Photodissociation of van der Waals clusters of isoprene with oxygen, C5H8-O2, in the wavelength range 213-277 nm

The speed and angular distribution of O atoms arising from the photofragmentation of C5H8-O-2, the isoprene-oxygen van der Waals complex, in the wavelength region of 213-277 nm has been studied with the use of a two-color dissociation-probe method and the velocity map imaging technique. Dramatic enhancement in the O atoms photo-generation cross section in comparison with the photodissociation of individual O-2 molecules has been observed. Velocity map images of these "enhanced" O atoms consisted of five channels, different in their kinetic energy, angular distribution, and wavelength dependence. Three channels are deduced to be due to the one-quantum excitation of the C5H8-O-2 complex into the perturbed Herzberg III state ((3)Delta(u)) of O-2. This excitation results in the prompt dissociation of the complex giving rise to products C5H8+O+O when the energy of exciting quantum is higher than the complex photodissociation threshold, which is found to be 41740 +/- 200 cm(-1) (239.6 +/- 1.2 nm). This last threshold corresponds to the photodissociation giving rise to an unexcited isoprene molecule. The second channel, with threshold shifted to the blue by 1480 +/- 280 cm(-1), corresponds to dissociation with formation of rovibrationally excited isoprene. A third channel was observed at wavelengths up to 243 nm with excitation below the upper photodissociation threshold. This channel is attributed to dissociation with the formation of a bound O atom C5H8-O-2 hv -> C5H8-O-2((3)Delta(u)) -> C5H8O + O and/or to dissociation of O-2 with borrowing of the lacking energy from incompletely cooled complex internal degrees of freedom C5H8*-O-2 + hv -> C5H8*-O-2((3)Delta(u)) -> C5H8 + O + O. The kinetic energy of the O atoms arising in two other observed channels corresponds to O atoms produced by photodissociation of molecular oxygen in the excited a (1)Delta(g) and b (1)Sigma(+)(g) singlet states as the precursors. This indicates the formation of singlet oxygen O-2(a (1)Delta(g)) and O-2(b (1)Sigma(+)(g)) after excitation of the C5H8-O-2 complex. Cooperative excitation of the complex with a simultaneous change of the spin of both partners X-1-O-3(2) + hv -> X-3-O-1(2) -> X-3 + O-1(2) is suggested as a source of singlet oxygen O-2(a (1)Delta(g)) and O-2(b (1)Sigma(+)(g)). This cooperative excitation is in agreement with little or no vibrational excitation of O-2(a (1)Delta(g)), produced from the C5H8-O-2 complex as studied in the current paper as well as from the C3H6-O-2 and CH3I-O-2 complexes reported in our previous paper [Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. The formation of O-2(a (1)Delta(g)) from C5H8-O-2 was observed at lambda(pump) = 213-277 nm with the yield going down towards the long wavelength edge of this interval. This spectral profile is interpreted as the red-side wing of the band of a cooperative transition X-1-O-3(2) + hv -> X-3(T-2)-O-1(2)(a (1)Delta(g)) in the C5H8-O-2 complex. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737856]