title: Threshold photoelectron spectroscopy of the methyl radical isotopomers, CH3, CH2D, CHD2 and CD3: synergy between VUV synchrotron radiation experiments and explicitly correlated coupled cluster calculations.
abstract: Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.
authors.affiliation: Laboratoire de Chimie Physique, UMR 8000, CNRS et Université Paris-Sud 11, Bât. 350, Centre Universitaire Paris-Sud, F-91405 Orsay, France. barbara.miranda@u-psud.fr
authors.forename: Bárbara K
authors.initials: BK
authors.lastname: Cunha de Miranda
authors.forename: Christian
authors.initials: C
authors.lastname: Alcaraz
authors.forename: Mohamed
authors.initials: M
authors.lastname: Elhanine
authors.forename: Bastian
authors.initials: B
authors.lastname: Noller
authors.forename: Patrick
authors.initials: P
authors.lastname: Hemberger
authors.forename: Ingo
authors.initials: I
authors.lastname: Fischer
authors.forename: Gustavo A
authors.initials: GA
authors.lastname: Garcia
authors.forename: Héloïse
authors.initials: H
authors.lastname: Soldi-Lose
authors.forename: Bérenger
authors.initials: B
authors.lastname: Gans
authors.forename: Luiz A Vieira
authors.initials: LA
authors.lastname: Mendes
authors.forename: Séverine
authors.initials: S
authors.lastname: Boyé-Péronne
authors.forename: Stéphane
authors.initials: S
authors.lastname: Douin
authors.forename: Jan
authors.initials: J
authors.lastname: Zabka
authors.forename: Peter
authors.initials: P
authors.lastname: Botschwina