Multiphoton ionization spectroscopy of the 3s Rydberg state in the deuterated acetaldehydes

The two-photon resonant n→3s Rydberg transitions in CH3CHO, CH3CDO, CD3CHO, and CD3CDO have been studied by multiphoton ionization in a parallel plate cell with 0.4 cm−1 resolution. Vibrational analysis confirms that the molecule remains planar in the resonant state, and that the methyl group torsional barriers have values of 750±25, 770±25, 750±25, and 780±25 cm−1 in the respective molecules compared to ground state values of 413, 413, 410, and 408 cm−1. The methyl group has an s–cis configuration in both the ground and excited states. Experiments using a supersonic expansion of the sample show that hot bands other than those arising from the low frequency methyl torsion are readily suppressed. Evidence for saturation effects in the MPI spectrum are also presented.