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Acta Physica Sinica

Direct measurement of the rotational constant of 0u+(6S1/2+6P1/2) long-range state via double-pass spectroscopy

Hu Chen-Yang, Wenliang Liu, Xu Run-Dong, Jizhou Wu, Jie Ma, Liantuan Xiao, Jia Suo-Tang

Published Jan 1, 2015• 0 citations
Publisher

Abstract

In this paper, we obtain the rotational constant and the distortion constant of v=187 belonging to 0u+ state below the 6S1/2+6P1/2 disassociation limit. In our experiment, we first prepare the ultra-cold cesium sample in the MOT (magneto-optical trap) by six beams of pumping laser, one beam of repumping laser, and a pair of anti-Helmholtz coils. Then we construct a high-resolution frequency reference using the double-pass photoassociation technique. The double-pass photoassociation technique is a creative and robust method. We use a polarization beam splitter to split one laser beam from the laser to two beams-Laser Ⅰ and Laser Ⅱ; Laser Ⅱ then passes twice through an acousto-optic modulator (AOM) whose central frequency is 110 MHz, using a reflecting mirror and a convex lens before illuminating the MOT. We use two shutters-S1 and S2 to control Laser Ⅰ and Laser Ⅱ. Open S1 while keep S2 close to make Laser I interact with the MOT; and after the rotational spectroscopy of J=0-6 is observed, turn off S1 and turn on S2 immediately. Let laser II interact with MOT and obtain another part of spectroscopy that is exactly the same with J=6; we define this part of spectroscopy as J'=6. The frequency interval between J=6 and J'=6 is exactly 220 MHz for the scan process is strictly linear, and that can be an accurate frequency interval in our experiment. The laser intensities of these two laser beams have to be strictly equal in case of the laser-induced frequency shift. Using the frequency interval of 220 MHz, we can calculate the frequency interval of J=0-6. The detection method we used here is the trap loss spectroscopic technology by modulating fluorescence of cold atoms in the MOT, which allows a direct spectroscopy detection at the rovibrational levels for a very weak transition probability. With the frequency intervals of each rotational quantum number, we can fit the frequency intervals to the non-rigid model to derive the rotation constant B and distortion constant D which are crucial to precisely measure the full molecule potential curves as well as deepen our understanding of molecular formation. This kind of double-pass photoassociation technique not only can direct obtain the precise value of rotation constant B and distortion constant D as compared with the traditional photoassociation method, but also can obtain a relatively accurate potential energy curve. And another great advantage is that we are able to calculate the frequency intervals easily without the wavelength meter which is rather expensive and difficult to control.

DOI
10.7498/aps.64.143302

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Acta Physica Sinica•Jan 1, 2015•2 citations
In this paper, we obtain the rotational constant and the distortion constant of v=187 belonging to 0u+ state below the 6S1/2+6P1/2 disassociation limit. In our experiment, we first prepare the ultra-cold cesium sample in the MOT (magneto-optical trap) by six beams of pumping laser, one beam of repumping laser, and a pair of anti-Helmholtz coils. Then we construct a high-resolution frequency reference using the double-pass photoassociation technique. The double-pass photoassociation technique is a creative and robust method. We use a polarization beam splitter to split one laser beam from the laser to two beams-Laser Ⅰ and Laser Ⅱ; Laser Ⅱ then passes twice through an acousto-optic modulator (AOM) whose central frequency is 110 MHz, using a reflecting mirror and a convex lens before illuminating the MOT. We use two shutters-S1 and S2 to control Laser Ⅰ and Laser Ⅱ. Open S1 while keep S2 close to make Laser I interact with the MOT; and after the rotational spectroscopy of J=0-6 is observed, turn off S1 and turn on S2 immediately. Let laser II interact with MOT and obtain another part of spectroscopy that is exactly the same with J=6; we define this part of spectroscopy as J'=6. The frequency interval between J=6 and J'=6 is exactly 220 MHz for the scan process is strictly linear, and that can be an accurate frequency interval in our experiment. The laser intensities of these two laser beams have to be strictly equal in case of the laser-induced frequency shift. Using the frequency interval of 220 MHz, we can calculate the frequency interval of J=0-6. The detection method we used here is the trap loss spectroscopic technology by modulating fluorescence of cold atoms in the MOT, which allows a direct spectroscopy detection at the rovibrational levels for a very weak transition probability. With the frequency intervals of each rotational quantum number, we can fit the frequency intervals to the non-rigid model to derive the rotation constant B and distortion constant D which are crucial to precisely measure the full molecule potential curves as well as deepen our understanding of molecular formation. This kind of double-pass photoassociation technique not only can direct obtain the precise value of rotation constant B and distortion constant D as compared with the traditional photoassociation method, but also can obtain a relatively accurate potential energy curve. And another great advantage is that we are able to calculate the frequency intervals easily without the wavelength meter which is rather expensive and difficult to control.
In this paper, ultracold cesium molecules are formed through photoassociation technology, which is carried out in a magneto-optical trap. High resolution photoassociaion spectra with the rotational progressions up to J = 7 are obtained. Three rovibrational levels of the long-range 0 u + state of Cs 2 below the (6S 1/2 + 6P 1/2 ) dissociation limit are specifically investigated. By fitting their binding energy intervals to the non-rigid rotational model, the rotational constant of the long-range 0 u + state is determined. A proportional dependence of the value of the rotational constant on the vibrational quantum number is demonstrated.
The high resolution spectrum of ultracold Cs2 pure long range 0g- state is experimentally measured using modulated trap loss spectroscopic technology. Based on Double photoassociation spectroscopic technique, precise frequency difference reference signals are constructed to accurately calibrate the resonant frequency intervals of the ro-vibrational levels, thus obtaining the relationship between the frequency interval of rotational level and the rotational quantum number. The experimental data are fitted by the non-rigid rotational model, and the molecular rotational constants corresponding to different vibrational levels in the ultracold Cs2 pure long range 0g- state are acquired. The experimental results demonstrate a linear decrease of the rotational constant with the increase of vibrational quantum number, and the linear decreasing rate is -0.41m MHz0.01 MHz.
We report on a quantitative experimental determination of the rotational constants for the high-lying vibrational levels of the ultracold pure long-range Cesium molecules formed via photoassociation. The scheme relies on a precise reference of frequency difference in a double photoassociation spectroscopy, which is induced by two laser beams based on an acoustic-optical modulator. The rotational constants are obtained by fitting a nonrigid rotor model into the frequency intervals of the neighboring rotational levels deduced from the reference.
This article demonstrates new observation of the high-resolution ro-vibrational bichromatic photoassociation spectra (BPAS) of Cs2 in the 0u+ long-range state below the asymptotes 6S1/2 + 6P1/2. By combining with a modulation spectroscopic technique, precise references of the frequency differences have been engineered through the BPAS, with which the rotational constants of low-lying vibrational levels of the Cs20u+ long-range state have been accurately determined by fitting the frequency differences to the non-rigid-rotor model. The rotational constants for the newly observed seven ro-vibrational levels are summarized and disagreement for the level ῦ = 498 is clarified. The rotational constants of different vibrational levels demonstrate strong perturbations of the related energy structures. A simple analysis is performed and shows good agreement with experimental results.
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.