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Direct multi-photon ionizations of H+2 in intense laser fields

We have discussed a weak ionization channel characterized by the kinetic energy release spectra of the protons from Coulomb explosion with the sum energy over 10 eV from a hydrogen molecular ion for various vibrational states in infrared pulses. After Franck–Condon averaging of the contributions for various vibrational states, a ‘shoulder’-like cutoff structure has been obtained which is consistent with the experiment. For longer pulses, a multi-peak structure is obvious for the v = 0 state and attosecond extreme-ultraviolet pulses were adopted to search the essential physical dynamics. The reproductions of the kinetic energy release spectra from extreme-ultraviolet light indicate that the high kinetic energies of the protons must be due to the direct multi-photon ionizations. The origin of the multi-peak structure has also been investigated by the extreme-ultraviolet pulses. Time-dependent spectra reveal the nuclear wavepacket motion and the time region when charge-resonance-enhanced ionization takes place. Competitions between direct multi-photon ionizations and charge-resonance-enhanced ionizations have also been discussed for various intensities and pulse durations for the v = 0 state.