Ro-vibrational excitation, alignment and orientation distributions of fast non-dissociatively scattered molecules

surface s c i e n c e ELSEVIER

Surface Science 392 (1997) 153 162

Ro-vibrational excitation, alignment and orientation distributions of fast non-dissociatively scattered molecules R. Harder, K.J. Snowdon

*

Department of Physics, University ol Newcastle, Newcastle upon Tvne, NE1 7RU, UK Received 20 October 1996: accepted for publication 30 June 1997

Abstract

The ro-vibrational distribution of fast diatomic molecules scattered from an uncorrugated surface under strongly dissipative glancing incidence conditions is calculated. The classical trajectory simulation includes potential surface switching associated with hot-electron scattering processes. Both ro-vibrational excitation and strong alignment of the classical angular momentum vector in the surface plane ("cartwheel motion") are observed, independent of the occurrence of potential surface switching. Ro-vibrational excitation is enhanced strongly by transitions between potential surfaces. The resultant larger proportion of molecules in highly rotationally excited states leads to a higher fraction of cartwheel-aligned molecules in the scattered molecule ensemble. The molecules which dissociate in the simulation are characterised by surface normal peaked internuclear axis orientation distributions. This is in agreement with the results of recent experiments [A. Nesbin et al., Surf. Sci. 331-333 (1995) 321]. We observe, in addition, an enhanced rotational population of "topspin" oriented molecules, which arises from differences in the surface parallel oriented friction forces acting on each atom of the molecule. Glancing incidence scattering from well-prepared close-packed metal surfaces would appear to provide an efficient, general method to obtain a beam of preferentially aligned fast neutral diatomic molecules.
Kevwords: Energy dissipation; Ion-solid interactions, scattering, channeling; Molecular dynamics; Molecule solid scattering and diffraclion- inelastic

I. Introduction

In a series of recent investigations [1 3], we have scattered fast (vth. . . . l<
* Corresponding author. E-mail: [email protected] 0039-6028/97/$17.00 (4; 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 6 0 2 8 ( 9 7 ) 0 0 5 3 9 - 6

[4]. We also found that the orientation distribution of the internuclear co-ordinate of the fragments of dissociatively scattered diatomic molecules is often peaked about the surface normal direction. Noting that the energy transfer appears primarily in the electron-hole-pair channel for such particle velocities [5], we proposed in Ref. [6] a mechanism for dissociative scattering via a dissociation induced by multiple electronic transitions (DIMET)-like [7] process. In this model, transient attachment of excited electrons to negative-ion-like resonances [8,9] of the molecule is assumed to occur. This leads to fluctuating forces on the atoms of the molecule which can induce vibrational excitation

154

R. Harder, K.J. Snowdon / SurJace Science 392 (1997) 153 162

both in the intra-molecular and atom-surface co-ordinates. The results of classical model simulations of this electron attachment process were found to be consistent with our experimental observations on the dissociatively scattered fraction [1-3], provided we choose model potential energy surface (PES) topographies which preclude the molecule from accessing that region of phasespace corresponding to dissociative chemisorption. The purpose of this paper is to examine, within the same multiple PES framework, the rovibrational, alignment and orientation distributions of those molecules which are scattered nondissociatively. There have been several previous studies of ro-vibrational, alignment and orientation distributions at much lower molecule energies, where the dynamics is most likely governed by a single adiabatic ground state PES. Sitz et al. [10, 11] reported a strong alignment of N 2 molecules after scattering hyperthermal energy beams from A g ( l l l ) . They found the final angular momentum vector of N 2 to lie preferentially in the surface plane ("cartwheel" motion). Theoretical investigations of Corey and Alexander [12] and Wolf et al. [13] were able to explain those results and the influence of corrugation on the alignment. More recently, Lahaye et al. [14] investigated the reactive heteronuclear system NO/Pt(111). We provide here a conceptual extension of earlier models by investigating, within a classical framework, the influence on the internal energy and the alignment and orientation distributions of transitions between PESs.

2. Model and simulation technique We have shown in Ref. [1] that for a model to reproduce the detailed experimental findings on the dissociatively scattered portion of all molecules, excluding H2, which we have studied to date (N2, 02, NO, CO, C4HI0, CH), it must contain a mechanism to hinder a significant increase in the internuclear separation of the atoms in the molecule while the molecule is in the vicinity of the turning point of its trajectory. This means that we must devise a general and effective means to transfer energy into the vibrational co-ordinate of

the molecule while simultaneously preventing significant bond stretching during the interaction. However, many of the molecule-surface systems we have studied are characterised by adiabatic ground state PESs with activation barriers to dissociative chemisorption which are small or even negligible in comparison with the incident surface normal energies of our neutral molecular beams ( ~ 1-4 Ev). We showed in Ref. [1] that low or even non-existent barriers to dissociative chemisorption can, nevertheless, be made effectively opaque by assuming partial propagation of the system, in the vicinity of the transition state, on an excited state PES whose topography in that region is attractive in the intramolecular co-ordinate. These considerations have dictated the general properties of the PESs defined below. The sensitivity of the scattering dynamics to the detailed form of the potential surfaces has been discussed elsewhere [ 1]. To influence the scattering dynamics, such excited electronic states must be populated, at least transiently, during the interaction of the molecule with the surface. Our experiments have shown [1] that the fast surface parallel motion of many molecules is strongly dissipative, and that electron-hole pair excitation represents the principle means of translational energy loss in the velocity regime relevant here. The presence of excited electrons of the metal in states above the Fermi level opens the possibility that such "hot" electrons can scatter through broadened, unoccupied negative-ion-like states of the nearby molecule. Studies of electron emission spectra by Kempter and co-workers [15] have shown that such states are indeed efficiently populated in fast molecule-surface interactions. Such electron scattering processes correspond classically to a transient population of the negativeion-like state and to a consequent switch in the molecule-surface interaction potential. The molecule-surface system can thus propagate for a short time on the negative-ionqike PES before returning to the ground state [7,9]. This DIMET-Iike process [7] provides both an efficient means to transfer energy into the molecule vibrational co-ordinate and a way to modify significantly the effective intramolecular forces in the transition state region of the ground state PES.

R. Harder, K.J. Snowdon / SurI'ace Science 392 (1997) 153 162 The simulation technique is described in detail in Ref. [1]. It is similar to the hard rod/hard cube simulations in earlier publications e.g. Ref. [16], except that we have removed the restriction that the internuclear distance in the diatomic molecule remains fixed. Furthermore, we have simulated the effect of inelastic electron scattering through negative-ion-like resonances by repeatedly switching between two laterally uncorrugated PESs during scattering. Each PES has been constructed from the sum of pair potentials U describing the intramolecular ( A - A ) and atom-surface ( A - S ) interactions, viz. PES1 ( z l , z 2 , / ' ) =

2 E i=1

glA-s(Zi)~- gl-A(r)'

(la)

UZAS(Zi)+U2A_A(r),

(lb)

PES 2 6 5

,=

3

2

2

P E S 2 ( z l , z e , r)= ~

155

1

I

I

1

2

3

4

3

4

i=l

where r is the molecule internuclear separation co-ordinate, the z~ are the atom-surface separation co-ordinates of the atoms 1 and 2 of the molecule and the superscripts 1 and 2 signify the different forms of the pair potentials used to generate the lower and upper PESs PES1 and PES2, respectively. We have used Morse functions for U~_s, UIA_A and UZA, and an exponential function for UZ_s. The form of these PESs are illustrated in Fig. 1 for the surface parallel molecular axis orientation and the potential parameters listed in Table 1. The lower PES (PES1) is chosen to represent in a simple way the ground state of a molecularly chemisorbed system. The upper PES (PES2) is chosen to represent a bound negative molecular ion state exhibiting a repulsive interaction in the molecule-surface co-ordinate. The inclusion of a dissociative chemisorption channel in the groundstate PES at large separations r would have no significant effect on the properties of the nondissociatively scattered molecules, as the latter portion of the scattered particle flux does not access such regions of phase-space. The calculations presented in this paper assume average propagation times of 10 fs on the groundstate PES (PES1) and only 1 fs on the negativeion-like PES (PES2). These times are rough estimates from the expected energy width of the molecular state and the density of excited substrate

I __

PES ! 6 5

N

3

2 1

j I

2

r[A] Fig. 1. Contour plots illustrating, for the surface parallel molecular axis orientation, the PESs used for the classical trajectory calculations (zcmis the distance of the molecule centre-of-mass from the surface and r is the internuclear separation in the molecule). The contour separation is 1 Ev. The pair potentials used in the construction of these PESs are defined in Table 1. The shape of these PESs changes in our calculation as the molecular axis orientation changes.

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R. Harder, K.J. Snowdon / Surface Science 392 (1997) 153-162

Table 1 Definition of functions and parameters used in the simulation

velocities using the relations

U~ S(Zl)=DAs{1-exp[-2~,_S(Zl-z~ s)]} 2

j2 Erot = - 21

where i= 1,2, D},_s=2.38 Ev, 2~s =0.75 A ', Z~s = 1.8 A

and

Pair potentials

U J A (r) =/~A A {1 -- exp[-- 2~ A(r-- r~_A)]}2 where D~_A=4.75 Ev, 2~, A= 1.95 ,~-', r~ A = 1.11 A; D~_A=S.Y5Ev, 22 A=2.65A ', r~_A=I.lSA

Evib : lfl

Vr'~

+ Epot (r)-

(4)

The product in the brackets of the first term in Eq. (4) is the projection of the relative velocity vector vr of the two atoms of the assumed homonuclear diatomic molecule A 2 on to the internuclear axis of A 2, where r is the internuclear co-ordinate and /~ is the reduced mass of the molecule. The classical angular momentum vector J and the moment of inertia I of the molecule are defined by

/fA-s(Z/)= U~O,As exp( 22 sZl) where i=1,2, U~o.a_s=50.0Ev, 22s=0.9A. 1 Friction parameters

/~As=0.156 EVA-',/~= 10.0A-', ~,=5.0 h

electrons [3,8]. Furthermore, we have assumed that transitions to the upper state can only occur when the molecule centre of mass lies within 2.5 A of the surface [8]. As mentioned above, the fast surface parallel motion of the molecule excites electron-hole pairs in the substrate, causing translational energy loss. This process is represented in our classical calculation by surface parallel oriented friction forces FA_S(Zi) acting individually on the two atoms of the molecule, viz. FA-S(Zi) = ~A-S [ea(z' - 7) + 1] - 1.

(3)

(2)

The values of the parameters I~A_Sl, /~ and 7 are estimated both from experimental energy loss data and to provide a smooth decrease of FA_s(Z~) to zero at atom-surface separations exceeding ~ 5 A [17]. They are defined in Table 1. The positions and velocities of the two atoms of the non-rigid rotor are calculated at all points and times along the trajectory by numerical solution of the classical equations of motion under the combined influence of the PESs defined by Eqs. (la) and ( l b ) , the surface parallel oriented frictional forces defined by Eq. (2) and random Franck-Condon-like switching between the two PESs. The rotational and vibrational energies, Erot and Ev~b, are calculated from these positions and

J=#'r×pr

(5)

and I=/~" (r2>.

(6)

The mean value of r 2 over all vibrational phases of the molecule is denoted in Eq. (6) by (r2). The vibrational energy is taken to be the sum of the kinetic energy ½1~(v"r/[rl) 2 and the A-A internuclear pair-potential Epot(r). The final rotational and vibrational excitation of A 2 is calculated at or beyond a molecule-surface separation of about l0 A, where the interaction with the surface has essentially ceased and these quantities are timeinvariant. The population distribution P(v, J) of the vibrational and rotational quantum states is calculated from the energies Evib and Erot using the equations [181

Evib =h~o(V + ½)-h%xo(v + ½)2

(7)

and Ero t = B , , J ( J + 1 ),

(8)

with By defined as h2 O~-

-

-

2p(r2)~ "

In Eq. (7), e)0 and Xo are constants derived [18]

R. Harder, K.J. Snowdon / Surlace Science 392 (1997) 153-162

from the parameters of the internuclear Morse potential U~ A of the free molecule. The rotational constant B,, is calculated from ( r 2) for each vibrational state v.

157

--- incident beam - - no transitions - - transitions

--

no transitions I transitions i

3. Results We have performed simulations of surface scattering of vibrationally cold ( v = 0 ) homonuclear diatomic molecules A 2 of assumed mass 28 amu whose initial rotational energy distribution corresponds to a temperature T = 3 7 3 K. This latter distribution, including the statistical weight (2J + 1 ), but divided by any nuclear spin degeneracy, is peaked at an energy 13.8 meV and has a mean value ( E r o t . i n i t i a l ) = 29 mev. An isotropic orientation distribution is assumed for the rotational axis of the incident molecules. The selection of these initial conditions was motivated [19] by our use of a plasma ion source in our earlier dissociative scattering experiments. Assuming an incident translational energy E i = 3 keV and the parameters of Table 1, we find that 90% of all molecules scatter non-dissociatively at an incident beam normal energy E ~ i = 4 Ev. This fraction was found to increase to 95% when the incident beam normal energy was reduced to 0.7 Ev. If we suppress transitions between the two PESs (so that the system propagates exclusively on the electronic ground state), all molecules are found to scatter non-dissociatively. The ro-vibrational energy and state distributions of an ensemble of 4 x 104 non-dissociatively scattered molecules which possessed an incident normal energy E l i = 0 . 7 E v are reproduced in Figs. 2 and 3. For the potential parameters we have used, the highest lying bound vibrational state is v =64. In the absence of electronic transitions, we see that the rotational energy spectrum is peaked at Erot, r ~ 0.02 Ev with a small tail extending to Erot,f~0-6Ev and a mean value (Erot,f)=0.061 Ev. This represents considerable rotational excitation of the incident beam ((Erot.i) =0.029 Ev). When we include in the simulation the transitions between PESs, the contribution to the spectrum of states with E r o t , f < 0 . 1 Ev

0.2

0.4

0.6

0.8

1.0

0.2

Erot.f[eV]

0.4

0.6

0.8

1.0

E .ib,f [eV]

Fig. 2. Calculated rotational Erot.f and vibrational Evibx energy distributions of non-dissociatively scattered molecules for an incident beam energy E i = 3 k e V and normal energy E±~-0.7 Ev. Franck-Condon transitions (solid line) lead to the high energy tail in both distributions extending well beyond 1 Ev. The curves are normalised to the same integrated area under the distribution.

---

incident beam

- "---q

- -

0

30

.... no ---

- no transitions

,i.

transltlons

transitions

60 J

90

120

10

20

30

sU J

40

v

Fig. 3. Calculated rotational P(J) (left) and vibrational P(v) (right) state population distributions of non-dissociatively scattered molecules for an incident beam energy E~=3 key and normal energy E±i = 0.7 Ev. The highest vibrational state below the dissociation limit occurs at v = 64. The curves are normalised to the same integrated area under the distribution.

is depressed, whereas the tail at higher rotational energies is enhanced. This tail extends to rotational energies considerably in excess of 1 Ev, leading to a mean rotational energy (Erot,f)= 0.38 Ev. In the absence of transitions the vibrational energy spectrum is peaked at zero energy and decreases to zero intensity at E, ib,f~0.7 Ev, with a mean value ( E v i b , f ) = 0 . 1 2 Ev. If we include transitions

158

R. Harder, K.J. Snowdon / SurJkwe Science 392 (1997) 153-162

this spectrum is significantly broadened, exhibiting a tail which extends beyond the dissociation limit. The mean vibrational energy in this latter case is =0.93Ev. We observe a small positive correlation between rotational and vibrational excitations. This correlation is independent of whether or not we include transitions in the calculation.

4. Alignment and orientation

@Q no transitions

transitions

E±,= 4 eV

8 0°

E±i= 0.7 eV

The distributions of the polar and azimuth angles 0 and ~ (Fig. 4) between the surface normal n and the angular momentum vector J are plotted in Fig. 5a,b for ensembles of 1 x 105 and 4 x 104 molecules and incident molecule normal energies E_u=4 Ev and 0.7 Ev, respectively. The fine structure of these curves is statistical in origin. Alignment and orientation are more conventionally described in terms of multipole moments A~. Alignment describes the extent to which the angular momentum vector J is directed along a given axis, whereas orientation is a measure of the sense

0= 180°

(a) transitions

no transitions

q~= 90~

1VOW@+-"°

E±i=0.7eV

d

= 270°

(b)

/

Y

Fig. 5. Probability of occurrence of (a) the polar angle 0 and (b) the azimuth angle ~ (plotted for constant sin 0 dO and &b, respectively) of the angular m o m e n t a J = (J, 0, ~b)of an ensemble of non-dissociatively scattered molecules for an incident beam energy E l = 3 kev. All distributions are normalised to their maxima. The angular scales are defined on the lower right-hand plots. The peaks at 0 = 9 0 ° in (a) imply that m a n y molecules leave the surface exhibiting a preferential "cartwheel-like"

motion.

h\\\\\\\\\\\\\\\\W/ X Fig. 4. The alignment and orientation of the classical angular m o m e n t u m vector J of the scattered molecules are defined in terms of the polar angle 0 between J and the surface normal n and by the azimuth angle 4~. The surface parallel motion of the incident beam is in the positive x-direction. The azimuth angles ~ = 90 ° and ~ = 270 ° represent "topspin" and "backspin" rotation respectively.

of J [10]. In the classical limit, the moments A~ are, apart from a normalisation factor, the spherical harmonics Y~(0, ~b) representing the expansion of J [20, 21 ]. We have used the same normalisation as Sitz et al. [10,11]. The even moments A~ (k even) describe the alignment of the molecule. The quadrupole and hexadecapole alignment moments A~ and A4 are

159

R. Harder, K.J. Snowdon /' Sur[ace Science 392 (1997) 153 162

defined as A2(j)=

~

4

A20(J) 1

(3 cos 2 0 - 1),

(9a) >.;

1 1 A4(j) = --~ (3 - 30 cos z 0 + 35 cos 4 0), NAs 8 As

A o (J)

°iI!!it;

LLI4 ¢n

(9b) where N~s is the number of angular m o m e n t u m values binned into the quantum state J of width AJ. These moments are zero for isotropic distributions. The values A g = - 1 and A 4 = +0.375 correspond to perfect alignment of J parallel to the surface plane (cartwheel motion). The values A 2 = 2 and A 4 = l correspond to helicopter rotation of the molecules. The odd moments A~ (k odd) describe the orientation of the molecules. The dipole and octopole moments A I - and A3_, which describe the orientation along the y-axis defined in Fig. 4, are defined as A]_(J)= ~

1

1

~ (sin 0 cos ~b),

(lOa)

3

A 3 _ ( j ) _ N~s. 5 ~As( 5 cos 2 0 - 1 ) sin 0 cos ¢.

(10b) If J were oriented along the positive y-direction ("topspin" orientation) A]_ and A~ would become + 1 and - 0 . 6 , respectively. The symmetry properties of the 0 and ~bdistributions reproduced in Fig. 5 ensure that all moments which do not depend on sin(~b) or on an even power of cos(0) are zero [20,21]. The moments defined above can illustrate all alignment and orientation effects arising within our model calculation, despite the fact that non-zero higher-order moments can occur. The dependence on J of all four moments defined above has been determined for the ensembles used to generate Fig. 5 and the results are plotted in Figs. 6 and 7. A clear correlation between rotational excitation and cartwheel-type alignment is seen. We see from Fig. 6 that the features at 0 ~ 90 ° in Fig. 5a, which lead to alignment moments A 0 2 ~ - 1 and A 4 ~ + 0 . 3 7 5

o

1

>=o ~ ,

,::5

,

.

•

,

.

.

,

,

.

ajg 1 c

0 -1 0

30

60

J

90

120

0

30

60

90

120

J

Fig. 6. Calculated distributions of the quadrupote AZo(J) and hexadecapole A4o(J) alignment moments of non-dissociatively scattered molecules. Every data point shows the mean value for all moleculeswith J-states inside a range of AJ= 10. The error bars denote the standard deviation of the moments of these molecules. The extreme values (0 in the case of an isotropic distribution of J in space and - 1 (+0.375) for A2 (Ao4) in the case that all J lie parallel to the surface plane) are indicated by dashed lines. (Eqs. (9a) and (9b)) arise primarily from molecules with significant rotational excitation ( J > 3 0 ) . Those molecules which possess little rotational excitation ( J < 3 0 in Fig. 6) contribute to the remaining much broader features of the 0-distribution. The occurrence of electronic transitions does not significantly influence the alignment distributions (Fig. 6). However, at low molecule surface normal energies, electronic transitions do increase the fraction of highly rotationally, and therefore cartwheel-aligned, molecules. There is a very slight tendency for the molecules which experience significant rotational excitation ( J > 2 0 ) to possess topspin orientation.

5. Discussion

Our simulations demonstrate how energy transfer into the internal mechanical degrees of

R. Harder, K.J. Snowdon / SurJace Science 392 (1997) 153-162

160

A~_(J)

A I.(J) 1

0,6

=2 o

o

>~ ~ ~.~ ,T

-o.~

uJ o~ 0.5

0.3

:~

o

o

-o.s

-o3

0.5

0.3

"~

0 . . . . . . . . . . . . .

> o-0.5 r--* d

0.3

" tttttt}tttt 0

O -0.3

,.u ~ o.s I T o "~

it-t-t-tt-tttt+t

.

.

.

.

.

.

0

.~.5

-1

30

60

90

120 -0.6

30

60

90

120

J

Fig. 7. Calculated distributions of the dipole A~ (J) and octopole A~_(J) orientation moments of non-dissociatively scattered molecules. Every data point shows the mean value for all molecules with J-states inside a range of AJ= 10. The error bars denote the standard deviation of the moments of these molecules. The extreme values are 0 in the case of an isotropic distribution of J in space and + 1 ( - 1) for A~_ and -0.6 (+0.6) for A3 for the case that all Jare oriented parallel (antiparallel) to the y-axis. freedom of the molecule is influenced by D I M E T like " h o t " electron scattering processes. Let us neglect for a moment the influence of the surface-parallel-directed friction force. In this approximation, in the absence of electronic transitions, and provided the molecules are scattered on a ground-state PES with a topography like PES1 in Fig. 1, only the m o m e n t u m components associated with the motion of the atoms normal to an uncorrugated surface can be effective in exciting the internal mechanical degrees of freedom of the molecules. We have shown in Figs. 2 and 3 that in the absence of electronic transitions, rotational and vibrational excitation of the incident molecules does occur. However, under the conditions of our simulation, the rotational and vibrational energy gained by the scattered molecules does not then exceed the incident beam normal energy. The inclusion of transitions between PESs provides a simple and efficient means to transfer

energy from the translational motion of the molecule to its internal mechanical degrees of freedom (Figs. 2 and 3). Some molecules gain sufficient internal energy via this process to scatter dissociatively. The remaining fraction leaves the surface with a broad vibrational population distribution. In our simulation, the transfer of energy to the rotational degree of freedom serves only to increase the surface parallel component of the angular m o m e n t u m vector. This occurs because the change in angular m o m e n t u m results alone from the action of surface-normal-directed force components exerted on the atoms of the molecule. This leads naturally to the observed "cartwheel motion" alignment of the rotational axis (Fig. 5a). The alignment becomes stronger the higher the rotational excitation, and it is nearly perfect for final J-states higher than J ~ 7 0 (Fig. 6). This strong effect is a consequence of our choice of laterally uncorrugated PESs, which we originally chose because of the relative insensitivity of our dissociative scattering experiments to the surface corrugation [2]. Measurements by Sitz et al. [10] of the internal state distributions and the alignment moments A02 and A 4 of scattered N 2 molecules incident with hyperthermal energies on A g ( l 11) have shown a preferred "cartwheel motion" alignment. Classical [13] and quantum mechanical [12] calculations investigating the influence of corrugation have reported a suppression of this alignment effect with increasing corrugation. When transitions between PESs are included, our calculation demonstrates that the fraction of molecules possessing alignment of the angular m o m e n t u m vector parallel to the surface plane is enhanced strongly, especially if the surface normal m o m e n t u m of the incident molecule is small. The alignment effect is not a direct consequence of the electronic transitions themselves, but of the increase in rotational excitation accompanying this efficient translational-to-internal energy transfer channel. There is little connection between the alignment effect reported in this paper and the molecular axis orientation effect we have seen for dissociatively scattered diatomic molecules [ 1-3], where the fragment internuclear co-ordinate is often strongly oriented along the surface normal direction. For

R. Harder. K.J. Snowdon / SutT[itceScience 392 (1997) 153- 162

non-dissociatively scattered molecules, the component of momentum transfer directed along the internuclear co-ordinate r of the molecule plays no role in aligning the final angular momentum vector. In contrast, for dissociatively scattered molecules it is the dominance of the difference in the surface normal momentum transfer to the two atoms over the difference in the surface parallel momentum transfer to the two atoms which causes the dominating surface normal component - and, therefore, the surface normal orientation of the internuclear co-ordinate. The influence of surface-parallel-directed friction forces, in particular of small differences in the friction forces acting on the atoms of the molecule, is to cause the slight propensity of topspin (~b= 90 ) orientations in the angular momentum distribution (Fig. 5b), leading to small positive values of the orientation moment AI (Fig. 7). The resultant torque exerted on the molecule increases the magnitude of the angular momentum in the direction q~=90. This effect is accompanied by rotational excitation of the molecule, in addition to that discussed in the preceding paragraphs. Sitz et al. [ 11 ] have measured the orientation moments A I and A 3_ for the Nz/Ag( 1 11 ) system at hyperthermal incident INI2 energies. They observed "topspin" orientations for J > 15 which they explained by a frictional hard-cube hard-ellipsoid model, The origin of the slight orientational anisotropy in Figs. 5b and 7 is the same as the one we have seen experimentally and in simulations of dissociative scattering of molecules such as H2, which possess an unusually high translational energy loss along their trajectory in the near-surface region [6,17]. There we find after dissociative scattering that the "backward leaning" orientation of the internuclear co-ordinate is preferred over the "tbrward leaning" orientation. In each case the effects are caused by a significant difference, during the scattering process, in the surface-parallel-oriented forces acting on each atom of the molecule.

6. Conclusions

We have investigated ro-vibrational excitation in a classical simulation of the scattering of fast

161

diatomic molecules from a surface under glancing incidence conditions. We find that the transfer of translational energy to internal mechanical degrees of freedom of the molecule can be enhanced strongly by electron scattering processes, provided the latter are accompanied by transitions between PESs of the molecule-surface system possessing differing topographies. We confirm that rotational excitation is accompanied by a propensity toward alignment of the molecular rotational axis in the plane of the surface ("cartwheel motion"), and show that this effect is enhanced by such electron scattering processes, In addition to the ~'cartwheellike" motion of those scattered molecules which are rotationally excited, we see a small preponderance of "'topspin" orientations over "backspin" orientations. This orientational anisotropy is caused by differences in the surface-parallel-oriented friction forces which act on the constituent atoms of the |:ast moving molecules. It would be interesting to test the predictions we have made by measuring directly, using laser spectroscopy [20,21], the ro-vibrational distributions P(J,v) and the alignment (A 2, A~) and orientation (A~_, A3_) moments of fast nondissociatively scattered molecules. Experimental verification would also provide further support of the model we have proposed to describe the dynamics of last molecule surface interactions under glancing incidence conditions. Finally, we note that glancing incidence scattering from well-prepared close-packed metal surfaces would appear to provide an efficient, general method to obtain a beam of preferentially aligned, fast neutral diatomic molecules.

References Ill K.J. Snowdon, R. Harder, A. Nesbitt. Surf. Sci. 363

I 1996) 42. [2] A. Nesbitt, R. Harder, A. Golichowski,G. tterrmann, K.J. Snowdon, Surf. Sci. 307-309 (1994) 147. [3] A. Nesbitt, R. Harder, K.J. Snowdon, Surf. Sci. 331 333 (1995) 321. [4] K.J. Snowdon, Commun. At. Mol. Phys. 30 (1994) 93. [5] A. Bilic, B. Gumhalter, W. Mix, A. Golichowski, S. Tzanev. K.J. Snowdon, Surf. Sci. 307309 (1994) 165.

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R. Harder, K.J. Snowdon / SurJace Science 392 (1997) 153-162

[6] R. Harder, A. Nesbitt, A. Golichowski, G. Herrmann, K.J. Snowdon, Surf. Sci. 316 (1994) 47. [7] J.A. Prybyla, T.F. Heinz, J.A. Misewich, M.M.T. Loy, J.H. Glownia, Phys. Rev. Lett. 64 (1990) 1537. [8] J. Los, J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. [9] J.W. Gadzuk, Phys. Rev. B 44 ( 1991 ) 13466. [10] G.O. Sitz, A.C. Kummel, R.N. Zare, J. Chem. Phys. 89 (1988) 2558. [11] G.O. Sitz, A.C. Kummel, R.N. Zare, J. Chem. Phys. 89 (1988) 2572. [12] G.C. Corey, M.H. Alexander, J. Chem. Phys. 87 (1987) 4937. [13] R.J. Wolf, D.C. Collins, Jr., H.R. Mayne, Chem. Phys. Lett. 119 (1985) 533. [14] R.J.W.E. Lahaye, S. StoRe, S. Holloway, A.W. Kleyn, J. Chem. Phys. 104 (1996) 8301.

[15] H. MUller, D. Gador, V. Kempter, Surf. Sci. 338 (1995) 313. [16] J.C. Polanyi, R.J. Wolf, Ber. Bunsenges. Phys. Chem. 86 (1982) 356. [17] R. Harder, A. Nesbitt, G. Herrmann, K. Tellioglu, J.-H. Rechtien, K.J. Snowdon, Surf. Sci. 316 (1994) 63. [18] G. Herzberg, Molecular Spectra and Molecular Structure, Vol. I, Spectra of Diatomic Molecules, Van Nostrand, New York, 1967. [19] J.-H. Rechtien, R. Harder, G. Herrmann, A. Nesbitt, K. Tellioglu, K.J. Snowdon, Surf. Sci. 282 (1993) 137. [20] A.C. Kummel, G.O. Sitz, R.N. Zare, J. Chem. Phys. 85 (1986) 6874. [21] A.C. Kummel, G.O. Sitz, R.N. Zare, J. Chem. Phys. 88 (1988) 67O7.