INTENSITY DEPENDENCE OF THE MULTIELECTRON DISSOCIATIVE IONIZATION OF N2 AT 305 AND 610 NM

We have studied the laser intensity dependence of the multielectron ionization of N2 in the range of 10135×1015 W/cm2 (laser wavelength 305 and 610 nm; pulse duration 2 ps). At 610 nm, we detect four charge states of atomic fragments (N+, N2+, N3+, and N4+). The fragment kinetic energies and population intensity dependences indicate that they are produced by a sequential ionization occurring during the dissociation of the molecular ions (from N22+ to N28+) and/or formed by subsequent atomic ionization. In the first case our findings imply that the ionization occurs at well-defined internuclear distances. Moreover, the initial kinetic energies of the fragments are independent of the laser intensity. The results are interpreted in terms of laser-pulse development and volume effects. At 305 nm the dissociation processes show a different tendency. We detect two charge states of nitrogen fragments (N+ and N2+) arising from a vertical excitation of the N2 molecules. © 1990 The American Physical Society.