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 ga uche (C-1) conformer which is known to be 250 cm(-1) lower in energy than t he more symmetric trans (C-s) conformer. The spectra are dominated by the l ow-frequency formyl torsion which appears in combination with the two methy l torsions (in-phase and out-of-phase) and with several skeletal modes. Vib rational assignments have been aided by ab initio calculations of the (n, 3 s) Rydberg state. (C) 2001 Academic Press.