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−O2, 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 O2 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−O2 complex into the perturbed Herzberg III state (3Δu) of O2. 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−O2 + hv → C5H8−O2(3Δu) → C5H8O + O and/or to dissociation of O2 with borrowing of the lacking energy from incompletely cooled complex internal degrees of freedom C5H8*−O2 + hv → C5H8*−O2(3Δu) → 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 1Δg and ${\rm b\,}^1 \Sigma _g^ +$b1Σg+ singlet states as the precursors. This indicates the formation of singlet oxygen O2(a 1Δg) and O2(${\rm b\, }^1 \Sigma _g^ +$b1Σg+) after excitation of the C5H8−O2 complex. Cooperative excitation of the complex with a simultaneous change of the spin of both partners 1X−3O2 + hν → 3X−1O2 → 3X + 1O2 is suggested as a source of singlet oxygen O2(a 1Δg) and O2(${\rm b \,}^1 \Sigma _g^ +$b1Σg+). This cooperative excitation is in agreement with little or no vibrational excitation of O2(a 1Δg), produced from the C5H8−O2 complex as studied in the current paper as well as from the C3H6−O2 and CH3I−O2 complexes reported in our previous paper [Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. The formation of O2(a 1Δg) from C5H8−O2 was observed at λpump = 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−3O2 + hν → 3X(T2)−1O2(a 1Δg) in the C5H8−O2 complex.