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Europhysics Letters (EPL)

Theory of Core-Resonant Ionization

Lars G. Hanson, J Zhang, P. Lambropoulos

Published Apr 10, 1995• 17 citations
Publisher

Abstract

We present the theory of photoelectron energy spectra in two-electron atoms strongly driven by a field near resonance with the ionic core. We show that the unavoidable simultaneous excitation of the atomic resonance transition and the ever present a.c. Stark shifts leads to rich structure in the spectrum not predictable by simplified models. We present quantitative results for calcium which illustrate the theory and also pertain to ongoing experiments.

DOI
10.1209/0295-5075/30/2/004

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We present the theory of photoelectron energy spectra in two-electron atoms strongly driven by a field near resonance with the ionic core. We show that the unavoidable simultaneous excitation of the atomic resonance transition and the ever present a.c. Stark shifts leads to rich structure in the spectrum not predictable by simplified models. We present quantitative results for calcium which illustrate the theory and also pertain to ongoing experiments.
Photoelectron energy spectra resulting from two-photon ionization of calcium atoms by 180 fs pulses have been studied as a function of wavelength (380--405 nm) and intensity ( $9\ensuremath{-}900\phantom{\rule{0ex}{0ex}}\mathrm{GW}/{\mathrm{cm}}^{2}$). When the wavelength is tuned near the $4s\ensuremath{\rightarrow}4p$ or $4s\ensuremath{\rightarrow}5s$ ionic core transition, the photoelectron peaks display a characteristic splitting proportional to the field strength and assignable to a two-electron continuum-continuum Autler-Townes effect. Spectra obtained by an essential-state model involving three coupled continua are compared to the experiment.
For pt.I see ibid., vol.28, p.5057-75 (1995). In the previous paper, a non-perturbative time-dependent theory of resonant multiphoton ionization by both smooth and rectangular pulses was derived, and as a first step, applied to the two-photon ionization of caesium in weak fields. In the present paper, we continue the analysis of the same system, extending the weak-field results and considering the case of stronger fields. At higher intensities the spectra exhibit energy modulations of a different character from those predicted for low power. Here, for stronger fields, the structure arises from Rabi oscillations, which, for the smooth pulses assumed here, are triggered by time-dependent Stark shifts. In order to obtain a better understanding of the energy oscillations, we also look at the photoelectron energy spectrum produced by a rectangular pulse, a problem which is solved exactly. We show that there are other sources of strong-field structure.
We investigate resonant strong field ionization of atomic hydrogen with respect to the 1s–2p transition. By 'strong' we understand that Rabi periods are executed on a femtosecond time scale. Ionization and AC Stark shifts modify the bound state dynamics severely, leading to nonperturbative signatures in the photoelectron spectra. We introduce an analytical model, capable of predicting qualitative features in the photoelectron spectra such as the positions of the Autler–Townes peaks for modest field strengths. Ab initio solutions of the time-dependent Schrödinger equation show a pronounced shift and broadening of the left Autler–Townes peak as the field strength is increased. The right peak remains rather narrow and shifts less. This result is analysed and explained with the help of exact AC Stark shifts and ionization rates obtained from Floquet theory. Finally, it is demonstrated that in the case of finite pulses as short as 20 fs the Autler–Townes duplet can still be resolved. The fourth generation light sources under construction worldwide will provide bright, coherent radiation with photon energies ranging from a tenth of an meV up to tens of keV, hence covering the regime studied in the paper so that measurements of nonperturbative, relative AC Stark shifts should become feasible with these new light sources.
We calculate photoelectron spectra for a two-electron atom under the influence of two external driving fields, using an essential states formalism. We focus on the regime of coherence transfer, in which electron-electron correlation transfers field-induced photocoherence from one electron to the other. In the case studied here, two laser fields are resonant with coupled atomic transitions, in the manner familiar from three-level dark-state spectroscopy. Dynamical two-electron effects are monitored via the photoelectron energy spectrum. We show that the distribution of the photoelectron energies can be singly, doubly, or triply peaked depending on the relative laser intensities. The electron spectra are independent of the turn-on sequence of the fields. \textcopyright{} 1996 The American Physical Society.
The authors calculate photoelectron spectra for a two-electron atom under the influence of two external driving fields, using an essential states formalism. They focus on the regime of so-called coherence transfer, in which electron-electron correlation transfers field-induced photo-coherence from one electron to the other. In the case studied here, two laser fields are resonant with coupled atomic transitions, in the manner familiar from three-level dark-state spectroscopy. Dynamical two electron effects are monitored via the photoelectron energy spectrum. The authors show that the distribution of the photoelectron energies can be singly, doubly or triply peaked depending on the relative laser intensities. The electron spectra are independent of the turn-on sequence of the fields.
We present results from a time-dependent two-level-plus-continuum model of multiphoton ionization in order to better understand the short-pulse photoelectron spectrum resulting from multiphoton ionization and the role of intermediate states. Both resonant and nonresonant population transfer to the upper state are found to play an important role in the multiphonon ionization process. The dynamic modeling reveals the potential for broadening and shifting peaks in the photoelectron spectrum. However, we conclude that despite the complex dynamics of the excitation, short-pulse photoelectron spectra can be used to measure ac Stark shifts of both ponderomotively and nonponderomotively shifted states in strong laser fields.