Dispersed Fluorescence Spectroscopy of Jet-Cooled Isobutoxy and 2-Methyl-1-butoxy Radicals

Abstract

We report dispersed fluorescence (DF) spectra of the isobutoxy and 2-methyl-1-butoxy radicals produced by photolysis of corresponding nitrites in supersonic jet expansion. Different vibrational structures have been observed in the DF spectra when different vibronic bands in the laser-induced fluorescence (LIF) spectra of each radical were pumped, which suggests that those vibronic bands be assigned to different conformers. Spectra simulated using calculated vibrational frequencies and Franck-Condon factors well reproduce the experimentally observed ones and support the assignment of the vibronic bands in the LIF spectra to the two lowest-energy conformers of each radical. DF spectra obtained by pumping the B̃ ← X̃ origin bands of the LIF spectra are dominated by CO stretch progressions because of the large difference in CO bond length between the ground (X̃) and the second excited (B̃) electronic states. Furthermore, with non-CO stretch bands pumped, the DF spectra are dominated by progressions of combination bands of the CO stretch and the pumped modes as a result of Duschinsky mixing. Ã-X̃ separation of both conformers of the isobutoxy radical has also been determined in the experiment.

Laser-Induced Fluorescence Spectroscopy of Large Secondary Alkoxy Radicals: Part I. Spectral Overviews and Vibronic Analysis

“…Alkoxy radicals (RO • ), where R stands for an alkyl group, are key reaction intermediates in the oxidation of hydrocarbons in both the combustion of fossil fuels and degradation of organic molecules in the atmosphere. − In our labs, they are studied comprehensively by laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectroscopy. − LIF spectra of both primary and secondary alkoxy radicals have been recorded with two different resolutions: the moderate-resolution spectra reveal the vibronic structure, and the high-resolution ones show the rotational and fine structures. The methoxy radical (CH 3 O), the smallest alkoxy radical, is a prototypical Jahn–Teller (JT) molecule and has been subject to extensive theoretical and experimental studies. − In its ground electronic state ( X̃ 2 E), CH 3 O demonstrates both linear and quadratic JT effects, although its dynamic geometry on the zero-point level retains its 3-fold symmetry and still belongs to the C 3 v point group.…”

Laser spectroscopy investigations of jet-cooled metal-containing free radicals.

“…Since the fluorescence is observed as a means of monitoring the absorption process, the total fluorescence can be detected. Fluorescence was collected, perpendicularly to the laser beam, via an f/1.5 lens and imaged onto a photomultiplier tube (PMT, Hamamatsu, H10721-01) 60,61 . Positioning the PMT at right angles to the beam helped to minimize scattered light noise and, therefore, gave an improved sensitivity.…”

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Laster spectroscopic investigations of organic alkoxy and peroxy radicals.

“…The experimental setup used to obtain DF spectra has also been discussed in detail elsewhere. 34,[93][94][95][96][97][98][99] . For the DF measurement, the fluorescence was focused on and dispersed by a monochromator (Acton Research, SpectraPro 300i) with an 1800 grooves/mm grating with 300 μm entrance size slit and then imaged onto an intensified charge-coupled device (CCD, Princeton Instruments, PI-MAX 512).…”

“…(28598 cm −1 ), B0 (28821 cm −1 ), and C (29058 cm −1 ) are assigned to the origin bands, The cause of the red-shift in the DF spectra within each group has been discussed in our previous publications. 35,93,96 In brief, the large change in the CO bond length upon the B̃ Nevertheless, we assign the A Bands in the experimental LIF spectrum to this conformer based on two facts. First, the calculated (and scaled) origin transition of the GG' conformer matches the frequency of the A0 band.…”