Laser Spectroscopy Studying Organic and Inorganic Intermediates in The Atmospheric Oxidation Process

Laser spectroscopy is a powerful tool to study the structure and photochemistry of molecules at selected electronic states. Under jet-cooled conditions, the rotational spectrum can be simplified with less congestion on the rotational contour owing to the low J population on the ground electronic state. Laser induced fluorescence (LIF), stimulated emission pumping (SEP), and cavity ringdown spectroscopy (CRDS) apparatuses are used to measure the methoxy(CH₃O·), β-hydroxyethylperoxy (HOC₂H₄OO·) and nitrate (NO₃·) radicals. The novel laser spectroscopy apparatuses are designed for the direct measurement to the forbidden electronic state attributed to the selection rules or low absorption cross section. The spectroscopic analysis applies the least-square fitting to determine the molecular parameters in Hamiltonians as well as the vibronic coupling induced distortion of molecular structure. Furthermore, in the collaboration with Dr. W. Leo Meerts (Radboud University, The Netherlands), the quantum mechanical model is also adapted to the evolutionary algorithm to investigate the biological evaluation on spectrum simulation.The ~A²A₁-~X²E_3/2 and ~X²E_1/2-~A²A₁ electronic transitions of methoxy and all four deuterium isotopologues are measured with a LIF/SEP apparatus that is both high-resolution (Δν ∼ 250 MHz) and and high-accuracy (Δσ ∼ 50 MHz). Since the ~X²E_1/2 state is not thermally populated in a jet-cooled environment, the complementary SEP experiment directly interrogates the ~X²E_1/2 level of methoxy by depleting the fluorescence from the ~A²A₁ state. The global analysis of the microwave, LIF, and SEP data breaks correlations in the microwave data and provides better determinations for the ~X and ~A states^′ parameters.

The jet-cooled ~A-~X origin band spectra of ₁G₂G₃ conformer of β-hydroxyethylperoxy have been recorded by a CRDS appratus with a laser source linewidth  ∼ 70 MHz in the near IR region. The spectra of four deuterium-substituted isotopologues have been analyzed and successfully simulated with an evolutionary algorithm, confirming the cyclic structure of the molecule responsible for the observed origin band. The broadened l inewidth on the absorption spectrum (Δν > 2 GHz) is due to the shortened lifetime of the ~A state attributed to excitation induced hydrogen transfer from the OH to the OO site of the molecule. The vibronic spectra of nitrate radical are recorded with the CRDS apparatus under jet-cooled conditions for the ~A²E^′′-~X²A₂^′ electronic transition. The rotationally resolved 2¹₀, 4¹₀, and 4²₀ bands have been recorded for studying the perpendicular (ν₂) and parallel (ν₄) band types. In addition, the vibronically forbidden 0⁰₀ band is also found to be different from either perpendicular or parallel band type and is likely due to the magnetic dipole transition. Ground state combination differences are used to analyze the vibronic bands, and the preliminary assignments are utilized for the comprehensive understanding of the NO₃ structure in the ~A state.