Photophysics of O2 excited by tunable laser radiation around 193 nm

The photodissociation/ionization dynamics of O2 around 193 nm have been studied using a narrowband tunable ArF excimer laser and the velocity map imaging technique. Angular and kinetic energy distributions of the product O+ ions and O(3P2) atoms are recorded and analyzed. The production of O(3P2) atoms is resonance enhanced on the one-photon level by the B 3Σu−(v=4) state, which is part of the B 3Σu−←X 3Σg− Schumann–Runge bands. Angular distribution measurements for individual rotational levels of the B state yield values for the anisotropy parameter, βSR, which are in good agreement with the values predicted by independent measurements of predissociation lifetimes from spectral linewidths. An average value of β=0.48 is found for the underlying Herzberg continuum at 193 nm. O2+ production is enhanced on the two-photon level via members of the nsσg(n−1)dπg1Σg+ Rydberg series terminating at higher vibrational levels of the ion. The high Rydberg states autoionize into the O2+ X 2Πg ground state or absorb one more photon and then autoionize into the A 2Πu and b 4Σu− states of O2+, which subsequently fluoresce. Production of O+ from one- and two-photon dissociation of the O2+ formed after two-photon absorption is also observed and characterized.