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Journal of Physics B Atomic and Molecular Physics

AC Stark effect in resonant multiphoton ionisation. I. Light statistics and focusing influence: displacement law

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

Published Oct 28, 1984• 15 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/17/20/012

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The influence of ac Stark shifts on the three-photon ionization of cesium in a moderate-intensity laser beam is investigated. The ionization process involves a two-photon resonance with the $8d$ fine-structure levels. The work consists of two related parts. In the first part, atomic parameters, such as the two-photon Rabi rate and the ac Stark shift, are calculated in the nonrelativistic Hartree-Fock approximation. In the second part, the atomic parameters thus obtained are used as coefficients in a system of coupled differential equations representing the time evolution of the density matrix. Ionization line shapes are generated for a transform-limited pulsed laser of moderate intensity with a single-mode transverse structure. As the time history of the intensity profile is varied from square to smooth, the ionization line shape becomes asymmetric due to the time dependence of the ac Stark shift. The effects of varying the spatial intensity distribution of the laser light to include transverse centering and longitudinal focusing are also studied.

Physical Review A•Oct 1, 1984•8 citations
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.
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.
Presents a theoretical analysis of the effect of laser light statistics on two-photon resonant three-photon ionisation of sodium. The ionisation probability and the population of the atom in the resonant state are compared in the presence of an intensity stabilised laser with a diffusing phase (phase diffusion model) and light undergoing Gaussian amplitude fluctuations (chaotic field). In the limit of large bandwidth fields the resonant two-photon excitation is statistically decoupled from the following ionisation step. A decrease of the bandwidth results in a partial correlation between these two steps manifesting itself in an enhancement of the ionisation probability in the chaotic field. With increasing intensity and the onset of saturation of the bound-bound transition, the ionisation probability and resonant excited-state population in the chaotic field may become smaller than the one in the phase diffusing light. In the large-bandwidth limit the Stark shift in the chaotic field is enhanced, which has a pronounced effect on the effective power-law dependence of the ionisation probability.
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.
The author presents a theory of resonant multiphoton ionisation by multimode laser light. He investigates N photon resonant N+1 photon ionisation for a high order bound-bound transition with the resonance dominated by the Stark shift. Two aspects of the problem are considered. He studies the bandwidth dependence for the resonant ionisation probability, approximating the multimode laser light by a chaotic field. Secondly, he investigates the effects of finite mode numbers of the multimode laser. Finally, the theory is compared with the recent experimental results of Lompre et al (1981).
Laser temporal-coherence effects are investigated in four-photon ionisation of caesium atoms using a tunable wavelength Nd-glass laser pulse and tuning the frequency onto the resonant three-photon transition 6S to 6F. The laser can be operated in either single or multimode. In the latter case, the laser bandwidth can be varied between 3*10 -2 and 1.5 cm -1 . The resonance peaks obtained with incoherent laser pulses are enhanced, shifted and broadened with respect to those induced by coherent pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses is statistically enhanced by a factor of 1.8+or-0.2 compared with the shift induced by coherent pulses, in good agreement with calculations. The resonant multiphonon ionisation rate induced by multimode pulses depends both on intensity and bandwidth effects. It is enhanced by a factor of 2.5 for pulses with bandwidths less than 0.2 cm -1 , while it becomes smaller than that induced with a coherent pulse of the same intensity when the bandwidth becomes larger than 1 cm -1 . Finally, the resonance width depends both on intensity an bandwidth effects. At low intensity (2*10 7 W cm -2 ) it is governed only by the laser bandwidth, while at high intensity (10 8 W cm -2 ) it varies linearly with the intensity and is statistically enhanced.
The influence of ac Stark shifts on the three-photon ionization of cesium in a moderate-intensity laser beam is investigated. The ionization process involves a two-photon resonance with the 8d8d8d fine-structure levels. The work consists of two related parts. In the first part, atomic parameters, such as the two-photon Rabi rate and the ac Stark shift, are calculated in the nonrelativistic Hartree-Fock approximation. In the second part, the atomic parameters thus obtained are used as coefficients in a system of coupled differential equations representing the time evolution of the density matrix. Ionization line shapes are generated for a transform-limited pulsed laser of moderate intensity with a single-mode transverse structure. As the time history of the intensity profile is varied from square to smooth, the ionization line shape becomes asymmetric due to the time dependence of the ac Stark shift. The effects of varying the spatial intensity distribution of the laser light to include transverse centering and longitudinal focusing are also studied.