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

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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.