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Journal of Physics B Atomic and Molecular Physics

The theory of stepwise electron and laser excitation of atoms. II. Strong optical excitation case

C J Webb, W. R. MacGillivray, M. C. Standage

Published Jun 28, 1984• 9 citations
Publisher

Abstract

For pt.I see ibid., vol.17, p.1675 (1984). A theory of stepwise excitation of atoms is presented in which electron excitation is followed by strong single-mode laser excitation. A model of the strong laser excitation processes is considered and theoretical expressions are obtained for line polarizations and electron-photon coincidence signals applicable to both axial and planar symmetry stepwise excitation experiments.

DOI
10.1088/0022-3700/17/12/024

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For pt.I see ibid., vol.12, no.12, p.1939 (1979). The modulated fluorescence in an optical double resonance experiment, i.e. when an atom interacts with two intense monochromatic laser beams each of which is weakly modulated in amplitude at frequency Omega , is studied in detail. The modulation rate corresponding to the steady-state population of the highest atomic level exhibits resonant structures at the modulation frequencies Omega =0, +or- alpha 0 , +or-2 alpha 0 due to the dynamical Stark splitting of the various energy levels; alpha 0 being related to the Rabi frequencies alpha 1 and alpha 2 associated with the two lasers and the two atomic transitions, by alpha 0 2 = mod alpha 1 mod 2 + mod alpha 2 mod 2 . The peak at Omega =+or- alpha 0 is the most intense and exhibits splitting when the applied fields are detuned from exact resonance with the respective atomic frequencies. Resonances are observed in the modulation rate of the intermediate-level population at the modulation frequencies Omega =0, +or-2 alpha 0 . In the presence of detunings of the applied fields, additional resonant structures occur at Omega =+or- alpha 0 which increase in magnitude with higher detunings and split. Explicit expressions for the positions, widths and weights of these resonant components are obtained analytically for intense, resonant fields. Numerical results for general values of the laser field strengths and the detunings are given.

Journal of Physics B Atomic and Molecular Physics•Feb 14, 1980•21 citations
For pt.I see ibid., vol.17, p.1675 (1984). A theory of stepwise excitation of atoms is presented in which electron excitation is followed by strong single-mode laser excitation. A model of the strong laser excitation processes is considered and theoretical expressions are obtained for line polarizations and electron-photon coincidence signals applicable to both axial and planar symmetry stepwise excitation experiments.
A theoretical description is presented for the process in which photons emitted from atoms which have been stepwise excited firstly by electron impact and then by resonant interaction with coherent radiation are detected in coincidence with the inelastically scattered electrons. The optical excitation is modelled using a full quantum electrodynamic theory. Calculations of the Stokes parameters are performed for the Hg (I=0) scheme; electron excitation from the 6 1 S 0 ground state to the 6 1 P 1 state followed by resonant laser excitation to the 6 1 D 2 state, with analysis being performed on the fluorescence from the 6 1 D 2 -6 3 P 1 relaxation channel. It is found that for this case, even though manifestations of optical pumping are evident, the Stokes parameters do not vary by more than +or-4% from the weak intensity limit.
A new type of electron/photon coincidence experiment is presented which utilises a laser -electron stepwise excitation technique to prepare atoms in an excited state. An electron collision initially excites an atom from the ground state to an intermediate state. Following this electron excitation process, coherent single mode laser radiation further excites the atom to an upper state, from which the fluorescence is observed. The nature of the laser excitation transfers both amplitude and phase information about the electron excited state to the upper excited state in a coherent way. This information is extracted from the upper state fluorescence by analysis of the fluorescence polarization in coincidence with the electron scattered from the atom for particular polarizations of the laser radiation. The coincidence technique allows an identically prepared sub -ensemble of excited atoms to be selected for measurement. The electron excited state studied was the 61131 state of mercury, excited by a beam of unpolarized electrons at an impact energy of 16 eV. The mercury atomic beam was isotopically selected following electron excitation using the narrow bandwidth of the laser radiation. Excitation from the electron excited 61P1 state was via 579 nm laser radiation to the 61D2 state and fluorescence was monitored from this state to the 63P1 state via a 313 nm transition. Stokes parameters were measured for two different laser polarization angles, since this allowed a complete set of collision parameters to be deduced for a single experimental geometry. Results for three different scattering angles at this energy are presented. The laser excitation process was modelled in both the weak and the strong laser excitation regimes. A perturbation approach developed to model single photon laser excitation allowed the collision parameters to be extracted from the measured coincidence Stokes parameters analytically, whereas a Quantum Electrodynamic model allowed high laser intensity effects on the coincidence signal to be explored theoretically. The Quantum Electrodynamic model indicated that the weak excitation model could be used to extract the collision parameters from the Stokes parameters at the laser intensities used in the experiment. Effects due to variation of the laser intensity were predicted from the Quantum Electrodynamic model and experiments conducted to test these predictions are presented. Collision parameters deduced from the measured stepwise coincidence Stokes parameters following optimization of the experimental conditions are presented. These parameters are compared qualitatively with existing data from heavy atomic systems and with a theoretical model based upon the Distorted Wave Born Approximation. Similarities between the experimental data are found, however significant discrepancies are noted between the parameters obtained here and with the predictions of the theoretical model.
A discussion is given of the requirements for observing strong level-crossing signals in stepwise fluorescence, where the state under investigation is optically excited via an intermediate state, the atom absorbing in the process two photons of different wavelengths. Data obtained using this technique are presented for fine-structure crossings in the 32D{3}^{2}D32D term of Li.
For pt.I see ibid., vol.12, no.12, p.1939 (1979). The modulated fluorescence in an optical double resonance experiment, i.e. when an atom interacts with two intense monochromatic laser beams each of which is weakly modulated in amplitude at frequency Omega , is studied in detail. The modulation rate corresponding to the steady-state population of the highest atomic level exhibits resonant structures at the modulation frequencies Omega =0, +or- alpha 0 , +or-2 alpha 0 due to the dynamical Stark splitting of the various energy levels; alpha 0 being related to the Rabi frequencies alpha 1 and alpha 2 associated with the two lasers and the two atomic transitions, by alpha 0 2 = mod alpha 1 mod 2 + mod alpha 2 mod 2 . The peak at Omega =+or- alpha 0 is the most intense and exhibits splitting when the applied fields are detuned from exact resonance with the respective atomic frequencies. Resonances are observed in the modulation rate of the intermediate-level population at the modulation frequencies Omega =0, +or-2 alpha 0 . In the presence of detunings of the applied fields, additional resonant structures occur at Omega =+or- alpha 0 which increase in magnitude with higher detunings and split. Explicit expressions for the positions, widths and weights of these resonant components are obtained analytically for intense, resonant fields. Numerical results for general values of the laser field strengths and the detunings are given.