Internal rotation of the methyl group in the S0 and S1 (3s Rydberg) electronic states of N-methylpyrrole

The one-photon and two-photon REMPI spectra of the ((1)A(2))S-1 <-- ((1)A(1))S-o Rydberg transition of jet-cooled N-methylpyrrole have been investigated, The transition is one-photon symmetry forbidden and a number of well-resolved low-frequency bands are present in the one-photon resonant MPI spectrum, on both sides of the absent origin, These spectral features. assigned to pure torsional transitions of the methyl group, gain intensity from an electronic motion-internal rotation coupling mechanism. The analysis afforded the potentials for the internal rotation of the methyl group in both states: V-6(n) = -45 cm(-1), B-n = 5.37 cm(-1) for the ground state and V (n)(6) = 17 cm(-1). B' = 5.24 cm(-1) for the S-1 excited state. (C) 2002 Elsevier Science B.V. All rights reserved.