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 (1A2)S1←(1A1)S0 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=−45 cm−1, B″=5.37 cm−1 for the ground state and V′6=17 cm−1, B′=5.24 cm−1 for the S1 excited state.