Slow Photoelectron Spectroscopy of δ‐Valerolactam and Its Dimer

Abstract

We studied the single-photon ionization of gas-phase δ-valerolactam (piperidin-2-one) and of its dimer using vacuum-ultraviolet (VUV) synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer. The slow photoelectron spectrum (SPES) of the monomer is dominated by the vibrational transitions to the ͠X state. Moreover, several weaker and complex bands are observed, corresponding to the population of the vibrational bands (pure or combination) of the electronically excited states of the cation arising from their mutual vibronic interactions. For the dimer, we measure a unique large band. These spectra are assigned with the help of theoretical calculations dealing with the equilibrium geometries, electronic-state patterns and evolutions, harmonic and anharmonic wavenumbers of the monomer and dimer, either neutral or positively charged. The state energies of the [δ-valerolactam](+) cation in the ͠X ground, ͠A, ͠B, ͠C, excited electronic states, and of the [δ-valerolactam](2) (+) cation's lowest states are determined. After its formation, [δ-valerolactam](2) (+) is subject to intramolecular isomerization, H transfer and then unimolecular fragmentation processes. Close to the ionization thresholds, the photoionization of these molecules is found to be mainly dominated by a direct process whereas the indirect route (autoionization) contributes at higher energies.

Photoionization Spectroscopy of Nucleobases and Analogues in the Gas Phase Using Synchrotron Radiation as Excitation Light Source

“…We display in Fig. 5 the CASSCF/aug-cc-pVDZ one-dimensional evolutions of the potential energy surfaces of the lowest doublet electronic states of [δ-valerolactam] + along the planarization angle θ and along the normal coordinate relative to the out-of-plane torsion mode 41 [95]. This figure reveals that the ground electronic state e X 2 A and the first excited state e A 2 A become close to each other for θ~190 and Δτ~14 .…”

“…Their spectra are composed of well-resolved bands extending over~1 eV each which were attributed to ionization from π N , n O for the first two bands and to the inner valence shell ionization for the higher ones. Recently, Mahjoub et al [95] studied the single photon ionization of gas phase δ-valerolactam and the state-to-state fragmentation of [δ-valerolactam] + by means of VUV synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer [97]. These authors have also performed extensive state-of-the-art computations on equilibrium geometries, electronic state patterns, and evolutions, harmonic, and anharmonic frequencies of neutral and positively charged δ-valerolactam, and for over 170 possible dissociation channel products.…”

“…The synthetic spectrum (black line) is obtained after a rough adjustment of the experimental spectra assuming few Gaussian profiles: the narrow bands at the left side are for the ground state and the larger ones are for the two contributing autoionizing states (the dotted and dashed blue lines). Lower panel: SPES spectrum of delta-valerolactam + in the vicinity of the cationic ground state and its tentative assignment (vertical lines) by the help of the theoretical results [95]. Copyright © Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim T + cation.…”

“…6 as an inset which corresponds to an enlargement of the SPES spectrum in the vicinity of the cationic ground state. The tentative assignment of the observed bands (vertical lines) is achieved with the help of the theoretical results [95].…”

“…Several structural and spectroscopic studies were performed on the neutral δ-valerolactam molecule. They are reviewed in [95]. For the cation, Potapov et al [131] and Treschanke and Rademacher [132,133] respectively by means of HeI photoelectron spectroscopy.…”