Jet-cooled multiphoton ionization spectroscopy of the iso-butanal 3s←n Rydberg transition

We report the results of a study on the vibrational structure in the (n, 3s) Rydberg state of iso-butanal, carried out via 2+1 multiphoton ionization spectroscopy in a supersonic expansion. Spectra were recorded for both the normal species, (CH3)(2)CHCHO, and the deuterated isotopomer, (CH3)(2)CDCHO. The spectra show a complicated vibrational pattern attributable to the gauche (C-1) conformer which is known to be 250 cm(-1) lower in energy than the more symmetric trans (C-s) conformer. The spectra are dominated by the low-frequency formyl torsion which appears in combination with the two methyl torsions (in-phase and out-of-phase) and with several skeletal modes. Vibrational assignments have been aided by ab initio calculations of the (n, 3s) Rydberg state. (C) 2001 Academic Press.