FRACTIONAL POPULATION OF NO(UPSILON-1) FROM VIBRATIONAL-RELAXATION OFNO(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 thermosphe re, for which the relaxation rates and pathways are largely governed b y 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 o f 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 b ranching fraction chi(0)(v = 3 --> 2) = 0.35 +/- 0.12 for O-atom colli sions. 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 r ecent quasiclassical trajectory calculation. For collisions with NO, c hi(NO)(v = 3 --> 2) = 0.73 +/- 0.19, indicating a significant multiqua ntum component. The branching fraction chi(NO)(v = 2 --> 1) = 1.19 +/- 0.31 can be considered an effective value only, since its interpretat ion relies on an assumption regarding the relaxation mechanism. The ra te 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 1 0(-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 th is laboratory.