A state-specific two-laser technique is used to investigate the collisional removal of O 2 molecules in the b 1 ⌺ g ϩ (vϭ1,2) levels, where we directly excite O 2 and then probe the populations by resonance-enhanced multiphoton ionization. We find general agreement with earlier 300 K values for vϭ1 removal by O 2 , and show that vϭ2 removal is slower by a factor of 5.6Ϯ0.6 than vϭ1 removal. Only upper limits are obtained for N 2 as a collider. For removal of vϭ1 in the atmosphere, N 2 is unimportant compared to O 2 , but it might be competitive for vϭ2. For CO 2 as a collider, addressing O 2 (b 1 ⌺ g ϩ ) removal in the atmospheres of Mars and Venus, the removal rate coefficients of the vibrationally-excited levels are similar to that for vϭ0. The significance of the large difference in the vϭ1 and vϭ2 rate coefficients for O 2 collisions will be discussed as it relates to the modeling of recent earth nightglow observations.

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The ground-state rotational spectra of the four isotopomers H, ' 5N-* .79Br35CI, H,' 5N* -.81 Br3%I, H, ' 5N* -* 79Br37CI and H, ' 5N. -.*l Br37CI of a complex formed by ammonia and bromine monochloride have been observed by pulsed-nozzle, FT microwave spectroscopy. A fast-mixing nozzle was used to prevent a chemical reaction between the components and allow the encounter complex to be isolated. The nature of the spectra establishes that the observed isotopomers are symmetric-top molecules. Assignment and analysis of the spectrum in each case led to the rotational constant 8, , the centrifugal distortion constantsD, and D,, , the halogen nuclear quadrupole coupling constants X(Br) and x(CI), and the component M,, (Br) of the bromine spin-rotation coupling tensor. The rotational constants allowed an r,-type value of !he distance N.-.Br of 2.627 A to be established. An r,-type method led to the distances r ( N . -.Br) = 2.59(1) A and r(Br-CI) = 2.186 A, the former requiring a value of B, for H,14N-9 -79Br35CI obtained from unperturbed transition centres estimated without a full hyperfine structure analysis. A consideration of the intermolecular stretching force constant k, , determined from D,, provides evidence of a relatively strong interaction between the subunits. However, an analysis of the x(X) (X = Br, CI) values reveals that the molecular interaction is mainly electrostatic in origin, with probably only a small extent of intermolecular electric charge redistribution on complex formation.

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