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Journal of Physics B Atomic and Molecular Physics

AC Stark effect in resonant multiphoton ionisation. II. Non-quadratic dynamical shift of the 6d D2levels in atomic mercury

Jürgen Reif, Michel Poirier, J. Morellec, Didier Normand

Published Oct 28, 1984• 9 citations
Publisher

Abstract

For pt.I see ibid., vol.17, p.4135 (1984). Using a previously described new method to evaluate light-induced level shifts, it is experimentally demonstrated that the two possible intermediate 6D 2 states in four-photon resonant five-photon ionisation of mercury are mixed at high laser intensities. Due to this mixing, the AC Stark shifts of these levels are no longer quadratic in field amplitude. This is in good agreement with a simple theoretical model. This makes a direct determination of the oscillator strengths for two mercury lines possible.

DOI
10.1088/0022-3700/17/20/013

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The polarization dependences of some resonant multiphoton ionizations in atomic mercury have been measured with a broadband (bandwidth ≈ 1.5 cm−1) multimode dye laser at moderate light intensities (≈ 500 MW∙cm−2). The multiphoton processes studied were the absorption of four photons to a resonant 1S0, 1D2, or 3D2 level by one-photon ionization. Complete saturation of the one-photon ionization step results in the ionization of all atoms excited to the resonant level. Because of the saturation of the ionization step, the polarization dependence of the four-photon excitation to the resonant level is measured. The theory developed by Dalby et al. is shown to give good agreement with the experiment when it is used to calculate the polarization dependence of the multiphoton transition to the resonant level. For the 6d1D2 resonance, distortions in the polarization dependence and an unusual linewidth dependence on the light polarization were observed. We relate these observations to the alternating current Stark effect and to the production of the third harmonic of the laser light in the focal volume.

Canadian Journal of Physics•Jan 1, 1988•1 citation
For pt.I see ibid., vol.17, p.4135 (1984). Using a previously described new method to evaluate light-induced level shifts, it is experimentally demonstrated that the two possible intermediate 6D 2 states in four-photon resonant five-photon ionisation of mercury are mixed at high laser intensities. Due to this mixing, the AC Stark shifts of these levels are no longer quadratic in field amplitude. This is in good agreement with a simple theoretical model. This makes a direct determination of the oscillator strengths for two mercury lines possible.
A new analytical treatment is developed for resonant multiphonon ionisation which takes into account the physical properties of the exciting laser such as the spatial intensity distribution of a focused Gaussian beam and light statistics effects in the narrow bandwidth limit. The ion number as a function of laser intensity and wavelength follows a 'displacement law', which means that it may be expressed as a single variable function and provides a more reliable way to determine light-induced level shifts than previous methods. Three-photon resonant five-photon ionisation of mercury in a chaotic field serves as an experimental test and agrees excellently with the displacement law. However, the derived dynamical Stark shift differs by about a factor of three from that calculated from currently known oscillator strengths.
The authors report a high-resolution electron spectroscopic study of three-photon resonant, five-photon ionisation of xenon. The results are analysed in terms of a dynamic resonance of three photons with an AC Stark-shifted intermediate level (6s), followed by a saturated two-photon transition to the continuum. They demonstrate a linear relationship between detuning from the zero-field resonance position and the local momentary light intensity causing the five-photon ionisation. Thus, the AC Stark shift acts as a probe which selects a particular intensity from the wide range present in the laser focus. They determined the AC Stark shift of the 6s level the distribution function of the light intensities and the intensity dependence of the ionisation probability. High-resolution electron spectra are used in arguments to support the validity of their analysis.
An electron spectroscopic study is reported of multiphoton ionisation of Xe and Cs using two laser beams. One of the two beams is tuned to multiphoton resonance with an intermediate state and causes a considerable AC Stark shift. The second beam acts as a probe of the population and shift of the intermediate state. In Xe the authors deal with a three-photon resonance with the 6s level ( lambda approximately=440 nm); in Cs with two-photon resonances with Rydberg levels of the nd series. As probes, beams of either lambda =1064 or 532 nm have been used.
Density-matrix formalism is applied to construct a model for third- and fifth-harmonic generation in a four-photon resonant medium. The transition rates and coherent superposition of states associated with the resonant levels are treated rigorously, while the adiabatic following approximation is used to calculate the contribution of all nonresonant levels to the interaction. Expressions for the quadratic and quartic Stark shift and multiphoton ionization emerge naturally from the theory. At the power level required for appreciable four-photon absorption, the intensity-dependent Stark shift and ionization become overwhelming and limit the efficiency of harmonic generation. Because of the Stark shift, an intense pulse suffers a ``self-detuning'' effect and propagates through the resonant medium with minimal absorption. The interference of the phase-dependent Raman scattering with the multiphoton transition is found to be responsible for the vanishing of ionization in the presence of the generated third-harmonic field, an effect which was observed in earlier experiments.
For pt.I see ibid., vol.8, p.1795 (1975) where the theory of Doppler-free two-photon transitions was discussed for the case of one single laser frequency. Here, the effects of two different frequencies are considered. When the residual Doppler width of the two-photon transition is less than the natural linewidth, the previous results are essentially regained. For large residual Doppler shifts, the absorption profile is a power-broadened Lorentzian if the frequency of the probe field is larger than the frequency of the strong field. In the opposite case, the absorption peak splits because of the AC Stark effect. The high-intensity distortions are minimized when equal frequencies can be used. Power-dependent shifts do not ordinarily occur with a resonantly tuned intermediate level. Some modifications of the spectra are introduced by phase-disturbing collisions.
The polarization dependences of some resonant multiphoton ionizations in atomic mercury have been measured with a broadband (bandwidth ≈ 1.5 cm−1) multimode dye laser at moderate light intensities (≈ 500 MW∙cm−2). The multiphoton processes studied were the absorption of four photons to a resonant 1S0, 1D2, or 3D2 level by one-photon ionization. Complete saturation of the one-photon ionization step results in the ionization of all atoms excited to the resonant level. Because of the saturation of the ionization step, the polarization dependence of the four-photon excitation to the resonant level is measured. The theory developed by Dalby et al. is shown to give good agreement with the experiment when it is used to calculate the polarization dependence of the multiphoton transition to the resonant level. For the 6d1D2 resonance, distortions in the polarization dependence and an unusual linewidth dependence on the light polarization were observed. We relate these observations to the alternating current Stark effect and to the production of the third harmonic of the laser light in the focal volume.