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

The vibrational structure was recorded for the S 1 ( π * - n) state of acetaldehyde (CH 3 CHO) and itsdeuterated derivatives (CH 3 CDO, CD 3 CHO and CD 3 CDO up to ∼3500 cm -1 above the 0-0 bands withthe LIF (laser induced fluorescence) and PHOFEX (photofragment excitation) techniques. In thePHOFEX spectroscopy, the yield of the formyl radical, HCO( X ˜ ) or DCO( X ˜ ), produced in the photodissociationof acetaldehyde is measured against the excitation frequency. The yield was determined bymonitoring 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 locatev 4(the C=O stretching) and 2 v 4bands unequivocally; thev 4 fundamentals at 1217, 1189, 1210 and 1178 cm -1 above the 0-0 band for CH 3 CHO, CH 3 CDO, CD 3 CHO and CD 3 CDO, respectively. The correspondingovertone ( 2 v 4 )bands are observed, respectively, at 2414, 2350, 2399 and 2334cm -1 . All the vibronicbands newly observed in the PHOFEX spectra are found to be interpreted as the combinations ofv 4and 2 v 4 with the modes appearing in the lower frequency (< 1200 cm -1 ) region, which consist ofv 10(theC—C—O in-plane bending),v 14(the C=O out-of-plane wagging) andv 15 (the methyl torsion).