## 抄録

We investigate the quantal dynamics of the electronic and nuclear wave packet of H_{2}^{+} in strong femtosecond pulses (≥ 10^{14}W/cm^{2}). A highly accurate method which employs a generalized cylindrical coordinate system is developed to solve the time-dependent Schrödinger equation for a realistic three-dimensional (3D) model Hamiltonian of H_{2}^{+}. The nuclear motion is restricted to the polarization direction z of the laser electric field E(t). Two electronic coordinates z and ρ and the internuclear distance R are treated quantum mechanically without using the Born-Oppenheimer approximation. As the 3D packet pumped onto 1 σ_{u} moves toward larger internuclear distances, the response to an intense laser field switches from the adiabatic one to the diabatic one; i.e., electron density transfers from a well associated with a nucleus to the other well every half optical cycle, following which interwell electron transfer is suppressed. As a result, the electron density is asymmetrically distributed between the two wells. Correlations between the electronic and nuclear motions extracted from the dynamics starting from 1 σ_{u} can be clearly visualized on the time-dependent "effective" 2D surface obtained by fixing ρ in the total potential. The 2D potential has an ascending and descending valley along z = ±R/2 which change places with each other every half cycle. In the adiabatic regime, the packet starting from 1 σ_{u} stays in the ascending valley, which results in the slowdown of dissociative motion. In the diabatic regime, the dissociating packet localized in a valley gains almost no extra kinetic energy because it moves on the descending and ascending valleys alternately. Results of the 3D simulation are also analyzed by using the phase-adiabatic states |1〉 and |2〉 that are adiabatically connected with the two states 1 σ_{g} and 1 σ_{u} as E(t) changes. The states |1〉 and |2〉 are nearly localized in the descending and the ascending valley, respectively. In the intermediate regime, both |1〉 and |2〉 are populated because of nonadiabatic transitions. The interference between them can occur not only at adiabatic energy crossing points but also near a local maximum or minimum of E(t). The latter type of interference results in ultrafast interwell electron transfer within a half cycle. By projecting the wave packet onto |1〉 and |2〉, we obtain the populations of |1〉 and |2〉, P_{1} and P_{2}, which undergo losses due to ionization. The two-state picture is validated by the fact that all the intermediates in other adiabatic states than |1〉 and |2〉 are eventually ionized. While E(t) is near a local maximum, P_{2} decreases but P_{1} is nearly constant. We prove from this type of reduction in P_{2} that ionization occurs mainly from the upper state |2〉 (the ascending well). Ionization is enhanced irrespective of the dissociative motion, whenever P_{2} is large and the barriers are low enough for the electron to tunnel from the ascending well. The effects of the packet's width and speed on ionization are discussed.

本文言語 | English |
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ページ（範囲） | 11152-11165 |

ページ数 | 14 |

ジャーナル | Journal of Chemical Physics |

巻 | 110 |

号 | 23 |

DOI | |

出版ステータス | Published - 1999 6 15 |

## ASJC Scopus subject areas

- 物理学および天文学（全般）
- 物理化学および理論化学