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Physical Review A

ac Stark shifts for cesium and their effect on ionization line shapes

Michael Stuart Pindzola, Alan H. Glasser, M. G. Payne

Published Oct 1, 1984• 8 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.30.1800

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In order to perform multi-photon ionization experiments at a particular light intensity, the AC Stark-shift of the xenon 6s level is used as a probe to select that intensity from the range of intensities present in the laser focus. Our analysis provides an accurate, wide-range distribution function of the intensities in our multi-mode laser pulses. Using this method we determined there al order of non-linearity of the resonant ionization process and the probability for additional photon absorption.

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Resonance shifts in the multiphoton ionization of cesium atoms

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Physical Review A•Jul 1, 1976•107 citations
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.
Summary form only given. We investigated the effect of temporal and spatial characteristics not only on the ionization spectrum but on the excited-state population. From the calculational results in which Gaussian temporal and spatial variation was assumed, it was found that the nonuniformity of a laser made the shift of ionization peak frequency different from that of excitation peak frequency. This result indicates that measurement of an ac Stark shift cross section by detecting the fluorescence emitted from an excited state might give the different results comparing with the ionization spectrum measurement case.
We present ionization probability and lineshape calculations for the two-step three-photon ionization process, , of the ground state of hydrogenic atoms in a non-monochromatic laser field with a time-dependent amplitude. Within the framework of a three-level model, the AC Stark shifts and non-zero ionization rates of all states involved were taken into account, together with spatial and temporal inhomogeneities of the laser signal. In contrast with the usual perturbative technique, the time evolution of the atomic states was simulated by directly solving the system of coupled time-dependent inhomogeneous differential equations numerically, the equations being equivalent to the appropriate non-stationary Schrödinger equation. Particular numerical results were obtained for typical parameters of the pulsed laser field that are employed in a new experiment to measure the 1S-2S energy separation with muonium at the Rutherford Appleton Laboratory. The shifts and asymmetries of the photoionization lineshapes revealed may be of relevance for ultra-high-precision experiments in hydrogen in CW laser fields.
In order to perform multi-photon ionization experiments at a particular light intensity, the AC Stark-shift of the xenon 6s level is used as a probe to select that intensity from the range of intensities present in the laser focus. Our analysis provides an accurate, wide-range distribution function of the intensities in our multi-mode laser pulses. Using this method we determined there al order of non-linearity of the resonant ionization process and the probability for additional photon absorption.
This work reports the multiphoton ionization of cesium atoms, via resonance with the 6F6F6F level, by interaction with a single transverse and longitudinal mode neodymium-glass laser beam. At a low rate of ion production, when the laser frequency is tuned through resonance, the laser intensity necessary to create a constant number of ions passes through a minimum, as expected, for a three-photon energy equal to the unperturbed $6S\ensuremath{\rightarrow}6F$ transition energy. But for a higher ionization rate, involving larger laser intensities, a departure from expectation is observed which is interpreted as a consequence of an energy shift of the transition. The level shifts are demonstrated in another way by their strong effect on the experimental order of nonlinearity of the interaction, Kexpt{K}_{\mathrm{expt}}Kexpt​. A sharp variation of Kexpt{K}_{\mathrm{expt}}Kexpt​ is observed when the energy shift of the $6S\ensuremath{\rightarrow}6F$ transition exactly compensates the wavelength detuning of the laser. The latter method gives shift values which are found to be independent of the large variations of the interaction volume under conditions of resonance. Both methods give a linear dependence of the shifts on the laser intensity. Identification of these shifts with energy shifts of the $6S\ensuremath{\rightarrow}6F$ transition is subject to theoretical reservations and needs further experimentation.