Ionization of the H2O molecule by intense ultrashort laser pulses

Complete text of publication follows. In the present work calculations for the ionization of the water molecule by intense half-cycle electric pulses were presented. Single active electron classical (CTMC- classical trajectory Monte Carlo) and quantum mechanical (numerical solution of the time dependent Schroedinger equation (TDSE)) models were employed in the framework of the hydrogenic approximation. In the hydrogenic approximation the 1b1 electrons are initialized on a 2pz hydrogenic orbital. The effective charge of the core (Zeff ) is calculated using the experimental value of the ionization energy Ei = 0.463 a.u. Calculations were performed using electric pulses with duration τ of 1, 3, and 5 a.u. at two different field intensities (E0 = 0.44 a.u. and E0 = 1 a.u.). Ionization probability densities are calculated the TDSE and VOLKOV expansion coefficients along with the orthogonalized TDSE (TDSE-O model) and Volkov (VOLKOV-O model) expansion coefficients. The CTMC ionization probability densities are calculated from the simulated classical trajectories. The probability densities predicted by quantum mechanical and classical models are qualitatively the same. In each approach the electrons are ejected with maximum probability along the polarization vector with momentum value around A-bar(τ), which is the momentum gained by the electrons from the external electric field. A double-peak structure in the ionization probability densities can be observed applying both the Volkov and TDSE models. This is the imprint of the double lobe structure of the initial state wave function. In the TDSE, TDSE-O and VOLKOV-O probability densities a parallel 'ridge' structure was observed, which were identified as single-photon ionization peaks attributed to the shape of the half-cycle pulse. The ionization spectra can be calculated from the ionization probability density by integrating over the electron ejection angles. We found a good agreement between the CTMC and TDSE results (see figure 1) only for pulses with high net momentum transfer toward the active electron, where the dominant ionization mechanism is the over-the barrier ionization. Both the TDSE and VOLKOV models provide qualitatively the same results. In both cases, the dP=dk curve has two maxima around the same electron momenta. Beside these similarities, significant discrepancies exist. In the case of the Volkov model both maxima have the same high, while in the case of TDSE model the first maximum is considerably smaller than the second one. This difference in the high of the maxima can be also observed for the CTMC model and can be explained as a result of the Coulomb interaction between the core and the electron, which is absent for the Volkov model. Acknowledgements This work was supported by the Romanian National Plan for Research (PNII) under contract No. ID 539, the European COST Action CM0702, the grant Bolyai from the Hungarian Academy of Sciences, and the Hungarian Research Found OTKA (K72172).