Chapter 20
Collision-Induced Dissociation of Highly Excited NO in the Gas Phase and on MgO (100) Surfaces 1
2
3
4
5
Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: June 10, 1997 | doi: 10.1021/bk-1997-0678.ch020
A. Sanov , D. W. Arnold , M . Korolik, H. Ferkel , C. R. Bieler , C. Capellos , C. Wittig, and Hanna Reisler Department of Chemistry, University of Southern California, SSC 619, Los Angeles, CA 90089-0482
Collision-induced dissociation (CID) of NO in highly excited mixed A/B states is studied in crossed molecular beams at collision energies of~2000cm and on crystalline MgO(100) at collision energies of ~2000 and 4400 cm . The yield spectra obtained by scanning the excitation laser wavelength while monitoring NO fragments show features identical to those in the fluorescence excitation spectrum of NO , but the yield of CID decreases exponentially with the increase of the amount of energy required to reach the threshold for the monitored NO state. The results are discussed in terms of a mechanism in which highly excited NO undergoes further activation by collisions, followed by unimolecular decomposition. The NO product spin-orbit excitations are sensitive to the chemical identity of the collider and bear the imprints of exit -channel interactions, which are more significant on the MgO(100) surface than in the gas-phase. 2
2
2
1
2
-1
-1
2
2
Studies of energy transfer are central to the understanding of unimolecular and bimolecular interactions. In collisional environments and at high temperatures, the high internal and/or translational energies that molecules acquire via collisions in the gas phase and with surfaces eventually lead to decomposition (1-3), but such collisions also lead to relaxation (4-6) and reactions (7-11). In the work described here, our approach has been to study the final decomposition step by exciting molecules to well defined internal and translational energies. By using molecular beams under single-collision conditions and state-selected laser detection of products, 1
Present address: Joint Institute for Laboratory Astrophysics, University of Colorado, Boulder, CO 80309
2
Present address: Sandia National Laboratories, Livermore, CA 94551 Present address: Technische Universität, Clausthal, 38678 Clausthal-Zellerfeld, Germany Present address: Department of Chemistry, Albion College, Albion, MI 49224 Permanent address: U.S. Army ARDEC, Picatinny Arsenal, AMSTA-AR-AEE, Dover, NJ 07801-5001
3
4
5
© 1997 American Chemical Society In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
291
2
292
HIGHLY EXCITED MOLECULES
it is possible to investigate these processes with good energy and quantum state resolution. We describe here our studies of CID of internally excited N 0 (hereafter denoted NO2*) with atomic and molecular colliders and with crystalline surfaces. The studies involving activation by gas-phase colliders are compared to the results of Hartland and Dai who observed that in the deactivating collisions of NO2*, the average energy transferred per collision is much higher for triatomic colliders, compared with atomic and diatomic colliders (72). It is also known that the quenching rates of NO2* by molecular colliders such as NH3 and H2O are particularly high (13, 14). A n important issue concerns the influence of attractive forces and reactive pathways in collisions of highly excited molecules. For example, HC1 has a permanent dipole moment which may enhance long-range interactions. Also, with HC1 and NH3, reactive channels producing HONO are energetically possible for the high excitation energies of NO2* used in this study. Would such attractive interactions manifest themselves in the CID channel? In reactions with surfaces, considerable energy transfer to the surface can slow down the departing molecules, which experience also long-range forces by the surface. Moreover, binding energies of the excited parent, as well as the fragments, to the surface may be significant. For example, will one or both of the fragments be captured (if only temporarily) by the surface? Alternatively, will the momentum of the rebounding parent succeed in carrying the products away from the surface, albeit with energy transfer in the exit channel? At the very least, a significant percentage of die N O product may experience forces during its escape that may alter its internal states. The results obtained in this work show that the probability of CID is significant when NO2 is excited to energies approaching its dissociation threshold (Do), and that the average energy transferred per activating collision with polyatomic colliders is smaller than that obtained with the atomic and diatomic colliders studied so far. In addition, no participation of reactive pathways could be discerned, suggesting that the probability of inelastic scattering is much larger than that for reaction. Signatures of exit-channel interactions with the collider or the surface are revealed primarily in the spin-orbit population ratios of the NO fragment.
Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: June 10, 1997 | doi: 10.1021/bk-1997-0678.ch020
2
Experimental Methodologies Gas-Phase Crossed Beams Experiments. The gas-phase experiments are performed in a crossed beams arrangement described in detail elsewhere (75). Briefly, it consists of a main collision chamber and two adjacent differentially pumped beam source chambers. A pulsed supersonic molecular beam of 2% NO2 seeded in 1.5 atm of He or A r carriers, and a beam of neat collider gas expanded under similar conditions are introduced into the collision chamber through skimmers. The rotational temperature2 of NO2 in the beam (estimated from the rotational distribution of background NO( n 1 / 2 ) always present in NO2 samples) is < 5 K . The relative arrangement of the molecular and laser beams in the collision chamber is shown schematically in Figure la. The molecular beams cross at 90° approximately 50 mm from the skimmers, creating an overlap region of ~ 1 cm . Number density estimates indicate that single-collision conditions prevail (75). The relative collision energies are evaluated under the assumption of fully expanded beams (16); these estimates are confirmed experimentally. Jet-cooled N 0 is excited into mixed B / A j molecular eigenstates using an excimer-laser pumped dye-laser system. It is crucial for laser excitation to precede collisions in order to minimize the probability of photodissociating collisionally excited N 0 molecules (75). Therefore, the excitation laser beam (15 ns; ~ 5 mJ; 396 - 414 nm) crosses the NO2 beam 20-30 mm upstream from the collider beam overlap region (see Figure la). The frequency-doubled output from a second, similar laser system is used to probe NO produced in NO2 CID. The probe beam (~ 226 nm; 3
2
2
2
2
2
In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
20. SANOV ET AL.
Dissociation of Highly Excited N0
293
2
15 ns; ~ 150 μΤ) crosses the two molecular beams at the center of the overlap region. Both pump and probe laser beams are loosely focused with 1 m focal-length lenses. Since the laser excitation takes place outside the collision region, a pump-probe delay (typically 14-20 μβ) is required to allow NO2* to reach the collision/detection region. The long lifetime of NO2* (-50 μβ) (77) ensures that a significant fraction of molecules remain in the excited state upon arrival at the interaction region. N O is detected state-selectively by 1+1 (one-color) resonantly enhanced multiphoton ionization (REMPI) via the Α Σ V , R energy transfer involving internal degrees of freedom of the molecular colliders competes with conversion of translational energy to NO2 internal energy. Regarding the molecule-surface CID results, it appears that NO2 is different than larger polyatomics, which are known to travel far from the surface before decompo sition occurs. Depending upon the incident translational energy, (and thus the velocity of the departing activated NO2Î and/or N O fragments), exit-channel interactions with the surface result in variable NO spin-orbit state distributions. Finally, in all cases, the CID yield spectra are very similar to the spectrum observed with inert Ar as the collider, and no signatures of chemical reactions are discerned. CID is thus the dominant channel at the collision and excitation energies employed. It is noteworthy that the family of small polyatomics (3 or 4 atoms) undergoing potentially reactive molecule-surface and molecule-molecule scattering includes many technically important systems. Thus, we anticipate that the issues which have emerged in the present study have relevance in a large number of systems.
Acknowledgments The authors wish to thank the U.S. Army Research Office, the National Science Foundation and Air Force Office of Scientific Research for supporting this research. We thank the other participants in these experiments, Martin Hunter, Lori Hodgson, and James Singleton for their significant contributions to this research and Professor George Flynn for his valuable comments.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8)
Forst, W. Theory of Unimolecular Reactions; Academic: New York, NY, 1973. Robinson, P. J.; Holbrook, K. A. Unimolecular Reactions; Wiley: New York, NY, 1972. Gilbert, R. G.; Smith, S. C. Theory of Unimolecular and Recombination Reactions; Blackwell Scientific Publ.: Oxford, UK, 1990. Hippler, H.; Troe, J. In Bimolecular Collisions; Bagott, J. E.; Ashfold, M. N. R. Eds.; The Royal Society of Chemistry: London, UK, 1989. Flynn, G. W.; Weston, R. E., Jr. J. Phys. Chem. 1993, 97, 8116. Yardley, J. T. Introduction to Molecular Energy Transfer; Academic Press: New York, NY, 1980. Tardy, D. C.; Rabinovitch, B. S. Chem. Rev. 1977, 77, 396. Metz, R. B.; Pfeiffer, J. M.; Thoemke, J. D.; Crim, F. F. Chem. Phys. Lett. 1994, 221, 347.
In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: June 10, 1997 | doi: 10.1021/bk-1997-0678.ch020
20. SANOV ET AL.
Dissociation of Highly Excited N0
2
303
(9) Metz, R. B.; Thoemke, J. D.; Pfeiffer, J. M.; Crim, F. F., J. Chem. Phys. 1993, 99, 1744. (10) Sinha, Α.; Thoemke, J. D.; Crim, F. F., J. Chem. Phys. 1992, 96, 372. (11) Sinha, Α.; Hsiao, M. C.; Crim, F. F., J. Chem. Phys. 1991, 94, 4928. (12) Hartland, G. V.; Qin, D.; Dai, H.-L. J. Chem. Phys. 1995, 102, 8677. (13) Donnelly, V. M.; Keil, D. G.; Kaufman, F. J. Chem. Phys. 1979, 71, 659. (14) Capellos, C. Proceedings of the Army Science Conference; Orlando, FL, 1994; Assistant Secretary of the Army, Office of the Director of Research, Development and Acquisition, Contract number DAAH04-93-C10048: vol. 1, pp 51-58. (15) Sanov, Α.; Bieler, C. R.; Reisler, H. J. Phys. Chem. 1995, 99, 7339. (16) Kolodney, E.; Amirav, A. Chem. Phys. 1983, 82, 269. (17) Patten, Jr., K. O.; Burley, J. D.; Johnston, H. S. J. Phys. Chem. 1990, 94, 7960. (18) Joswig, H.; Andresen, P.; Schinke, R. J. Chem. Phys. 1986, 85, 1904. (19) Bieler, C. R.; Sanov, Α.; Reisler, H. Chem. Phys. Lett. 1995, 235, 175. (20) Kolodney, E.; Baugh, D.; Powers, P. S.; Reisler, H.; Wittig, C. J. Chem. Phys. 1989, 90, 3883. (21) Kolodney, E.; Powers, P. S.; Hodgson, L.; Reisler, H.; Wittig, C. J. Chem. Phys. 1991, 94, 2330. (22) Powers, P. S.; Kolodney, E.; Hodgson, L.; Ziegler, G.; Reisler, H.; Wittig, C. J. Phys. Chem. 1991, 95, 8387. (23) Ferkel, H.; Singleton, J. T.; Reisler, H.; Wittig, C. Chem. Phys. Lett. 1994, 221, 447. (24) Troe, J. J. Chem. Phys. 1977, 66, 4745. (25) Hunter, M.; Reid, S. Α.; Robie, D. C.; Reisler, H. J. Chem. Phys. 1993, 99, 1093. (26) Reid, S. Α.; Robie, D. C.; Reisler, H. J. Chem. Phys. 1994, 100, 4256. (27) Reid, S. Α.; Reisler, H. J. Phys. Chem. 1996, 100, 474, and references therein. (28) Sanov, A. Ph.D. Thesis; University of Southern California, Los Angeles, CA, 1996. (29) Chou, J. Z.; Hewitt, S. Α.; Hershberger, J. F.; Flynn, G. W. J. Chem. Phys. 1990, 93, 8474. (30) Hartland, G. V.; Qin, D.; Dai, H.-L. J. Chem. Phys. 1994,101,8554. (31) Zheng, L.; Chou, J. Z.; Flynn, G. W. J. Phys. Chem. 1991, 95, 6759. (32) Ionov, S. I.; Brucker, G. Α.; Jaques, C.; Chen, Y.; Wittig, C. J. Chem. Phys. 1993, 99, 3420. (33) Kolodney, E.; Baugh, D.; Powers, P. S.; Reisler, H.; Wittig, C. Chem. Phys. Lett. 1988, 145, 177. (34) Kahler, C. C.; Ansell, E.; Upshur, C. M.; Green, Jr., W. H. J. Chem. Phys. 1984, 80, 3644. (35) Redpath, A. E.; Menzinger, M.; Carrington, T. Chem. Phys. 1978, 27, 409.
In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
