STATE-TO-STATE, ROTATIONAL ENERGY-TRANSFER DYNAMICS IN CROSSED SUPERSONIC JETS - A HIGH-RESOLUTION IR ABSORPTION METHOD

A high-resolution IR absorption method is presented for the experiment al determination of state-to-state, integral and differential cross se ctions for rotationally inelastic energy transfer. An infrared chromop hore, cooled into its lowest rotational state(s) in a pulsed supersoni c expansion, is rotationally excited with low collision probability by a gas pulse from a second supersonic jet. The initial and final popul ations of the infrared absorber are monitored as a function of J state and of Doppler detuning, via direct absorption of narrow bandwidth li ght from a continuously tunable, CW infrared laser. The scattered and unscattered species are detected with Doppler-limited spectral resolut ion (less than or similar to 0.01 cm(-1)), providing quantum-state sel ectivity not attainable with time-of-flight energy-loss methods. The i nfrared-based probe also permits study of a much wider class of absorb ing species inaccessible to ultraviolet/visible laser-induced fluoresc ence (LIF) or resonance-enhanced multiphoton ionization (REMPI) method s. From fractional IR absorbances and Beer's law, the column-integrate d number densities in each jet are measured directly, which allows abs olute, state-to-state, integral cross sections to be determined. Furth ermore, the correspondence between the molecular velocity and the obse rved Doppler shift can be used to extract state-to-state differential cross sections from the high-resolution line shapes. Details of the ex perimental technique are demonstrated via sample studies of state-to-s tate integral and differential scattering in rare-gas collisions with CH4.