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Acta Physica Sinica

Experimental study of ultracold cesium atom photoassociation spectrum using an amplitude modulation technique

Wang Li-Rong, Jie Ma, Linjie Zhang, Liantuan Xiao, Jia Suo-Tang, 量子光学与光量子器件国家重点实验室，山西大学物理电子工程学院，太原 030006

Published Jan 1, 2007• 7 citations
Publisher

Abstract

A high resolution photoassociation spectrum of ultracold cesium atoms is reported in this paper. A photoassociation laser with its frequency red detuned below 6S1/2+6P3/2 dissociation limit acts on the cold cesium atoms in magnetic optical trap and the excited ultracold molecules generated in the photoassociation process are observed. In order to obtain a high signal-to-noise ratio, the trapping laser is amplitude modulated using acousto-optic modulator and the fluorescence signal is demodulated by lock-in technique. The results are discussed using the density matrix equations.

DOI
10.7498/aps.56.6373

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A high resolution photoassociation spectrum of ultracold cesium atoms is reported in this paper. A photoassociation laser with its frequency red detuned below 6S1/2+6P3/2 dissociation limit acts on the cold cesium atoms in magnetic optical trap and the excited ultracold molecules generated in the photoassociation process are observed. In order to obtain a high signal-to-noise ratio, the trapping laser is amplitude modulated using acousto-optic modulator and the fluorescence signal is demodulated by lock-in technique. The results are discussed using the density matrix equations.
In the present paper, ultracold cesium atoms were obtained in magnetic optical trap using the laser cooling technology. The number of ultracold atoms is 10(7). The temperature of ultracold atoms is about 200 μK and the diameter of the ultracold atoms cloud is about 400 μm. The ultracold cesium molecules of excited states were formed using the photoassociation of ultracold atoms. The resolution of vib-rotational spectrum was studied under different scan rates of photossociation laser in the experiment. The resolution of vib-rotational spectrum becomes high when the scan rate of photossociation laser becomes slow. The cold atoms fluorescence was obtained using the high sensitive avalanche photo detector and the high resolution vib-rotational spectrum of ultracold cesium molecule in its first excited state of with dissociation limit long range state was obtained. The ultracold ground molecule was formed by controlled Raman photoassociation and the photoassociation laser must be locked to atom-molecular hyperfine transitions. The ultracold atoms photoassociation spectrum was modulated using ultra-low frequency. The first-order differential signal was optimized by changing modulation amplitude and modulation frequency. It provides a feedback signal to correct error of the laser. The laser frequency satisfies experimental requirement to form ultracold ground molecules using the controlled Raman photoassociation. This work is important for studies of ultracold atoms and molecules in confined space.
Photoassociation spectroscopy with high resolution for 0u+(6P3/2) long-range state of ultracold cesium molecules has been measured experimentally using the modulated trap loss fluorescence spectroscopy technology. The spectral range has been extended over 60 cm-1 below the 6S1/2 + 6P3/2 dissociation limit as compared with other groups. twenty-five new observations of the Cs2 0u+ in long-range state are reported. The vibrational binding energies of these states are analyzed by using the LeRoy-Bernstein formula. The long-range parameter C3 in molecular 0u+(6P3/2) state is derived for 16.1030.010, and the corresponding molecular potential curve is depicted.
We report on new observations of the photoassociation spectroscopy of ultracold cesium molecules using a highly sensitive detection technique and a combined analysis with all observed electronic states. The technique is achieved by directly modulating the frequency of the trapping lasers of a magneto-optical trap. New observations of the Cs2 0g(-), 0u(+), and 1g states at the asymptotes 6S1/2 + 6P1/2 and 6S1/2 + 6P3/2 are reported. The spectral range is extended to the red detuning of 112 cm(-1) below the 6S1/2 + 6P3/2 dissociation limit. Dozens of vibrational levels of the ultracold Cs2 0g(-), 0u(+), and 1g states are observed for the first time. The available experimental binding energies of these states are analyzed simultaneously in a framework of the generalized LeRoy-Bernstein theory and the almost degenerate perturbation theory by Marinescu and Dalgarno [Phys. Rev. A: At., Mol., Opt. Phys. 52, 311 (1995)]. The unique atomic-related parameter c3 governing the dispersion forces of all the molecular states is estimated as (10.29 ± 0.05) a.u.
An ultrahigh resolution photoassociation spectrum of caesium atoms in a magneto-optical trap is presented. Hyperfine structure of the excited state molecule is obtained by using the lock-in method based on modulated cold atoms in this spectrum. Amplitude of resonant lines related to the rotational levels increases with photoassociation laser intensity, and saturation effect of photoassociation of cold atoms is observed in our experiment. The saturation intensity of photoassociation is deduced by fitting the experimental data to a saturation model based on scattering theory. Differences among saturation intensities of different rotational progressions in the ν = 55 vibrational state of the caesium molecular long-range 0 g − state have been found.
High precision photoassociation spectroscopy is performed in ultracold cesium gas, with detunings as large as 51 cm(-1) below the Cs(6S(1/2))+Cs(6P(3/2)) asymptote. Trap-loss fluorescence detection is used for detecting the photoassociation to excited state ultracold molecules. Long vibrational progressions are assigned to electronic states of 0(g) (-), 0(u) (+), and 1(g) symmetry. The spectral data are fitted to a LeRoy-Bernstein equation, in order to obtain the effective coefficients of the leading long-range interaction term (C(3)/R(3)) and the relative vibrational quantum numbers measured down from dissociation. Additionally we present evidence for perturbations between the 0(g) (-) state and the dark 2(u) state.
Ultracold atom photoassociation spectroscopy is significant for studying the long-range molecular potential energy structure. The signal-noise ratio of ultracold Cesium Molecule photoassociation Rovibronic Spectrum is one of the indicators as a measure of detection technology which directly affect the distinguishing ability and detection sensitivity of the spectra. We obtain an long-range ultracold cesium molecular hyperfine rovibration spectra by modulating the fluorescence spectroscopy. We find that demodulation parameters have a significant impact to the signal-noise ratio of spectra by studying the demodulation parameters such as integration time and sensitivity, which show nonlinear relation with the ratio. We achieve the optimal control of the signal-noise ratio for spectrum according to our experiment, which laid an important foundation for further studying the long-range state rovibration level of ultracold cesium molecule.