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Photofragmentation and Recombination Dynamics of Partially Solvated Anionic Clusters

TitlePhotofragmentation and Recombination Dynamics of Partially Solvated Anionic Clusters
Publication TypeThesis
Year of Publication2012
AuthorsMartin, JP

Photofragmentation studies of mass-selected, partially-solvated anionic clusters are performed to investigate solvent-number specific perturbations to the dissociation dynamics of anionic chromophores. Two anion solutes are employed in the studies reported here: IBr− and the chemically-related ICN−. While the former has been the subject of earlier photodissociation studies, ICN− has not been studied previously. Replacing Br with CN allows the possibilities of isomerization to INC− and rotational excitation of a photofragment. Two quite different solvents, CO2 and Ar, are utilized in the experiments. The CO2 solvent is bound to the chromophore by ~200 meV. Its charge distribution gives rise to substantial electric quadrupole and higher moments that can modify the electronic structure of the solute ion. The Ar solvent is much more weakly bound to the solute (~50 meV), has no permanent multipole moments, and no possibility of rotational, vibrational or electronic excitation in the dissociation process. The contrasting photodissociation dynamics associated with the different combinations of these solutes and solvents reported in this Dissertation both provides new understanding and raises questions concerning photoprocesses in partially solvated ionic complexes.

Following 430 to 650 nm excitation of ICN− to its 2Π1/2 excited state, the predominant ionic photoproduct is I−, with a minor CN− component. Photodissociation of ICN−(Ar)n, n = 1–5, results in dominant I− and I−(Ar)n ionic photoproducts, the observation of single-solvent cage recombination for n = 1, and highly solvated I− and ICN− photoproducts. The experimental results, electronic structure calculations, and quantum dynamics calculations together indicate that efficient transfer of a significant amount (> 0.3 eV) of the photoexcitation energy into rotation of the CN diatom occurs following dissociation of the chromophore.

Photofragmentation studies of ICN−(CO2)n, n = 1–18, following 400, 500, and 600 nm excitation examine the influence of long-range solute-solvent interactions on the dissociation dynamics. While the dominant fragment from ICN− excitation is I−, the addition of even one CO2 solvent results in dominant CN−-based photoproducts. Significant cage recombination products are observed for n = 3, 4, and 7 following 600, 500, and 400 nm excitation. Solvent evaporation experiments show that the average CO2 solvent binding energy to the ICN− chromophore is ~200 meV, consistent with earlier experiments on diatomic solutes.