| Research Abstract |
To understand the ultrafast electronic dynamics in intense fields, we have been developing an efficient grid point method, the dual transformation method, for solving the time-dependent Schrodinger equation for the electronic degrees of freedom of a molecule. We have applied this method to small molecular systems such as H_2. The vibrational degree of freedom is also incorporated in the calculation of H^+_2 without resorting to the Born-Oppenheimer approximation.In a long wavelength case, for H_2, the localized ionic (bond) states H^+H^- and H^-H^+ appear alternately according to the optical cycle, through which tunnel ionization proceeds. After one-electron ionization from H_2, H^+_2 is prepared around the equilibrium internuclear distance of H_2. The bond distance of H^+_2 is then stretched by the applied field. With the help of ab initio molecular orbital calculations, we also demonstrate molecular structure deformations characteristic of intense-field dynamics such as symmetric two-bond stretching of CO_2 cations. The wave packet approach enables visualization of electron-nucleus correlation and two-electron dynamics in intense laser fields.
Electronic motion can be controlled by adjusting the intensity, frequency, and pulse length. As an example, we have examined the electronic dynamics of H_2 in a short wavelength case (〜100 nm). Contrary to the long wavelength case, the electron pair is created near the proton at which the dipole interaction energy becomes higher. The nonstationary states H^+H^- and H^-H^+ appear alternately with a period inherent to the molecule even after the incident pulse has fully decayed. Ultrafast electron hopping between the nuclei can be controlled by a second pulse. Using this two-pulse scheme, we are able to enhance or suppress the ionization.
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