Methyl rotor effects on acetone Rydberg spectra. II. The 1B2(3s←n)←1A1 transition

Abstract

Both a2(ν12) and b1(ν17) methyl torsion fundamentals and their overtones are found to be active in two-photon resonance multiphoton ionization jet spectra of the acetone 1B2(3s←n)←1A1 Rydberg transition. The acetone 3s Rydberg state torsional fundamental frequencies determined from fundamental and sequence band measurements are 118 cm−1 (ν′12) and 175 cm−1 (ν′17) compared to ground state values 77.8 and 124.5 cm−1, respectively. Corresponding values in fully deuterated acetone are 83 and 132 cm−1 compared to 53.4 and 96.0 cm−1 in the ground state. Our measured frequencies differ significantly (greatly so for the gearing torsional fundamental ν′17) from published values for both acetone-h6 and -d6. The 3s state 2ν′12, 2ν′17, and ν′12 + ν′17 frequencies are also measured allowing determination of excited state methyl torsion potential constants. The important increase (∼250 cm−1) in the 3s state V3 constant from its ground state value leads to a large increase in the potential barrier height to internal rotation, i.e., ≊1250 cm−1, 50% greater than the ≊800 cm−1 ground state barrier. This result is similar to the large increase in barrier height for eclipsed–eclipsed→staggered–staggered synchronous rotation recently found for the 1A2(3px←n) Rydberg state. There are, however, important differences in potential curvature between the 3s and 3px state internal rotation potential functions.