ZEKE electron spectroscopy of azulene and azulene-argon

Mass-selected ion-current spectra and zero-kinetic-energy (ZEKE) electron s pectra were obtained for azulene and its van der Waals (vdW) complex with A r in supersonic jets by two-photon (1 + 1') resonant ionization through the second singlet electronic excited states (S-2). Ab initio calculations wer e also carried out to study the optimized geometries and vibrational modes for azulene in the neutral and cation ground states (S-2 and D-0). Lennard- Jones (LJ) potential energy calculations including 'charge-charge-induced-d ipole interactions' were also made for azulene-Ar. The main results may be summarized as follows. (1) The adiabatic ionization energies have been dete rmined as: I-a(azulene) = 59781 +/- 5 cm(-1) and I-a(azulene-Ar) = 59708 +/ - 5 cm(-1). The difference in I-a is 73 cm(-1). (2) Several vibrational fre quencies of (azulene)+ have been observed and identified on the basis of ab initio theoretical calculations. (3) A vibrational progression with a spac ing of 9-10 cm(-1) in the ZEKE spectra of azulene-Ar, has been assigned exp erimentally and theoretically to the vdW bending vibration b(x)(+1) along t he long axis of azulene. (4) From the calculated LJ potential energy minima , it has been found that Ar is shifted by 0.10 Angstrom along the long axis of azulene from the position in the neutral electronic ground state. (5) T he observed vdW vibrational progressions have been reproduced by Franck-Con don calculations, suggesting that Ar is shifted by 2 degrees for (azulene-A r)(+) with respect to their neutral S-2 state. (C) 1998 Elsevier Science B. V. All rights reserved.