Mechanism of enhanced ionization of linear H3+ in intense laser fields -: art. no. 043411

We investigate the mechanism of enhanced ionization that occurs at a critical internuclear distance R-c in the two-electron symmetric linear triatomic molecule H-3(+) subjected to an ultrashort, intense laser pulse by solving exactly the time-dependent Schrodinger equation for a one-dimensional model of H-3(+). Results of the simulations are analyzed by using three essential adiabatic field states \1], \2], and \3] that are adiabatically connected with the lowest three electronic states X(1)Sigma (+)(g), B(1)Sigma (+)(u), and E(1)Sigma (+)(g) of the field free ion. We give also a simple MO (molecular orbital) picture in terms of these three states to illustrate the important electronic configurations in an intense field, The states \1], \2], and \3] are shown to be composed mainly of the configurations HHH+, HH+H, and H+HH, respectively in the presence of the field. We conclude that the overall level dynamics is governed mainly by transitions at the zero-field energy quasicrossings of these three states. The response of H-3(+) to a laser field can be classified into two regimes. In the adiabatic regime (R<R-c), the electron transfers from one end of the molecule to the other end every half optical cycle thus creating the ionic component H+H+H-. In the diabatic regime (R>R-c), internuclear electron transfer is suppressed due to electron repulsion and laser induced localization. In the intermediate (R similar or equal toR(c)) region, where enhanced ionization occurs, the state \3] is most efficiently created by the field-induced nonadiabatic transitions between the states at quasi-crossing points. The "quasistatic" laser-induced potential barriers are low enough for the electron to tunnel from the ascending (upper) well, thus confirming the quasistatic model at high intensities. Analytic expressions for the critical distance R-c are obtained from this model and collective electron motion is inferred from the detailed time-dependent two-electron distributions.