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Physical Review A

Observation of parity-unfavored transitions in the nonresonant photoionization of argon

B. Langer, Jens Viefhaus, Oliver A. Hemmers, Alexander Menzel, R. Wehlitz, U. Becker

Published Feb 1, 1995• 18 citations
Publisher

Abstract

Single-photon ionization of an atom or molecule can be subdivided into parity-favored and -unfavored transitions, the latter characterized by electron emission, preferentially perpendicular to the electric vector. The nonresonant existence of these transitions is shown experimentally and studied over an extended energy range for a variety of satellite transitions in atomic argon. The spectra exhibit several clearly resolved satellite lines with strongly negative \ensuremath{\beta} values close to -1, independent of the photon energy. The results confirm the corresponding predictions of angular-momentum transfer theory.

DOI
10.1103/physreva.51.r882

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Single-photon ionization of an atom or molecule can be subdivided into parity-favored and -unfavored transitions, the latter characterized by electron emission, preferentially perpendicular to the electric vector. The nonresonant existence of these transitions is shown experimentally and studied over an extended energy range for a variety of satellite transitions in atomic argon. The spectra exhibit several clearly resolved satellite lines with strongly negative \ensuremath{\beta} values close to -1, independent of the photon energy. The results confirm the corresponding predictions of angular-momentum transfer theory.
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When a bound electron is photoionized from an atom or molecule, the remaining multielectron ion with excess internal energy will relax towards lower-energy states. The subsequent transverse angular momentum transfer between the parent ion and the ejected electron can lead to an electron parity-unfavored transition. Here we perform a self-calibrated timing experiment to measure the time delay of the angular momentum transfer of parity-unfavored transitions during the multiphoton ionization. We use a strong linearly polarized 400-nm light field to trigger a parity-unfavored transition in the channel of the spin-orbit excited ionic state of krypton atoms and then selectively probe the phases of the electron wave functions with different magnetic quantum numbers with a weak parallelly (orthogonally) polarized 800-nm light field. The parallel and orthogonal probes serve as the start and the stop of a stopwatch, respectively. Hence, this allows us to access the characteristic time of such an ultrafast intra-atom multielectron process.
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Abstract Ionization of three-photon excited polarized states of argon atoms by circularly polarized probe light was studied in a supersonic beam. The revealed oscillation structure in the observed photoionization signals due to the Larmor precession of excited atomic states in a magnetic field allowed finding novel data for the Landé g-factors. We derived analytical formulas for the photoelectron current and explained the various types of the detected oscillations in terms of the principal lines among multiplet components of optical transitions. Our results indicate the possibility of implementing Doppler-free spectroscopy involving bound-free transitions.
Abstract The dipole radiation emitted by an atom excited by a unidirectional electron beam has a nonuniform angular distribution which is simply related to the percentage polarization P of the radiation emitted perpendicular to the beam. P was first calculated using the OppenheimerPenney (O.-P.) theory. In this theory the probability of excitation of an upper quantum state and the probability of subsequent emission of a polarized photon from such a state are considered independently. P is finally expressed in terms of the cross-sections QM for excitation of states of definite component of angular momentum along the direction of the electron beam. In general, P is dependent on detailed numerical calculations of QM j, but the selection rule A = 0 removes this dependence at threshold. In the O.-P. theory allowance may be made for fine structure and hyperfine structure, but the theory is ambiguous when the f.s. or h.f.s. separations are comparable with the line width. A theory is therefore developed which is based on the calculation of the probability of a polarized photon being emitted by the complete system of atom + electron. The ambiguity of the O.-P. theory is removed by integration over line profiles, but the expressions reduce to O.-P. expressions when the f.s. or h.f.s. separations are much smaller or much larger than the line width. The Lyot line of hydrogen is an intermediate case for which the line widths and the h.f.s. separations are comparable. Assuming the validity of the Born approximation, a simple expression is obtained which allows the QM to be calculated from the angular distribution of the scattered electrons. Theoretical predictions are compared with experimental results. For the Na£) lines the predicted polarization is small enough to escape experimental detection. Polarizations observed by - Skinner & Appleyard in 1927 for various Hg lines rise to maxima with decreasing electron energy, and then tend to values close to zero at threshold. These experimental results at low energies appear to be inexplicable in terms of the reactions considered, but if the polarization curve above the maximum is extrapolated to threshold, the theory and experiment are found to be in reasonable agreement. Further experimental work is thought to be desirable