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Orientation dependence of rotational excitation in no scattering from Ag(111)
Volume 168, number 1
CHEMICAL PHYSICS LETTERS
ORIENTATION DEPENDENCE OF ROTATION& IN NO SCATTERING FROM Ag( 111)
20 April 1990
EXCITATION
Manfred G. TENNER, Frank H, GEUZEBROEK, Edgar W. KUIPERS ’ Arjan E. WISKERKE, Aart W. IUEYN FOM-Institute for Atomicand MolecularPhysics.Kruislaan40 7, 1098 SJAmsterdam, The Netherlands
Steven STOLTE Laser F&&ties of theDepanmentof Chemistryof theFree University, DeBoelelaan 1083, log1 HVAtnsterdam,The Netherlands
and Akira NAMIIU Departmentof Electricaland ElectronicEngineering. ToyohashiUniversityofTechnology, Tempaku-cho,Toyohashi440, Japan Received 19 January 1990; in final form 1 February 1990
Scattering experiments with oriented NO beams on Ag( 111)have been performed. The scattered molecules are detected by a resonantly enhanced muttiphoton ionization. A dependence of the rotational rainbow upon the scattering angle has been observed. The first results of the steric effect measured for single rotational (J) levels of scattered molecules are presented. The J= 18.5 state is produced preferentially when the O-end collides first with the surface. For the J= 8.5 state we find that the Ndown geometry is preferable. These results are in qualitative agreement with theory.
1. Introduction The rotation-energy distribution of molecules directly scattered from a surface gives detailed information about the dynamics of the molecule-surface interaction. Such experimental results can serve as a stringent test for dynamical scattering calculations and the potential energy surfaces (PES) used therein. For several molecule-surface systems the rotational distributions for direct inelastic scattering show clear deviations from a thermal Boltzmann distribution at high J levels [l-3]. The rotation of the scattered molecule is generated by the dependence of the PES upon the orientation of the molecular axis. A rotational rainbow is caused by an extremum in the dependence of J on the initial molecular orientation. ’ Present address: Department of Physical Chemistry, University of Cambridge, Cambridge Cl32 1EP, UK.
The observed deviation from a thermal distribution for high J states has been explained as a manifestation of such a rotational rainbow [ 1,4-61. Angular distributions measured with rotationalstate-specific detection techniques reveal a connection between the final rotational state Yand the scattering angle 0,. For NO and Nz scattered from Ag( 1 I1 ) the scattering angle with maximal intensity, @,, is observed to shift with increasing J more towards the surface [ 1,2,7]. This, and the rotational rainbow picture suggest that there is a conaection between the reflection angle and the orientation of the molecule just before the collision. The similar behavior of the rotational excitation spectra and the angular distribution of scattered intensity observed for both N2 and NO suggests a similar anisotropy in the interaction potential.’ However, for the homonuclear N2 the PES will be symmetric with respect to a plane through the center
0009-26 14/90/S 03.50 Q Elsevier Science Publishers B.V. (North-Holland )
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of mass of the molecule and parallel to the surface. But for molecules consisting of two different atoms, we cannot exclude that the two ends of the molecule will act in a different way. In that case, for NO scattering different results may be obtained depending on which end of the molecule collides first with the surface. In previous experiments, the orientational dependence of the NO surface interaction has been tested directly by orienting the NO molecule before the collision with either its N atom or its 0 atom pointing preferentially towards the surface, For both orientations we measured the angular distribution of the scattered intensities from Ag ( 111) and Pt ( 111) with a quadrupole mass spectrometer (QMS) [ 8-101. The angular distributions observed show a steric effect: the distribution measured for a oriented beam with the O-end preferentially in front was shifted towards the surface with respect to that of the N-end. From these observations, we concluded that there is an orientational dependence for NO scattering which cannot be present for Nz. Different PESs have been proposed for NO on Ag. In several studies the rotational distributions computed using these PESs have been compared with experimental data. Voges and Schinke [ 51 proposed a two-dimensional potential containing both cosine and cosine-cubed terms for the orientational dependence of the repulsive part. This potential yields a different maximal torque for the two ends of the molecule which leads to two rotational rainbows with different J. The translational-energy dependence of the position of the high J rainbow measured by Kleyn et al. [ 11 could essentially be reproduced by quanta1 calculations using this PES. Calculations for initially oriented NO molecules by Holloway and Halstead, with the potential of Voges and Schinlce show explicitly a difference in rotational excitation for the two ends [ 111. The potential used has an attractive well depth of 0.078 eV which is too small compared to the experimental value of 0.2 eV [ 121. Another potential is given by Muhlhausen et al. [ 13 1, which is a multi-dimensional one and includes the interaction with a number of surface atoms. The anisotropy of this potential is mainly introduced by a strong attraction for the N atom, causing an anisotropy in the repulsion as well. In their classical trajectory calculations Muhlhausen et al. [ 131 re46
20 April 1990
produce the experimental rotational distributions and their initial translational-energy dependence. The high J states are populated mainly by O-end orientations. Quantum calculations performed by Corey and Lcmoine [ 141 for essentially the same potential but assuming oriented NO molecules confirmed the higher rotational excitation for the O-end. In contrast to the potential of Voges and Schinke [ 51 the PES introduced by Muhlhausen [ 131 and used by Corey and Lemoine [ 141 possesses an attractive well which is too deep (>0.58 eV). This was corrected in later calculations, for which no rotational excitation spectra have been shown [ 7 1. The Voges and Schinke potential has a different anisotropy for the two ends of the molecule. From the analysis of our steric effect measurements Kuipers et al. concluded that the O-end corresponds to the most anisotropic end of the molecule [ 15,161. Thus both potentials have in common that they predict larger rotational excitation of the O-end orientation of NO on Ag( 111). Most theoretical studies on the steric effect in rotational excitation assume normal incidence and reflection along the normal. Due to the physical size of our detectors blocking the incident beam we cannot perform experiments for this geometry. Nor could be measure up to now rotational energy distributions. Instead only the steric effect of angular intensity distributions has been measured. Within a cube model the connection between the rotational excitation and the scattering angle S, is straightforward. As long as the angular distribution of the directly scattered molecules is lobular around the specular angle the cube model is more or less valid and calculated rotational rainbows will be transformed in angular rainbows. The broadening of the angular distributions is also caused by the interaction with phonons, corrugation, multiple bounces and energy transfer to the rotational degree of freedom. For low “normal” initial translational energy E"iO.2 eV no rotational rainbows could be observed experimentally [ 171. The rainbows are probably washed out by the broadening processes. However there may exist a connection between the final J state and the reflection angle which can be studied experimentally. To verify the theoretical predictions of the steric effects for the final rotational state distribution, we present the first results of experiments with a beam of oriented NO molecules scattering of
Volume 168, number 1
CHEMICAL PHYSICSLETTERS
Ag( 111) measuring the rotational-energy distribution of the scattered molecules with a resonantly enhanced multiphoton ionization (REMPI) detector.
2. Experimental The experimental set-up of the molecular beam source and the UHV system with the surface sample and the QMS detector is described in detail in refs. [ 18,191. Briefly, by use of an electrostatic hexapole deflector a pulsed NO beam is state selected in the 2II,,2 (J~0.5, 51~0.5, M~0.5) state and subsequently oriented in a uniform electric field applied just in front of the surface sample, which is mounted on a two-axis goniometer in our UHV scattering chamber. In previous experiments we detected the scattered molecules with a total density detector (the QMS). In the present experiments we used a detector yielding NO ions produced by photo ionization, which will be described in detail elsewhere [ 201. NO ions are formed by ( 1 + 1) REMPI of NO molecules via the A’Z(v=O, J’)tX21&,~(v=0, J’) transitions in the wavelength region around 225 nm. It turned out that the output signal of the detector was initially affected by fringing fields of the strong “orientation” field of 15 kV/cm and that very careful screening was necessary. Angular scans are made by rotating the REMPI detector around the surface sample, which can be done independently of the also rotatable differentially pumped QMS. The REMPI signal 1,, is corrected for the rotational-transition strength S,., for the AtX transition. When ln{lJ,JISJ,( W+ 1) J} is plotted as function of rotational energy (the so-called Boltzmann plot ), one expects a straight line for a thermal distribution. From its slope the rotational temperature ;r, can be calculated. Indeed, spectra obtained for 10m9mbar NO background gas yield a straight line with rotational temperature T,= 300f 30 K.
20April1990
EizO.44 eV. For the incoming angle @i= 15”, the “normal” energy En= Ei COS2@=0.41 eV is comparable to the E,, for which Kleyn et al. [ 1] observed clearly rotational rainbows. For our experiment with S,= 15”) in fig. 1 is shown the Boltzmann plot for the QI,tP2, branch (squares) and the Qll tRll branch (triangles) of the lower spin-orbit manifold. Due to the linewidth of the excitation laser (0.2 cm-’ ) some lines could not be separated from lines of other transitions and have been omitted. Good agreement is found with the data of ref. [ 1 ] for similar conditions, plotted as a solid line in the same figure. A clear deviation from a straight line for J> 20.5 indicative of a rotational rainbow is found in both data sets. The fall-off for lower Jstates gives for both experiments a rotational temperature T,= 300 + 30 K. The same data are plotted in fig. 2, but nor corrected for the 2Jt 1 degeneracy. Although there is some noise in the data, for J> 15.5 no further decrease occurs which is indicative of rotational rainbow scattering. In previous rotationally unresolved experiments the steric effect showed around the specular angle a rotational auantum number J
eiir+Te
iniernaiener~y ( eV )
3. Results and discussion The first goal of our experiments was to reproduce the rotational rainbow as found by Kleyn et al. [ 1 ] _ The fastest beam of oriented NO molecules we could produce in our apparatus has an initial energy of
Fig. 1. Rotational-state distribution for scattered NO molecules presented in a Boltzmann plot as a function of internal energy (bottom) or rotational quantum number J (top). The squares are the populations derived from the Q,, +P2, transitions of the Z+C%,,~ band and the triangles from the Q2, +R,, transitions of the same band. The scattering conditions are EizO.44 eV, Bi=~,=15”.Thesolidlineisafittothedataofref. [il.
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CHEMICAL PHYSICS LETTERS
20 April 1990
rotational quantum number J 10.5 I”
5.5
10.5
15.5
20.5
20.5
’
I
I
30.5
I
25.5
rotational quantum number J Fig. 2. Rotational-state distribution for scattered NO molecules presented in a linear plot as a function of rotational quantum number J. Same data and symbols used as in fig. 1.
linear dependence on the reflection angle 6, [ 8- 101. For the scattering conditions of Ei=0,44 eV and @i= 45 ’ we have measured the rotational distribution for a subspecular angle (&=30”) and a supraspecular angle ( E+ 55’ ) . These measurements were not feasible for @i= 15 ‘. However, for 8, = 45 ‘, E”zO.24 eV suff%zesto show an onset of the rotational rainbow [ 11, In our experiments we observe a small relative increase for I> 20.5 for the supraspecular reflection, squares in fig. 3, but no significant increase for the subspecular scattering, triangles in fig. 3. For this 0, the intensity for Er>0.12 eV was below our detection limit. The small increase compared to the linear dependence shows the onset of a rotational rainbow which evidently depends on @,. A similar behavior for the rotational rainbow of the reflection angle for N,/Ag( 1 I 1) was found by Sitz et al. [ 21. They observed much sharper features, possibly induced by their much lower surface temperature T,=90K compared to T,=575K in our experiment. The main goal of our investigations was to measure for the same scattering conditions the scattered rotational-state distribution for preferentially oriented NO molecules. However, the statistical accuracy of the datapoints such as shown in fig. 3 is about 20% which is consequently too low to detect steric effects on the order of a few percent as measured with the QMS detector. In contrast, the paper of Holloway and Halstead [ 111 predicts a steric effect which 48
Fig. 3. Rotational-state distribution of Q,, + P1, transitions of the C++-2111,2band of scattered NO molecules presented as a Boltzmann plot as a function of internal energy (bottom) or rotationaI quantum number J (top). The scattering conditions are &=0.44 eV, &=45’ and @,=30” (triangles) and 0,=5S0 (squares).
is, at least for the high J states, so large that we certainly should be able to detect it. However, they performed strictly two-dimensional calculations in which no account is taken of the full spatial distribution of the molecules as present in the real experiment. Therefore these calculations may underestimate the contribution of the more broadside orientations but the effect is rather small [21]. Moreover these calculations are done at a value of E,, twice the E,, used in our experiments and do not include surface motions. In order to suppress the statistical error, we improved the REMPE detector and took longer measuring times. In the first instance we put all our efforts into a limited set of final rotational states of the Q11+Pzl branch and one reflection angle. We changed the initial beam energy to Ei=0.10 eV. At this energy the hexapole focuser yields a maximum intensity of state-selected molecules. In spite of the low collision energy and the absence of rotational rainbows we observed steric effects in experiments with oriented beams measuring the scattered mole-
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CHEMICAL PHYSICS LETTERS
cules with a QMS [8,9]. We define RJfor a single rotational state J in our usual way as R
.I=
Z(Z,;,-I$,) zsJ+Isf,
’
I
I
I
cu
d
’P
P
&? d
d
9
!I------
G-7
4.5
Table 1 The values of R,, (&), n,/%., and (E,) for the observedrotational quantum number J, measured for @=45”, 8,=60’, Er=O. 1eV and T,=575 K J
with ZJ,~denoting the intensity obtained for the laser wavelength tuned to the transition of J to J’ for the preferential oriented beam with the O-end pointing towards the surface, and ZsJ with that .for the N-end towards the surface. For these experiments we have chosen 8i=45” and T,= 575 K and set the detector to &=60’. For this angle the steric effect for QMS measurements, which averages over all final J states, give R =0.05, i.e. a larger intensity for the O-end orientation. We did not observe a rotational rainbow as was expected from the literature [ 1,171. The R, values for the J states measured are plotted in fig. 4 and listed in table 1. The lowest J values have been probed via the bandhead, to which J= 1.5-4.5 contribute all nearly equally. In this ligure the value of R for the bandhead is plotted at 3.5 on the horizontal axis. Also given in table 1 are the rotational and the final translational energy. For the latter the same value for all J states is taken [22] as is confirmed experimentally by Kimman et al. [ 71. The sum of rotation and translation is always larger than the initial translational energy, so energy transfer from the surface motion is needed. For J= 18.5 we found R,=0.26 which is significantly larger than the value of the rotationally unresolved measurements. Indeed, this indicates that the larger J states originates relatively more from the O-end orientations as can be concluded from the
8.5
------h. 10.5
16.5
rotational quantum number J
Fig. 4. Steric effect R,as function of rotational quantum number J. TheR,for the bandbead is plotted atJz3.5. 8,=45”, @,=60°, Er=O.l eVand T,=575 K.
20 April 1990
RI
(J&I>
nJ/nO.s
(W
2.5-4.5 a.5 11.5 15.5 18.5 total (QMS)
+ 0.018 f 0,020 -0.0241tO.017 +0.024f0.024 +0.166*0.035 +0.263 eO.032 +0.050f0.01
0.003 0.017 0.030 0.054 0.076 -
6%) (W
1.0 11.6 10.6 7.9 5.4 -
0.12 0.12 0.12 0.12 0.12 0.12
positive sign of the effect. Interestingly, R, does not increase monotonically as a function of J as can be seen in fig. 4. The value for J= 8.5 is the lowest and even below zero, indicating that this rotational state is preferentially populated in collisions with the Nend in front. The rotational distribution can be fitted with a Boltzmann distribution with T,= 450 K. The ratio of nJ, the number of molecules in rotational level J, divided by no.5,the number in level J=OS for that distribution is given in table 1. Weighting the steric effect RJ with nJ/Cn,, the sum running over all detected values of J in table 1, one finds that the averaged steric effect is about (R) = 0.07 _+0.0 15. This value agrees quite well and within the error with the steric effect measured with the QMS, R,,,=0.05 ?O.Ol. For the single supraspecular reflection reported in this Letter, the observed RJ values are in qualitative agreement with the theoretical predictions. For this scattering geometry the highest rotational states originate as expected from the O-end of the NO molecule. The negative sign of R/ for J=8.5 can be interpreted as the rotational rainbow for the N-end orientation as predicted by the potential of Voges and Schinke [ 15,16 3. Although the two-rainbow picture can be reconciled with the data, the absolute values of R, are far too large. The effect calculated by Holloway and Halstead [ 111 is much larger, also for the highest experimental value, J= 18.5. The high rotational energy for the J= 18.5 state cannot be reached in this experiment without energy transfer from the heat bath of the surface. In the Voges and Schinke [ 51 calculations the motion of the surface is not included and this J state would lie above the classical rainbow value associated with the initial beam energy of Ej= 0.1 eV. One notes as a function of J that 49
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CHEMICAL
PHYSICS
only at its minimum (J= 8.5), is R, slightly negative. The Voges and Schinke calculations would predict a singular intensity around J= 8.5 which would result in a very large negative steric effect. Because of many averaging processes and the fact that three orientations classically give rise to J= 8.5, the N-end rotational rainbow is heavily diluted and the corresponding steric effect is only slightly negative. Preliminary measurements for other reflection angles show a similar angular dependence of RJ as for the unresolved experiments. This indicates that other processes like phonon excitation or multiple collisions are important and will have steric effects as well. 4. Conclusions We have successfully observed steric effects in the rotational excitation of NO molecules scattering from an Ag ( 111) surface with our new REMPI detector. With this detector we were also able to reproduce the rotational rainbow structure observed by Kleyn et al. [ 11. We observed an angular shift of the rotational rainbow to the supraspeculti angle, similar to the observations for N2 on Ag ( 111) [ 2 1. The first results for the steric effect obtained by orienting the NO molecules in the initial beam, show qualitative agreement with calculated predictions using the PES proposed by Voges and Schinke [ 5 1. The J= 8.5 rotational level is preferentially produced by molecules with N-end colliding first, while the higher J= 18.5 is produced preferentially by the O+nd collisions_ This is in agreement with the model of Voges and Schinke, yielding two different rotational distributions for the two ends of the molecule. Finally, the steric effects observed previously by a QMS have been interpreted in terms of rotational excitation. Because the QMS is a density detector, an interpretation in terms of different energy to phonons for both ends of the molecule could not be definitely excluded. The present observation of a strong steric effect of individual J states corroborates our earlier inferences about the importance of orientation-dependent rotational excitation. Acknowledgement This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Mate-
50
LETTERS
20 April 1990
rie (Foundation for Fundamental Research of Matter) and was made possible by financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Dutch Organization for Advancement of Research).
References [ 1] A.W. Kleyn, AC. Luntz and D.J. Auerbach, Phys. Rev. Letters47 (1981) 1169; Surface Sci. 117 (1982) 33. [2] G.O. Sitz, AC. Kummel and R.N. Zare, J. Chem. Phys. 89 (1988) 2558; 2572. [ 31 A. M&U, T. Gritsch, F. Budde, T.J. Chuang and G. Ertl, Phys. Rev. Letters 57 (1986) 384. [4] J.A. Barker, A.W. Kleyn and D.J. Auerbach, Chem. Phys. Letters 97 (1983) 9. [ 51H. Voges and R. Schinke, Chem. Phys. Letters 100 ( 1983) 245. [ 61 T. Bnmner, R. Brake and W. Brenig,Phys. Rev. A 35 (1987) 5266. [ 71 J. Kimman, CT. Rettner, D.J. Auerbach , J.A. Barker and J.C. Tully, Phys. Rev. Letters 57 (1986) 2053. [S] E.W. Kuipers, M.G. Tenner, A.W. Kleyn and S. Stolte, Nature 334 (1988) 420. 191 M.G. Tenner, E.W. Kuipers, A.W. Kleyn and S. Stolte, Surface Sci, to be submitted for publication. [lo] E.W. Kuipers, M.G. Tenner, A.W. Kleyn and S. Stolte, Phys. Rev. Letters 62 (1989) 2152. [ 111 S. Holloway and D. Halstead, Chem. Phys. Letters 154 (1989) 181. [ 121 R.J. B&m and CR Bruudle, J. Vacuum Sci. Techn. A 2 (1984) 1040; J.A. Barker and D.J.Auerbach, Surface Sci. Rept. 4 ( 1984) 1. [ 131 C.W. Muhlhausen, L.R. Williams and J.C. Tully, J. Chem. Phys. 83 (1985) 2594. [ 141 G.C. Corey and D. Lemoine, Chem. Phys. Letters 160 (1989) 324. [15]M.G. Tenner, E.W. Kuipcrs, A.W. Kleyn and S. Stolte, J. Chem. Phys. 89 (1988) 6552. [ 161 E.W. Kuipers, M.G. Tenner, A.W. Kleyn and S. Stolte, SurfaceSci. 21 l/212 (1989) 819; Chem. Phys. 38 (1989) 451. [ 171 G.D. Kubiak, J.E. Hurst, H.G. Rennagel, G.M. McClelland andR.N. Zare, J. Chem.Phys. 79 (1983) 5163. [ 181 M.E.M. Spruit, E.W. Kuipcrs, M.G. Tenner, J. Kimman and A.W. Kleyn, J. Vacuum Sci. Techn. A 5 (1987) 496. [ 191 M.G. Tenner, E.W. Kuipers, W.Y. Langhout, A.W. Kleyn, G. Nicolasen and S. Stolte, J. Vacuum. Sci. Techn. B., to be submitted for publication. [ZO] F.H. Geuzebroek, AE. Wiskerke, M.G. Temer, A.W. Kleyn and A. Namiki, to be published. [ 211 S. Holloway, private communication. [22] E.W.Kuipers, M.G.Tc~er,M.E.M.SpruitandA.W. Kleyn, Surface Sci. 189/ 190 (1987) 669.
CHEMICAL PHYSICS LETTERS
ORIENTATION DEPENDENCE OF ROTATION& IN NO SCATTERING FROM Ag( 111)
20 April 1990
EXCITATION
Manfred G. TENNER, Frank H, GEUZEBROEK, Edgar W. KUIPERS ’ Arjan E. WISKERKE, Aart W. IUEYN FOM-Institute for Atomicand MolecularPhysics.Kruislaan40 7, 1098 SJAmsterdam, The Netherlands
Steven STOLTE Laser F&&ties of theDepanmentof Chemistryof theFree University, DeBoelelaan 1083, log1 HVAtnsterdam,The Netherlands
and Akira NAMIIU Departmentof Electricaland ElectronicEngineering. ToyohashiUniversityofTechnology, Tempaku-cho,Toyohashi440, Japan Received 19 January 1990; in final form 1 February 1990
Scattering experiments with oriented NO beams on Ag( 111)have been performed. The scattered molecules are detected by a resonantly enhanced muttiphoton ionization. A dependence of the rotational rainbow upon the scattering angle has been observed. The first results of the steric effect measured for single rotational (J) levels of scattered molecules are presented. The J= 18.5 state is produced preferentially when the O-end collides first with the surface. For the J= 8.5 state we find that the Ndown geometry is preferable. These results are in qualitative agreement with theory.
1. Introduction The rotation-energy distribution of molecules directly scattered from a surface gives detailed information about the dynamics of the molecule-surface interaction. Such experimental results can serve as a stringent test for dynamical scattering calculations and the potential energy surfaces (PES) used therein. For several molecule-surface systems the rotational distributions for direct inelastic scattering show clear deviations from a thermal Boltzmann distribution at high J levels [l-3]. The rotation of the scattered molecule is generated by the dependence of the PES upon the orientation of the molecular axis. A rotational rainbow is caused by an extremum in the dependence of J on the initial molecular orientation. ’ Present address: Department of Physical Chemistry, University of Cambridge, Cambridge Cl32 1EP, UK.
The observed deviation from a thermal distribution for high J states has been explained as a manifestation of such a rotational rainbow [ 1,4-61. Angular distributions measured with rotationalstate-specific detection techniques reveal a connection between the final rotational state Yand the scattering angle 0,. For NO and Nz scattered from Ag( 1 I1 ) the scattering angle with maximal intensity, @,, is observed to shift with increasing J more towards the surface [ 1,2,7]. This, and the rotational rainbow picture suggest that there is a conaection between the reflection angle and the orientation of the molecule just before the collision. The similar behavior of the rotational excitation spectra and the angular distribution of scattered intensity observed for both N2 and NO suggests a similar anisotropy in the interaction potential.’ However, for the homonuclear N2 the PES will be symmetric with respect to a plane through the center
0009-26 14/90/S 03.50 Q Elsevier Science Publishers B.V. (North-Holland )
45
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of mass of the molecule and parallel to the surface. But for molecules consisting of two different atoms, we cannot exclude that the two ends of the molecule will act in a different way. In that case, for NO scattering different results may be obtained depending on which end of the molecule collides first with the surface. In previous experiments, the orientational dependence of the NO surface interaction has been tested directly by orienting the NO molecule before the collision with either its N atom or its 0 atom pointing preferentially towards the surface, For both orientations we measured the angular distribution of the scattered intensities from Ag ( 111) and Pt ( 111) with a quadrupole mass spectrometer (QMS) [ 8-101. The angular distributions observed show a steric effect: the distribution measured for a oriented beam with the O-end preferentially in front was shifted towards the surface with respect to that of the N-end. From these observations, we concluded that there is an orientational dependence for NO scattering which cannot be present for Nz. Different PESs have been proposed for NO on Ag. In several studies the rotational distributions computed using these PESs have been compared with experimental data. Voges and Schinke [ 51 proposed a two-dimensional potential containing both cosine and cosine-cubed terms for the orientational dependence of the repulsive part. This potential yields a different maximal torque for the two ends of the molecule which leads to two rotational rainbows with different J. The translational-energy dependence of the position of the high J rainbow measured by Kleyn et al. [ 11 could essentially be reproduced by quanta1 calculations using this PES. Calculations for initially oriented NO molecules by Holloway and Halstead, with the potential of Voges and Schinlce show explicitly a difference in rotational excitation for the two ends [ 111. The potential used has an attractive well depth of 0.078 eV which is too small compared to the experimental value of 0.2 eV [ 121. Another potential is given by Muhlhausen et al. [ 13 1, which is a multi-dimensional one and includes the interaction with a number of surface atoms. The anisotropy of this potential is mainly introduced by a strong attraction for the N atom, causing an anisotropy in the repulsion as well. In their classical trajectory calculations Muhlhausen et al. [ 131 re46
20 April 1990
produce the experimental rotational distributions and their initial translational-energy dependence. The high J states are populated mainly by O-end orientations. Quantum calculations performed by Corey and Lcmoine [ 141 for essentially the same potential but assuming oriented NO molecules confirmed the higher rotational excitation for the O-end. In contrast to the potential of Voges and Schinke [ 51 the PES introduced by Muhlhausen [ 131 and used by Corey and Lemoine [ 141 possesses an attractive well which is too deep (>0.58 eV). This was corrected in later calculations, for which no rotational excitation spectra have been shown [ 7 1. The Voges and Schinke potential has a different anisotropy for the two ends of the molecule. From the analysis of our steric effect measurements Kuipers et al. concluded that the O-end corresponds to the most anisotropic end of the molecule [ 15,161. Thus both potentials have in common that they predict larger rotational excitation of the O-end orientation of NO on Ag( 111). Most theoretical studies on the steric effect in rotational excitation assume normal incidence and reflection along the normal. Due to the physical size of our detectors blocking the incident beam we cannot perform experiments for this geometry. Nor could be measure up to now rotational energy distributions. Instead only the steric effect of angular intensity distributions has been measured. Within a cube model the connection between the rotational excitation and the scattering angle S, is straightforward. As long as the angular distribution of the directly scattered molecules is lobular around the specular angle the cube model is more or less valid and calculated rotational rainbows will be transformed in angular rainbows. The broadening of the angular distributions is also caused by the interaction with phonons, corrugation, multiple bounces and energy transfer to the rotational degree of freedom. For low “normal” initial translational energy E"iO.2 eV no rotational rainbows could be observed experimentally [ 171. The rainbows are probably washed out by the broadening processes. However there may exist a connection between the final J state and the reflection angle which can be studied experimentally. To verify the theoretical predictions of the steric effects for the final rotational state distribution, we present the first results of experiments with a beam of oriented NO molecules scattering of
Volume 168, number 1
CHEMICAL PHYSICSLETTERS
Ag( 111) measuring the rotational-energy distribution of the scattered molecules with a resonantly enhanced multiphoton ionization (REMPI) detector.
2. Experimental The experimental set-up of the molecular beam source and the UHV system with the surface sample and the QMS detector is described in detail in refs. [ 18,191. Briefly, by use of an electrostatic hexapole deflector a pulsed NO beam is state selected in the 2II,,2 (J~0.5, 51~0.5, M~0.5) state and subsequently oriented in a uniform electric field applied just in front of the surface sample, which is mounted on a two-axis goniometer in our UHV scattering chamber. In previous experiments we detected the scattered molecules with a total density detector (the QMS). In the present experiments we used a detector yielding NO ions produced by photo ionization, which will be described in detail elsewhere [ 201. NO ions are formed by ( 1 + 1) REMPI of NO molecules via the A’Z(v=O, J’)tX21&,~(v=0, J’) transitions in the wavelength region around 225 nm. It turned out that the output signal of the detector was initially affected by fringing fields of the strong “orientation” field of 15 kV/cm and that very careful screening was necessary. Angular scans are made by rotating the REMPI detector around the surface sample, which can be done independently of the also rotatable differentially pumped QMS. The REMPI signal 1,, is corrected for the rotational-transition strength S,., for the AtX transition. When ln{lJ,JISJ,( W+ 1) J} is plotted as function of rotational energy (the so-called Boltzmann plot ), one expects a straight line for a thermal distribution. From its slope the rotational temperature ;r, can be calculated. Indeed, spectra obtained for 10m9mbar NO background gas yield a straight line with rotational temperature T,= 300f 30 K.
20April1990
EizO.44 eV. For the incoming angle @i= 15”, the “normal” energy En= Ei COS2@=0.41 eV is comparable to the E,, for which Kleyn et al. [ 1] observed clearly rotational rainbows. For our experiment with S,= 15”) in fig. 1 is shown the Boltzmann plot for the QI,tP2, branch (squares) and the Qll tRll branch (triangles) of the lower spin-orbit manifold. Due to the linewidth of the excitation laser (0.2 cm-’ ) some lines could not be separated from lines of other transitions and have been omitted. Good agreement is found with the data of ref. [ 1 ] for similar conditions, plotted as a solid line in the same figure. A clear deviation from a straight line for J> 20.5 indicative of a rotational rainbow is found in both data sets. The fall-off for lower Jstates gives for both experiments a rotational temperature T,= 300 + 30 K. The same data are plotted in fig. 2, but nor corrected for the 2Jt 1 degeneracy. Although there is some noise in the data, for J> 15.5 no further decrease occurs which is indicative of rotational rainbow scattering. In previous rotationally unresolved experiments the steric effect showed around the specular angle a rotational auantum number J
eiir+Te
iniernaiener~y ( eV )
3. Results and discussion The first goal of our experiments was to reproduce the rotational rainbow as found by Kleyn et al. [ 1 ] _ The fastest beam of oriented NO molecules we could produce in our apparatus has an initial energy of
Fig. 1. Rotational-state distribution for scattered NO molecules presented in a Boltzmann plot as a function of internal energy (bottom) or rotational quantum number J (top). The squares are the populations derived from the Q,, +P2, transitions of the Z+C%,,~ band and the triangles from the Q2, +R,, transitions of the same band. The scattering conditions are EizO.44 eV, Bi=~,=15”.Thesolidlineisafittothedataofref. [il.
47
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rotational quantum number J 10.5 I”
5.5
10.5
15.5
20.5
20.5
’
I
I
30.5
I
25.5
rotational quantum number J Fig. 2. Rotational-state distribution for scattered NO molecules presented in a linear plot as a function of rotational quantum number J. Same data and symbols used as in fig. 1.
linear dependence on the reflection angle 6, [ 8- 101. For the scattering conditions of Ei=0,44 eV and @i= 45 ’ we have measured the rotational distribution for a subspecular angle (&=30”) and a supraspecular angle ( E+ 55’ ) . These measurements were not feasible for @i= 15 ‘. However, for 8, = 45 ‘, E”zO.24 eV suff%zesto show an onset of the rotational rainbow [ 11, In our experiments we observe a small relative increase for I> 20.5 for the supraspecular reflection, squares in fig. 3, but no significant increase for the subspecular scattering, triangles in fig. 3. For this 0, the intensity for Er>0.12 eV was below our detection limit. The small increase compared to the linear dependence shows the onset of a rotational rainbow which evidently depends on @,. A similar behavior for the rotational rainbow of the reflection angle for N,/Ag( 1 I 1) was found by Sitz et al. [ 21. They observed much sharper features, possibly induced by their much lower surface temperature T,=90K compared to T,=575K in our experiment. The main goal of our investigations was to measure for the same scattering conditions the scattered rotational-state distribution for preferentially oriented NO molecules. However, the statistical accuracy of the datapoints such as shown in fig. 3 is about 20% which is consequently too low to detect steric effects on the order of a few percent as measured with the QMS detector. In contrast, the paper of Holloway and Halstead [ 111 predicts a steric effect which 48
Fig. 3. Rotational-state distribution of Q,, + P1, transitions of the C++-2111,2band of scattered NO molecules presented as a Boltzmann plot as a function of internal energy (bottom) or rotationaI quantum number J (top). The scattering conditions are &=0.44 eV, &=45’ and @,=30” (triangles) and 0,=5S0 (squares).
is, at least for the high J states, so large that we certainly should be able to detect it. However, they performed strictly two-dimensional calculations in which no account is taken of the full spatial distribution of the molecules as present in the real experiment. Therefore these calculations may underestimate the contribution of the more broadside orientations but the effect is rather small [21]. Moreover these calculations are done at a value of E,, twice the E,, used in our experiments and do not include surface motions. In order to suppress the statistical error, we improved the REMPE detector and took longer measuring times. In the first instance we put all our efforts into a limited set of final rotational states of the Q11+Pzl branch and one reflection angle. We changed the initial beam energy to Ei=0.10 eV. At this energy the hexapole focuser yields a maximum intensity of state-selected molecules. In spite of the low collision energy and the absence of rotational rainbows we observed steric effects in experiments with oriented beams measuring the scattered mole-
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cules with a QMS [8,9]. We define RJfor a single rotational state J in our usual way as R
.I=
Z(Z,;,-I$,) zsJ+Isf,
’
I
I
I
cu
d
’P
P
&? d
d
9
!I------
G-7
4.5
Table 1 The values of R,, (&), n,/%., and (E,) for the observedrotational quantum number J, measured for @=45”, 8,=60’, Er=O. 1eV and T,=575 K J
with ZJ,~denoting the intensity obtained for the laser wavelength tuned to the transition of J to J’ for the preferential oriented beam with the O-end pointing towards the surface, and ZsJ with that .for the N-end towards the surface. For these experiments we have chosen 8i=45” and T,= 575 K and set the detector to &=60’. For this angle the steric effect for QMS measurements, which averages over all final J states, give R =0.05, i.e. a larger intensity for the O-end orientation. We did not observe a rotational rainbow as was expected from the literature [ 1,171. The R, values for the J states measured are plotted in fig. 4 and listed in table 1. The lowest J values have been probed via the bandhead, to which J= 1.5-4.5 contribute all nearly equally. In this ligure the value of R for the bandhead is plotted at 3.5 on the horizontal axis. Also given in table 1 are the rotational and the final translational energy. For the latter the same value for all J states is taken [22] as is confirmed experimentally by Kimman et al. [ 71. The sum of rotation and translation is always larger than the initial translational energy, so energy transfer from the surface motion is needed. For J= 18.5 we found R,=0.26 which is significantly larger than the value of the rotationally unresolved measurements. Indeed, this indicates that the larger J states originates relatively more from the O-end orientations as can be concluded from the
8.5
------h. 10.5
16.5
rotational quantum number J
Fig. 4. Steric effect R,as function of rotational quantum number J. TheR,for the bandbead is plotted atJz3.5. 8,=45”, @,=60°, Er=O.l eVand T,=575 K.
20 April 1990
RI
(J&I>
nJ/nO.s
(W
2.5-4.5 a.5 11.5 15.5 18.5 total (QMS)
+ 0.018 f 0,020 -0.0241tO.017 +0.024f0.024 +0.166*0.035 +0.263 eO.032 +0.050f0.01
0.003 0.017 0.030 0.054 0.076 -
6%) (W
1.0 11.6 10.6 7.9 5.4 -
0.12 0.12 0.12 0.12 0.12 0.12
positive sign of the effect. Interestingly, R, does not increase monotonically as a function of J as can be seen in fig. 4. The value for J= 8.5 is the lowest and even below zero, indicating that this rotational state is preferentially populated in collisions with the Nend in front. The rotational distribution can be fitted with a Boltzmann distribution with T,= 450 K. The ratio of nJ, the number of molecules in rotational level J, divided by no.5,the number in level J=OS for that distribution is given in table 1. Weighting the steric effect RJ with nJ/Cn,, the sum running over all detected values of J in table 1, one finds that the averaged steric effect is about (R) = 0.07 _+0.0 15. This value agrees quite well and within the error with the steric effect measured with the QMS, R,,,=0.05 ?O.Ol. For the single supraspecular reflection reported in this Letter, the observed RJ values are in qualitative agreement with the theoretical predictions. For this scattering geometry the highest rotational states originate as expected from the O-end of the NO molecule. The negative sign of R/ for J=8.5 can be interpreted as the rotational rainbow for the N-end orientation as predicted by the potential of Voges and Schinke [ 15,16 3. Although the two-rainbow picture can be reconciled with the data, the absolute values of R, are far too large. The effect calculated by Holloway and Halstead [ 111 is much larger, also for the highest experimental value, J= 18.5. The high rotational energy for the J= 18.5 state cannot be reached in this experiment without energy transfer from the heat bath of the surface. In the Voges and Schinke [ 51 calculations the motion of the surface is not included and this J state would lie above the classical rainbow value associated with the initial beam energy of Ej= 0.1 eV. One notes as a function of J that 49
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PHYSICS
only at its minimum (J= 8.5), is R, slightly negative. The Voges and Schinke calculations would predict a singular intensity around J= 8.5 which would result in a very large negative steric effect. Because of many averaging processes and the fact that three orientations classically give rise to J= 8.5, the N-end rotational rainbow is heavily diluted and the corresponding steric effect is only slightly negative. Preliminary measurements for other reflection angles show a similar angular dependence of RJ as for the unresolved experiments. This indicates that other processes like phonon excitation or multiple collisions are important and will have steric effects as well. 4. Conclusions We have successfully observed steric effects in the rotational excitation of NO molecules scattering from an Ag ( 111) surface with our new REMPI detector. With this detector we were also able to reproduce the rotational rainbow structure observed by Kleyn et al. [ 11. We observed an angular shift of the rotational rainbow to the supraspeculti angle, similar to the observations for N2 on Ag ( 111) [ 2 1. The first results for the steric effect obtained by orienting the NO molecules in the initial beam, show qualitative agreement with calculated predictions using the PES proposed by Voges and Schinke [ 5 1. The J= 8.5 rotational level is preferentially produced by molecules with N-end colliding first, while the higher J= 18.5 is produced preferentially by the O+nd collisions_ This is in agreement with the model of Voges and Schinke, yielding two different rotational distributions for the two ends of the molecule. Finally, the steric effects observed previously by a QMS have been interpreted in terms of rotational excitation. Because the QMS is a density detector, an interpretation in terms of different energy to phonons for both ends of the molecule could not be definitely excluded. The present observation of a strong steric effect of individual J states corroborates our earlier inferences about the importance of orientation-dependent rotational excitation. Acknowledgement This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Mate-
50
LETTERS
20 April 1990
rie (Foundation for Fundamental Research of Matter) and was made possible by financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Dutch Organization for Advancement of Research).
References [ 1] A.W. Kleyn, AC. Luntz and D.J. Auerbach, Phys. Rev. Letters47 (1981) 1169; Surface Sci. 117 (1982) 33. [2] G.O. Sitz, AC. Kummel and R.N. Zare, J. Chem. Phys. 89 (1988) 2558; 2572. [ 31 A. M&U, T. Gritsch, F. Budde, T.J. Chuang and G. Ertl, Phys. Rev. Letters 57 (1986) 384. [4] J.A. Barker, A.W. Kleyn and D.J. Auerbach, Chem. Phys. Letters 97 (1983) 9. [ 51H. Voges and R. Schinke, Chem. Phys. Letters 100 ( 1983) 245. [ 61 T. Bnmner, R. Brake and W. Brenig,Phys. Rev. A 35 (1987) 5266. [ 71 J. Kimman, CT. Rettner, D.J. Auerbach , J.A. Barker and J.C. Tully, Phys. Rev. Letters 57 (1986) 2053. [S] E.W. Kuipers, M.G. Tenner, A.W. Kleyn and S. Stolte, Nature 334 (1988) 420. 191 M.G. Tenner, E.W. Kuipers, A.W. Kleyn and S. Stolte, Surface Sci, to be submitted for publication. [lo] E.W. Kuipers, M.G. Tenner, A.W. Kleyn and S. Stolte, Phys. Rev. Letters 62 (1989) 2152. [ 111 S. Holloway and D. Halstead, Chem. Phys. Letters 154 (1989) 181. [ 121 R.J. B&m and CR Bruudle, J. Vacuum Sci. Techn. A 2 (1984) 1040; J.A. Barker and D.J.Auerbach, Surface Sci. Rept. 4 ( 1984) 1. [ 131 C.W. Muhlhausen, L.R. Williams and J.C. Tully, J. Chem. Phys. 83 (1985) 2594. [ 141 G.C. Corey and D. Lemoine, Chem. Phys. Letters 160 (1989) 324. [15]M.G. Tenner, E.W. Kuipcrs, A.W. Kleyn and S. Stolte, J. Chem. Phys. 89 (1988) 6552. [ 161 E.W. Kuipers, M.G. Tenner, A.W. Kleyn and S. Stolte, SurfaceSci. 21 l/212 (1989) 819; Chem. Phys. 38 (1989) 451. [ 171 G.D. Kubiak, J.E. Hurst, H.G. Rennagel, G.M. McClelland andR.N. Zare, J. Chem.Phys. 79 (1983) 5163. [ 181 M.E.M. Spruit, E.W. Kuipcrs, M.G. Tenner, J. Kimman and A.W. Kleyn, J. Vacuum Sci. Techn. A 5 (1987) 496. [ 191 M.G. Tenner, E.W. Kuipers, W.Y. Langhout, A.W. Kleyn, G. Nicolasen and S. Stolte, J. Vacuum. Sci. Techn. B., to be submitted for publication. [ZO] F.H. Geuzebroek, AE. Wiskerke, M.G. Temer, A.W. Kleyn and A. Namiki, to be published. [ 211 S. Holloway, private communication. [22] E.W.Kuipers, M.G.Tc~er,M.E.M.SpruitandA.W. Kleyn, Surface Sci. 189/ 190 (1987) 669.