Rotationally resolved infrared-ultraviolet double resonance spectroscopy in molecular D₂CO and HDCO

Rotationally resolved infrared-ultraviolet double resonance (IRUVDR), consisting of sequentially excited rovibrational and rovibronic transitions sharing a common intermediate molecular level, is demonstrated. The technique employs pulsed CO₂ and tunable dye lasers and is applied to the molecules D₂CO and HDCO. For D₂CO, infrared pumping produces 100fold population enhancement in specific rotational sublevels of the v₄ = 1 level of the X̃¹A₁ electronic ground state, followed by ultraviolet excitation in the 365-nm 4₁⁰ band of the à ¹A₂ ← X̃ ¹A₁ electronic system. This ultraviolet excitation occurs at a specific set of dye laser frequencies, determined by the preceding rovibrational transition, and is detected by molecular fluorescence in the visible region. Similar effects observed in HDCO involve rovibrational pumping to either the v₆ = 1 or v₅ = 1 levels and give rise to enhanced rovibronic transitions in the 6₁⁰ and 5₁⁰ bands of the à ← X̃ system, respectively. The resulting IRUVDR spectra enable detailed spectroscopic assignments to be made and are consistent with previous results from infrared and ultraviolet absorption, laser Stark, and infrared-radiofrequency double resonance spectroscopy. Collision-induced satellite structure, arising from rotational relaxation of the intermediate rovibrational level in the IRUVDR sequence, is also reported.