Excitation energies, photoionization cross sections, and asymmetry parameters of the methyl and silyl radicals

Producción CientíficaVertical excitation energies of the methyl and silyl radicals were inferred from ab initio electron propagator calculations on the electron affinities of CH3 + and SiH3 +. Photoionization cross sections and angular distribution of photoelectrons for the outermost orbitals of both CH3 and SiH3 radicals have been obtained with the Molecular Quantum Defect Orbital method. The individual ionization cross sections corresponding to the Rydberg channels to which the excitation of the ground state’s outermost electron gives rise are reported. Despite the relevance of methyl radical in atmospheric chemistry and combustion processes, only data for the photon energy range of 10–11 eV seem to be available. Good agreement has been found with experiment for photoionization cross section of this radical. To our knowledge, predictions of the above mentioned photoionization parameters on silyl radical are made here for the first time, and we are not aware of any reported experimental measurements. An analysis of our results reveals the presence of a Cooper minimum in the photoionization of the silyl radical. The adequacy of the two theoretical procedures employed in the present work is discussed.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA330U13

Gas-phase infrared spectra of cationized nitrogen-substituted polycyclic aromatic hydrocarbons

Gas-phase infrared spectra of several ionized nitrogen substituted polycyclic aromatic hydrocarbons (PANHs) have been recorded in the 600-1600 cm(-1) region via IR multiple-photon dissociation (IRMPD) spectroscopy. The UV photoionized PANH ions are trapped and isolated in a quadrupole ion trap where they are irradiated with an IR free electron laser. The PANHs were studied in their radical cation (PANH(+)) and protonated (H+ PANH) forms, and include quinoline, isoquinoline, phenanthridine, benzo[h] quinoline, acridine, and dibenzo[f,h] quinoline. Experimental IRMPD spectra were interpreted with the aid of density functional theory methods. The PANH(+) IR spectra are found to resemble those of their respective non-nitrogenated PAH cations. The IR spectra of H+ PANHs are significantly different owing to the NH inplane bending vibration, which generally couples very well with the aromatic CH bending and CC stretching modes. Implications of the NPAH (+, H+) laboratory spectra are discussed for the astrophysical IR emissions and, in particular, for the band at 6.2 mu m