Electronic, vibrational, and torsional couplings in N-methylpyrrole: Ground, first excited, and cation states

The electronic spectrum associated with the S-1 <- S-0 (A1A2 <- X1A1) one-photon transition of jet-cooled N-methylpyrrole is investigated using laser-induced fluorescence (LIF) and (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy; in addition, the (2 + 2) REMPI spectrum is considered. Assignment of the observed bands is achieved using a combination of dispersed fluorescence (DF), two-dimensional LIF (2D-LIF), zero-electron-kinetic energy (ZEKE) spectroscopy, and quantum chemical calculations. The spectroscopic studies project the levels of the S-1 state onto those of either the S-0 state, in DF and 2D-LIF spectroscopy, or the ground state cation (D-0(+)) state, in ZEKE spectroscopy. The assignments of the spectra provide information on the vibrational, vibration-torsion (vibtor), and torsional levels in those states and those of the S-1 levels. The spectra are indicative of vibronic (including torsional) interactions between the S-1 state and other excited electronic states, deduced both in terms of the vibrational activity observed and shifts from expected vibrational wavenumbers in the S-1 state, attributed to the resulting altered shape of the S-1 surface. Many of the ZEKE spectra are consistent with the largely Rydberg nature of the S-1 state near the Franck-Condon region; however, there is also some activity that is less straightforward to explain. Comments are made regarding the photodynamics of the S-1 state.

Automated summary

The electronic spectrum of N-methylpyrrole was investigated using various spectroscopic techniques and quantum chemical calculations, revealing vibrational-electronic interactions between the S-1 state and other excited states, as well as vibrational, vibration-torsion, and torsional levels in those states.