Laser-induced fluorescence spectroscopy of the alkoxy radicals

Laser-induced fluorescence (LIF) spectra of large primary and secondary straight-chain alkoxy radicals and cyclohexoxy (c-C6H11O) were recorded under jet-cooled conditions (T~1K). For 1-hexoxy (1-C6H13O), 1-heptoxy(1-C7H15O) and 2-butoxy (2-C4H9O), rotational constants for both the ground electronic (X) state and the second excited (B) state, and components of the spin-rotational tensor for the X state were obtained by an analysis of the resolved rotational and fine structure of the vibronic bands. Comparing these experimental results with quantum chemistry calculations permitted unambiguous conformational and vibrational assignments for the bands. Such techniques have been extended to larger straight-chain alkoxy radicals. A high-resolution LIF apparatus has been improved by the addition of a high-accuracy calibration system. With this apparatus we have studied the methoxy radical (CH3O) and its deuterated isotopomers (CH2DO, CHD2O and CD3O). A spectral resolution (Full Width at Half Maximum, FWHM) of ~250MHz in the UV spectral region was achieved and individual lines have been measured with an absolute accuracy (one standard deviation) ~50MHz at a frequency of ~10⁹MHz, i.e., ~0.05ppm. Vibronic bands of different rotational types of transitions from the lower-energy spin-orbit component of the ground electronic state (X^2E_{3/2}) to the first excited state(A^2A_1) have been recorded. A global fitting combining the LIF and previously observed microwave and stimulated emission pumping (SEP) spectra, which involves the higher-energy spin-orbit component of the ground electronic state (X^2E_{1/2}), to a rotational and fine structure Hamiltonian has been achieved. All of the above data reveal the structure and properties of the alkoxy radicals, which are important intermediate species in the oxidation of hydrocarbons, a reaction that is critical in both combustion and atmospheric chemistry. The spectroscopy is a prerequisite for subsequent dynamic studies. Our work is also of special theoretical interest due to the fact that methoxy radical provides a benchmark case of the Jahn-Teller effect coupled with spin-orbit interaction, giving rise to a potential energy surface which stringently tests modern ab initio calculations.