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Journal of Physics B Atomic Molecular and Optical Physics

The interference phenomena at negative ion photodecay in a bichromatic field

V A Pazdzersky, V I Usachenko

Published Aug 14, 1997• 9 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/30/15/012

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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.
A novel kind of coherent control of various atomic and molecular photoprocesses proceeding in a bichromatic electromagnetic (EM) laser field, angular coherent control , is proposed and demonstrated here theoretically by studying the particular case of negative-ion photodetachment in the field of a linearly polarized fundamental laser frequency and its second harmonic of arbitrary spatial configuration and orientation of the unit polarization vectors e 1 and e 2 of the respective harmonic fields. Due to the inherent spatial asymmetry of the applied bichromatic field, the ejection of detached photoelectrons is also generally asymmetric in respect of the detached photoelectron momentum replacement p →- p . As follows from our gauge-invariant analytical calculations within the framework of perturbation theory on EM interaction and a zero-range potential model for the negative ion, this field asymmetry results in a photocurrent, the magnitude and direction of which are strongly dependent on both the relative phase shift Φ between the harmonics of the ionizing bichromatic field and on the angle θ between the polarization vectors e 1 and e 2 . The latter strong dependence on the angle θ is the background for angular control of the direction and magnitude of the produced photocurrent by means of an appropriate choice and proper adjustment of the angle between the polarization vectors of the incident bichromatic field harmonics.
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.
Coherent control of the photoelectron angular distribution (PAD) in ionization by a bichromatic circularly polarized field is theoretically studied in neon in the photon energy range 16--20 eV. The breakdown of the PAD axial symmetry with respect to the light propagation direction is analyzed. The coherent control of the PAD manifests itself through: (1) a change from a one-lobe to a three-lobe shape, respectively, for co- and counter-rotating harmonics, (2) a variation of the polar asymmetry with the light frequency, and (3) a rotation of the PAD around the direction of the beam depending on the relative phase between the harmonics. Tuning the lowest harmonic at the 2p53s2{p}^{5}3s2p53s or 2p54s2{p}^{5}4s2p54s intermediate excited states strongly influences the PAD. In contrast to atomic hydrogen and the alkali atoms, the cases of equal and opposite helicity are not related in neon. Features of a conjugate polarization setup, when one harmonic is linearly polarized while the other one is circularly polarized, are also discussed.
We study dichroic effects in the two-photon ionization of hydrogen from its ground state due to the interaction with a bichromatic field of commensurate frequencies. The two field components have different polarization: one is linearly polarized and the other is circularly polarized (CP). Circular dichroism (CD) in the angular distribution of the photoelectrons appears if the helicity of the CP field is reversed. Numerical results reveal the influence of the photon frequencies chosen on the CD in the azimuthal angular distribution of the ejected photoelectrons.