Photodissociation of singlet oxygen in the UV region

Abstract

Photodissociation of singlet oxygen, O₂ a¹Δ_g, by ultraviolet radiation in the region from 200 to 240 nm has been investigated using velocity map imaging of the atomic oxygen photofragments. Singlet oxygen molecules are generated in a pulsed discharge and studied by one-laser photodissociation and detection around 226 nm as well as two color photodissociation at various wavelengths in the range from 200 to 240 nm. A simple model of the discharge on and off signal indicates efficient conversion of O₂ X³Σ_g⁻(v = 0) in the parent beam to O₂ a¹Δ_g(v = 0–2). Minute amounts of highly excited vibrational levels of ground state O₂ X³Σ_g⁻(v > 0) are detected but no evidence is found for production of the O₂ b¹Σ_g⁺ state. Over the decreasing wavelength range 240–200 nm the a¹Δ_g-state signal relative to the X³Σ_g⁻(v = 0) signal decreases strongly. Around 226 nm the a¹Δ_g(v = 0–2) states averaged branching ratio percentage for O(³P_jj = 2 : 1 : 0) is found to be 56 : 36 : 8 (±5%), respectively. The anisotropy parameter for photodissociation of a¹Δ_g(v = 0–2) averages to β = 1.3 ± 0.4. The a¹Δ_g(v = 0) photodissociation cross section is found to 3–10 times stronger than theory predicts. Furthermore, the photodissociation image shows a strong parallel character, (i.e., transition moment parallel to the molecular axis) while theory predicts a predominantly negative character.