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

This review concentrates on a time-resolved infrared-ultraviolet doubleresonance 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 4?, 6? or 42 vibronic band in the A«-X 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 lOnsTorr), 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:./-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.