Physics
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusmsa22a..10b&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #SA22A-10
Physics
0310 Airglow And Aurora, 0317 Chemical Kinetic And Photochemical Properties, 0340 Middle Atmosphere: Composition And Chemistry, 0342 Middle Atmosphere: Energy Deposition
Scientific paper
The fluorescence of vibrationally excited, ground electronic state hydroxyl radical (OH) in the airglow originates in the mesosphere-lower thermosphere (MLT) region of Earth's atmosphere. Spectroscopic measurements of this infrared emission are being made by the TIMED satellite to characterize the dynamics, temperature profiles, and HOy chemistry in the region 80-100 km. In the atmosphere, hydroxyl radicals in υ = 6-9 are formed in the reaction of hydrogen atoms with ozone; lower vibrational levels are populated through subsequent collisional deactivation by molecular oxygen. The lifetimes of the lower levels (υ <= 4) are significantly affected by collisions with atomic oxygen, as collisions with molecular oxygen are less efficient at relaxation than at higher levels. Given the importance of O-atom collisions in the atmosphere, we have developed an experimental approach and performed experiments on the collisional removal of OH(υ = 2, 3, 4) by atomic oxygen. In this work, the reaction of OH with atomic oxygen is studied using a two-laser method. Ozone is photolyzed in nitrogen with a pulsed excimer laser to generate O(1D), a portion of which reacts with either hydrogen to form OH(υ <= 4) or with water vapor to form OH(υ <= 3); the remainder is rapidly deactivated by collisions with N2 to produce ground state O(3P). A second, tunable dye laser pulse probes the OH population in a specific rovibrational state as a function of reaction time, using fluorescence from the A 2}Σ {+ - X 2Π { i} system. By adjusting the composition of the reactant gas mixture and by varying the photolysis laser fluence to control the ozone dissociation fraction, the dominant relaxation partner can be varied systematically from ozone and water or hydrogen to atomic oxygen. Experimentally determined rate constants for the removal of OH(υ = 2, 3, 4) by O(3P) are obtained at room temperature, with values of 6 x 10-11, 1.0 x 10-10 and 1.6 x 10-10 for υ = 2, 3 and 4, respectively, and 2-σ uncertainties of approximately 30%. We find that our measured rate constant for the removal of OH(υ = 2) by O(3P) is more than a factor of two slower than the single previous, indirect measurement of the relaxation rate constant for OH(υ = 1)[1]. However, the measured deactivation rate constants increase with vibrational energy and speculative extrapolations to OH in higher vibrational states (υ = 6-9) indicate that collisions of OH(υ) with O(3P) may have a significant influence on the total removal rate for these highly excited levels in the MLT region. [1] Spencer J. E, and Glass, G. P., Int. J. Chem. Kinetics, 9, 97-109 and 111-122, 1977. This work is supported by the Aeronomy Program of the National Science Foundation (ATM-9909807).
Boulter James E.
Copeland Richard A.
Marschall Jochen
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