Photodissociation of van der Waals clusters of isoprene with oxygen, C _5H _8-O _2, in the wavelength range 213-277 nm

摘要

The speed and angular distribution of O atoms arising from the photofragmentation of C _5H _8-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 C _5H _8-O _2 complex into the perturbed Herzberg III state (Δ _u~3) of O _2. This excitation results in the prompt dissociation of the complex giving rise to products C _5H _8OO 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 C _5H _8-O _2 hv → C _5H _8-O _2(Δ _u~3) → C _5H _8O O andor to dissociation of O _2 with borrowing of the lacking energy from incompletely cooled complex internal degrees of freedom C _5H _8 -O _2 hv → C _5H _8-O _2(Δ _u~3) → C _5H _8 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 Δ _g~1 and b1 Σ _(g+) singlet states as the precursors. This indicates the formation of singlet oxygen O _2(a Δ _g~1) and O _2(b1 Σ _(g+)) after excitation of the C _5H _8-O _2 complex. Cooperative excitation of the complex with a simultaneous change of the spin of both partners ~1X- O _2~3 hν → ～3X- O _2~1 → ~3X O _2~1 is suggested as a source of singlet oxygen O _2(a Δ _g~1) and O _2(b1 Σ _(g+)). This cooperative excitation is in agreement with little or no vibrational excitation of O _2(a Δ _g~1), produced from the C _5H _8-O _2 complex as studied in the current paper as well as from the C _3H _6-O _2 and CH _3I-O _2 complexes reported in our previous paper Baklanov, J. Chem. Phys. 126, 124316 (2007). The formation of O _2(a Δ _g~1) from C _5H _8-O _2 was observed at λ _(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 ~1X- O _2~3 hν → ~3X(T _2)- O _2~1(a Δ _g~1) in the C _5H _8-O _2 complex.