ZEKE electron spectroscopy of alkylbenzene-argon van der Waals complexes

Van der Waals (vdW) complexes of a series of alkylbenzenes (benzene, toluen e, ethylbenzene, and n-propylbenzene) formed with Ar in supersonic jets hav e been studied with two-color zero-kinetic-energy (ZEKE) electron spectrosc opy combined with a PFI (pulsed field ionization) method. The shifts in adi abatic ionization energies (I-a) due to complex formation have been determi ned as -128 and -112 cm(-1) for ethylbenzene-Ar and n-propylbenzene-Ar, res pectively, with respect to the bare molecules. These shifts have been repro duced qualitatively in terms of atom-atom Lennard-Jones (LJ) potentials inc orporating charge-charge-induced-dipole interactions. Some low-frequency ZE KE bands due to vdW Vibrations have been observed for benzene-Ar for the fi rst time. However, no vdW vibrational structures appear in ZEKE electron sp ectra both for ethylbenzene-Ar and n-propylbenzene-Ar. The absence of vdW v ibrational structure in the spectra may be probably due to a steric hindran ce between Ar and the alkyl group. This has been supported also by the pres ent LJ potential calculations. Furthermore, for toluene-Ar, it has been fou nd that upon ionization the relative position of Ar is shifted along the di rection of the C-C(H-3) bond. (C) 2000 Elsevier Science B.V. All rights res erved.