The C=O Stretching Frequency in the S₁ (π^* - n) State of Acetaldehyde and its Deuterated Derivatives Determined With the Photofragment Excitation Spectroscopy

Abstract

The vibrational structure was recorded for the S₁ (π^* - n) state of acetaldehyde (CH₃CHO) and its deuterated derivatives (CH₃CDO, CD₃CHO and CD₃CDO up to ∼3500 cm^-1above the 0-0 bands with the LIF (laser induced fluorescence) and PHOFEX (photofragment excitation) techniques. In the PHOFEX spectroscopy, the yield of the formyl radical, HCO(X ˜) or DCO(X ˜), produced in the photodissociation of acetaldehyde is measured against the excitation frequency. The yield was determined by monitoring one of the rotational lines in the B˜ ← X˜ transition of the formyl radical by the LIF technique. This is the first measurement of the vibronic structure of acetaldehydes in supersonic jets above the dissociation threshold where they are non-fluorescent. The results have made it possible to locate v4 (the C=O stretching) and 2v4 bands unequivocally; the v4 fundamentals at 1217, 1189, 1210 and 1178 cm^-1 above the 0-0 band for CH₃CHO, CH₃CDO, CD₃CHO and CD₃CDO, respectively. The corresponding overtone (2v4) bands are observed, respectively, at 2414, 2350, 2399 and 2334cm^-1. All the vibronic bands newly observed in the PHOFEX spectra are found to be interpreted as the combinations of v4 and 2v4 with the modes appearing in the lower frequency (< 1200 cm^-1) region, which consist of v10 (the C—C—O in-plane bending), v14 (the C=O out-of-plane wagging) and v15 (the methyl torsion).