Vibrational predissociation spectroscopy of mass-selected ionic clusters

[摘要] This thesis presents structural investigations of molecular ions and ionic clustersusing vibrational predissociation spectroscopy. Experimentally, a pulsed beam of themass-selected ion is crossed by a tunable infrared laser beam generated by a Nd:YAGpumped LiNbO_3 optical parametric oscillator. The resulting fragment ion is mass-analyzedand detected, with its intensity as a function of the laser wavelength being the"action" spectrum of the parent ion. In the case of SiH_7^+, we observed a vibrational bandcentered at 3866 cm^(-1) with clear P, Q, R branches, which is assigned as a perturbed H_2stretch. The absence of a second H_2 band suggests that the ion forms a symmetriccomplex with a structure H_2•SiH_3^+•H_2 , in contrast to the species CH_7^+, which has thestructure CH_5^+•H_2. The infrared spectra of NO_2^+(H_2O)_n clusters exhibit a marked changewith cluster size, indicating that an intracluster reaction occurs with sufficient solvation.Specifically, in NO_2^+(H_2O)_n clusters where n≤3, H_2O binds to a nitronium ion core; butat n=4 the NO_2^+ reacts, transforming the cluster to a new structure ofH_3O^+•(H_2O)_(n_2)•HNO_3. For protonated chlorine nitrate, we have observed two distinctisomers previously predicted by ab initio calculations: NO_2^+•(HOC1), the lowest energyisomer, and (ClO)(HO)NO^+, a covalently bonded isomer about 20 kcal/mol higher inenergy. Both isomers decompose to NO_2^+ and HOCl upon photo-excitation. Theseresults for HClONO_2^+ lend strong support to the involvement of an ionic mechanism inthe reaction of ClONO_2 on polar stratospheric cloud surfaces, a critical step in thedramatic springtime depletion of ozone over Antarctica. Current research activities onhalide-solvent clusters and metal-ligand complexes as well as technological improvementsof the apparatus are also discussed.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics