| Abstract
| - A high-resolution IR absorption method is presented for theexperimental determination of state-to-state,integral and differential cross sections for rotationally inelasticenergy transfer. An infrared chromophore,cooled into its lowest rotational state(s) in a pulsed supersonicexpansion, is rotationally excited with lowcollision probability by a gas pulse from a second supersonic jet.The initial and final populations of theinfrared absorber are monitored as a function of J state andof Doppler detuning, via direct absorption ofnarrow bandwidth light from a continuously tunable, CW infrared laser.The scattered and unscattered speciesare detected with Doppler-limited spectral resolution (≲0.01cm-1), providing quantum-state selectivitynotattainable with time-of-flight energy-loss methods. Theinfrared-based probe also permits study of a muchwider class of absorbing species inaccessible to ultraviolet/visiblelaser-induced fluorescence (LIF) or resonance-enhanced multiphoton ionization (REMPI) methods. From fractional IRabsorbances and Beer's law, thecolumn-integrated number densities in each jet are measured directly,which allows absolute, state-to-state,integral cross sections to be determined. Furthermore, thecorrespondence between the molecular velocityand the observed Doppler shift can be used to extract state-to-statedifferential cross sections from the high-resolution line shapes. Details of the experimental technique aredemonstrated via sample studies of state-to-state integral and differential scattering in rare-gas collisionswith CH4.
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