Title: Multiphoton processes in an intense laser field. V. The high-frequency regime
Abstract: We present results of Floquet calculations of shifts and widths of the 1s and 2s energy levels of atomic hydrogen irradiated by intense linearly or circularly polarized light whose frequency \ensuremath{\omega} is above the (weak-field) threshold ${\mathrm{\ensuremath{\omega}}}_{\mathrm{thr}}^{(\mathit{i})}$ for one-photon ioinzation from state i. We have studied the dependence of the shifts and widths on \ensuremath{\omega} and on the intensity I. Where possible, we compare our results with those obtained from a high-frequency theory [M. Pont and M. Gavrila, Phys. Rev. Lett. 65, 2362 (1990)] that yields shifts that depend only on ${\mathrm{\ensuremath{\alpha}}}_{0}$ (\ensuremath{\propto} \ensuremath{\surd}I /${\mathrm{\ensuremath{\omega}}}^{2}$), the excursion amplitude of a free electron, rather than on I and \ensuremath{\omega} separately. As I increases, with \ensuremath{\omega} fixed, the width reaches a maximum value ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{max}}$ at an intensity ${\mathit{I}}_{\mathrm{max}}$ for which \ensuremath{\surd}(\ensuremath{\Elzxh}\ensuremath{\omega}/2P) \ensuremath{\approxeq}1, where P==2\ensuremath{\pi}I/\ensuremath{\mu}c${\mathrm{\ensuremath{\omega}}}^{2}$, the ponderomotive shift. As I increases beyond ${\mathit{I}}_{\mathrm{max}}$, the width decreases toward zero, in accord with the high-frequency theory, and the shift approaches the result of that theory. (For different fixed \ensuremath{\omega}, the shifts first cross the \ensuremath{\omega}=\ensuremath{\infty} curve as ${\mathrm{\ensuremath{\alpha}}}_{0}$ increases, and they intersect, almost at a common value of ${\mathrm{\ensuremath{\alpha}}}_{0}$, before approaching the \ensuremath{\omega}=\ensuremath{\infty} curve.) As \ensuremath{\omega} increases, ${\mathit{I}}_{\mathrm{max}}$ increases as roughly ${\mathrm{\ensuremath{\omega}}}^{3}$, and ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{max}}$ decreases. If \ensuremath{\omega} is sufficiently large, we find that (2\ensuremath{\pi}/\ensuremath{\omega})${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{max}}$/\ensuremath{\Elzxh}1, so that, for very high frequencies, ionization takes place over more than one cycle even at large intensities. At frequencies below ${\mathrm{\ensuremath{\omega}}}_{\mathrm{thr}}^{(\mathit{i})}$, we detect states that emerge from ``shadow'' states; these states allow for a continuous variation of the shift and width across the threshold. Furthermore, we conjecture that the rise and fall of the width, in the vicinity of ${\mathit{I}}_{\mathrm{max}}$, occurs through an interaction between shadow and real (or ``dominant'') states.
Publication Year: 1991
Publication Date: 1991-04-01
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 116
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