Six-photon ionisation of H2via the E, F,1Σg+, νE=0 level. Absorption of photons in the ionisation continuum

The authors have observed the six-photon ionisation of H 2 with four-photon resonances on the E, F 1 Sigma g + , nu E =0 levels, using laser intensities in the range of 10 11 W cm -2 and photon energies around 3.07 eV. Contrary to all expectations, there is evidence for significant H + production; both H + and H 2 + ion resonance spectra are dominated by the Q lines of the four-photon transitions E, nu E =0 from X, nu =0. From the electron energy spectra, which show a strong nonFranck-Condon behaviour, it appears that the H + ions are formed by photodissociation of H 2 + ( nu + =2). At higher intensity (10 12 W cm -2 ) the authors demonstrate the absorption of two extra photons by the H 2 molecule into the X 2 Sigma g + ionisation continuum.