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Physical Review A

Coherently excited atoms in external electric fields

M. Lombardi, Marc Giroud, J. H. Macek

Published Mar 1, 1975• 13 citations
Publisher

Abstract

Various manifestations of coherent mixtures of even- and odd-parity eigenstates of collision-excited atomic hydrogen in external electric fields are discussed on general grounds. We show that when even-and odd-parity states are coherently excited, application of an electric field perpendicular to the velocity axis induces orientation in the originally unoriented atoms. Circular polarization of the decay radiation is an observable consequence of the orientation.

DOI
10.1103/physreva.11.1114

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Physical Review A•Nov 1, 1977•19 citations
Various manifestations of coherent mixtures of even- and odd-parity eigenstates of collision-excited atomic hydrogen in external electric fields are discussed on general grounds. We show that when even-and odd-parity states are coherently excited, application of an electric field perpendicular to the velocity axis induces orientation in the originally unoriented atoms. Circular polarization of the decay radiation is an observable consequence of the orientation.
We present a detailed theoretical model and experimentally measured signals of alignment to orientation conversion (AOC) in the 7D3/2 and 9D3/2 states of cesium atoms in the presence of an external dc electric field [1]. Alignment produced by linearly polarized light can be converted into orientation by the quadratic Stark effect in an electric field if the angle between the electric field and the polarization vector of the exciting radiation is neither 0 or 90 degrees and if there are magnetic sublevels with Δm = ±1 that cross as the electric field is scanned. The appearance of orientation is verified by the appearance of a net circular polarization in the laser induced fluorescence. The theoretical model is based on the optical Bloch equation and extends models previously developed to solve the rate equations for Zeeman coherences in single-step [2] and double step laser excitation [3] of atoms. The agreement between the simulated and measured signals is excellent.
We report the experimental observation of alignment to orientation conversion in the 7D3/2 and 9D3/2 states of Cs in the presence of an external dc electric field and without the influence of magnetic fields or atomic collisions. Initial alignment of angular momentum states was created by two-step excitation with linearly polarized laser radiation. The appearance of transverse orientation of angular momentum was confirmed by the observation of circularly polarized light. We present experimentally measured signals and compare them with the results of a detailed theoretical model based on the optical Bloch equations. The effect is odd under time reversal and should be taken into account in ever more sensitive searches for an electron electric dipole moment.
A density-matrix approach has been developed for the angular distribution and polarization of radiative emission during single-photon atomic transitions for a general set of steady-state excitation processes in an arbitrary arrangement of static (or quasistatic) electric and magnetic fields. Particular attention has been directed at spectroscopic observations in the intense fields of the high-power ion diodes on the Particle Beam Fusion Accelerator II (PBFA II) and SABRE devices at Sandia National Laboratories and at magnetic-field measurements in tokamak plasmas. The field-dependent atomic eigenstates are represented as expansions in a complete basis set of field-free bound and continuum eigenstates. Particular emphasis has been given to directed-electron collisional excitations, which may be produced by an anisotropic incident-electron velocity distribution. We have allowed for the possibility of the coherent excitation of the nearly degenerate field-dependent atomic substates, which can give rise to a complex spectral pattern of overlapping Stark-Zeeman components. Coherent excitations may be produced by a beam of electrons that are spin-polarized at an angle with respect to the propagation direction or by nonparallel electric and magnetic fields. Our main result is a general expression for the matrix elements of the photon-polarization density operator representing the total intensity, angular distribution, and polarization of the atomic radiative emission. For the observation of radiative emission in the direction of the magnetic field, the detection of linearly polarized emission, in addition to the usual circularly polarized radiation, can reveal the presence of a perpendicular electric field or a coherent excitation mechanism.
We present experimental and theoretical studies of coherent excitation of magnetic sublevels in nD states of cesium that cross in an external electric field. Coherent excitation of mFmagnetic sublevels of hyperfine F levels with DeltamF= plusmn2 leads to resonances in the linearly polarized laser induced fluorescence (LIF), while coherent excitation of magnetic sublevels with DeltamF= plusmn1 leads to alignment to orientation conversion (AOC). From the resonance signals we were able to measure atomic constants, such as the hyperfine constant A and the tensor polarizability alpha2(Auzinsh et al., 2006). AOC involves the deformation of the spatial distribution of an atom's angular momentum and is of interest as a background in certain types of experiments that search for a permanent electric dipole moment (EDM) of the electron.
We consider a gas of hydrogen atoms illuminated by a broadband, unpolarized radiation with zero anisotropy. In the absence of external fields, the atomic J-levels are thus isotropically populated. While this condition persists in the presence of a magnetic field, we show instead that electric fields can induce the alignment of those levels. We also show that this electric alignment cannot occur in a two-term model of hydrogen (e.g., if only the Ly-alpha transition is excited), or if the level populations are distributed according to Boltzmann's law.
In a measurement of emission of Balmer-$\ensuremath{\alpha}$ flux from atomic hydrogen or deuterium excited by electron impact in a crossed-beam experiment, a longitudinal electric field which enhances the Balmer-$\ensuremath{\alpha}$ flux by Stark-mixing components of like MJ{M}_{J}MJ​ within the n=3n=3n=3 shell, does so asymmetrically depending on whether the field is applied parallel or antiparallel to the electron beam. The effect is attributed to coherent excitation of states of opposite parity.