Slow Photoelectron Spectroscopy of δ-Valerolactam and Its Dimer

• Published in: Chemphyschem 2011-07, Vol.12 (10), p.1822-1832
• Main Authors: Mahjoub, Ahmed, Hochlaf, Majdi, Poisson, Lionel, Nieuwjaer, Nicolas, Lecomte, Frédéric, Schermann, Jean-Pierre, Grégoire, Gilles, Manil, Bruno, Garcia, Gustavo A., Nahon, Laurent
• Format: Article
• Language: English
• Subjects: Atomic and Molecular Clusters; Cations - chemistry; Dimerization; dimers; ionization; Models, Theoretical; Photoelectron Spectroscopy; Physics; Piperidones - chemistry; Quantum Theory; Vibration; vibrational transitions; δ-valerolactam
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201100090

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 ${{\rm{\tilde X}}}$ 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 ${{\rm{\tilde X}}}$ ground, ${{\rm{\tilde A}}}$, ${{\rm{\tilde B}}}$ and ${{\rm{\tilde C}}}$ 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. The single‐photon ionizations of gas‐phase δ‐valerolactam (piperidin‐2‐one) and its dimer are studied using vacuum‐ultraviolet 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 ${{{\rm{\tilde {\rm X}}}$ state. For the dimer, a unique large band is observed.