title: Theoretical studies on tunneling ionizations of helium atom in intense laser fields.
abstract: Our generalized Keldysh theory is applied to the simplest many-electron atom, helium atom. For the single ionization (He-->He(+)+e) we derive a compact rate formula, which does not contain any series summation or integral and thus is as simple as the Ammosov-Delone-Krainov ionization rates. In addition to its simplicity, our formula can explicitly show the wavelength dependence. Furthermore a simple form of the angular distribution of the photoelectron is available. Our compact formula agrees well with both the exact numerical calculations [A. Scrinzi et al., Phys. Rev. Lett. 83, 706 (1999)] and experimental data [B. Walker et al., Phys. Rev. Lett. 73, 1227 (1994)] in the intensity range of I<5x10(15) Wcm(2). In higher intensity regions, we suggest to utilize another simple formula which is valid in the tunneling limit.
authors.affiliation: Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 106, Taiwan, Republic of China. nagaya@gate.sinica.edu.tw
authors.forename: K
authors.initials: K
authors.lastname: Nagaya
authors.forename: K
authors.initials: K
authors.lastname: Mishima
authors.forename: M
authors.initials: M
authors.lastname: Hayashi
authors.forename: S H
authors.initials: SH
authors.lastname: Lin