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AIP conference proceedings

Line shape study of three-photon ionization through intermediate states of Rb atoms

A. Fioretti

Published Jan 1, 2001• 0 citations
Publisher

Abstract

We report a line shape study in a multiple level atomic system under the perturbation of a strong and pulsed laser field. The line shapes have been observed in the ion spectra during an experiment aimed to produce and detect translationally cold molecules in a rubidium magneto-optical trap and compared with the results of computer simulations of the atom-radiation dynamics.

DOI
10.1063/1.1370641

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Physical Review Letters•Mar 27, 2000•191 citations
We report a line shape study in a multiple level atomic system under the perturbation of a strong and pulsed laser field. The line shapes have been observed in the ion spectra during an experiment aimed to produce and detect translationally cold molecules in a rubidium magneto-optical trap and compared with the results of computer simulations of the atom-radiation dynamics.
We report on the analysis of the line shapes in a two-color, three-photon ionization process of rubidium atoms in a magneto-optical trap. A cw and a pulsed laser are tuned almost at resonance with the ${5}^{2}{S}_{1/2}{\ensuremath{-}5}^{2}{P}_{3/2}$ and the ${5}^{2}{P}_{3/2}{\ensuremath{-}6}^{2}{D}_{5/2}$ transitions, respectively. The line shapes, recorded as functions of the pulsed laser wavelength, are strongly asymmetric with respect to the near resonance peak positions: in fact, they present long tails only on one side, according to the sign of the detuning of the cw laser. Moreover, the signal is overall stronger for negative than for positive detunings. Theoretical considerations based on the Autler-Townes scheme and computer simulations of the excitation-ionization dynamics show that the main source of asymmetry is the coupling of the two ionization pathways going through the 62D3/2{6}^{2}{D}_{3/2}62D3/2​ and 62D5/2{6}^{2}{D}_{5/2}62D5/2​ levels. In order to reproduce the experimental line shapes one must take into account the variability of the pulsed laser intensity and its finite coherence time.
The spectra of three-photon laser excitation (according to the 5 S 1/2 → 5 P 3/2 → 6 S 1/2 → 37 P 3/2 scheme) of mesoscopic ensembles of cold Rb Rydberg atoms in a magneto-optical trap, using cw single-frequency lasers in each stage, have been investigated. The ensembles consisted of 1 to 5 atoms, and their presence was recorded by the selective field ionisation method with postselection according to the number of atoms. The dependence of the spectral shape on the excitation laser pulse duration has been studied. For 2-μs pulses, the minimum spectral width was found to be 1.3 MHz; it was determined by the laser linewidth and the spectral width of laser pulse. For pulses shorter than 0.5 μs, additional spectral broadening and occurrence of Rabi oscillations in the wings of three-photon resonances have been observed, a fact indicative of implementation of coherent three-photon laser excitation of Rydberg atoms. An analysis of the spectra within the four-level theoretical model based on optical Bloch equations showed good agreement between the experimental and theoretical results. The dependence of the shape of three-photon excitation spectrum on the number of recorded atoms was also studied. An increase in the average number of atoms leads to the occurrence of a dip in the single-atom excitation spectra; this dip is due to the specificity of the excitation and detection statistics of Rydberg atoms. The results obtained are important for applications of Rydberg atoms in quantum informatics.
The spectra of the three-photon laser excitation 5 S 1/2 → 5 P 3/2 → 6 S 1/2 nP of cold Rb Rydberg atoms in an operating magneto-optical trap based on continuous single-frequency lasers at each stage are studied. These spectra contain two partly overlapping peaks of different amplitudes, which correspond to coherent three-photon excitation and incoherent three-step excitation due to the presence of two different ways of excitation through the dressed states of intermediate levels. A four-level theoretical model based on optical Bloch equations is developed to analyze these spectra. Good agreement between the experimental and calculated data is achieved by introducing additional decay of optical coherence induced by a finite laser line width and other broadening sources (stray electromagnetic fields, residual Doppler broadening, interatomic interactions) into the model.
Experiments are carried out on the spectroscopy of the Förster resonance lines Rb(37 P ) + Rb(37 P ) → Rb(37 S ) + Rb(38 S ) and microwave transitions nP → n ′ S , n ′ D between Rydberg states of cold rubidium atoms in a magneto-optical trap (MOT). Under ordinary conditions, all spectra exhibit a linewidth of 2–3 MHz irrespective of the interaction time between atoms or between atoms and microwave radiation, although the limit resonance width should be determined by the inverse interaction time. The analysis of experimental conditions has shown that the main source of line broadening is the inhomogeneous electric field of cold photoions that are generated under the excitation of initial nP Rydberg states by broadband pulsed laser radiation. The application of an additional electric-field pulse that rapidly extracts photoions produced by a laser pulse leads to a considerable narrowing of lines of microwave resonances and the Förster resonance. Various sources of line broadening in cold Rydberg atoms are analyzed.
We study photoionization of cold rubidium atoms in a strong infrared laser field using a magneto-optical trap (MOT) recoil ion momentum spectrometer. Three types of cold rubidium target are provided, operating in two-dimension (2D) MOT, 2D molasses, and 3D MOT with densities in the orders of 10710^7107 atoms/cm3^33, 10810^8108 atoms/cm3^33, and 10910^9109 atoms/cm3^33, respectively. The density profile and the temperature of 3D MOT are characterized using the absorption imaging and photoionization. The momentum distributions of Rb+^++ created by absorption of two- or three-photon illuminate a dipole-like double-peak structure, in good agreement with the results in the strong field approximation. The yielding momentum resolution of 0.12±0.030.12 \pm 0.030.12±0.03 a.u. is achieved in comparison with theoretical calculations, exhibiting the great prospects for the study of electron correlations in alkali metal atoms through interaction with strong laser pulses.
We report the first observation of translationally cold ( $\ensuremath{\sim}90\ensuremath{\mu}\mathrm{K}$) Rb2{\mathrm{Rb}}_{2}Rb2​ molecules. They are produced in a magneto-optical trap in their triplet ground state. The detection is performed by selective mass spectroscopy after two-photon ionization into Rb2+{\mathrm{Rb}}_{2}^{+}Rb2+​, resonantly enhanced through the intermediate $a{}^{3}{\ensuremath{\Sigma}}_{u}^{+}\ensuremath{\rightarrow}2{}^{3}{\ensuremath{\Pi}}_{g}$ molecular band. The two rubidium isotopes present very different types of behavior that are interpreted in terms of their respective collisional properties.