One- and two-photon spectroscopy of the 3s←n Rydberg transition of propionaldehyde (propanal)

Abstract

Vacuum-UV (VUV) absorption and two-photon laser photoionization spectra of propionaldehyde vapour have been recorded at ambient temperature in the region of the 3s←n Rydberg transition and have been compared with resonance-enhanced multiphoton ionization (REMPI) spectra of the jet-cooled species. The vibrational analysis of the spectra has confirmed 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 excited-state conformers, and an increase to 1290 cm ⁻¹ of the three-fold barrier to internal rotation of the methyl group on excitation of the (n,3s) state has been deduced. The unusual temperature dependence of the spectra has been explained by transitions from higher thermally populated levels of the aldehyde torsional vibration of the gauche conformers.