Methyl rotor effects on acetone Rydberg spectra. I. The 1A2(3p←n)←1A1 transition

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Abstract
Two‐photon spectra of the acetone 1A2(3px←n)←1A1 Rydberg transition obtained by injection seeded excitation determines a2(ν12) and b1(ν17) acetone 1A2 Rydberg state torsional fundamental frequencies as 70.8 cm−1 (ν’12) and 129.4 cm−1 (ν’17) (compared to the ground‐state values 77.8 and 124.5 cm−1, respectively). Corresponding values in (CD3)2CO are 49.1 and 98.2 cm−1, compared to 53.4 and 96.0 cm−1 in the ground state. The 1A2 state 2ν’12, 3ν’12, and ν’12 + ν’17 energies are also assigned allowing determination of excited‐state potential constants. There are large changes (65–200 cm−1) in these constants from those for the ground state causing significant perturbation in the shape of the 1A2 state potential from that of the ground state. The 1A2 constants are V3=551, V33=349, V’33=−221, and V6=0 cm−1. The effective potential barrier height to internal rotation (202 cm−1) is decreased from the ground state by about 15%, i.e., by ≊35 cm−1; but the barrier height for eclipsed–eclipsed→staggered–staggered synchronous‐rotation is increased by ≊300 cm−1 (i.e., ≊35%) to 1103 cm−1.