Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsa22a0719a&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SA22A-0719
Physics
0310 Airglow And Aurora, 0317 Chemical Kinetic And Photochemical Properties, 0340 Middle Atmosphere: Composition And Chemistry
Scientific paper
The O2(5Πg) state plays a key role in O + O recombination. The 5Πg state has the highest spin-orbit degeneracy and at large internuclear distance is the lowest energy electronic state of O2. Previous studies in our laboratories have shown that despite its shallow potential well, once formed, population in the O2(5Πg) state relaxes to vibrational levels of more strongly bound O2 states rather than dissociating back into O + O. These experiments were conducted with O2, N2, and CO2 colliders over a wide temperature range (155-300 K), allowing us to conclude that such relaxation processes are crucial for O-atom recombination in the atmospheres of Venus and Earth. Here we report on continuing studies of the O2(5Πg) state using our two-laser experimental approach of first exciting O2 to high vibrational levels of the A3Σ u+ state, followed by probing O2 (5Πg) via resonance enhanced multiphoton ionization (REMPI). To determine the energy position of the O2 (5Πg, v=0) level, relative yields for the production of O2(5Πg,v=0) were measured following excitation to different vibrational levels in the O2(A3Σu+,v=7-10) state at temperatures between 150 and 300 K. The O2 (5Πg, v=0) yield from those A-state vibriational levels that are below the origin of the quintet state should decrease dramatically as temperature is lowered due to the decrease in thermal energy. We find that the yield of O2(5Πg,v=0) from initially produced A(9) and A(10) is relatively insensitive to temperature, while the yield from A(8) has a strong temperature dependence, decreasing by a factor of four compared to A(9) when going from room temperature to 155 K. Production of O2 (5Πg, v=0) from A(7) was unobservable below room temperature. Our results are consistent with the previously estimated location of the O2(5Πg,v=0) in the energy range between vibrational levels v=8 and v=9 in O2 (A3Σu+) or roughly 1000 cm-1 below the dissociation limit. Kinetics studies at different temperatures were also carried out for O2(5Πg,v=1) using O2 and N2. For both O2 and N2 the extracted rate coefficients are roughly independent of temperature and 30 to 100% larger than corresponding rates for v=0. In order to better understand the mechanism of electronic energy transfer we investigated the structureless colliders He and Ar. We found the rate coefficients for removal of O2(5Πg,v=0) to be about a factor of three smaller than those for O2, with little or no temperature dependence. This work was supported by a grant from the NASA Geospace Sciences (ITM) program. Participation of W. Lapcharoensap was made possible by a grant from the NSF Physics Research Experiences for Undergraduates (REU) program.
Amaral Gabriel
Copeland Richard A.
Huestis David L.
Lapcharoensap W.
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