VUV mass analyzed threshold ionization: A single photon ionization technique applied to several different spectroscopic problems.

摘要

Mass Analyzed Threshold Ionization (MATI) spectroscopy has been applied to the investigation of several different problems in molecular spectroscopy. Using a single photon excitation scheme, the spectroscopy of molecular cations and the related potential energy surfaces of three very different molecules have been investigated. The MATI technique has been applied to a simple cyclic hydrocarbon (1,4-dioxane), two molecules with methyl rotors with low torsional barriers (propene, trans-2-butene), as well as Jahn-Teller active molecules (benzene and perdeuterobenzene). From the MATI spectra collected from these molecules a great deal has been learned about the structure of the cation, as well as how the normal modes of the cation vary with respect to the neutral molecule.; By using a single photon excitation scheme for these MATI experiments, as opposed to a pump-probe excitation scheme, there was no longer a prerequisite requirement that the molecule posses an accessible intermediate state through which the ionization continuum could be reached. This facilitated the analysis of molecules with short-lived or difficult to access intermediate states. In addition, it allows for the comparison of spectra collected via the single photon excitation scheme with spectra collected via other excitation schemes. By comparing spectra collected from different electronic levels, different spectral features could be uncovered based on the respective selection rules related to each excitation schemes.; A new MATI technique was also implemented in this laboratory whereby the MATI spectra were collected originating from an excited triplet state. The MATI spectra from the 3B_1u state of benzene and perdeuterobenzene were collected for the first time, with the triplet state being generated in a HV electric discharge. This enabled the aforementioned change in selection rules due to the change in electronic transition, and allowed for the identification of previously unidentified Jahn-Teller active normal modes in the benzene MATI spectra. By obtaining new spectral information about the JT active modes of benzene, improvements in the understanding of the JT coupling parameters were obtained that incorporated three of the four e_2g JT active normal modes of the benzene ground state cation.