Mass-Analyzed-Threshold-Ionization-Spectroscopy of Pyrazine and Pyrazine-Ar

Abstract

Vibrational spectra of the pyrazine and the pyrazine-Ar cation in the electronic ground state (D 0) have been measured via several intermediate states of the first excited state (S 1) by mass-analyzed threshold ionization spectroscopy. Additionally, ab-initio calculations at the DFT, CIS and MP2 level of theory have been conducted to compare experimental findings with theory. For the pyrazine monomer a negative anharmonic behavior for combination bands containing the 16a or the 1 vibration could be observed for the first time. It was found that the 16a vibration has to be considered when discussing the nature of the vibronic coupling in the first excited state (S 1) as well as in the ionic ground state (D 0). For some vibrations a new assignment is given in the first excited state (S 1) and the ionic ground state (D 0). An earlier discussed appearance of internal vibrational redistribution (IVR) by activating the 516a1 vibrational state in the first electronically excited state (S 1) is rejected due to the results of the presented measurements. For the pyrazine-Ar complex the ionization energy, as well as the binding energy in the ionic and the neutral ground state could be determined. These values are given by 74632 ± 5 cm-1, 630 cm-1 ± 20 cm-1 and 349 cm-1 ± 20 cm-1, respectively. The van-der-Waals vibrations in the first excited state could be reassigned and the frequencies of these vibrations have been found to be 12 cm-1, 27 cm-1 and 39 cm-1.