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

Significant populations of NO in excited vibrational levels v greater than or equal to 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 chi(0)(v = 3 --> 2) = 0.35 +/- 0.12 for O-atom collisions. The chi(0) value is consistent with a long-lived NO2* 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, chi(NO)(v = 3 --> 2) = 0.73 +/- 0.19, indicating a significant multiquantum component. The branching fraction chi(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(0)(v = 3) = (3.0 +/- 0.6) x 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) x 10(-12) cm(3) s(-1) and k(NO)(v = 3) = (3.4 +/- 0.7) x 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.