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Journal of Physics B Atomic Molecular and Optical Physics

Resonant ionisation polarisation spectroscopy of complex atoms

P. Thornton Greenland

Published Dec 28, 1988• 10 citations
Publisher

Abstract

A simple expression for the ion yield from resonant three-photon ionisation through an autoionising level of a complex atom is obtained. In contrast to some treatments it is assumed that coherent, pulsed lasers of fixed polarisation are used to ionise the atoms, and effects which arise from the coherence are identified. The dependence of ion yield on the laser polarisations is exposed, and the possibility of a spectroscopy based on this dependence is discussed. As an example, it is shown how the spin of the autoionising level in a complex atom may be determined by only relative ionisation measurements.

DOI
10.1088/0953-4075/21/24/012

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A simple expression for the ion yield from resonant three-photon ionisation through an autoionising level of a complex atom is obtained. In contrast to some treatments it is assumed that coherent, pulsed lasers of fixed polarisation are used to ionise the atoms, and effects which arise from the coherence are identified. The dependence of ion yield on the laser polarisations is exposed, and the possibility of a spectroscopy based on this dependence is discussed. As an example, it is shown how the spin of the autoionising level in a complex atom may be determined by only relative ionisation measurements.
A theory of two-photon ionisation of atomic systems is developed incorporating into the treatment details of the laser spectrum. A general expression for the time-integrated two-photon ionisation probability is obtained together with general expressions for the induced shifts and widths of the atomic levels. These expressions are valid near resonance, when in the ordinary perturbation treatment the energy denominators vanish. The general expressions are then evaluated for the special case of a Lorentzian laser spectrum, when simple analytic results are obtained. The treatment is not confined to the two-level atom or the rotating-wave approximation.
The wavelength dependence of resonant two-photon ionisation of metastable He (21S) and He(23S) via the 3P intermediate level is reported. In both the singlet and triplet systems an additional complex, but reproducible, structure is seen in the resonance profile of the ion signal. This occurs for moderate laser intensities (<1 GW cm-2) strong enough to saturate the resonant transition. The physical reason for the structure is not understood, but measurements point to a delicate dependence on the time development of the atomic states and the laser pulse.
An improved method for the angular momentum measurement of autoionizing levels of complex atoms based on photoionization polarization spectroscopy is reported. In this method, a variation of the ion yield is measured as a function of the combination of the polarizations of the lasers used to excite the autoionizing level through a ladder of intermediate resonant states. Here, eight combinations of different polarizations in three lasers are used, which include not only circular but also linear polarizations. It has been confirmed that the angular momentum of the autoionizing level can be determined uniquely by relative ion-yield measurements for any combination of angular momenta of the levels of the excitation ladders by choosing appropriate intensities of the lasers.
General expressions are presented and investigated for the degree of polarisation of electrons produced in the resonant two-photon ionisation of unpolarised atoms from the ground S state. The exciting and ionising electromagnetic fields are assumed to have arbitrary directions of propagation and arbitrary degrees of polarisation. It is shown that completely polarised electrons can be obtained when the matrix element of the P to S transition from the resonant state to the continuum is larger than the matrix element of the P to D transition. Such cases are realised in sodium and potassium atoms.
Two-laser multiphoton ionisation under the autoionising-like resonance condition is studied theoretically within an extended atomic model dictated by the experiment of Feldmann et al (1986). Ordinary coupling between a selected pair of bound states via the continuum only is supplemented in the author's model by Raman-type coupling involving the atomic spectrum below the ionisation threshold. In the case considered this additional Raman coupling enhances the ionisation rate to a much greater extent than the coupling via the continuum can do. The Raman coupling modifies the autoionising-like shape parameter and is decisive as far as the resonance peak profile is concerned. Exploiting this Raman coupling within the atomic model with four effective bound states, the author gives explanations for all the details of the ionisation spectrum detected by Feldmann et al, i.e. the number of resonance peaks, their strength, fine structure, shape, relative heights and sensitivity to the laser field.