Multiphoton Excitation and Ionisation in the 9–12 eV Region of O 2 by a 1.06 μm Glass Laser

In this letter we report results of a selective multiphoton excitation of O 2 in the 9–12 eV region by a neodymium glass laser. The main feature of this experiment is the major presence of the ion specie O 2 + in the interaction volume with the values 10 ± 1 for the nonlinear interaction order. This result can be interpreted as a double mechanisms namely: a one step process, the direct absorption from the ground state or a two step process via an intermediate resonant state which has been attributed as the second ( B ′ 3 Σ u − , v ′=2) state of O 2 located at 10.56 ± 0.03 eV. The probability of multiphoton formation of O 2 + for a flux value of 1,5 × 10 12 W/cm 2 is equal to 2 × 10 7±1 sec −1 .