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 experimental determination of state-to-state, integral and differential cross sections for rotationally inelastic energy transfer. An infrared chromophore, cooled into its lowest rotational state(s) in a pulsed supersonic expansion, is rotationally excited with low collision probability by a gas pulse from a second supersonic jet. The initial and final populations of the infrared absorber are monitored as a function of J state and of Doppler detuning, via direct absorption of narrow bandwidth light from a continuously tunable, CW infrared laser. The scattered and unscattered species are detected with Doppler-limited spectral resolution (less than or similar to 0.01 cm(-1)), providing quantum-state selectivity not attainable with time-of-flight energy-loss methods. The infrared-based probe also permits study of a much wider class of absorbing species inaccessible to ultraviolet/visible laser-induced fluorescence (LIF) or resonance-enhanced multiphoton ionization (REMPI) methods. From fractional IR absorbances and Beer's law, the column-integrated number densities in each jet are measured directly, which allows absolute, state-to-state, integral cross sections to be determined. Furthermore, the correspondence between the molecular velocity and the observed Doppler shift can be used to extract state-to-state differential cross sections from the high-resolution line shapes. Details of the experimental technique are demonstrated via sample studies of state-to-state integral and differential scattering in rare-gas collisions with CH4.