Zero Kinetic Energy (ZEKE) Photoelectron Study of Fluorobenzene−Argon van der Waals Complexes

Abstract

Two kinds of van der Waals (vdW) complexes of fluorobenzene (FB) with argon, FB−Ar and FB−Ar2, produced in supersonic jets were studied by mass-selected ion-current spectra and zero kinetic energy (ZEKE) photoelectron spectra by using two-photon resonant ionization through the first singlet electronic excited states (S1). The results obtained from the ZEKE photoelectron spectra may be summarized as follows. (1) The adiabatic ionization energies have been determined as Ia(FB) = 74 238 ± 4 cm-1 (9.2043 ± 0.0005 eV), Ia (FB−Ar) = 74 011 ± 4 cm-1 (9.1762 ± 0.0005 eV), and Ia (FB−Ar2) = 73 816 ± 4 cm-1 (9.1521 ± 0.0005 eV). (2) The decreases in Ia have been found to be 227 and 422 cm-1 for FB−Ar and FB−Ar2, respectively, this fact indicating that the increases in the dissociation energy upon photoionization are 227 and 422 cm-1, respectively. (3) Well-resolved vibrational progressions with frequencies of 12 and 9 cm-1 have been observed for the vdW cations (FB−Ar)+ and (FB−Ar2)+, respectively, and assigned to the vdW bending vibrations bx+1 and bxs+1, respectively, along the x axis (the C−F bond), by selecting some appropriate S1 vdW vibrational levels as intermediate resonant states. (4) Two more bands appearing at 31 and 50 cm-1 above the origin for (FB−Ar)+ may be assigned to the vdW bending (by+2) and the vdW stretching (sz+1), respectively. (5) From Franck−Condon calculations, it has been found that Ar is shifted by 7° and 6° for (FB−Ar)+ and (FB−Ar2)+, respectively, with respect to their neutral S1 states.