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Conference on Lasers and Electro-Optics

Laser nonuniformity and ac Stark shift effect on two-photon resonant three-photon ionization process of cesium atom

Jonghoon Yi, Hyunmin Park, Jaemin Han, Yongjoo Rhee, Jongmin Lee, Moon‐Gu Baik

Published Nov 27, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1109/iqec.1998.680141

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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.
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.
The temporal effects of multiphoton ionization are investigated using a two-color method which determines both the time and intensity at which the process occurs. We show that the total ionization efficiency depends strongly on the time at which an atom makes a transition to an excited state during an intense laser pulse. This result clearly shows that the ac-Stark-shifted bound-state resonances not only enhance the cross section for photoionization, but that the excited bound states provide temporary storage states for the atomic population during the photoionization process.
We present a theoretical analysis of a recent experiment [ J. Opt. Soc. Am. B6, 1513 ( 1989)] on two-color three-photon ionization of Xe through resonant even-parity Rydberg states. We calculate second-order ac-Stark shifts, two-photon Rabi frequencies, and ionization widths by using wave functions obtained from a multichannel quantum-defect-theory analysis of the Rydberg series in Xe. We perform a detailed description of the experiment, using the density-matrix formalism, accounting for the temporal and spatial distributions of the laser fields. The agreement with the experiment is good except for the ac-Stark shift of the Rydberg states, which, in our calculation is close to the ponderomotive energy, whereas in the experiment it is measured to be half of this value.
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.
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