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Physical Review A

ac-Stark-shift-induced photoelectron spin polarization in resonant multiphoton ionization

S. N. Dixit

Published Sep 1, 1982• 2 citations
Publisher

Abstract

The influence of the ac-Stark shifts on the spin polarization (SP) of the photoelectron in a resonant multiphoton ionization is discussed. It is shown that the Stark shifts of various levels are different and this removes any initial fine-structure degeneracy of the excited states. This increased dynamic separation between the states increases the SP of the ejected photoelectron. Numerical results are presented to illustrate this effect. By expressing the equations of motion in the spin basis set the precise conditions necessary to observe nonzero SP are identified. It is shown on the basis of these conditions that although the removal of the fine-structure degeneracy occurs independent of whether the atom has any spin-orbit coupling, this does not induce any spin polarization if the atom has no spin-orbit coupling.

DOI
10.1103/physreva.26.1505

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The influence of the ac-Stark shifts on the spin polarization (SP) of the photoelectron in a resonant multiphoton ionization is discussed. It is shown that the Stark shifts of various levels are different and this removes any initial fine-structure degeneracy of the excited states. This increased dynamic separation between the states increases the SP of the ejected photoelectron. Numerical results are presented to illustrate this effect. By expressing the equations of motion in the spin basis set the precise conditions necessary to observe nonzero SP are identified. It is shown on the basis of these conditions that although the removal of the fine-structure degeneracy occurs independent of whether the atom has any spin-orbit coupling, this does not induce any spin polarization if the atom has no spin-orbit coupling.
The influence of the dynamic Stark effect on photoelectron angular distributions for two-photon-resonant three-photon ionization of sodium by way of the 42D4^{2}D42D fine-structure components has been investigated. Owing to the dynamic Stark shift of the atomic transition frequency, the angular distributions expected at low laser intensities were modified as different transitions were intensity tuned into and out of resonance with the laser frequency. The distributions are in good qualitative agreement with theory.
In this paper, we present the theory of photoelectron angular distributions in resonant multiphoton ionization that rigorously takes into account the saturation, ac Stark-shift, and the laser-linewidth effects. The influence of these effects on the distribution is incorporated through the coupling equations between the angle-resolved ionization probabilities and the (time-dependent) bound-state density-matrix elements. General expressions for the angular distributions are derived and several properties of these distributions are arrived at from these expressions. Finally, numerical results are presented to illustrate the effects of saturation, of ac Stark shifts, and of the laser line shape.
The behavior of photoelectron angular distributions in a strongly driven resonant multiphoton ionization process is discussed. It is shown that the angular distribution changes drastically with increasing intensity as a result of the ac Stark shifting of the levels. Moreover, under strong saturation conditions the distribution reaches a limiting form significantly different from the form predicted by perturbation theory. The effects of the laser bandwidth on the angular distribution are also discussed briefly.
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.
The energy levels of atoms and molecules exposed to intense, short, high-frequency laser pulses undergo time-dependent distortion known as dynamic Stark shift (DSS). These level shifts are induced by the coupling with nonresonant (nonessential) states and follow the temporal intensity profile of the laser pulse. Owing to the different DSSs of the individual atomic levels, transient resonance suppressions and enhancements significantly modify the multiphoton ionization pathway, leaving clear fingerprints (asymmetry, splitting, and shifting) in the energy spectrum of emitted photoelectrons. Here we investigate this phenomenon by solving the time-dependent Schr\"odinger equation of the valence electron of Na and Li in increasing levels of complexity: (i) developing minimal models for two-photon transition and (2+12+12+1)-photon ionization, (ii) applying a spectral method without continuum-continuum couplings, and (iii) propagating the electron wave packet on a large spatial grid accounting for all possible couplings. We show that appropriately detuned transform limited pulses provide a high level of control in strong-field excitation, when the atomic levels are dynamically shifted. Furthermore, we demonstrate the role of DSSs in the multipeak structure of the Autler-Townes doublet, found when several Rabi oscillations are induced during the strong-field resonant ionization.
As it is well known now1, electron energy spectra from multiphoton ionization (MPI) and Above-Threshold Ionization (ATI) of atoms by subpicosecond pulses reveal Stark-induced resonances and provide an elegant and unique method to investigate AC-Stark shifts of strongly perturbed excited states. In this work we have systematically studied such spectra and their interpretation in the case of MPI of Rare gases with 100 fs, 617 nm pulses. The intensity ranges from 1013 (for xenon) to 1015 W.cm−2 (for helium). The corresponding shifts range from .3 eV to 30 eV and may be, in principle, larger than the atomic ionization potential. Besides the measurement of AC-Stark shifts, the photoelectron spectra provide valuable information about the ionization process itself in a highly non-perturbative regime.