Time-resolved infrared-ultraviolet double resonance in D 2CO and HDCO: Spectroscopic applications

Abstract

Spectroscopic results, from an extensive study of rotationally resolved infrared-ultraviolet double resonance (IRUVDR) effects in the molecules D 2CO and HDCO, are reported. The experiments involve rovibrational pumping (in the ν 4 or ν 6 bands of D 2CO and the ν 5 or ν 6 bands of HDCO) and rovibronic probing of the A 1A 2 ← X 1A 1 electronic system (in the 4 0 1 or 6 0 1 bands of D 2CO and the 5 0 1 or 6 0 1 bands of HDCO) with visible-fluorescence detection, using sequential pulsed excitation by CO 2 and dye lasers. A remarkable result is the observation of IRUVDR rovibronic features in the very weak and heavily overlapped 6 0 1 band of D 2CO, which has not previously been detected by conventional means. Further experiments which probe the 4 1 2 band of D 2CO are interpreted in terms of single-photon rovibrational pumping in the (2 ν 4 - ν 4) hot band, rather than two-photon pumping in the 2 ν 4 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 D 2CO and HDCO to coincidences between CO 2-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 CO 2-laser line; relevance to mechanisms of infrared multiple-photon excitation of D 2CO; inconsistencies between observed IRUVDR signal intensities and those anticipated from rovibronic transition probabilities.