59th Annual Report on Research 2014

Experimental:  An apparatus for laser-induced fluorescence/dispersed fluorescence (LIF/DF) spectroscopy of reaction intermediates under supersonic-jet expansion conditions has been built. LIF spectra map out the excited-state vibronic structure of the target molecules, while DF spectra reveal the ground-state vibrational energy levels. Free radicals are generated using either laser photolysis or pulsed electric discharge of appropriate precursors. DF spectra of free radicals from three categories have been obtained: (1) molecules with high symmetry that are subject to Jahn-Teller (JT) interaction, for example, the t-butoxy radical, whose spectra are further complicated by internal rotation of its three methyl groups; (2) asymmetrically deuterated JT-active molecules such as CH₂DO and CHD₂O (Figure 1). Asymmetric deuteration parametrically separates the potential and kinetic parts of the vibronic coupling and is hence an effective method for investigation of such interactions; (3) molecules with low or no symmetry in nearly degenerate electronic states, which demonstrate the pseudo-Jahn-Teller (pJT) effect. For example, energy separation between the ground (~X) and the first excited (~A) electronic state (ΔE) of iso-pentoxy is predicted to be ~10 cm^-1 (or 1.24 meV), resulting in its complex LIF and DF spectra (Figure 2). The energetics of the pJT molecules is sensitive to their conformations as well. Figure 3 compares the DF spectra of four isomers of methylcyclohexoxy (MCHO) with the methyl group on the 1-, 2-, 3-, and 4- position of the six-membered ring. While the vibrational structures of 2-, 3-, and 4-MCHOs are similar to each other, dominated by a CO stretch progression, the vibrational structures of 1- (or tertiary) MCHO conformers do not show such progression and thus require further theoretical examination. Furthermore, the ~A-~X separation (ΔE) of 2-MCHO is significantly larger than the other conformers. Such difference is attributed to the influence of the methyl group to the pJT-active vibrational mode (-CHO wagging).

Theoretical: Analyzing experimentally obtained spectra of the target molecules and understanding the mechanism of vibronic interactions relies on accurate quantum chemistry calculations and spectroscopic modelling of these interactions. Numerous ab initio calculations have been carried out, based on which experimentally observed LIF/DF bands were assigned and simulated. Collaboration with computational chemists has been established to understand the complicated spectra of asymmetrically deuterated JT molecules, namely CH₂DO/CHD₂O, in the high-frequency region. Moreover, it has been found that rotational and fine-structure analysis of LIF transitions involving the nearly degenerate electronic states may aid in interpretation of the vibronic structure. For instance, in all pJT molecules hitherto studied in the project, the spin-orbital (SO) interaction contributes to the experimentally measured ~A-~X separation. Analysis of previously obtained high-resolution LIF spectra of these molecules unravels the interplay between two intramolecular interactions: the JT/pJT effect and the SO effect. A new spectroscopic model that describes the rotational and fine structure of pJT-active molecules has been proposed and used to simulate and fit the LIF spectra of such molecules (Figure 4). Contributions from the relativistic SO effect and the non-relativistic electrostatic interaction and zero-point energy difference between the ~A and ~X states were determined separately and quantitatively.

The proposed research has not only fundamental interests but also direct applications in chemical kinetics research. For instance, it is well known that rate constants of certain reactions of alkoxy radicals, e.g., unimolecular dissociation (decomposition by C-C bond fission) and isomerization via 1,5 H-shift, are highly sensitive to molecular structure. Our DF spectra of MCHO (Figure 3) illustrate significantly different vibrational structures of the lowest electronic states of its four isomers (with five observed conformations), which will help when modelling alkoxy reactions. The location and degree of substitution of alkoxy radicals also affects formation of secondary organic aerosols (SOAs) in the atmosphere. Spectroscopic investigation of alkoxy radicals therefore provides experimental information that is tremendously useful for  understanding the governing role of the structure of alkoxy radicals in SOA formation.

Impact of PRF DNI Award

The PRF DNI award has had a significant and positive impact on Dr. Liu’s career. This award has provided financial support to a postdoctoral research fellow and will be used to support a graduate student. A laser spectroscopy lab has been built that was partially aided by the PRF awarded. In addition to the two papers that have been published in peer-reviewed journals, several manuscripts based on the results summarized above have been submitted or are in preparation. The achievements reported here provide a solid foundation for future work in the second year of the proposed project and many years to come.

Dr. Liu’s group now consists of five graduate students, working in two laser labs which are dedicated to gas-phase frequency-domain laser spectroscopy and ultrafast laser spectroscopy related to renewable energy research. Ten undergraduate students have worked in the research labs, including a Research Experience for Undergraduates (REU) student from a traditionally black college. One of the undergraduate students won the University of Louisville Undergraduate Research Scholar Grant that supports computational study of potential energy surfaces of pJT molecules.

Future work:

Spectroscopic research will be expanded along several dimensions: First, more varieties of molecules with low symmetry and strong vibronic interactions will be investigated, including small organometallic compounds: ethyl calcium (CaCH₂CH₃), isopropyl calcium (CaCH(CH₃)₂), calcium ethoxide (CaOCH₂CH₃), calcium isopropoxide (CaOCH(CH₃)₂), etc. Second, spectroscopic techniques other than LIF/DF will be applied, especially mass-selective photoelectron/photoionization spectroscopy. Finally, we expect to combine the gas-phase spectroscopy apparatus and the ultrafast laser system in our labs so that molecular dynamics can be monitored directly.