Production and State-Selective Detection of Ultracold RbCs Molecules

Using resonance-enhanced two-photon ionization, we detect ultracold, metastable RbCs molecules formed in their lowest triplet state $a{}^3\Sigma ^{+}$ via photoassociation in a laser-cooled mixture of ${}^{85}\mathrm{Rb}$ and ${}^{133}\mathrm{Cs}$ atoms. We obtain extensive bound-bound excitation spectra of these molecules, which provide detailed information about their vibrational distribution, as well as spectroscopic data on several RbCs molecular states including $a{}^3\Sigma ^{+}$, $(2){}^3\Sigma ^{+}$, and $(1){}^1\Pi$. Analysis of this data allows us to predict strong transitions from observed levels to the absolute vibronic ground state of RbCs, potentially allowing the production of stable, ultracold polar molecules at rates in excess of ${10}^6{\mathrm{s}}^{-1}$.

Figure 1

Ultracold RbCs formation and detection processes. (a) Colliding ground state atom pairs are excited into weakly bound levels of ${\mathrm{RbCs}}^{\star }$. (b) Excited molecules can decay into the $a{}^3\Sigma ^{+}$ and $X{}^1\Sigma ^{+}$ states. (c) Metastable $a{}^3\Sigma ^{+}$ molecules are excited by a weak infrared laser pulse predominantly to the $(2){}^3\Sigma ^{+}$ and $(1){}^1\Pi$ states (in the range shown by the shaded rectangle), and are then (d) ionized by an intense 532 nm pulse. (e) From coupled $(2){}^3\Sigma ^{+}$ and $(1){}^1\Pi$ vibrational levels observed in this work, we predict that transitions to $X{}^1\Sigma ^{+}(v=0)$ should be strong.

Figure 2

Detection and spectroscopy of ultracold RbCs molecules. (a) Time-of-flight mass spectrum. The channeltron current, averaged over 20 shots, is plotted vs delay time after the laser pulse. The PA laser is tuned to the ${\mathrm{RbCs}}^{\star }$ level at $\Delta =-38.019{\mathrm{cm}}^{-1}$ having $\Omega =0^{-},J=1$ [7], which can decay only to $a{}^3\Sigma ^{+}$ levels with $\Omega \in 0^{-},1$ and $J\in 0,2$ [15]. (b) Bound-bound spectrum, showing a portion of the total scan range indicated by the shaded rectangle in Fig. 1. The strong progression beginning around $8770{\mathrm{cm}}^{-1}$ is associated with the $(2){}^3\Sigma ^{+}$ excited state, whose $v=0$ level corresponds to the sudden onset of the series. The doubling of peaks occurs due to the spin-orbit splitting of its $\Omega =0^{-},1$ components. The smaller features at lower energies arise from transitions to the coupled $A{}^1\Sigma ^{+}$ and $b{}^3\Pi$ states.

Figure 3

Substructure of the $(2){}^3\Sigma ^{+}\leftarrow a{}^3\Sigma ^{+}$ transition. The two sets of peaks arise from the $\Omega =0^{-},1$ components of $(2){}^3\Sigma ^{+},v=3$. The dashed lines show the predicted vibrational structure of the $a{}^3\Sigma ^{+}$ state [16], and the solid circles indicate the predicted relative populations. The absolute position of this pattern was adjusted to match the observed features, which fixes the $(2){}^3\Sigma ^{+}$ level position. Note that the $\Omega =0^{-},1$ components of $a{}^3\Sigma ^{+}$ levels and the hyperfine/rotational substructure of both $a{}^3\Sigma ^{+}$ and $(2){}^3\Sigma ^{+}$ are not resolved within the $\sim 0.05{\mathrm{cm}}^{-1}$ linewidth of our laser.