Selective infrared excitation in D₂CO: Rotational assignments and the role of collisions

A time-resolved infrared-ultraviolet double-resonance technique is used to characterize specific rovibrationa] states pumped by a pulsed CO₂ laser within the v₄ vibrational ladder of D₂CO. The roles of rotational and quasiresonant rovibrational relaxation, requiring, respectively, ≤1 and ~3 gas-kinetic collisions, are studied. The rotational specificity of the latter suggests that a collision-assisted sequence of infrared absorption steps is able to account for the selective initial stages of multiple-photon excitation in formaldehyde.