A COMBINED EXPERIMENTAL AND THEORETICAL-STUDY OF ROTATIONAL ENERGY-TRANSFER IN COLLISIONS BETWEEN NO(X-(2)PI(1 2), V=3,J) AND HE, AR AND N-2 AT TEMPERATURES DOWN TO 7 K/

Infrared-ultraviolet double resonance (IRUVDR) experiments have been i mplemented in the ultra-cold environment provided by a CRESU (Cinetiqu e de Reaction en Ecoulement Supersonique Uniforme) apparatus. With thi s technique rate coefficients of two kinds have been measured fop rota tional energy transfer in collisions between NO and He, Ar and N-2: (a ) rate coefficients for total removal from specific states of NO(X (2) Pi(1/2); upsilon = 3; J = 0.5, 3.5 or 6.5) and (b) state-to-state rate coefficients for rotational energy transfer from these levels to spec ific final states. Using different Laval nozzles, results have been ob tained at several different temperatures: for He as collision partner, 295, 149, 63, 27, 15 and 7 K; for Ar, 139, 53, 44 and 27 K; and for N -2, 86 and 47 K. The thermally averaged cross-sections for total remov al show remarkably little variation, either with temperature or with i nitial rotational state. The variation of state-to-state rate coeffici ents with Delta J shows three general features: (i) a decrease with in creasing Delta J; (ii) a propensity to favor even Delta J transitions over odd Delta J changes; and (iii) at lower temperatures, decreases i n J are increasingly favored over increases in J and the distribution of rate coefficients against Delta J becomes narrower. The experimenta l rate coefficients for collisions with He and Ar are compared with th ose from both close coupled and coupled states calculations based on p otential energy surfaces determined within the coupled electron pair a pproximation (CEPA) with a large atomic orbital basis set. The agreeme nt between theory and experiment of both the total and the state-to-st ate rate coefficients is excellent over the complete range of temperat ures covered in the experiments. (C) 1998 American institute of Physic s. [S0021-9606(98)02034-0].