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Physical Review A

Nondipole ionization dynamics of excited atoms in intense femtosecond pulses

Morten Førre

Published Dec 6, 2006• 20 citations
Publisher

Abstract

The photoionization of the $4s$ Rydberg state in hydrogen by an intense femtosecond $400\text{\ensuremath{-}}\mathrm{nm}$ light pulse is studied beyond the dipole approximation. We show that a recently predicted nondipole, high-frequency ionization mechanism [M. F\o{}rre et al., Phys. Rev. Lett. 97, 043601 (2006)] has a low-frequency counterpart in excited atomic systems. This opens the possibility that the effect could be measured in the near future.

DOI
10.1103/physreva.74.065401

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The photoionization of the 4s4s4s Rydberg state in hydrogen by an intense femtosecond 400\ensuremath-nm400\text{\ensuremath{-}}\mathrm{nm}400\ensuremath-nm light pulse is studied beyond the dipole approximation. We show that a recently predicted nondipole, high-frequency ionization mechanism [M. F\o{}rre et al., Phys. Rev. Lett. 97, 043601 (2006)] has a low-frequency counterpart in excited atomic systems. This opens the possibility that the effect could be measured in the near future.
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We have investigated multiphoton ionization of Ba 6snl6snl6snl Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the 6sns6sns6sns and 6snd6snd6snd 15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of Ba+{\mathrm{Ba}}^{+}Ba+ Rydberg states and Ba2+{\mathrm{Ba}}^{2+}Ba2+ photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto Ba+{\mathrm{Ba}}^{+}Ba+ Rydberg states.
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Recent advances in strong x-ray laser techniques enable the study of nonlinear multiphoton ionization in extreme high-frequency fields. Although the stabilization regime in such fields is theoretically established, its modified properties in the nondipole regime for long laser pulses remains unknown. Here, we numerically investigate the strong-field ionization of an atom in a long XUV laser pulse in the nondipole regime. Our study of the time-dependent quantum dynamics reveals a quasiperiodic modulation of the ionization yield as a function of pulse duration. We demonstrate that the Coulomb-field-induced slow oscillation of the ionized electron wave packet during the interaction is responsible for the observed modulation of the ionization yield. Furthermore, we scrutinize the unusual photon momentum sharing between the photoelectron and the ion in this extreme regime. These effects are observable in upcoming x-ray free-electron laser facilities.
Synopsis We develop a time-dependent perturbation theory beyond the dipole approximation to analyze the two-photon ionization dynamics of atoms exposed to short laser pulses with an arbitrary polarization. Subse-quent applications of the PT method in the nonresonant regime allow us to analyze one of the most important nondipole effects, i.e., the linear-momentum-transfer law in the two-photon ionization process and to unveil the important role of the interferences between the dipole and nondipole transition pathways to the photoelectron momentum shift along the laser-propagation direction.