利用双光缔合光谱技术直接测量超冷铯分子0_u^+(6S_(1/2)+6P_(1/2))长程态的转动常数的实验研究

Direct measurement of the rotational constant of 0_u^+(6S_(1/2)+6P_(1/2)) long-range state via double-pass spectroscopy

实验获得了超冷铯分子6S1/2+6P1/2离解限下0+u长程态振动能级v=187的转动常数.首先利用调制的荧光光谱技术获得了高分辨的转动量子数J=0—6的超冷铯分子纯转动光谱.利用双光缔合光谱技术,构建了高精度的频率参考信号,进而得到了相邻转动能级间的频率间隔.最后通过非刚性转子模型拟合得到了转动常数和离心畸变常数.

In this paper, we obtain the rotational constant and the distortion constant of v = 187 belonging to 0＋ 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 Y＇ = 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.