ZEKE electron spectroscopy of alkylbenzene–argon van der Waals complexes

Abstract

Van der Waals (vdW) complexes of a series of alkylbenzenes (benzene, toluene, ethylbenzene, and n-propylbenzene) formed with Ar in supersonic jets have been studied with two-color zero-kinetic-energy (ZEKE) electron spectroscopy combined with a PFI (pulsed field ionization) method. The shifts in adiabatic ionization energies ( I a ) due to complex formation have been determined as −128 and −112 cm −1 for ethylbenzene–Ar and n-propylbenzene–Ar, respectively, with respect to the bare molecules. These shifts have been reproduced qualitatively in terms of atom–atom Lennard–Jones (LJ) potentials incorporating charge–charge-induced-dipole interactions. Some low-frequency ZEKE bands due to vdW vibrations have been observed for benzene–Ar for the first time. However, no vdW vibrational structures appear in ZEKE electron spectra both for ethylbenzene–Ar and n-propylbenzene–Ar. The absence of vdW vibrational structure in the spectra may be probably due to a steric hindrance between Ar and the alkyl group. This has been supported also by the present LJ potential calculations. Furthermore, for toluene–Ar, it has been found that upon ionization the relative position of Ar is shifted along the direction of the C–C(H 3 ) bond.