Zero kinetic energy photoelectron spectroscopy of polycyclic aromatic hydrocarbons Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8g84mp07s

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  • In this dissertation, I present electronic spectra of a few polycyclic aromatic hydrocarbons (PAHs): tetracene, pentacene, pyrene, benzo[g,h,i]perylene and benzo[a]pyrene using resonantly enhanced multiphoton ionization (REMPI) and zero kinetic energy (ZEKE) photoelectron spectroscopy. The work of tetracene and pentacene also combine a laser desorption source with a ZEKE spectrometer, demonstrating our capability for studies of thermally labile species. The experiment involves two tunable ultraviolet laser sources, one to excite the vaporized PAH molecules to the first excited electronic state with different levels of vibrational energy, and the other to further reach the Rydberg states just below the ionization threshold of a vibrational level of the cation. Ultimate ionization and detection of ZEKE electrons are achieved using a delayed pulsed electric field. With this approach, the adiabatic ionization potential of each molecule is obtained. Several skeletal vibrational modes of the first electronically excited state of the neutral species and those of the cation are assigned, with the aid of ab initio and density functional theory calculations. In addition to giving a fundamental understanding of the photophysics of this type of compounds, another major motivation of this study is to offer a database for astrophysical modeling, in terms of both direct line identification and modeling of the chemical and radiation balance of the interstellar medium. The distinctive set of infrared (IR) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 μm are ubiquitously seen in a wide variety of astrophysical environments. They are generally attributed to polycyclic aromatic hydrocarbons. However, not a single PAH species has yet been identified. Zero kinetic energy photoelectron spectroscopy presents information on the vibrational modes of a cation in the far-infrared (FIR) region, and the FIR modes are sensitive to the skeletal characteristics of a molecule and hence are critical for chemical identification. Although ZEKE is governed by the Franck-Condon principle, some IR active bands can be probed through vibronic coupling between the two lowest intermediate electronic states in highly symmetric PAHs. In low symmetry PAHs such as benzo[a]pyrene, the Franck-Condon allowed total symmetric modes are already IR active. Both IR active and forbidden modes are necessary for astrophysical modeling. With the frequencies from ZEKE, calibration of theoretical calculations with experiments becomes possible, particularly for the FIR region of cations where other types of experiments suffer from lack of light sources, insensitive detectors, and low particle concentrations.
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