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Acta Physica Sinica

The temperature measurement for the ultracold Cs2 molecules formed by photoassociation

Yanting Zhao, Yuan Jin-Peng, Ji Zhong-Hua, Zhonghao Li, Tengfei Meng, Tao Liu, Liantuan Xiao, Jia Suo-Tang

Published Jan 1, 2014• 1 citation
Publisher

Abstract

Ultracold Cs2 molecules have been formed by photoassociation. Using the multiphoton ionization technique, we measure the time-evolution of expanding process in ultracold Cs2 molecule system and obtain the decay curve of photoionization signals. Based on a simple case, where the initial distribution of atomic or molecular sample is a Gaussian function of position and of velocity, we get the sample's temperature by theoretical simulation. The result shows a reasonable agreement with the result of release-recapture method in cold atom sample. This method avoids the disadvantage of detecting the weakly fluorescence and can be widely used for measuring the temperature in cold atom or cold molecule system.

DOI
10.7498/aps.63.193701

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Ultracold Cs2 molecules have been formed by photoassociation. Using the multiphoton ionization technique, we measure the time-evolution of expanding process in ultracold Cs2 molecule system and obtain the decay curve of photoionization signals. Based on a simple case, where the initial distribution of atomic or molecular sample is a Gaussian function of position and of velocity, we get the sample's temperature by theoretical simulation. The result shows a reasonable agreement with the result of release-recapture method in cold atom sample. This method avoids the disadvantage of detecting the weakly fluorescence and can be widely used for measuring the temperature in cold atom or cold molecule system.
A simple and reliable method is used for the temperature measurement of ultracold molecules prepared in a magnetooptical trap (MOT). This method is based on the case where the initial distribution of an atomic or molecular sample is a Gaussian function of position and velocity. The atoms or molecules are detected by a sensitive photoionization detection technique. We obtain the temperature of molecules by analyzing the evolution of a photoionization signal. We compare the results of atomic temperature with the measurements by the release recapture technique (R&R), and find reasonable agreement. This provides a complementary method of determining the temperature of ultracold atoms and molecules.
In recent years several groups of researchers have succeeded in making samples of cold molecules, at temperatures below 1 K, and of ultracold molecules, at temperatures below 1 mK. We concentrate on the formation of Cs2 dimers. Starting with laser-cooled atoms, photoassociation forms molecules in an excited electronic state. Various mechanisms are presented for stabilizing these molecules by spontaneous emission into a bound level of the ground electronic state. The design of new formation schemes requires precise knowledge of molecular potential curves that is well beyond the accuracy of ab initio calculations. We show that analytical long-range potentials can be fitted on photoassociation spectra and yield, through the determination of the C3 long-range coefficient, a value of the radiative lifetime of the 6P3/2 cesium level with an unprecedented accuracy. For short-range potentials it is possible to avoid explicit calculations by using asymptotic methods that generalize to long-range molecules the quantum-defect concepts developed for Rydberg atoms. Considering two coupled channels, we show how generalized Lu–Fano plots can extract precise information from experimental spectra.
Photoassociation of ultracold atoms is an important method for producing stable ultracold molecules with extensive applications. Ion spectroscopy technique, which has higher sensitivity compared with trap loss spectroscopy and fragment spectroscopy, plays an important pole in the research of photoassociation spectroscopy. Based on ion spectroscopy technique, a complete experimental system is employed to produce and detect the ultracold ground state molecules, including the equipment of photoionization and time-of-fight mass spectroscopy. Ultra-cold ground state cesium molecules are demonstrated by the ionization detection technique. The number of the ground-state molecules produced in our MOT is measured. In addition, the relationship of the time interval of the cesium atom ion and the cesium molecular ion to the metal grid voltage is studied for the optimization of the experimental parameters.
We report on the formation of translationally cold Cs2 ground-state molecules through photoassociation of laser-cooled cesium atoms in a magneto-optical trap. The cold molecules are obtained after spontaneous decay of photoassociated molecules, and detected after pulsed-laser photoionization into Cs2+ ions. Photoassociation spectra of cesium attractive molecular states below the 6S1/2+6P3/2 dissociation limits are reported, obtained with both ion detection and trap-loss method. A temperature as low as 20−5+15 has been measured for the molecular cloud.
We use time-of-flight mass spectroscopy to detect the ultracold ground state Cs2 molecules formed directly by a photoassociation due to the trapping laser beams and repumping laser beam in a magneto-optical trap. We investigate the dependences of the Cs2+ intensity on operating voltage of micro-channel-plate, accelerating field intensity and duration time of the accelerating field. The experimental results are consistent with theoretical fitting. We obtain the best optimal parameters suited to our experimental condition, which is important for the future experiment on ultracold molecule photoassociation and photoionization spectroscopy.
We have performed photoassociative spectroscopy of cold Cs atoms in a vapor-cell magneto-optical trap, within $10{\mathrm{cm}}^{\ensuremath{-}1}$ below the dissociation limit 6s2S1/2+6p2P3/26s^{2}S_{1/2}+6p^{2}P_{3/2}6s2S1/2​+6p2P3/2​. Our detection method is based on pulsed-laser photoionization of Cs2{\mathrm{Cs}}_{2}Cs2​ molecules, selectively detected through a time-of-flight mass spectrometer. Temporal and spatial analysis of Cs2+{\mathrm{Cs}}_{2}^{+}Cs2+​ ions show for the first time the formation of translationally cold Cs2{\mathrm{Cs}}_{2}Cs2​ triplet ground state molecules, at a temperature $T\ensuremath{\sim}300\ensuremath{\mu}\mathrm{K}$.