ZEKE electron spectroscopy of azulene and azulene–argon

Abstract

Mass-selected ion-current spectra and zero-kinetic-energy (ZEKE) electron spectra were obtained for azulene and its van der Waals (vdW) complex with Ar in supersonic jets by two-photon (1+1′) resonant ionization through the second singlet electronic excited states (S2). Ab initio calculations were also carried out to study the optimized geometries and vibrational modes for azulene in the neutral and cation ground states (S2 and D0). Lennard-Jones (LJ) potential energy calculations including `charge–charge-induced-dipole interactions' were also made for azulene–Ar. The main results may be summarized as follows. (1) The adiabatic ionization energies have been determined as: Ia(azulene)=59781±5 cm−1 and Ia(azulene–Ar)=59708±5 cm−1. The difference in Ia is 73 cm−1. (2) Several vibrational frequencies of (azulene)+ have been observed and identified on the basis of ab initio theoretical calculations. (3) A vibrational progression with a spacing of 9–10 cm−1 in the ZEKE spectra of azulene–Ar, has been assigned experimentally and theoretically to the vdW bending vibration bx+1 along the long axis of azulene. (4) From the calculated LJ potential energy minima, it has been found that Ar is shifted by 0.10Å along the long axis of azulene from the position in the neutral electronic ground state. (5) The observed vdW vibrational progressions have been reproduced by Franck–Condon calculations, suggesting that Ar is shifted by 2° for (azulene–Ar)+ with respect to their neutral S2 state.