Time-resolved infrared-ultraviolet double resonance in D₂CO and HDCO: Spectroscopic applications

Spectroscopic results, from an extensive study of rotationally resolved infrared-ultraviolet double resonance (IRUVDR) effects in the molecules D₂CO and HDCO, are reported. The experiments involve rovibrational pumping (in the ν₄ or ν₆ bands of D₂CO and the ν₅ or ν₆ bands of HDCO) and rovibronic probing of the A¹A₂ ← X¹A₁ electronic system (in the 4⁰ ₁ or 6⁰ ₁ bands of D₂CO and the 5⁰ ₁ or 6⁰ ₁ bands of HDCO) with visible-fluorescence detection, using sequential pulsed excitation by CO₂ and dye lasers. A remarkable result is the observation of IRUVDR rovibronic features in the very weak and heavily overlapped 6⁰ ₁ band of D₂CO, which has not previously been detected by conventional means. Further experiments which probe the 4¹ ₂ band of D₂CO are interpreted in terms of single-photon rovibrational pumping in the (2ν₄ - ν₄) hot band, rather than two-photon pumping in the 2ν₄ band. The results reported are obtained under effectively collision-free conditions, achieved by maintaining the product of pump-probe delay and sample pressure below ∼10 nsec Torr; collision-induced rotational and rovibrational relaxation is apparent when this product is increased. Other aspects of the IRUVDR technique which are considered include: the relation of IRUVDR results for D₂CO and HDCO to coincidences between CO₂-laser and molecular rovibrational frequencies, previously demonstrated by sub-millimeter optically pumped laser emission, infrared-radiofrequency double resonance, and Lambdip absorption studies; the role of saturation broadening in promoting multiple coincidences of molecular rovibrational transitions with a given CO₂-laser line; relevance to mechanisms of infrared multiple-photon excitation of D₂CO; inconsistencies between observed IRUVDR signal intensities and those anticipated from rovibronic transition probabilities.