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 (³Δ_u) 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(³Δ_u) → 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(³Δ_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 ¹Δ_g and b1 Σ+gb1Σg+ singlet states as the precursors. This indicates the formation of singlet oxygen O₂ (a ¹Δ_g) and O₂(b1Σ+gb1Σg+) 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 ¹Δ_g) and O₂(b1Σ+gb1Σg+). This cooperative excitation is in agreement with little or no vibrational excitation of O₂(a ¹ Δ_g), 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 ¹Δ_g) from C₅H₈− O₂ 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−³O₂ + hν → ³X(T₂)−¹O₂(a ¹ Δ_g) in the C₅H₈−O₂ complex.