Measurement of the Dissociation Energies of Gas-Phase Neutral Dimers by a Photoionization Technique: Values for trans-2-Butene/Sulfur Dioxide, (trans-2-Butene)₂, and Benzene/Sulfur Dioxide

A method has been developed for the measurement of the dissociation energies of gas-phase neutral dimers using photoionization spectra of mass selected ions from molecular beams generated by jet expansion. This measurement involves the determination of the appearance potential for dissociative photoionization of the dimer using a difference technique. The 0 K dissociation energies obtained are 3.85 ± 0.23, 2.9 ±1.1, and 4.40 ± 0.28 kcal/mol for trans-C₄H₈.SO₂, (trans-C₄H₈)₂, and C₆H₆.SO₂, respectively. The method is applicable for dimers in which the photoionization efficiency function of the partner of lower ionization potential begins with a distinct step at threshold, a condition that is satisfied for many molecules. The technique is best for heterodimers and less good for homodimers. At its best the experimental work can be accomplished within a day for a given dimer, and the data can be analyzed unambiguously in a model-independent fashion.