above or below the dissociation limit of the initial configuration. The effective height, location and shape of such barriers are expected to be dependent on the "impact site" of
tive adsorption [6]. Despite this intense effort several questions remain unanswered. A critical review of the available data on this system has recently been provided by Michelsen and Auerbach [6]. They point out a distirct paucity of knowledge on the role of rotation in adsorption and desorption dynamics in general, and for this system in particular. They also point out that such effects may be most important at incident beam energies close to the translational threshold required to traverse the activation barrier. In this paper we report measurements of the molecular axis orientation dependence of the dissociative scattering of hyd:ogen from a C u ( l l l ) surface at perpendicular beam energies close to the expected barrier height. Such measurements are expected to probe the dependence of the llUk.tlVU;
rotational and vibrational state of the molecule, and on the molecular axis orientation. While experimental determination of all these dependences is a formidable if not impossible task, state and orientation selective measurements are nevertheless technically feasible [1-5]. The H 2 / C u system is perhaps the mo~';t thoroughly investigated example of activated dissocia-
dLtlVdtlUll
Odlll~l
IIKJI~Ilt
UII
IIlUIU~UUI~I
axis orientation relative to the surface.
2. Technique and apparatus The detection technique we use to determine the final molecular axis orientation is described in detail elsewhere [7,8]. In principle, we measure
1)(139-t'~1)2,~/92/$1)5.1)0 ~i) 1992 - Elsevier Science Publishers B.V. All rights reserved
J. -H. Rechtien et aL ,/Dissociatit'e scattering of hydrogen from Cu (111)
the final laboratory-frame velocities v,,,~b and VZ~.b of the atoms 1 and 2 which result from the dissociative scattering of a diatomic molecule from a surface. We use these measured laboratory' frame velocities to calculate the relative velocity
241
v =v2,~ab - v u ~ t , of the atoms. The direction of the relative velocity vector v is identical with the final orientation of the molecular axis after all interaction between the molecular fragments and the surface has ceased. In addition, we obtain the ,.,
....... Ol = 1.05 °
....,.... . . . . . . . . . . . . . . . . . . . .
j,
r.
(
(.... ~
._
30 ""-"''""b
O~
o
/
r.,..
--
6o
0 30 so 0
2
,~
o[.,~]
90
6 10
-,
i
,'
o
~,[ov]
1"
,
,
,
2
!
v',
,''"I
4
'
%
O"
4
'1'-,--
lO
[ev]
C
2
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8
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•-----.
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6
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~
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8
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60
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I
90
/
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10
.
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.
.
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I .
i
.
.
6
8
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,~[ev] 0 oi
flit.__ nlall~f~Ik
flit
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Jl
0
°
60
I
L
~r
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-¢ , - - , : ~, ', '
.............
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°'°'o' •
I
L::L.....
90
,
6
levi
8
10
0
2
4
6
8
10
[~v]
Fig. 1. Distribution of released kinetic energy e as a function of the final orientation 0 of the molecular axis to the surface normal for dissociative neu,ralization of 2.98 keV H~- at a C u ( l l l ) surface for incident beam perpendicular energies E ± i (to the "macroscopic surface", see text) of 1, 1.25, 1.5, 2 and 4 eV and azimuth & = - 20 ° to the [ti0] direction. The direction of the faster of the two fragments was chosen to lie halfway (in angle) between the surface plane and the maximum of the scattered particle angular distribution. Only dissociation events corresponding to "backward leaning" molecules were used to generate these plots. The detector is partially blind to the region of (e, 0) space near both the e and 0 axes. We have set the yield to zero in the affected region.
J.-H, Rechtien et aL / Dissociativescattering of hydrogenfrom Cu(l l I)
242
o
~
60
~°eld,d " 0
2
- - ' ~ - - ' ~ #
6
8
go
go
10
--
0
2
4
elev]
6
8
10
.Icy] 0
! o
3O
6O
0 eta]
,~[,v]
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2
4
6
8
10
e[ev]
Fig. 1. Continued.
kinetic energy released in the dissociation event from the magnitude of v, viz. e -
½Pv 2,
(1)
where t~ is the reduced mass of the molecule. We represent the data corresponding to true coincidence events as differential distributions dN(e, 0 ) / d e dta, where dr, represents a solid angle element whose size is independent of e and 0. For simplicity we will call these distributions N(e, 0) distributions. The experiments were performed using a 2.98 keV H~ ion beam incident at a grazing angle of incidence O~ to a carefully prepared Cu(111)surface. The zero of the incidence angle scale was determined before each measurement by monitoring the ion beam intensity with a detector placed at a scattering angle of zero degrees while rocking the crystal about nominal zero. This method allows u~ to determine the incidence angle to the "macroscopic surface" (defined by
the twc, "highest points" on that region of the surface exposed to the beam) to an accuracy and reprodueability of ~ 0.05°. The beam divergence is of a similar magnitude (,-0.04* half-cone angle). The relative uncertainty AE j.i,/E J.z in the perpendicular energy E J.i = Ei sin 2 0i of the beam to the surface is A E ~.i/E J.i = 2 AOilO i + A E i / E i,
(2)
where A0~ is a measure of the uncertainty in the incidence angle 0 i and AEi is the uncertainty in the incident beam energy ,El, The second t~rrn in (2) is negligible compared with the first. For E i = 2.98 keV, #i = 1.05" and A9 i = 0.05 °, we find that E l i -- 1 +_0.1 eV. The spread in perpendicular energy is therefore much less than the vibrational spacing in gas phase H 2 ( ~ 0.5 eV). However, the absolute error in ELi referenced to the (111) surface plane (rather than to the above defined "macroscopic surface") may be significant. A polishing error of ~ 0.2° would modify
J.-H. Rechtien et aL / Dissociatice scattering of hydrogen frorn Cu (111)
243 "
-' ..............
the perpendicular energy by ~ 0.3 eV in the above example. The beam was incident at an azimuthal angle 4~ = - 20° to the [1]0] direction. Its rotational and vibrational state population distributions were unknown, however the initial ionization of the beam occurred in a plasma ion source. We will assume an isotropic molecular axis orientation distribution for the incident beam.
3. Results We have measured N(~, 0) distributions (for coincidences between neutral hydrogen atoms) at beam incidence angles 0 i = 0.47, 0.88, 1.05, 1.17, 1.28, 1.48, 1.81, 1.96, 2.09 and 2.96°, corresponding to incident beam perpendicular energies (to the macroscopic surface defined above) of 0.2, 0.7, 1.0, 1.25, 1.5, 2.0, 3.0, 3.5, 4.0 and 8.0 eV. The spectra spanning the range E j_i = 1-2 eV, plus the spectrum obtained a t E_Li = 4 eV are shown in fig. 1.
4. Discussion Above an incident beam perpendicular energy E ±i = 3.5 eV ;he N(e, 0) distribution looks like that shown for E . t . i = 4 eV in fig. 1. It is characterized by an e-distribution peaking at or below 1 eV. As we reduce E ± i, a broad structure peaking at e = 2-3 eV, 0 = 60° superimposes on this distribution. The N(e, 0) distribution is dominated by this broad structure for E J_i -< 1 eV. We will assume in our analysis that the shape of the former distribution is independent of E . over the complete range of our experiment (0.2-8 eV). We know this to be true for the E!~ range 3.5-8 eV and for the (e, 0) range (0-10 eV, 0-15 °) not affected by the broad structure mentioned above. Provided this assumption is valid, we can subtract from all spectra (after suitable normalisation) any spectrum in which the broad structure is absent. We have used the (suitably normalized) spectrum corresponding to E L i = 4 eV (fig. 1) for this "background subtraction". This procedure provides us with a series of spec-
0
i
15
'
i
30
........
~
.
i
45 0 rdeg]
.
.
.
.
.
.
.
.
.
.
i
60
75
90
Fig, 2. Orientation angle dependence of the yield corresponding to the broad structure peaking at e = 2-3 eV, 0 = 600 in fig. 1, as a function of incident beam perpendicular energy (in eV), Note that the absolute shape of the curves is unreliable in the range 0 >i 70° since the detector is partially blind in this region (see caption to fig. 1).
tra of the broad structure alone, which illustrate its incident beam perpendicular energy dependence. Integration of these spectra with respect to the e coordinate provides the orientation angle dependence of the process leading to this structure, as a function of incident beam perpendicular energy. The results are shown in fig. 2. At E!~ = 0.2 eV, the 0 distribution peaks at (or possibly even beyond, see caption to fig. 2) 0---70 °. As E l i is increased to 1 eV, a slight reduction in the yield for orientations beyond 0 = 60° is observed. At E ~ = 1.25 eV, only the yield at orientations exceeding 0 = 30° is strongly suppressed. Only for E i i> 2 eV is the yield at all molecular axis orientations significantly reduced. We summarize the yield variation as a function of E.Li for several values of the orientation angle 0 in fig. 3. We observe efficient neutralization of the incident H + beam during scattering. Simple considerations suggest that at least the b 3y.~ and X ~~g+ states of H 2 should be energetically accessible for charge transfer on the incident trajectory by either resonant or Auger processes. Earlier studies [7,9] have led us (tentatively) to con3 "+ clude that direct charge transfer to the b ~2u state represents at most a minority dissociation channel. We do not at present believe that direct
J.-H. Rechtien et al. / Dissociatire scattering of hydrogen from Cu(l l l)
244
initial occupation of this state is responsible for any of the features seen in fig. 1. Instead we propose that charge transfer occurs first, on the incident trajectory, to a state correlating with the (surface modified) X I Zg..t- state of H 2. If we assume that electronic excitation of both the substrate and molecule during the interaction are insignificant, then it may be a good first approximation to attempt to describe the interaction with the help of the adiabatic PES describing the interaction of ground state H 2 with copper. We have in fact provided such a description in earlier publications [7,8]. We reproduce an approximation to the H 2/Cu PES (due to Harris [10]) in fig. 4. We noted in ref. [7] that the sudden change in the relative yields of the two structures in fig. 1 at E±~--- 1 eV may be correlated with traversal of the activation barrier (of height ~ 0.8 eV) separating the H2-Cu and 2(Cu-H) configurations in fig. 4. The analysis summarized in figs. 2 and 3 lends further support to this interpretation. Indeed, if our interpretation is correct, then figs. 2 and 3 may contain a direct measure of the orientation dependence of the effecive barrier height. The final molecular axis orientation distribution we measure (fig. 2) is influenced by the orientation dependence of both the initial charge 0
"~
1
2
3
,
,
,-
~ 'k~ \ \
>-
• 25-35
°
a 35-45
o
o 55-65 o o 65-,5o 75-85
0
1
2 E, l e V I
3
°
4
Fig. 3. V a r i a t i o n w i t h E , ~ o f the total yield in fig. 2 in the 0 w i n d o w s 2 5 - 3 5 ° , 3 5 - 4 5 ° , 4 5 - 5 5 ° , 5 5 - 6 5 ° , 6 5 _ 7 5 ° and 7 5 - 8 5 ° . T h e lower energy scale is that c o r r e s p o n d i n g to o u r nomina~ incidence angle c a l i b r a t i o n . T h e u p p e r scale is that o b ta in e d
by reducing all incidence angles by 0.2 °. The .:urves have been normalized.
6
5
£ N
2
0
2
3
Fig. 4. Contour plot of the adiabatic ground state potential energy surface proposed by Harris [10] to describe the interaction of molecular hydrogen with copper (z is the distance of the centre-of-mass of the molecule to the surface, x is the H - H separation, the H 2 axis is assumed to lie parallel to the surface, the contours extend from 0.1 to 2 eV in 0.1 eV intervals).
exchange step converting H~- to H 2, and finally of the dissociation process itself. We expect that the 0-dependence of the initial charge exchange step is (to first-order at least) independent of EI~. The observed change in the orientation distribution with EI~ therefore reflects the orientation dependence of the dissociation process. We see little change in these distributions up to E . i - - 1 eV, and for E t i > 3.5 eV. We therefore propose that independent of their orientation, up to E . i = 1 eV most molecules reflect from the repulsive "back-wall" of the H z - C u configuration, and that beyond E 1 ,: -- 2 eV, most molecules reflect from the repulsive "back-wall" of the 2(H-Cu) configuration. The fate of the molect!es in the intermediate regime 1
J. -H. Rechtien et al. / Dissociatit'e scattering of hydrogen from Cu (111)
effective activation barrier height on the H z / C u ( l l l ) PES describing the scattering. Vibrational and rotational excitation in the incident beam will determine both the position and width of the transition region, however the change in the threshold value with 0 (and therefore the 0 dependence of the effective barrier height extracted from it) should be independent of these (presently unknown) rotational and vibrational distribution'~. We infer that the effective barrier height decreases by ~ 0.9 eV with a change in the final molecular axis orientation from 0 = 30 ° (near-perpendicular to the surface) to 0 = 80 ° (near-parallel). The barrier is lowest for nearparallel orientations, in agreement with theoretical expectations [11,12]. An activation barrier height in the entrance channel of more than 1 eV (fig. 3) is not consistent with current experimental understanding of the adiabatic ground state of the H 2 / C u ( l l l ) system [6]. Although we sample a distribution of barrier heights over the full surface unit cell in our scattering experiment, calculations on the C u ( l l l ) surface imply a spread in barrier heights of only 0.2 eV [I3]. This is clearly insufficient to explain the discrepancy. We mentioned earlier that our E L~ scale is very sensitive to our zero calibration of the incidence angle 0~ to the (111) surface plane. We could in principle determine the absolute value of 0, to sufficient accuracy by rotating the crystal 180° and repeating the experiment. The threshold-like behaviour shown in fig. 3 could then be used to extract the absolute value of 0~. This experiment will be performed in the near tuture. The effect on our conclusions of an arbitrarily chosen 0.2 ° reduction in the absolute value of 0 i is shown by the upper energy scale in fig. 3. We conclude that either such a revision is necessary, or that the PES describing the interaction at our (total) beam energy is quite different from the adiabatic ground state PES describing dissociative chemisorption at thermal energies. If the two PES's are indeed identical, then our result should be independent of the incident beam (total) energy. Tt,~s experiment is also currently being performed. We note, however, that electron-hole pair excitation caused by the fast parallel motion of the beam should (to first-order) only
245
shift the adiabatic PES vertically upward, while conserving its topography, and it is this topography and not the zero of the energy scale in fig. 4. to which we are sensitive in our experiment. The relation of the final molecular axis orientation (which we measure) to the actual orientation of the molecular axis at critical points on its trajectory (e.g., at the barrier) is unknown. The collision time, however, can be estimated to be of the order of several tens of picoseconds. This is an order of magnitude less than the (classical) rotation time of H 2. We conclude that the rotational motion is effectively frozen on the timescale of the collision, and that the orientation we measure probably mirrors quite closely the orientation of the molecule throughout its interaction with the surface.
5. Conclusion
We have measured the molecular axis orientation dependence of the dissociative neutralization of 2.98 keV H + inciden~ at glancing ang!es of incidence to a C u ( l l 1) surface. We tentatively correlate the two distinct structures we see with scattering from two distinct electronic configurations of the H 2/Cu system. The relative contribution of these two spectral components to the total yield changes rapidly in the (nominal) incident beam perpendicular energy range 1-2 eV. Analysis reveals that the perpendicular energy at which this change occurs is dependent on the molecular axis orientation. We correlate this dependence with the molecular axis orientation dependence of the effective height of the barrier separating the two electronic configurations. We provisionally infer a barrier height change between the (near) perpendicular and (near) parallel axis orientation of approximately 0.9 eV. The barrier height is lowest for the surfacc-pa~allel axis oricntati¢~n. The t~bserved behaviour is qualitatively consistent with that expected for sc~,ttering from a PES whose topography is similar to that of the adiabatic ground state of the H 2/Cu(l 11) system. We have concentrated in this paper on the change in the shape of the orientation distribution with perpendicular beam energy. A discussion of the
246
J.-H. Rechtien et al. / Dissociative scattering o`f hydrogen from Cu(l l l)
shape of these distributions is presented else-
[31 T.J Curtiss and R.B. Bernstein, Chem. Phys. Lett. 161
where [8].
[41 M.G. Tenner, F.H. Geuzebroek, E.W. Kuipers, A.E.
Acknowledgements We wish to thank D. Auerbach for valuable discussions. This work was generously supported by the Deutsche Forschungsgemeinschaft.
References [1] E.W. Kuipers, M.G. Tenner, A.W. Kleyn and S. Stolte, Nature 334 (1988) 420. [2] G. Fecher, M. Volkmer, N. B/Swering, B. Pawlitzky and U. Heinzmann, J. Chem. Soc. Faraday Trans. II, 85 (1989) 1364.
(1989) 212. Wiskerke, A.W. Kleyn, S. Stoite and A. Namiki, Chem. Phys. Lett. 168 (1990) 45. [51 A. Hodgson, J. Moryl and H. Zhao, Chem. Phys. Lett. 182 (1991) 152. [61 H.A. Michelsen and D.J. Auerbach, J. Chem. Phys. 94 (1991) 7502. [7] J.-H. Rechtien, R. Harder, G. Herrmann, C. R6thig and K.J. Snowdon, Surf. Sci. 269/270 (1992) 213. [81 J.-H. Reehtien, R. Harder, G. Herrmann and K.J. Snowdon, to be published. [9] J.-H. Reehtien, W. Mix and K.J. Snowdon, Surf. Sci. 259 (1991) 26. [lO1 J. Harris. Surf. Sci. 221 (1989) 335. [111 P.K. Johansson, Surf. Sci. 104 (1981) 510. [12] U. Nielsen, D. Halstead, S. HoUoway and J.K. Nerskov, J. Chem. Phys. 93 (1990) 2879. [131 C. Engdahl, B.I. Lundquist, U. Nielsen and J.K. Nlarskov, to be published.
surface s c i e n c e
Orientation and perpendicular energy dependence of dissociative scattering of hydrogen from Cu(111) J.-H. Rechtien, R. H a r d e r , G. H e r r m a n n and K.J. S n o w d o n Fachbereich Physik, Unicersith't Osnabriick, D-4500 Osnabriick, Germany Received 29 October 1991; accepted for publication 28 November 1991
We report measurements of the final molecular axis orientation and kinetic energy released in the dissociative neutralization of 2.98 keV H + incident at glancing angles to a Cu(111) surface. We observe two distinct struclures which we correlate with scattering on two distinct regions of the potential energy hypersurface (PES) describing the interaction. The relative contribution of each of these structures to the total spectrum exhibits a threshold-like behaviour as a function o~" the perpendicular energy of the beam in the (nominal) range 1-2 eV. The perpendicular energy at which this threshold appears is dependent on the molecular axis orientation. We correlate this threshold with a barrier on the PES, and its dependence on orientation with an orientation dependent barrier height. The height variation is of order 0.9 eV, and the barrier is lowest for the surface-parallel axis orientation.
1. Introduction A prerequisite for a detailed understanding of the interaction of a molecule with a surface is a knowledge of the topography of the corresponding potential energy surface (PES), or surfaces, and of the coupling between such svrfaces. The PES of lowest energy corresponding to dissociative adsorption may contain regions corresponding to physisorbed a n d / o r molecularly chemisorbed species, in addition to a region corresponding to the dissociatively chemisorbed state. These regions may be separated by barriers which may lie either above or below the dissociation limit of the initial configuration. The effective height, location and shape of such barriers are expected to be dependent on the "impact site" of
tive adsorption [6]. Despite this intense effort several questions remain unanswered. A critical review of the available data on this system has recently been provided by Michelsen and Auerbach [6]. They point out a distirct paucity of knowledge on the role of rotation in adsorption and desorption dynamics in general, and for this system in particular. They also point out that such effects may be most important at incident beam energies close to the translational threshold required to traverse the activation barrier. In this paper we report measurements of the molecular axis orientation dependence of the dissociative scattering of hyd:ogen from a C u ( l l l ) surface at perpendicular beam energies close to the expected barrier height. Such measurements are expected to probe the dependence of the llUk.tlVU;
rotational and vibrational state of the molecule, and on the molecular axis orientation. While experimental determination of all these dependences is a formidable if not impossible task, state and orientation selective measurements are nevertheless technically feasible [1-5]. The H 2 / C u system is perhaps the mo~';t thoroughly investigated example of activated dissocia-
dLtlVdtlUll
Odlll~l
IIKJI~Ilt
UII
IIlUIU~UUI~I
axis orientation relative to the surface.
2. Technique and apparatus The detection technique we use to determine the final molecular axis orientation is described in detail elsewhere [7,8]. In principle, we measure
1)(139-t'~1)2,~/92/$1)5.1)0 ~i) 1992 - Elsevier Science Publishers B.V. All rights reserved
J. -H. Rechtien et aL ,/Dissociatit'e scattering of hydrogen from Cu (111)
the final laboratory-frame velocities v,,,~b and VZ~.b of the atoms 1 and 2 which result from the dissociative scattering of a diatomic molecule from a surface. We use these measured laboratory' frame velocities to calculate the relative velocity
241
v =v2,~ab - v u ~ t , of the atoms. The direction of the relative velocity vector v is identical with the final orientation of the molecular axis after all interaction between the molecular fragments and the surface has ceased. In addition, we obtain the ,.,
....... Ol = 1.05 °
....,.... . . . . . . . . . . . . . . . . . . . .
j,
r.
(
(.... ~
._
30 ""-"''""b
O~
o
/
r.,..
--
6o
0 30 so 0
2
,~
o[.,~]
90
6 10
-,
i
,'
o
~,[ov]
1"
,
,
,
2
!
v',
,''"I
4
'
%
O"
4
'1'-,--
lO
[ev]
C
2
I""'
8
' - z - _ ........
•-----.
C
"
6
"
~
6
8
"
60
O
I
90
/
"tL-,~
90 -,
10
.
0
2
r .
.
.
c .
4.
, [ev]
I .
i
.
.
6
8
10
,~[ev] 0 oi
flit.__ nlall~f~Ik
flit
1,28 °
Jl
0
°
60
I
L
~r
r'"
"-'-":-
-¢ , - - , : ~, ', '
.............
(":
30
°'°'o' •
I
L::L.....
90
,
6
levi
8
10
0
2
4
6
8
10
[~v]
Fig. 1. Distribution of released kinetic energy e as a function of the final orientation 0 of the molecular axis to the surface normal for dissociative neu,ralization of 2.98 keV H~- at a C u ( l l l ) surface for incident beam perpendicular energies E ± i (to the "macroscopic surface", see text) of 1, 1.25, 1.5, 2 and 4 eV and azimuth & = - 20 ° to the [ti0] direction. The direction of the faster of the two fragments was chosen to lie halfway (in angle) between the surface plane and the maximum of the scattered particle angular distribution. Only dissociation events corresponding to "backward leaning" molecules were used to generate these plots. The detector is partially blind to the region of (e, 0) space near both the e and 0 axes. We have set the yield to zero in the affected region.
J.-H, Rechtien et aL / Dissociativescattering of hydrogenfrom Cu(l l I)
242
o
~
60
~°eld,d " 0
2
- - ' ~ - - ' ~ #
6
8
go
go
10
--
0
2
4
elev]
6
8
10
.Icy] 0
! o
3O
6O
0 eta]
,~[,v]
10
0
2
4
6
8
10
e[ev]
Fig. 1. Continued.
kinetic energy released in the dissociation event from the magnitude of v, viz. e -
½Pv 2,
(1)
where t~ is the reduced mass of the molecule. We represent the data corresponding to true coincidence events as differential distributions dN(e, 0 ) / d e dta, where dr, represents a solid angle element whose size is independent of e and 0. For simplicity we will call these distributions N(e, 0) distributions. The experiments were performed using a 2.98 keV H~ ion beam incident at a grazing angle of incidence O~ to a carefully prepared Cu(111)surface. The zero of the incidence angle scale was determined before each measurement by monitoring the ion beam intensity with a detector placed at a scattering angle of zero degrees while rocking the crystal about nominal zero. This method allows u~ to determine the incidence angle to the "macroscopic surface" (defined by
the twc, "highest points" on that region of the surface exposed to the beam) to an accuracy and reprodueability of ~ 0.05°. The beam divergence is of a similar magnitude (,-0.04* half-cone angle). The relative uncertainty AE j.i,/E J.z in the perpendicular energy E J.i = Ei sin 2 0i of the beam to the surface is A E ~.i/E J.i = 2 AOilO i + A E i / E i,
(2)
where A0~ is a measure of the uncertainty in the incidence angle 0 i and AEi is the uncertainty in the incident beam energy ,El, The second t~rrn in (2) is negligible compared with the first. For E i = 2.98 keV, #i = 1.05" and A9 i = 0.05 °, we find that E l i -- 1 +_0.1 eV. The spread in perpendicular energy is therefore much less than the vibrational spacing in gas phase H 2 ( ~ 0.5 eV). However, the absolute error in ELi referenced to the (111) surface plane (rather than to the above defined "macroscopic surface") may be significant. A polishing error of ~ 0.2° would modify
J.-H. Rechtien et aL / Dissociatice scattering of hydrogen frorn Cu (111)
243 "
-' ..............
the perpendicular energy by ~ 0.3 eV in the above example. The beam was incident at an azimuthal angle 4~ = - 20° to the [1]0] direction. Its rotational and vibrational state population distributions were unknown, however the initial ionization of the beam occurred in a plasma ion source. We will assume an isotropic molecular axis orientation distribution for the incident beam.
3. Results We have measured N(~, 0) distributions (for coincidences between neutral hydrogen atoms) at beam incidence angles 0 i = 0.47, 0.88, 1.05, 1.17, 1.28, 1.48, 1.81, 1.96, 2.09 and 2.96°, corresponding to incident beam perpendicular energies (to the macroscopic surface defined above) of 0.2, 0.7, 1.0, 1.25, 1.5, 2.0, 3.0, 3.5, 4.0 and 8.0 eV. The spectra spanning the range E j_i = 1-2 eV, plus the spectrum obtained a t E_Li = 4 eV are shown in fig. 1.
4. Discussion Above an incident beam perpendicular energy E ±i = 3.5 eV ;he N(e, 0) distribution looks like that shown for E . t . i = 4 eV in fig. 1. It is characterized by an e-distribution peaking at or below 1 eV. As we reduce E ± i, a broad structure peaking at e = 2-3 eV, 0 = 60° superimposes on this distribution. The N(e, 0) distribution is dominated by this broad structure for E J_i -< 1 eV. We will assume in our analysis that the shape of the former distribution is independent of E . over the complete range of our experiment (0.2-8 eV). We know this to be true for the E!~ range 3.5-8 eV and for the (e, 0) range (0-10 eV, 0-15 °) not affected by the broad structure mentioned above. Provided this assumption is valid, we can subtract from all spectra (after suitable normalisation) any spectrum in which the broad structure is absent. We have used the (suitably normalized) spectrum corresponding to E L i = 4 eV (fig. 1) for this "background subtraction". This procedure provides us with a series of spec-
0
i
15
'
i
30
........
~
.
i
45 0 rdeg]
.
.
.
.
.
.
.
.
.
.
i
60
75
90
Fig, 2. Orientation angle dependence of the yield corresponding to the broad structure peaking at e = 2-3 eV, 0 = 600 in fig. 1, as a function of incident beam perpendicular energy (in eV), Note that the absolute shape of the curves is unreliable in the range 0 >i 70° since the detector is partially blind in this region (see caption to fig. 1).
tra of the broad structure alone, which illustrate its incident beam perpendicular energy dependence. Integration of these spectra with respect to the e coordinate provides the orientation angle dependence of the process leading to this structure, as a function of incident beam perpendicular energy. The results are shown in fig. 2. At E!~ = 0.2 eV, the 0 distribution peaks at (or possibly even beyond, see caption to fig. 2) 0---70 °. As E l i is increased to 1 eV, a slight reduction in the yield for orientations beyond 0 = 60° is observed. At E ~ = 1.25 eV, only the yield at orientations exceeding 0 = 30° is strongly suppressed. Only for E i i> 2 eV is the yield at all molecular axis orientations significantly reduced. We summarize the yield variation as a function of E.Li for several values of the orientation angle 0 in fig. 3. We observe efficient neutralization of the incident H + beam during scattering. Simple considerations suggest that at least the b 3y.~ and X ~~g+ states of H 2 should be energetically accessible for charge transfer on the incident trajectory by either resonant or Auger processes. Earlier studies [7,9] have led us (tentatively) to con3 "+ clude that direct charge transfer to the b ~2u state represents at most a minority dissociation channel. We do not at present believe that direct
J.-H. Rechtien et al. / Dissociatire scattering of hydrogen from Cu(l l l)
244
initial occupation of this state is responsible for any of the features seen in fig. 1. Instead we propose that charge transfer occurs first, on the incident trajectory, to a state correlating with the (surface modified) X I Zg..t- state of H 2. If we assume that electronic excitation of both the substrate and molecule during the interaction are insignificant, then it may be a good first approximation to attempt to describe the interaction with the help of the adiabatic PES describing the interaction of ground state H 2 with copper. We have in fact provided such a description in earlier publications [7,8]. We reproduce an approximation to the H 2/Cu PES (due to Harris [10]) in fig. 4. We noted in ref. [7] that the sudden change in the relative yields of the two structures in fig. 1 at E±~--- 1 eV may be correlated with traversal of the activation barrier (of height ~ 0.8 eV) separating the H2-Cu and 2(Cu-H) configurations in fig. 4. The analysis summarized in figs. 2 and 3 lends further support to this interpretation. Indeed, if our interpretation is correct, then figs. 2 and 3 may contain a direct measure of the orientation dependence of the effecive barrier height. The final molecular axis orientation distribution we measure (fig. 2) is influenced by the orientation dependence of both the initial charge 0
"~
1
2
3
,
,
,-
~ 'k~ \ \
>-
• 25-35
°
a 35-45
o
o 55-65 o o 65-,5o 75-85
0
1
2 E, l e V I
3
°
4
Fig. 3. V a r i a t i o n w i t h E , ~ o f the total yield in fig. 2 in the 0 w i n d o w s 2 5 - 3 5 ° , 3 5 - 4 5 ° , 4 5 - 5 5 ° , 5 5 - 6 5 ° , 6 5 _ 7 5 ° and 7 5 - 8 5 ° . T h e lower energy scale is that c o r r e s p o n d i n g to o u r nomina~ incidence angle c a l i b r a t i o n . T h e u p p e r scale is that o b ta in e d
by reducing all incidence angles by 0.2 °. The .:urves have been normalized.
6
5
£ N
2
0
2
3
Fig. 4. Contour plot of the adiabatic ground state potential energy surface proposed by Harris [10] to describe the interaction of molecular hydrogen with copper (z is the distance of the centre-of-mass of the molecule to the surface, x is the H - H separation, the H 2 axis is assumed to lie parallel to the surface, the contours extend from 0.1 to 2 eV in 0.1 eV intervals).
exchange step converting H~- to H 2, and finally of the dissociation process itself. We expect that the 0-dependence of the initial charge exchange step is (to first-order at least) independent of EI~. The observed change in the orientation distribution with EI~ therefore reflects the orientation dependence of the dissociation process. We see little change in these distributions up to E . i - - 1 eV, and for E t i > 3.5 eV. We therefore propose that independent of their orientation, up to E . i = 1 eV most molecules reflect from the repulsive "back-wall" of the H z - C u configuration, and that beyond E 1 ,: -- 2 eV, most molecules reflect from the repulsive "back-wall" of the 2(H-Cu) configuration. The fate of the molect!es in the intermediate regime 1
J. -H. Rechtien et al. / Dissociatit'e scattering of hydrogen from Cu (111)
effective activation barrier height on the H z / C u ( l l l ) PES describing the scattering. Vibrational and rotational excitation in the incident beam will determine both the position and width of the transition region, however the change in the threshold value with 0 (and therefore the 0 dependence of the effective barrier height extracted from it) should be independent of these (presently unknown) rotational and vibrational distribution'~. We infer that the effective barrier height decreases by ~ 0.9 eV with a change in the final molecular axis orientation from 0 = 30 ° (near-perpendicular to the surface) to 0 = 80 ° (near-parallel). The barrier is lowest for nearparallel orientations, in agreement with theoretical expectations [11,12]. An activation barrier height in the entrance channel of more than 1 eV (fig. 3) is not consistent with current experimental understanding of the adiabatic ground state of the H 2 / C u ( l l l ) system [6]. Although we sample a distribution of barrier heights over the full surface unit cell in our scattering experiment, calculations on the C u ( l l l ) surface imply a spread in barrier heights of only 0.2 eV [I3]. This is clearly insufficient to explain the discrepancy. We mentioned earlier that our E L~ scale is very sensitive to our zero calibration of the incidence angle 0~ to the (111) surface plane. We could in principle determine the absolute value of 0, to sufficient accuracy by rotating the crystal 180° and repeating the experiment. The threshold-like behaviour shown in fig. 3 could then be used to extract the absolute value of 0~. This experiment will be performed in the near tuture. The effect on our conclusions of an arbitrarily chosen 0.2 ° reduction in the absolute value of 0 i is shown by the upper energy scale in fig. 3. We conclude that either such a revision is necessary, or that the PES describing the interaction at our (total) beam energy is quite different from the adiabatic ground state PES describing dissociative chemisorption at thermal energies. If the two PES's are indeed identical, then our result should be independent of the incident beam (total) energy. Tt,~s experiment is also currently being performed. We note, however, that electron-hole pair excitation caused by the fast parallel motion of the beam should (to first-order) only
245
shift the adiabatic PES vertically upward, while conserving its topography, and it is this topography and not the zero of the energy scale in fig. 4. to which we are sensitive in our experiment. The relation of the final molecular axis orientation (which we measure) to the actual orientation of the molecular axis at critical points on its trajectory (e.g., at the barrier) is unknown. The collision time, however, can be estimated to be of the order of several tens of picoseconds. This is an order of magnitude less than the (classical) rotation time of H 2. We conclude that the rotational motion is effectively frozen on the timescale of the collision, and that the orientation we measure probably mirrors quite closely the orientation of the molecule throughout its interaction with the surface.
5. Conclusion
We have measured the molecular axis orientation dependence of the dissociative neutralization of 2.98 keV H + inciden~ at glancing ang!es of incidence to a C u ( l l 1) surface. We tentatively correlate the two distinct structures we see with scattering from two distinct electronic configurations of the H 2/Cu system. The relative contribution of these two spectral components to the total yield changes rapidly in the (nominal) incident beam perpendicular energy range 1-2 eV. Analysis reveals that the perpendicular energy at which this change occurs is dependent on the molecular axis orientation. We correlate this dependence with the molecular axis orientation dependence of the effective height of the barrier separating the two electronic configurations. We provisionally infer a barrier height change between the (near) perpendicular and (near) parallel axis orientation of approximately 0.9 eV. The barrier height is lowest for the surfacc-pa~allel axis oricntati¢~n. The t~bserved behaviour is qualitatively consistent with that expected for sc~,ttering from a PES whose topography is similar to that of the adiabatic ground state of the H 2/Cu(l 11) system. We have concentrated in this paper on the change in the shape of the orientation distribution with perpendicular beam energy. A discussion of the
246
J.-H. Rechtien et al. / Dissociative scattering o`f hydrogen from Cu(l l l)
shape of these distributions is presented else-
[31 T.J Curtiss and R.B. Bernstein, Chem. Phys. Lett. 161
where [8].
[41 M.G. Tenner, F.H. Geuzebroek, E.W. Kuipers, A.E.
Acknowledgements We wish to thank D. Auerbach for valuable discussions. This work was generously supported by the Deutsche Forschungsgemeinschaft.
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(1989) 212. Wiskerke, A.W. Kleyn, S. Stoite and A. Namiki, Chem. Phys. Lett. 168 (1990) 45. [51 A. Hodgson, J. Moryl and H. Zhao, Chem. Phys. Lett. 182 (1991) 152. [61 H.A. Michelsen and D.J. Auerbach, J. Chem. Phys. 94 (1991) 7502. [7] J.-H. Rechtien, R. Harder, G. Herrmann, C. R6thig and K.J. Snowdon, Surf. Sci. 269/270 (1992) 213. [81 J.-H. Reehtien, R. Harder, G. Herrmann and K.J. Snowdon, to be published. [9] J.-H. Reehtien, W. Mix and K.J. Snowdon, Surf. Sci. 259 (1991) 26. [lO1 J. Harris. Surf. Sci. 221 (1989) 335. [111 P.K. Johansson, Surf. Sci. 104 (1981) 510. [12] U. Nielsen, D. Halstead, S. HoUoway and J.K. Nerskov, J. Chem. Phys. 93 (1990) 2879. [131 C. Engdahl, B.I. Lundquist, U. Nielsen and J.K. Nlarskov, to be published.