ONE-PHOTON AND 2-PHOTON SPECTROSCOPY OF THE 3S[-N RYDBERG TRANSITION OF PROPIONALDEHYDE (PROPANAL)

Vacuum-UV (VUV) absorption and two-photon laser photoionization spectr a of propionaldehyde vapour have been recorded at ambient temperature in the region of the 3s <-- n Rydberg transition and have been compare d with resonance-enhanced multiphoton ionization (REMPI) spectra of th e jet-cooled species. The vibrational analysis of the spectra has conf irmed that the (n,3s) Rydberg state has cis and gauche conformational minima and that the electronic transition has two separate origins. An energy difference of 1260 cm(-1) has been determined between the exci ted-state conformers, and an increase to 1290 cm(-1) of the threefold barrier to internal rotation of the methyl group on excitation of the (n,3s) state has been deduced. The unusual temperature dependence of t he spectra has been explained by transitions from higher thermally pop ulated levels of the aldehyde torsional vibration of the gauche confor mers.