Time-resolved infrared–ultraviolet double-resonance spectroscopy of formaldehyde-d2

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Abstract
This review concentrates on a time-resolved infrared-ultraviolet double-resonance technique which has revealed many aspects of spectroscopic properties and energy-transfer processes involving the formaldehyde-d2 molecule, D2CO. The experiments comprise sequential pulsed excitation of D2CO by CO2 and dye lasers, with visible-fluorescence detection. The infrared Pump laser excites a transition in the v4, v6 or (2v4—v4) vibrational band, which prepares D2CO in a specific rovibrational quantum state. This is followed by rovibronic excitation by a tunable Probe laser, via the 40 1, 60 1 or 4 1/2 vibronic band in the à ←-[Xtilde] electronic absorption system of D2CO. Detailed spectroscopic information is obtained by keeping the product of sample pressure and Pump-Probe delay as small as possible (typically below 10 ns Torr), approaching collision-free conditions. Additional information on a range of collision-induced state-to-state energy transfer processes is obtained by varying the number of collisions experienced by the D2CO molecule in the interval between the Pump and Probe pulses. The following kinetic and mechanistic applications are reviewed: J-changing rotational relaxation arising from long-range molecular interactions; mode-to-mode vibrational energy transfer, with particular emphasis on the role of rotational energy states and intramolecular perturbations; the way in which collision-induced molecular processes may be modified by selecting the rovibrational quantum state of the formaldehyde molecule and by varying its collision partner; and infrared multiple-photon excitation and laser photochemistry.