Internal excitation of methyl radicals produced in the photolysis of acetone at 193 nm and the collisional enhancement of the infrared emission intensity in the .nu.3 spectral region

Abstract

Time-resolved I R emission spectroscopy has been used to monitor the fluorescence in the C-H stretch region of methyl radicals produced in the 193-nm photolysis of acetone. Spectra collected at 20-cm-l resolution in the u3 spectral region do not exhibit any structure. This indicates that the emission in this region is due to both the u3 fundamental of CH3 and combination bands of the radical which overlap each other. Modes other than ~~( 0 0 n O ) must contribute to the observed emission in the 3000-3350-cm-l region. Translationally hot methyl radicals are also found to undergo very fast T -V energy-transfer processes via collisions with various noble gases, resulting in enhanced infrared emission. The intensity of the enhanced emission is a factor of 4 or 5 times the emission intensity in the absence of the noble gases, suggesting that most of the radicals are formed in other vibrational states. The results are explained by assuming that the CH3 radical is initially produced in a broad range of vibrational states.

Low-energy electron-energy-loss spectroscopy of condensed acetone: Electronic transitions and resonance-enhanced vibrational excitations

“…This transition has also been observed by threshold electron-impact spectroscopy by van Veen et al 43 However, the maximum could not be reported in that study because of the strong n→3s transition to the 1 1 B 2 Rydberg state near 6.35 eV, 7,10,40,41,43 which is in agreement with various theoretical predictions. 2,4,42 Interestingly in gas-phase photolysis studies, [13][14][15][16][17][18][19][20][21][22] the dissociation of acetone into two methyl radicals and a CO has been attributed to this Rydberg transition.…”

“…The electronic spectroscopy of acetone 2-10 is of further particular interest to increase our understanding of the mechanisms of photolysis, 11,12 especially regarding three-body dissociation dynamics. [13][14][15][16][17][18][19][20][21][22] Strong dissociation channels leading to the formation of various molecular fragments have been reported for many electronic states. In the study of the electronic spectrum of acetone, both experiment 5,6,23 and theoretical calculations 2 have revealed the existence of a coupling between some Rydberg states and the →* valence state of the molecule.…”

Infrared emission accompanying the gas phase recombination of alkyl radicalsElectronic supplementary information (ESI) available: Appendix: Model for emission from the C2H6 recombination product of CH3 radicals. See http://www.rsc.org/suppdata/cp/b3/b303780k/

“…Reaction (1) is a clean source of CH 3 , with a quantum yield for formation of two methyl radicals close to unity (F ¼ 0.95). [4][5][6] Infrared emission, [7][8][9][10] laser induced fluorescence (LIF), 11 resonantlyenhancedmultiphotonionisation(REMPI), 11 photofragment spectroscopy 6 and diode laser absorption 12 have all been used to study features of the photodissociation dynamics, and the general consensus is that dissociation takes place in a stepwise fashion following initial formation of CH 3 CO and CH 3 and subsequent decomposition of the acetyl radical. [6][7][8]12 The methyl fragment emits in the C-H stretching region near 3050 cm À1 , and measurements of its internal energy distributions and energy transfer rates of the fragment have been inferred from the wavelength and time dependences of the emission.…”

“…[6][7][8]12 The methyl fragment emits in the C-H stretching region near 3050 cm À1 , and measurements of its internal energy distributions and energy transfer rates of the fragment have been inferred from the wavelength and time dependences of the emission. 7,9,13…”