Fluorescence-detected Raman-optical double-resonance spectroscopy of glyoxal vapor

A novel Raman-optical double-resonance (ODR) technique is described in detail and applied to studies of molecular glyoxal (C₂H₂O₂). The technique employs a pulsed excitation sequence, consisting of coherent Raman pumping of a molecular rovibrational transition followed by rovibronic probing through visible laser-induced fluorescence (LIF). The experiments demonstrate a 10³-fold enhancement of sensitivity, relative to established coherent Raman spectroscopic methods, and enable individual sets of O-, P-, Q-, R-, and S-branch Raman transitions to be distinguished with high specificity under effectively collision-free conditions. For trans-glyoxal, a typical excitation sequence is $\tilde{X}$, v = 0, (J″, K″) → $\tilde{X}$, v₂ = 1, (J, K) → Ã, v′ = 0, (J′, K′), where $\tilde{X}$ and à denote the ground (¹A_g) and first excited (¹A_u) electronic singlet states, respectively, and ν₂ is the symmetric CO stretching mode of vibration. There is also evidence of contributions from hot bands involving sequences in the low-frequency modes, ν₇ and ν₁₂. The Raman-ODR spectra are analyzed to yield new spectroscopic constants for the $\tilde{X}$, v₂ = 1 vibronic level of trans-glyoxal. Variation of the time delay between Raman-excitation and LIF-probe pulses has permitted direct observation of collision-induced rotational relaxation in the ground electronic manifold of glyoxal at a rate that is ~6.5 times gas-kinetic.