Fractional population of NO(v − 1) from vibrational relaxation of NO(v = 2, 3) by O and NO

Abstract

Significant populations of NO in excited vibrational levels v 2 have recently been observed in the lower thermosphere, for which the relaxation rates and pathways are largely governed by collisions with O atoms. Laboratory experiments can provide accurate kinetic parameters for modelling and interpreting such steady-state v-dependent population distributions. In this study, a two-laser, pump–probe arrangement has been used to measure the fractional population of NO(v − 1) arising from the collision-induced relaxation of NO(v = 3) by O atoms and, in an ancillary experiment, NO(v = 2, 3) by NO. The branching fraction χ _O (v = 3→2) = 0.35 ± 0.12 for O-atom collisions. The χ _O value is consistent with a long-lived NO ₂ ^* collision complex, in which the total energy is randomly distributed among the internal degrees of freedom prior to dissociation, and agrees with a recent quasiclassical trajectory calculation. For collisions with NO, χ _NO (v = 3→2) = 0.73 ± 0.19, indicating a significant multiquantum component. The branching fraction χ _NO (v = 2→1) = 1.19 ± 0.31 can be considered an effective value only, since its interpretation relies on an assumption regarding the relaxation mechanism. The rate constants k _O (v = 3) = (3.0 ± 0.6) × 10 ⁻¹¹ cm ³ s ⁻¹ for the vibrational relaxation of NO(v = 3) by O atoms, and k _NO (v = 2) = (2.7 ± 0.5) × 10 ⁻¹² cm ³ s ⁻¹ and k _NO (v = 3) = (3.4 ± 0.7) × 10 ⁻¹² cm ³ s ⁻¹ for the relaxation of NO(v = 2, 3) by NO have also been obtained, and are in good agreement with previous results from this laboratory.