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 X, v = 0, (J", K") X, V2 = 1, (J”, K”) Ã, ' = 0, (J', K'), where X and Ä denote the ground l Ag) and first excited (VA) electronic singlet states, respectively, and v% is the symmetric CO stretching mode of vibration. There is also evidence of contributions from hot bands involving sequences in the low-frequency modes, v’ and v12- The Raman-ODR spectra are analyzed to yield new spectroscopic constants for the 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.
|Number of pages||12|
|Journal||Journal of the Optical Society of America B: Optical Physics|
|Publication status||Published - 1985|