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Physical Review A

Photoionization by a bichromatic field: Adiabatic theory

V A Pazdzersky, Vladimir Yurovsky

Published Jan 1, 1995• 19 citations
Publisher

Abstract

Atom photoionization by the superposition of a fundamental field and its second harmonic is considered. The finite analytical expressions for the photoionization probability are obtained using the adiabatic approximation. They demonstrate that the photoelectron angular distribution has a polar symmetry when the electrical field strength has a maximal polar asymmetry and the distribution is asymmetrical when the field is symmetrical. A strict proof of the polar symmetry of the photoionization probability in the case of the electrical field with maximal asymmetry is deduced using the Keldysh-Faisal-Reiss theories. The obtained results are in agreement with the experimental data available.

DOI
10.1103/physreva.51.632

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Atom photoionization by the superposition of a fundamental field and its second harmonic is considered. The finite analytical expressions for the photoionization probability are obtained using the adiabatic approximation. They demonstrate that the photoelectron angular distribution has a polar symmetry when the electrical field strength has a maximal polar asymmetry and the distribution is asymmetrical when the field is symmetrical. A strict proof of the polar symmetry of the photoionization probability in the case of the electrical field with maximal asymmetry is deduced using the Keldysh-Faisal-Reiss theories. The obtained results are in agreement with the experimental data available.
A method alternative to the density matrix formalism is applied to derive a general expression for the radiative decay probability following the photoionization of polarized atoms in two-step approximation. The graphical technique of the angular momentum theory is used for the integration over angular and spin variables. The general expression joining both steps of photoionization and fluorescence contains the possibility to investigate the polarization of all particles in the initial and final states and the angular distributions as well as angular correlations of photoelectrons and radiation. A simple way to get the expressions for special cases of the angular distribution and polarization of fluorescence radiation in the photoionization of randomly orientated and polarized atoms is given as an example of the practical application of the general formula.
The interference phenomena which can take place at the negative ion photodecay in a bichromatic electromagnetic (EM) field of linear and circular polarization consisting of coherent superposition of commensurable th and th harmonics of the same fundamental field are studied within the confines of the theory of perturbation on EM interaction and a zero-range potential model for the negative ion. The case of both odd and and that of odd and even (or vice versa) have been considered. It has been shown that, in the first case, the angular distribution of the emitted photoelectrons is always completely polar symmetric (in respect of the replacement , here p is the photoelectron momentum) and that the total photodecay probability is a periodic function of phase shift between harmonics. However, in the second case of odd and even , the angular distribution has polar asymmetry the degree of which depends on the values of and ratio of the electric vector strengths of the harmonics. Unlike the first case however, the total photodecay probability does not depend on phase shift at all. Moreover, the importance of taking into account the intermediate intercontinuum EM transitions (accompanied with the photoelectron momentum and energy change) has been shown too.
The angular correlation between the photoelectron and subsequent polarized fluorescent radiation measured in coincidence is analysed theoretically for atomic photoionization. Using the density matrix and statistical tensor formalism the general expression for the correlation function is obtained for an arbitrary polarized incident photon beam. The particular geometry of a recent experiment (Beyer et al., 1995) is considered and the parameters which determine the angular correlation are expressed in terms of photoionization amplitudes. It is demonstrated that the photoelectron- (polarized) fluorescence coincidence measurements reveal strong circular and linear dichroism. As an example the photoionization of Ca atoms near the 4p 2P3/2 threshold is calculated within the framework of multichannel quantum defect theory.
Abstract The biharmonic (ω, 2ω) photoionization of atomic inner-shell electrons opens up new perspectives for studying nonlinear light–atom interactions at intensities in the transition regime from weak to strong-field physics. In particular, the control of the frequency and polarization of biharmonic beams enables one to carve the photoelectron angular distribution and to enhance the resolution of ionization measurements by the (simultaneous) absorption of photons. Apart from its quite obvious polarization dependence, the photoelectron angular distributions are sensitive also to the (relative) intensity, the phase difference and the temporal structure of the incoming beam components, both at resonant and nonresonant frequencies. Here, we describe and analyze several characteristic features of biharmonic ionization in the framework of second-order perturbation theory and (so-called) ionization pathways, as they are readily derived from the interaction of inner-shell electrons with the electric-dipole field of the incident beam. We show how the photoelectron angular distribution and elliptical dichroism can be shaped in rather an unprecedented way by just tuning the properties of the biharmonic field. Since such fields are nowadays accessible from high-harmonic sources or free-electron lasers, these and further investigations might help extract photoionization amplitudes or the phase difference of incoming beams.
Expressions for the angular distribution of photoelectrons produced in two-photon two-color atomic ionization are obtained within the statistical tensor formalism, taking into account a full multipole expansion of the radiation in electric and magnetic moments. The general expressions are reduced to a simpler form for a particular geometry used in experiments with XUV + optical radiation beams and analyzed within the first-order nondipole corrections for different polarization states of the photons. As a numerical example, the Ne 1s1s1s above-threshold ionization by two-color radiation is considered within the second-order perturbation theory. A large forward-backward asymmetry in the photoelectron angular distribution is predicted.
Synopsis Photoionization is a fundamental process in which an electron is emitted from an atom. The emission is traditionally considered to be symmetric with respect to the polarization axis, unless it is temporally confined to a period shorter than an optical cycle time. We demonstrate that this symmetry can still be broken by combining a train of a few attosecond pulses and a dressing laser field. The light fields act as temporal slits and phase modulator that releases electron wavepackets. The resulting photoelectron spectra differ for electrons emitted in opposite direction along the polarization.