Resonance enhanced two-photon ionization spectrum of ultracold 85Rb133Cs molecules in transitions
Introduction
Ultracold polar molecules have attracted great interests for both physicists and chemists due to their rich rovibrational structures, large permanent electric dipole moments and long coherent times [1], [2], [3], [4], [5]. These characteristics allow potential applications in ultracold chemistry [6], [7], quantum computation [8], [9], quantum simulation [10], [11], precise measurement [12], [13] and degenerate quantum gas [14].
All of these applications require efficient production of molecules in a well-defined ground state. Up to now such molecules may be produced in a variety of ways. One approach is to transfer pairs of ultracold atoms to a Feshbach state by ramping a magnetic field, and then coherently transfer to a vibronic level of molecular state by implementing stimulated Raman adiabatic passage [15]. In favourable cases, this method can produce molecules in a single hyperfine and Zeeman state [16], [17], [18], [19], [20], [21], [22]. However, this approach produce molecules only once during one experimental cycle, which usually takes around one minute. Contrastively, other two alternative approaches, direct laser cooling [23] and short-range photoassociation (PA) [24] allow continuously producing molecules. The former method has a rapid develop recently with a milestone of realizing molecular magneto-optical trapping [25], [26], [27]. As this method requires nearly closed laser-cooling transitions, it is still limited to a small class of molecules. The latter depends on resonant coupling of PA excited states, which have both appropriate Franck-Condon (F-C) factors with initially scattering atomic state and deeply bound molecular state. In short-range PA, the formed molecules are distributed in several vibrational levels, that is unfavorable for producing a pure quantum state. If transition information between the distributed ground state and an suitable excited state can be obtained, it can provide guides for implementing optical pumping to continuously accumulate molecules in one pure quantum state, just like the case of homonuclear Cs2 molecules [28].
In 2010 Stwalley et al. theoretically studied resonant coupling states for short-range PA in all 10 heteronuclear alkali metal dimers [24]. Since then such approach has been implemented in LiCs [29], NaCs [30], KRb [31], RbCs [32] and LiRb [33]. Among these dimers, RbCs molecule, especially 85Rb133Cs, attract interests benefiting from its special characteristics: sizable permanent electric dipole moment [34] enables easy alignment for quantum simulation [11]; avoidable immiscibility of its components, which is different from its isotopic components [35], provides possibility to realize molecular Bose-Einstein condensation; inelastic collision with co-trapped Cs atom also supports molecule purification in lowest vibronic state [36].
In Ref[24]., the authors proposed that (2)1Π1 electronic state, which has resonant coupling with (1)1Π1 (or (B)1Π1) state, appears to be a quite promising path for producing ultracold RbCs molecules in the lowest vibronic ground state. Over the past ten years, several short-range PA electronic states [32], [37], [38], [39], [40], [41], [42], [43], [44], [45], including the early proposed (2)1Π1 state [43], were used to produce ultracold ground state 85Rb133Cs molecules. Reference [43] also shows that most vibrational levels of 21Π1 state have relatively strong production rates, indicating that this state may provide a promising passway to implement optical pumping. Thus transition information between (2)1Π1 and states is required.
It is known that resonance enhanced two-photon ionization (RETPI) spectroscopy is an easy and quick method to obtain such information. In 2014 Bruzewicz et al. presented a portion of RETPI spectrum for RbCs molecules. The scanning range of photoionization (PI) laser frequency is 15180–15340 cm. They found that the formed molecules mainly distributed in levels. Except for the lowest vibronic state, other vibrational transitions are not assigned in that work. The finite assignments are insufficient to derive transition information between (2)1Π1 and states.
In this paper, the RETPI spectrum of ultracold ground state 85Rb133Cs molecules with a larger frequency range between 14,500 and 15850 cm is presented. With an assistant of optical pumping from a 1070nm laser, three electronic transitions, and are distinguished. On the focused transition where the formed molecules are excited from the single ground state, vibrational transitions among are assigned. The energy separation between these two electronic states Te, harmonic constant ω and anharmonic constants ωχ for each state, ωy for (2)1Π1 state, are derived simultaneously. Based on these derived spectroscopic constants, a map of F-C factors is plotted. These investigations would be meaningful for accumulating 85Rb133Cs molecules in the lowest vibronic ground state with further optical pumping.
Section snippets
Experimental setup
Our experimental setup and operation procedure are nearly the same as one of our publications [47], in which RETPI spectrum between 13,700 and 14600 cm of ultracold RbCs molecules was reported. In that paper molecules in the metastable ground state are photoionized, while here molecules lay in the single ground state .
Fig. 1 shows the formation and detection mechanisms of ultracold ground state 85Rb133Cs molecules we use. In a vacuum chamber with a pressure of 3 Pa, 1 × 107 85Rb
Results
Fig. 2(a) shows our measured RETPI spectrum between 14,500 and 15850 cm of ultracold RbCs molecules formed via rovibrational level. The PI laser frequency is scanned with a speed of 0.04 nm/s. As the linewidth of dye laser is 3 GHz and rotational constant of RbCs molecules in ground state is around 500 MHz [48], RETPI can only resolve vibrational transitions. Even there were published abundant literatures on structures and spectra of RbCs molecule, accurate assignments for
Conclusions
The RETPI spectrum of ultracold ground state 85Rb133Cs molecules between 14,500 and 15850 cm has been investigated. Optical pumping from one 1070 nm laser is used to distinguish and electronic transitions. Vibrational transitions among have been assigned. Based on these assignments, the following spectroscopic constants have been obtained simultaneously for both and (2)1Π1 states: harmonic constant ω and anharmonic constants
CRediT authorship contribution statement
Zhonghua Ji: Conceptualization, Methodology, Investigation, Writing - original draft. Ting Gong: Investigation, Data curation. Yanting Zhao: Investigation, Writing - review & editing, Funding acquisition. Chuanliang Li: Software, Visualization. Liantuan Xiao: Funding acquisition. Suotang Jia: Funding acquisition.
Declaration of Competing Interest
The auithors declare that they have no known competing financial interests or personel relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by National Key Research and Development Program of China (Grant No. 2017YFA0304203), Natural Science Foundation of China (Nos. 61675120, 61875110), NSFC Project for Excellent Research Team (No. 61121064), Shanxi ``1331 Project″ Key Subjects Construction, PCSIRT (No. IRT17R70), 111 project (Grant No. D18001).
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