Spectroscopic studies of the B̃ 2A′′-X̃ 2A′′ system of the jet-cooled vinoxy radical

Abstract

We apply several techniques to the study of the B̃ 2A′′-X̃ 2A′′ band system of the jet-cooled vinoxy radical, CH2CHO. Vibronically resolved excitation spectra are obtained using both laser-induced fluorescence (LIF) and a two-color resonant four-wave mixing (TC-RFWM) scheme that provides the nonlinear equivalent of hole-burning spectra. Rotationally resolved LIF spectra recorded at low temperatures (⩽3 K) provide rotational constants for 9 B̃-state levels. We also measure the fluorescence lifetimes of 19 B̃-state levels and obtain high-quality dispersed fluorescence (DF) spectra from seven of the most strongly fluorescing levels in the B̃ state. The excitation and DF spectra reveal far more vibrational levels in the two electronic states than have been previously observed. In total, we provide assignments for 54 levels observed in the first 3650 cm−1 of the B̃ state and for 57 levels in the first 3100 cm−1 of the X̃ state. These assignments include the identification of the a′ fundamentals for ν4 through ν9 and all three a′′ overtones, 2ν10 through 2ν12, in both states. The differences between the TC-RFWM and LIF spectra and the measured lifetimes indicate a dramatic increase in the predissociation rate of the B̃ state beginning at 1190 cm−1 above the origin. The predissociation rate is markedly mode-specific and is enhanced by out-of-plane excitation, possibly due to vibronic coupling with either the à 2A′ or C̃ 2A′ electronic states. The congestion and complexity of the DF spectra at high energies provides direct evidence of extensive intramolecular vibrational redistribution on the ground-state potential surface.

Photodissociation spectroscopy and dynamics of the CH2CFO radical

“…29 Spectroscopic investigations of the B 2 AЉ state reveal that at energies approximately 1400 cm Ϫ1 above the origin, fluorescence is quenched by predissociation. [21][22][23][24][25][26]28 In fact, all vibrational levels of the CH 2 CHO B 2 AЉ state predissociate to CH 3 ϩCO and CH 2 COϩH photoproducts with a branching ratio estimated to be 1:4. 28 Both product channels were attributed to dissociation on the ground state potential energy surface over a barrier following rapid internal conversion ͑IC͒ from the B state.…”

“…The vinoxy radical has two observed excited electronic states, which have been examined by absorption, 21 hole-burning spectroscopy, 22 LIF, [23][24][25][26][27] photodissociation, 28 and anion photoelectron spectroscopy. 29 Spectroscopic investigations of the B 2 AЉ state reveal that at energies approximately 1400 cm Ϫ1 above the origin, fluorescence is quenched by predissociation.…”

Electronic spectra of the jet-cooled 1-methylvinylthio radical

“…The C 2 -C 3 torsional mode (ν 19 ) is thus expected to be Franck-Condon active in the electronic spectra. Actually, in the spectra for theB −X band system of CH 2 CHS, whose planar geometry in theX state is twisted around the C-C bond by ∼25 • upon the electronic excitation, several bands associated with the C-C torsional mode have been observed with moderate intensities.…”

“…11,12 It is interesting that the methyl substitution effect of CH 2 CHS on theB −X origin position is compared with that of the oxygen analog, the vinoxy radical. The origin band positions of the vinoxy and its methyl-substituted species have been experimentally determined, [19][20][21] as summarized in Table VIII. It is seen that the origin band of the 1-methylvinoxy radical shifts from that of the vinoxy radical by −1503 cm −1 , while the shifts of 2-methylvinoxy radicals are much smaller.…”

Multiresonant Spectroscopy and the High-Resolution Threshold Photoionization of Combustion Free Radicals

“…52 Considerable spectroscopic work on this system has thoroughly characterized the ground state and the brightly fluorescent B 2 A" state. [53][54][55][56][57][58] This data provides the information necessary to benchmark ab initio calculations and develop diagnostics in order to probe the reaction dynamics of this radical.…”