Unimolecular reaction rate constants of NO{sub 2} just above D{sub 0}
Photoinitiated unimolecular decomposition on a barrierless potential energy surface (PES) has been studied for the reaction NO{sub 2} {yields} O({sup 3}P{sub 2}) + NO(X{sup 2}{Pi}{sub 1/2}, {nu} = 0) for excess energies up to approximately 17 cm{sup {minus}1} above the dissociation threshold (i.e., D{sub 0} for nonrotating molecules) by using expansion-cooled samples and the time-resolved pump-probe technique. To examine the threshold region with enough energy resolution to discern abrupt changes in the rate constant, should they occur, a pump-probe cross-correlation temporal width of {approximately}25 ps and a pump line width {le}2 cm{sup {minus}1} has been used. These are the first direct observations of the reaction rate constants in this energy regime. The rate constant was found to increase by an order of magnitude, varying from {approximately}2 x 10{sup 10} s{sup {minus}1} to {ge}10{sup 11} s{sup {minus}1}, the latter being a rough lower bound imposed by the experimental arrangement. The rate constant does not display the energy dependence predicted by using phase space theory, at least in detail. Rather, it appears to reflect the highly complex nature of the levels and the multiple PESs that are believed to be responsible for the anomalously high vibronic level density which has been observed just below D{sub 0}. These results bridge the gap between spectroscopic studies which have been carried out at energies just above D{sub 0} and ultrafast experiments which have measured rate constants in this energy region with pump laser spectral widths of {approximately}30 cm{sup {minus}1}.
- Research Organization:
- Univ. of Southern California, Los Angeles, CA (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG03-85ER13363
- OSTI ID:
- 20013063
- Journal Information:
- Journal of Physical Chemistry A: Molecules, Spectroscopy, Kinetics, Environment, amp General Theory, Vol. 103, Issue 49; Other Information: PBD: 9 Dec 1999; ISSN 1089-5639
- Country of Publication:
- United States
- Language:
- English
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