Time-dependent multiconfiguration theory for electronic dynamics of molecules in an intense laser field Tsuyoshi Kato * , Hirohiko Kono Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan Received 10 March 2004; in final form 16 April 2004 Available online 19 June 2004 Abstract A multiconfiguration theory for electronic dynamics of molecules in an intense laser field is developed based on the Dirac– Frenkel time-dependent variational principle. The equations of motion for spin–orbitals and configuration-interaction coefficients are explicitly given. Numerical calculations of electronic dynamics of a hydrogen molecule in an intense electronic field are per- formed as a practical application of the theory. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction The light intensity of I ¼ 3:5  10 16 W/cm 2 exerts on an electron a force that is as strong as the electron-nu- cleus interaction in a hydrogen atom. Interactions of atoms or molecules with near-infrared intense laser fields of light intensity of I > 10 12 W/cm 2 induce non- perturbative phenomena such as above-threshold ioni- zation, tunnel ionization, and higher-order harmonic generation. An epoch is marked by the advent of atto- second light pulses generated by superposition of higher- order harmonics, which opened the door to tracing ultrafast electronic dynamics in atoms or molecules with attosecond time resolution. A lifetime of 8 fs of M-shell vacancies of Kr has been measured by varying the delay between a few-femtosecond visible light pulse (probing Auger electrons) and a synchronized sub-femtosecond soft X-ray (generating a core hole) [1]. Interest has recently been shown in questions as to how ionization of a molecule in an intense field is af- fected by molecular structure [2] or by electronic struc- ture. Early measurements indicated that the ionization rates of diatomics in an intense field are nearly equal to those of atoms with similar electron binding energies. In recent works, however, the ionization yield of O 2 has been found to be greatly suppressed, relative to Xe, an atom with nearly the same ionization potential as O 2 [3,4]. Faisal and coworkers [5] attributed the sup- pression effect to a one-electron character, i.e., the symmetry of the highest occupied molecular orbital of O 2 ; the antibonding orbital causes a destructive inter- ference between the two subwaves of the ionizing elec- tron emerging from the two atomic centers. In order to elucidate above-mentioned experimentally observed phenomena, it is necessary to reveal the role of multi-electron dynamics in atoms or molecules inter- acting with intense laser fields. Theoretical treatments beyond the single active electron approximation have been begun. By incorporating the electron–electron re- pulsion term into the so-called intense-field many body S-matrix theory, Becker and Faisal [6] reproduced the observed momentum distribution of doubly charged ions in nonsequential double ionization of He. Effects of multi-electron dynamics on field-induced intramolecular electron transfer accompanied by ionization [7] have been investigated by solving the time-dependent Schr€ odinger equation using a grid point wave packet method [8]. It has been shown that the proper descrip- tion of ionization processes in intense fields requires an * Corresponding author. Fax: +81-22-217-7715. E-mail address: kato@mcl.chem.tohoku.ac.jp (T. Kato). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.05.106 Chemical Physics Letters 392 (2004) 533–540 www.elsevier.com/locate/cplett