E. A. Walters

Dissociation energy of the benzenewater van der Waals complex

Abstract The dissociation energy of the complex C 6 H 6 ·H 2 O was determined by the threshold difference method. These complexes were produced in molecular beams via the expansion of a mixture of helium, and benzene and water vapours in the ratio of 96:3:1. The method requires the ionization potential of C 6 H 6 and the appearance potential of C 6 H + 6 from C 6 H 6 ·H 2 O, the difference between which is the dissociation energy. The necessary values are 9.246 and 9.344 ± 0.012 eV, respectively, from which D (C 6 H 6 ·H 2 O) = 0.098 ± 0.012 eV = 2.25 ± 2.25 ± 0.28 kcal mol −1 . The ionization potential of C 6 H 6 ·H 2 O is 9.170 ± 0.014 eV, which leads to the dissociation energy of the complex ion of D ([C 6 H 6 ·H 2 O] + ) = 4.0 ± 0.4 kcal mol −1 .

A photoionization study of the van der Waals molecule C2H4 · HCl

The dissociation energy of the C2H4 · HCl van der Waals complex was determined to be 3.18±0.73 kcal mol−1 by a dissociative photoionization technique. C2H4 · HCl was produced by free expansion of a 1:4 mixture of C2H4 in HCl and the clusters were ionized with tunable synchrotron radiation. The photoionization efficiency function of (C2H4 · HCl)+ from C2H4 · HCl was determined between 600 and 1,300 Å and the onset for (C2H4 · HCl)+ was established as 1,163±2 Å = 10.66±0.02 eV; these values give ΔHf0(C2H4 · HCl) = −10.7±0.7 kcal mol−1 and ΔHf0(C2H4·HCl+)=235.1±0.9 kcal mol−1. A complex ion dissociation energyD0(C2H4 · HCl+) = −0.3±0.9 kcal mol−1 was calculated from the results. The major features on the PIE curve for C2H4 · HCl+ can be analyzed in terms of the known energetic features of C2H4+ and HCl. An extended energy diagram for the C2H4 + HCl system is presented.

Competitive production of weakly bound heterodimers in free jet expansions

Abstract The relative density of heterodimers in molecular beams formed from free jet expansions of gas mixtures is measured into the region where trimers and larger clusters dominate, using single-photon dissociative photoionization below the trimer threshold. Calculations based on simple kinetics and pseudoequilibrium models are consistent with the dimer beam intensities at low pressures, but predict intensities at pressures well above the intensity maxima that are far too small. Possible reasons for the continuation of substantial dimer intensities to high nozzle pressures are discussed.

Production of C2H4Cl+ by dissociative photoionization of weak molecular complexes in C2H4+HCl mixtures

The photoionization efficiency (PIE) spectrum from 600 to 1200 Å for the production of the ion C2H4Cl+ by dissociative photoionization of the products of room-temperature jet expansions of a 1:4 mixture of C2H4 and HCl was measured at several nozzle pressures. The results were resolved into the PIE yield curve for the heterodimer process C2H4·HCl+hv→C2H4Cl++H+e. This reaction is necessarily characterized by a large change in geometry between neutral complex and ionic product. The observed spectrum exhibits an unusual and conspicuous peak at 15.2 eV that is characterized by a sharp cutoff to the high energy side. This feature points to the onset of strongly nonstatistical channels for the production of C2H4Cl+ at this energy such that product formation proceeds through very few states. The observed onset of C2H4Cl+ at 11.92±0.24 eV is 17±6 kcal mol−1 above the true threshold. An important conclusion is that at all energies above the onset the yield of dissociative ionization of the heterodimer to the cation C2H4Cl+ is determined by dynamical factors.