Four-photon dissociation and ionization ofH2

We report a quantitative investigation of competition between dissociation and ionization in four-photon excited ${\mathrm{H}}_2$. The C ${}_{}{}^1{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ v=0–4, N=1–3 states are prepared by three-photon excitation from the ground state. Absorption of a fourth photon from the same laser pulse leads either to ${\mathrm{H}}_2$ ${\mathrm{}}^{+}$+e or H+H(n=3,4). This branching is monitored in photoelectron spectra, taken with a novel magnetic bottle electron spectrometer based on a permanent magnet design. The competition between dissociation and ionization shows a pronounced dependence on the symmetry, vibrational, and rotational quantum numbers of the C state. We attempt to explain this dependence in terms of excitation of dissociative ${\mathrm{H}}_2$ states in the ${\mathrm{H}}_2$ ${\mathrm{}}^{+}$ ionization continuum from which both autoionization and dissociation may occur.