Dissociative photoionisation of O2 with tunable 10, 20 and 30 eV photons

Atomic oxygen ions, O+, created by photoexcitation of a cold supersonic beam of O-2 by the output of a free-electron laser operating in the 128-122 nm region are detected by sliced velocity map imaging. Analysis of the richly structured high resolution kinetic energy and angular distributions obtained show that O+ production is due to the second and third harmonics present in the laser output. Excellent slicing and optimal velocity mapping allows separation of overlapping signal from dissociative photoionisation processes driven by 20 eV versus 30 eV photons. A two-colour experiment combining the free electron laser and a nanosecond dye laser output at 225.6 nm allowed detection of direct photodissociation of the metastable O(2 )(+ )b(4)S(g)(-) (v = 0-4) state is observed and assigned to a parallel b(4)S(g)(-) - (c(4)S(u)(-))4s Rydberg transition followed by dissociation to O+ (S-4) + O(P-3(2)). [GRAPHICS] .

Automated summary

A cold supersonic beam of O-2 is photoexcited by a free-electron laser to generate atomic oxygen ions, O+. The kinetic energy and angular distributions of the O+ ions produced reveal that the laser output contains second and third harmonics. By utilizing excellent slicing and velocity mapping techniques, signal separation from dissociative photoionization processes driven by 20 eV and 30 eV photons is achieved. Furthermore, a two-color experiment combining the free-electron laser and a dye laser allows for the detection of direct photodissociation of metastable O(2)(+) in a specific excited state.