MASS-ANALYZED THRESHOLD IONIZATION OF VAN-DER-WAALS COMPLEXES - BINDING-ENERGIES OF DIBENZOFURAN-CENTER-DOT-AR AND DIBENZO-P-DIOXIN-CENTER-DOT-AR

The dissociation behavior of the weakly bound van der Waals complexes dibenzofuran.Ar (DBF.Ar) and dibenzo-p-dioxin.Ar(DD.Ar) is investigate d by mass analyzed threshold ionization (MATI) spectroscopy. Adiabatic ionization energies (AIEs) were measured for the bare molecules and t he complexes, yielding 65422 cm(-1) (DBF), 65307 cm(-1) (DBF.Ar), 6128 3 cm(-1) (DD) and 61071 cm(-1) (DD.Ar). The breakdown of threshold ion signal at the complex mass and its appearance on the fragment mass ch annel leads to the following dissociation thresholds E-0 in the ionic ground state: E-0 less than or equal to 636 cm(-1) for (DBF.Ar)(+) and E-0 = 739 +/- 10 cm(-1) for (DD.Ar)(+). From the red-shifts of the AI Es of -115 cm(-1) (DBF.Ar) and -212 cm(-1) (DD.Ar) the dissociation en ergies D-0 in the neutral ground state complexes are found to be D-0 l ess than or equal to 521 cm(-1) for DBF.Ar and D-0 = 527 +/- 18 cm(-1) for DD.Ar. While the ionic dissociation energies differ for both van der Waals systems, their neutral dissociation energies are nearly the same. This points to large charge-induced contributions to the binding energy in DD Ar because of charge localization at the central, oxygen containing ring. (C) 1997 Elsevier Science B.V.