A NEW DIRECT INFRARED-LASER ABSORPTION METHOD FOR STATE-TO-STATE ROTATIONAL ENERGY-TRANSFER IN CROSSED SUPERSONIC JETS - EXPERIMENTAL RESULTS AND QUANTUM SCATTERING ANALYSIS FOR AR+CH4

A new method for measuring state-to-state rotational energy transfer i n crossed supersonic beams is described. The method is based on direct absorption of tunable, high-resolution infrared laser light by target molecules collisionally excited into final rotational states. The dir ect IR absorption approach offers high sensitivity, full quantum state resolution, a Doppler probe of final velocity components, and is appl icable to any target molecule that absorbs in the near IR. Preliminary results are presented for Ar + CH4 scattering in crossed supersonic b eams at a mean center-of-mass collision energy of 41 meV. Because of t he high spectral resolution, the method can readily distinguish rotati onal fine structure states of A, F, and E symmetry in the tetrahedral group, as well as the much more energetically separated final j states . The results are compared with full quantum close-coupling calculatio ns on two different Ar + CH4 potential energy surfaces. The state-to-s tate scattering results provide a sensitive measure of the potential a nisotropy, and in particular probe the relative magnitudes of the diff erent anisotropic terms in the potential (V3 and V4).