Detection of the O2 A′3ΔU Herzberg III state by photofragment imaging

Abstract

Photofragment imaging is shown to provide a sensitive method for detection of the O₂ A′³Δ_u Herzberg III state using a one-laser dissociation/O(¹D) resonance enhanced multiphoton ionization (REMPI) scheme with a focused nanosecond dye laser beam tuned to 203.8 or 205.2 nm, combined with velocity map imaging of the atomic oxygen photofragment. O₂ populated in the Herzberg states is generated by photodesorption at 250 nm of solid O₂ ice held at 15 K and by an electric discharge in a pulsed molecular beam of pure O₂. Ice photo-desorption results in Herzberg state products with higher translational, vibrational and rotational energy spreads, yielding the same signal as the discharge source but with lower velocity resolution. A clear signal with parallel character (β ∼ 0.9) assigned to dissociation of O₂ A′³Δ_u(v = 0, 1 Ω = 1) was observed when using a pulsed electric discharge source under specific ‘cold’ conditions with O(¹D) detection, driving one-photon dissociation around 205 nm. No products corresponding to O₂ A′³Δ_u state dissociation were observed for 225.625 or 200.32 nm dissociation with O(³P₂) detection, which implies that the O₂ A′³Δ_u state dissociates exclusively to the third (O¹D + O¹D) dissociation limit. Dissociation is suggested to take place through the 2¹Π_g upper state to the O¹D + O¹D limit where spin–orbit coupling of the A′³Δ_u state with the 1¹Π_u state accesses the allowed parallel ¹Π_u → ¹Π_g transition. While the absence of a parallel-type photodissociation signal from the c¹Σ⁻_u state may be expected, the A³Σ⁺_u should spin–orbit couple through the same pathway as the A′³Δ_u state. The fact that no clear A³Σ⁺_u signal is observed suggests a faster deactivation process compared to the A′³Δ_u state in the discharge and ice desorption process.