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Optics Express

Optics Express

  • Editor: J. H. Eberly
  • Vol. 4, Iss. 2 — Jan. 18, 1999
  • pp: 53–58
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SF6 scattering from graphite surfaces: comparison of effects induced by thermal and laser controlled vibrational excitation

M. van Opbergen, A. Boschetti, and S. Iannotta  »View Author Affiliations


Optics Express, Vol. 4, Issue 2, pp. 53-58 (1999)
http://dx.doi.org/10.1364/OE.4.000053


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Abstract

We report scattering experiments of multiphoton vibrationally excited SF6 molecules from graphite surfaces demonstrating ro-vibrational excitation in the collision. The beam scattering experiments were carried out at different initial kinematic conditions and as a function of the surface temperature. The energy transfer depends both on the initial state of the molecule and on the momentum transferred as well as on the temperature of the surface. The role of surface atomic corrugation is evidenced.

© Optical Society of America

1. Introduction

The understanding of the dynamics of the interaction of excited molecules with solid surfaces is very important to clarify physical and chemical processes of fundamental and applied interest such as physisorption, chemisorption and heterogeneous catalysis. The comprehension itself of gas-surface chemical dynamics relies on the understanding of the different elementary processes such as energy transfer, diffusion and reaction that are often strongly coupled. Studies focusing separately on the different steps in selected systems have therefore acquired an increasing importance. In this framework energy transfer between non-reactive partners has enjoyed relevant attention and great progresses have been made in the last decade or so even though these studies are still restricted to simple molecules and a limited number of surfaces. This has driven an increasing interest towards collision experiments of ro-vibrational excited molecules with well-characterised surfaces. Advances in the understanding of the role of molecular vibrations are reviewed by Houston and Merrill [1

1. P. L. Houston and R. P. Merrill, “Gas-Surface Interactions with Vibrationally Excited Molecules,” Chem. Rev. 88, 657–671 (1988). [CrossRef]

], and by Rettner et al [2

2. C. T. Rettner, D. J. Auerbach, J. C. Tully, and A. W. Kleyn, “Chemical Dynamics at the Gas-Surface Interface,” J. Phys. Chem. 100, 13021–13033 (1996). [CrossRef]

]. There are very few examples of studies involving molecules larger than diatomic or triatomic due to the difficulties inherent in the experimental analyses of the final ro-vibrational state of the molecule after scattering because of the too high vibrational densities of states and the rotational congestion in polyatomic molecules.

An interesting original approach has been adopted by Andersson et al. in studying the inelastic scattering and vibrational excitation of SF6 from graphite surfaces [3

3. M. B. Andersson and J. B. C. Pettersson, “Vibrational excitation of SF6 scattering from graphite,” Chem. Phys. Lett. 250, 555–559 (1995). [CrossRef]

]. They determined the final vibrational temperature of the scattered molecules by monitoring the ratio of the yields of the fragment ions produced by electron impact. The relative abundance of the fragments is in fact a sensitive indication of the vibrational excitation of the parent molecule. Besides the fundamental aspects, the study of the interaction of SF6 with solid surfaces is of particular interest since its excited species could be efficiently and selectively used in processes of technological interest [4

4. E. R. Fischer, B. L. Kickel, and P. B. Armentrout, “Collision-induced dissociation and charge transfer reactions of SFx+ (x=1-5): Thermochemistry of sulfur fluoride ions and neutrals,” J. Chem. Phys. 97, 4859–4870 (1992). [CrossRef]

,5

5. S. Yamamoto, K. Mochiji, and N. Mikami, “Trapping and low-energy extraction of photodissociated ions of SF6,” Jpn. J. Appl. Phys. 34, L393–L395 (1995). [CrossRef]

]. A relevant experimental point is that beams of highly vibrationally excited molecules are efficiently prepared by CO2 multiphoton excitation and have been studied in detail [6

6. A. Boschetti, M. Zen, D. Bassi, and M. Scotoni, “Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br,” Chem. Phys. 87, 131–138 (1984). [CrossRef]

,7

7. C. Liedenbaum, S. Stolte, and J. Reuss, “Multi-photon excitation of a beam of SF6 molecules pumped and probed by cw CO2 lasers,” Chem. Phys. 122, 443–454 (1988). [CrossRef]

].

In the present work on graphite (HOPG), we take full advantage of an optothermal detection scheme to study directly the energy transferred during the collision of multiphoton excited SF6 molecules. Our angular resolved measurements, giving information on both momentum and energy transfer, allow to investigate the coupling of the different degrees of freedom during the collision. In this framework the use of different type and degree of excitation of the incident molecule reveals the presence of significant state selectivity.

2. Experimental

A sample manipulator allows full control of the position of the surface along the three axes of rotation and the three translations. We perform high resolution He diffraction to obtain the required information about the orientation of the surface and its atomic corrugation. The He diffraction pattern is also used as a very sensitive measure of any adsorption, from the background gases or from the beams, that would pollute the surface.

The SF6 beam is produced by a variable temperature standard supersonic source with temperature stability better than 1%. Time of flight measurements are used to accurately determine the initial kinetic energy of the beam. By accurately tuning the source temperature and the percentage of seeding in He we can easily explore kinetic energies ranging from 100 meV to about 600 meV. It is important to recall that, in a free jet expansion, source temperatures and seedings change also the beam’s vibrational and, much more dramatically, rotational temperatures inducing the ro-vibrational cooling [10

10. G. Scoles, Atomic and molecular beam methods (Oxford Univ. Press, New York - Oxford, 1988).

]. Boschetti et al. [6

6. A. Boschetti, M. Zen, D. Bassi, and M. Scotoni, “Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br,” Chem. Phys. 87, 131–138 (1984). [CrossRef]

] showed that in the expansion of SF6 the final rotational and translational temperatures are very close and low, whereas the vibrational temperature can be assumed close to the source temperature.

We use a high power CO2 laser to induce a multiphoton vibrational excitation of the SF6 molecules. The laser beam crosses the molecular beam orthogonally in the second stage of the differentially pumped beam set-up. The laser light is focused on the molecular beam by a 5 cm focal length salt lens in order to increase the laser fluence achieving a higher vibrational excitation. The fluence experienced by the molecules ranges from 160 to 290 mJ/cm2 depending on the beam velocity keeping the laser power fixed at 90 W. SF6 beams at different source temperatures are characterised by a radiation absorption intensity that depends on the laser frequency [6

6. A. Boschetti, M. Zen, D. Bassi, and M. Scotoni, “Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br,” Chem. Phys. 87, 131–138 (1984). [CrossRef]

]. In the present experiments the CO2 laser is tuned to the 10P26 line that gives a high absorption signal for all the source temperatures used. Under the highest fluences and depending on the source conditions, the molecules irradiated by the laser typically absorb 5 CO2 photons each. This has been calculated using the method of Liedenbaum et al. [7

7. C. Liedenbaum, S. Stolte, and J. Reuss, “Multi-photon excitation of a beam of SF6 molecules pumped and probed by cw CO2 lasers,” Chem. Phys. 122, 443–454 (1988). [CrossRef]

] on the basis of the observed opto-thermal signals. We compared the amount of energy in the different degrees of freedom of the molecules in the beam regimes explored. The kinetic energy is varied from about 150 to 600 meV while the thermal excitation into vibrations of a pure beam is changed from about 280 to 700 meV (2300 to 5700 cm-1), evaluated as in [6

6. A. Boschetti, M. Zen, D. Bassi, and M. Scotoni, “Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br,” Chem. Phys. 87, 131–138 (1984). [CrossRef]

]. Since the typical laser induced excitation used is 600 meV, we really explore with the present experiments collision events where the role of the different degrees of freedom can be assessed.

3. Results and discussion

A first series of scattering experiments from graphite was devoted to monitor the energy transfer processes for thermal excited SF6 beams. We carried out a large number of measurements as a function of the surface temperature and initial beam conditions. Figure 1a shows the polar plot of the normalised angular distribution of the bolometric signal of the scattered particles. The data shown correspond to three different surface temperatures. The angular distributions are characterised by two contributions: a lobular part peaked at angles close to the specular angle and a very small diffuse cosine-like part. This diffuse scattering is usually attributed to indirect inelastic and adsorption-desorption processes while the lobular structure is due to direct inelastic processes. The large increment of the normalised intensity of the direct inelastic lobe with increasing surface temperature is due to the larger uptake of energy by the molecule scattered from the hotter surface. Moreover the lobe angular position shifts towards the surface normal as the surface temperature is increased. This is consistent with a picture where the molecule acquires perpendicular momentum. We can assess the contribution to the energy transferred in the collision by assuming that during the scattering only the kinetic energy of the molecule would be effected. In this case, and assuming that the parallel momentum is conserved, one can predict kinematically the shifts of the lobes due to the energy transferred perpendicularly with respect to the lowest surface temperature. We calculated in this way the position of the maximum of the lobes for the two higher surface temperatures. They are indicated in figure 1a by the signs on the angle scale drawn in the same colour as the corresponding sets of data. The experimental shifts are much less than the kinematic predictions obtained from the relative energy transferred. There should therefore be a relevant flow of energy into the ro-vibrational modes that competes with the purely kinetic channel. This is consistent with the observations in the experiments on the GaSe surface [8

8. A. Boschetti, A. Cagol, C. Corradi, R. Jacobs, M. Mazzola, and S. Iannotta, “Energy transfer processes and molecular degrees of freedom in the collision of SF6 molecules with the GaSe (001) surface,” Chem. Phys. 163, 179–191 (1992). [CrossRef]

].

Fig. 1. a) Normalised angular distributions of scattered SF6 molecules (Vbeam=520m/s) for three different surface temperatures. b) Distributions of SF6 beams scattered from the HOPG surface (Ts=178K) at different initial beam velocities. The arrows indicate the position of the incidence and specular angles (±40° respectively). The coloured signs on the degree scale of fig.1a show the angles calculated kinematically (see text).

The normalised angular distributions obtained for different beam source conditions are shown in figure 1b. These data are obtained at a surface temperature of 178K and show that for higher source temperatures the forward lobe grows considerably. This specific trend is similar to the one observed in the GaSe experiment. The molecular beams with more kinetic energy before the collision give rise to a higher energy uptake during the collision and an angular shift of the maximum of the lobe towards the surface. This trend is again inconsistent with a model involving only the kinetic energy transfer. In fact, still assuming parallel momentum conservation, there should be an increasing loss of perpendicular velocity of the molecule causing the observed lobe shifts as a function of the initial velocity. On the contrary the normalised intensities show more energy uptake of the molecule during the collision. This is another indication of the relevance of the internal degrees of freedom.

Fig. 2. a) Normalised angular distributions of laser multiphoton excited SF6 molecules scattered from the graphite surface (Ts=178K) at different initial beam velocities. b) Normalised intensity of the lobes for the laser induced scattering signals of two different beams as a function of the opto-thermal intensity of the incoming beam (see text). The open symbols represent the data obtained on the GaSe surface while the closed symbols show the results from the graphite surface. The lines are shown just to guide the eye.

We mentioned before that the angular position of the maximum of the lobe changes with the beam average velocity. To qualify further this dependence, the position of each direct inelastic lobe is determined by fitting the form of the angular distribution by an appropriate shape function. We have investigated the angular position of the maximum of the lobe as a function of the beam velocity and different molecular excitation. The results are shown in figure 3. These data show that a higher internal energy before the collision pushes the maximum of the lobe towards the surface. We believe that this is our major experimental result that demonstrates the coupling between internal excitation and momentum transfer in the collision. This behaviour is similar to that observed for GaSe. A major difference however is that for graphite the lobe position, at similar beam conditions, is shifted much more towards the surface. This could be due to the role played by the corrugation of the surface and the relative importance of parallel versus perpendicular momentum. As mentioned earlier, in the case of GaSe the conservation of parallel momentum is far from holding so that the energy exchanged is correlated to both components of the momentum. The larger scattering angles observed on graphite are consistent with substantial conservation of parallel momentum.

The different phonon structures between GaSe and graphite could also play a role in the energy transfer [12

12. S. Iannotta, C. Gravili, A. Boschetti, A. Cagol, and M. Cacciatore, “Energy transfer processes and surface corrugation: a combined collision dynamics and experimental study of the Xe-GaSe (001) system,” Chem. Phys. 194, 133–144 (1995). [CrossRef]

] and therefore deserves further investigation. We only mention here that the perpendicular surface Debye temperatures are not too different (θD⊥ ≈ 342K for GaSe [12

12. S. Iannotta, C. Gravili, A. Boschetti, A. Cagol, and M. Cacciatore, “Energy transfer processes and surface corrugation: a combined collision dynamics and experimental study of the Xe-GaSe (001) system,” Chem. Phys. 194, 133–144 (1995). [CrossRef]

] and θD⊥ ≈ 500K [13

13. S. Iannotta, G. Scoles, and U. Valbusa, “Effect of surface atom vibrations on the diffraction of 1H and 2H beams from the basal plane of graphite,” J. Phys. Chem. 89, 1914–1921 (1985). [CrossRef]

]) when compared to the temperature range explored in our experiments while the phonon spectra differ significantly [12

12. S. Iannotta, C. Gravili, A. Boschetti, A. Cagol, and M. Cacciatore, “Energy transfer processes and surface corrugation: a combined collision dynamics and experimental study of the Xe-GaSe (001) system,” Chem. Phys. 194, 133–144 (1995). [CrossRef]

].

Fig. 3. Position of the maximum of the lobes (degrees from surface normal) as a function of the average beam velocity. The open symbols are for the laser induced angular distributions, while the closed symbols are from the results with the thermal beams. The angle of incidence is 40° (Ts=178K).

4.Conclusions

Acknowledgements

The authors are very grateful to Professor J. Reuss and Dr. T. Toccoli for stimulating discussions and ideas. They are very thankful to C. Corradi and M. Mazzola for their precious technical support. This work has been realised in the framework of the EC-HCM network (ERB-CHR-XCT-94-0603) and is financially supported through an EC-TMR grant (ERB4001GT961231).

References and links

1.

P. L. Houston and R. P. Merrill, “Gas-Surface Interactions with Vibrationally Excited Molecules,” Chem. Rev. 88, 657–671 (1988). [CrossRef]

2.

C. T. Rettner, D. J. Auerbach, J. C. Tully, and A. W. Kleyn, “Chemical Dynamics at the Gas-Surface Interface,” J. Phys. Chem. 100, 13021–13033 (1996). [CrossRef]

3.

M. B. Andersson and J. B. C. Pettersson, “Vibrational excitation of SF6 scattering from graphite,” Chem. Phys. Lett. 250, 555–559 (1995). [CrossRef]

4.

E. R. Fischer, B. L. Kickel, and P. B. Armentrout, “Collision-induced dissociation and charge transfer reactions of SFx+ (x=1-5): Thermochemistry of sulfur fluoride ions and neutrals,” J. Chem. Phys. 97, 4859–4870 (1992). [CrossRef]

5.

S. Yamamoto, K. Mochiji, and N. Mikami, “Trapping and low-energy extraction of photodissociated ions of SF6,” Jpn. J. Appl. Phys. 34, L393–L395 (1995). [CrossRef]

6.

A. Boschetti, M. Zen, D. Bassi, and M. Scotoni, “Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br,” Chem. Phys. 87, 131–138 (1984). [CrossRef]

7.

C. Liedenbaum, S. Stolte, and J. Reuss, “Multi-photon excitation of a beam of SF6 molecules pumped and probed by cw CO2 lasers,” Chem. Phys. 122, 443–454 (1988). [CrossRef]

8.

A. Boschetti, A. Cagol, C. Corradi, R. Jacobs, M. Mazzola, and S. Iannotta, “Energy transfer processes and molecular degrees of freedom in the collision of SF6 molecules with the GaSe (001) surface,” Chem. Phys. 163, 179–191 (1992). [CrossRef]

9.

M. van Opbergen, A. Boschetti, and S. Iannotta, “Energy transfer processes in the scattering of multiphoton excited SF6 from the (001) surface of GaSe,” submitted to Surface Science.

10.

G. Scoles, Atomic and molecular beam methods (Oxford Univ. Press, New York - Oxford, 1988).

11.

M. B. Någård, N. Marković, and J. B. C. Pettersson, “Scattering and trapping dynamics of gas-surface interactions: Vibrational excitation of CF3Br on graphite,” J. Chem. Phys. 109, 10350–10360 (1998). [CrossRef]

12.

S. Iannotta, C. Gravili, A. Boschetti, A. Cagol, and M. Cacciatore, “Energy transfer processes and surface corrugation: a combined collision dynamics and experimental study of the Xe-GaSe (001) system,” Chem. Phys. 194, 133–144 (1995). [CrossRef]

13.

S. Iannotta, G. Scoles, and U. Valbusa, “Effect of surface atom vibrations on the diffraction of 1H and 2H beams from the basal plane of graphite,” J. Phys. Chem. 89, 1914–1921 (1985). [CrossRef]

OCIS Codes
(020.2070) Atomic and molecular physics : Effects of collisions
(020.4180) Atomic and molecular physics : Multiphoton processes
(290.5840) Scattering : Scattering, molecules

ToC Category:
Focus Issue: Laser controlled dynamics

History
Original Manuscript: December 6, 1998
Published: January 18, 1999

Citation
M. van Opbergen, A. Boschetti, and S. Iannotta, "SF6 scattering from graphite surfaces: comparison of effects induced by thermal and laser controlled vibrational excitation," Opt. Express 4, 53-58 (1999)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-4-2-53


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References

  1. P. L. Houston and R. P. Merrill, "Gas-Surface Interactions with Vibrationally Excited Molecules," Chem. Rev. 88, 657-671 (1988). [CrossRef]
  2. C. T. Rettner, D. J. Auerbach, J. C. Tully and A. W. Kleyn, "Chemical Dynamics at the Gas-Surface Interface," J. Phys. Chem. 100, 13021-13033 (1996). [CrossRef]
  3. M. B. Andersson and J. B. C. Pettersson, "Vibrational excitation of SF6 scattering from graphite," Chem. Phys. Lett. 250, 555-559 (1995). [CrossRef]
  4. E. R. Fischer, B. L. Kickel and P. B. Armentrout, "Collision-induced dissociation and charge transfer reactions of SFx + (x=1-5): Thermochemistry of sulfur fluoride ions and neutrals," J. Chem. Phys. 97, 4859- 4870 (1992). [CrossRef]
  5. S. Yamamoto, K. Mochiji and N. Mikami, "Trapping and low-energy extraction of photodissociated ions of SF6," Jpn. J. Appl. Phys. 34, L393-L395 (1995). [CrossRef]
  6. A. Boschetti, M. Zen, D. Bassi and M. Scotoni, "Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br," Chem. Phys. 87, 131-138 (1984). [CrossRef]
  7. C. Liedenbaum, S. Stolte and J. Reuss, "Multi-photon excitation of a beam of SF6 molecules pumped and probed by cw CO2 lasers," Chem. Phys. 122, 443-454 (1988). [CrossRef]
  8. A. Boschetti, A. Cagol, C. Corradi, R. Jacobs, M. Mazzola and S. Iannotta, "Energy transfer processes and molecular degrees of freedom in the collision of SF6 molecules with the GaSe (001) surface," Chem. Phys. 163, 179-191 (1992). [CrossRef]
  9. M. van Opbergen, A. Boschetti and S. Iannotta, "Energy transfer processes in the scattering of multiphoton excited SF6 from the (001) surface of GaSe," submitted to Surface Science.
  10. G. Scoles, Atomic and molecular beam methods (Oxford Univ. Press, New York - Oxford, 1988).
  11. M. B. N†g†rd, N. Markovi a and J. B. C. Pettersson, "Scattering and trapping dynamics of gas-surface interactions: Vibrational excitation of CF3Br on graphite," J. Chem. Phys. 109, 10350-10360 (1998). [CrossRef]
  12. S. Iannotta, C. Gravili, A. Boschetti, A. Cagol and M. Cacciatore, "Energy transfer processes and surface corrugation: a combined collision dynamics and experimental study of the Xe-GaSe (001) system," Chem. Phys. 194, 133-144 (1995). [CrossRef]
  13. S. Iannotta, G. Scoles and U. Valbusa, "Effect of surface atom vibrations on the diffraction of 1 H and 2 H beams from the basal plane of graphite," J. Phys. Chem. 89, 1914-1921 (1985). [CrossRef]

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