H-2 AND D-2 IN INTENSE SUBPICOSECOND LASER-PULSES - PHOTOELECTRON-SPECTROSCOPY AT 1053 AND 527 NM

We report multiphoton ionization experiments on H-2 and D-2 molecules at 1053- and 526.5-nm excitation wavelengths in the intensity range 5x 10(13)-5x10(14) W/cm(2). The intensity dependence of the total ion yie ld, the dissociation fraction, and the photoelectron spectrum is inves tigated. At 1053 nm we find a strong isotope effect in the dissociatio n fraction, whereas at 526.5 nm no such effect is observed. Up to 1x10 (14) W/cm(2) the photoelectron spectrum at 526.5 nm is dominated by re sonant ionization processes via Rydberg states of the molecules. They are shifted into resonance at intensities above similar to 10(13) W/cm (2). The spectra show that the potential energy curves of the resonant slates must have a shape very similar to the corresponding ionic ones . They are therefore mainly determined by the dipole coupling between the ion core orbitals 1s sigma(g) and 2p sigma(u). At 1053 nm two phot oionization regimes are observed: the multiphoton regime with Keldysh parameter gamma>1 showing resonance ionization structures, and the tun nel regime (gamma<1) at high intensity. The isotope effect in the diss ociation fraction at 1053 nm has no influence on the shape of the corr esponding photoelectron spectra at this wavelength.