Date on Master's Thesis/Doctoral Dissertation

12-2019

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Liu, Jinjun

Committee Co-Chair (if applicable)

Thompson, Lee

Committee Member

Thompson, Lee

Committee Member

Ramezanipour, Farshid

Committee Member

Yu, Ming

Author's Keywords

laser spectroscopy; molecular spectroscopy

Abstract

Metal-containing free radicals are important intermediates in metal-surface reactions and the interactions between metals and organic molecules. Among metal-containing free radicals, monovalent derivatives of alkaline earth metals have been extensively investigated by using spectroscopic techniques in a gas phase, mainly in Broida ovens or under supersonic-jet-cooled conditions. In the present study, the results of performed spectroscopic investigations of both metal-containing monoalkoxides (CaOR) and metal oxides (MO) radicals have been reported. Laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the electronic transition of the calcium CaOR radicals have been obtained under jet-cooled conditions. Complete active space self-consistent field (CASSCF) and coupled-cluster (CCSD) calculations on the free radical were performed to aid the assignment of vibronic transitions observed in the LIF/DF spectra. In addition to dominant spectral features that are well reproduced by vibrational frequencies and Franck-Condon (FC) factors calculated ab initio, the pseudo-Jahn-Teller interaction involving the state induces additional vibronic transitions that are not allowed under the harmonic oscillator approximation. A constant value for the spin-orbit splitting has been observed for all vibrational levels of the state accessed in the LIF experiment. Recently, alkaline earth monoalkyl (MR) and monoalkoxide (MOR) free radicals, e.g., CaCH3, CaOCH3, SrCH3, and SrOCH3, have been proposed as candidates for laser cooling of polyatomic molecules. The implication of the present spectroscopic investigations has been conferred in the context of the proposed scheme of laser-cooling MOR (M=alkaline earth metals) molecules. Accurate determination of Franck-Condon factors (FCFs) is critical to laser cooling. Traditionally, FCFs of MORs are determined in LIF/DF measurements. However, the accuracy of so determined FCFs is limited by the interference of the scattering of the excitation laser. In addition, when a pulsed excitation laser is used, the FC-favored transitions may easily be saturated, which leads to underestimated FCFs for these transitions. A direct absorption-based cavity ring-down spectroscopy (CRDS) apparatus is used to find the saturation and corrected measurement of FCFs. Dark states play a critical role in the laser cooling of atoms and molecules. Population loss due to the relaxation to dark states determines the maximum averaged a number of scattering events an atom or molecule can experience. In MOs, the dark electronic state relevant to laser-cooling is the state that is in close proximity. Since it is “dark”, the state doesn’t fluoresce significantly and cannot be detected by LIF spectroscopy with a high signal-to-noise ratio (SNR). However, it can be detected using cavity ring-down (CRD) spectroscopy. In an aim to study the dark states, the transition of the YO molecule which is one of the only three diatomic molecules that have been laser-cooled was detected using the pulsed-CRD spectroscopy. The sensitivity is comparable to the LIF spectroscopy. A cw-CRD spectroscopy apparatus is under construction, which is expected to increase the SNR by two to three orders of magnitude.

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