Resonance enhanced two-photon ionization spectrum of ultracold ⁸⁵Rb¹³³Cs molecules in ${\left(2\right)}^1{\Pi }_1\leftarrow X^1{\Sigma }^{+}$ transitions

Highlights

• With an assistant from an optical pumping laser, (2)${}^1{\Pi }_1\leftarrow X^1{\Sigma }^{+}$ electronic transition is distinguished from (4)${}^3{\Sigma }^{+}\leftarrow a^3{\Sigma }^{+}$ and (3)${}^3{\Pi }^{+}\leftarrow a^3{\Sigma }^{+}$ transi- tions in RETPI spectrum.

• Global assignments of vibrational transitions among (2)${}^1{\Pi }_1(v=3-20)X^1{\Sigma }^{+}(v=0-5)$ were presented.

• Spectroscopic constants of (2)¹Π₁ and $X^1{\Sigma }^{+},$ including energy separation, harmonic and anharmonic constants, are simultaneously derived.

• A map of Franck-Condon factors between X${}^1{\Sigma }^{+}(v=0-9)$ and (2)${}^1{\Pi }_1(v=o-25)$ vibrational transitions are plotted, supporting further optical pumping for accumulating absolutely ground state molecules.

Abstract

We present resonance enhanced two-photon ionization (RETPI) spectrum between 14,500 and 15850 cm${}^{-1}$ for ultracold ground state ⁸⁵Rb¹³³Cs molecules. With an assistant of optical pumping from one 1070nm laser, ${\left(2\right)}^1{\Pi }_1\leftarrow X^1{\Sigma }^{+}$ electronic transition is distinguished from ${\left(4\right)}^3{\Sigma }^{+}\leftarrow a^3{\Sigma }^{+}$ and ${\left(3\right)}^3\Pi \leftarrow a^3{\Sigma }^{+}$ transitions. Some observed RETPI spectra are globally assigned to vibrational transitions from $X^1{\Sigma }^{+}(v=0-5)$ to ${\left(2\right)}^1{\Pi }_1(v=5-20)$. Based on these assignments, the spectroscopic constants of $X^1{\Sigma }^{+}$ and (2)¹Π₁ are simultaneously derived, including energy separation, harmonic and anharmonic constants. Then a map of Franck-Condon factors between vibrational transitions of $X^1{\Sigma }^{+}(v=0-9)$ and ${\left(2\right)}^1{\Pi }_1(v=0-20)$ is plotted based on these constants. Our present work would be meaningful for continuous accumulation of ultracold ⁸⁵Rb¹³³Cs molecules in the lowest vibronic state with further optical pumping.

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 $X^1{\Sigma }^{+}$ 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 ${\left(2\right)}^1\Pi \leftarrow X^1{\Sigma }^{+}$ RETPI spectrum for RbCs molecules. The scanning range of photoionization (PI) laser frequency is 15180–15340 cm${}^{-1}$. They found that the formed molecules mainly distributed in $X^1{\Sigma }^{+}(v=0-5)$ 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 $X^1{\Sigma }^{+}$ states.

In this paper, the RETPI spectrum of ultracold ground state ⁸⁵Rb¹³³Cs molecules with a larger frequency range between 14,500 and 15850 cm${}^{-1}$ is presented. With an assistant of optical pumping from a 1070nm laser, three electronic transitions, ${\left(4\right)}^3{\Sigma }^{+}\leftarrow a^3{\Sigma }^{+},$ ${\left(2\right)}^1{\Pi }_1\leftarrow X^1{\Sigma }^{+}$ and ${\left(3\right)}^3\Pi \leftarrow a^3{\Sigma }^{+}$ are distinguished. On the focused $2^1{\Pi }_1\leftarrow X^1{\Sigma }^{+}$ transition where the formed molecules are excited from the single ground state, vibrational transitions among ${\left(2\right)}^1\Pi (v=5-20)\leftarrow X^1{\Sigma }^{+}(v=0-5)$ are assigned. The energy separation between these two electronic states T_e, 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.