Formation of ultracold $X{}^1{\Sigma }^{+}(v^{\prime \prime }=0)$ NaCs molecules via coupled photoassociation channels

We report production of ultracold $X{}^1{\Sigma }^{+}(v^{\prime \prime }=0)$ NaCs molecules via photoassociation. We utilize a combination of spectroscopic techniques to determine formation pathways and label ground-state samples. Efficient free-bound excitation occurs due to coupling between $B{}^1\Pi$ and neighboring electronic states in the $\text{Na}3S+\text{Cs}6P$ complex. An $f$-wave shape resonance contributes to formation of a rotationally pure sample of $X{}^1{\Sigma }^{+}(v^{\prime \prime }=0,J^{\prime \prime }=1)$ molecules.

Figure 1

REMPI scans detecting the absolute vibrational ground state. (a) RE3PI scan with labeled ground-state vibrational levels. (b) RE2PI scan where $D{}^1\Pi -f{}^3{\Delta }_{\Omega =1}\leftarrow X{}^1{\Sigma }^{+}(v^{\prime \prime }=0)$ transitions are indicated and compared to the $D{}^1\Pi$ Dunham expansion from Ref. [26]. The observation of two perturbed electronic states gives the appearance of doubling.

Figure 2

Photoassociation scans with REMPI detecting $v^{\prime \prime }=0$. Primes indicate $B{}^1\Pi$ rotational states. (a) PA scan of $B{}^1\Pi (v^{\prime }=17)$ and the inset depicts a scan of the nearly degenerate (2)0$+$ state revealed using a different REMPI frequency that is sensitive to $a{}^3{\Sigma }^{+}$ states. (b) PA scan of $B{}^1\Pi (v^{\prime }=13)$ where the inset resolves the hyperfine structure of a resonantly coupled state.

Figure 3

Potential energy curves and approximate wave functions for inhomogeneously coupled states producing $X{}^1{\Sigma }^{+}(v^{\prime \prime }=0)$ molecules. Ground-state potential curves are from Ref. [22] and the adiabatic $0^{+}$ states are approximated perturbatively from the diabatic PECs in Ref. [14] and long-range dispersion coefficients in Ref. [29]. The experimental $B{}^1\Pi$ state is from Ref. [15]. Wave functions were calculated using level 8.0[30].

Figure 4

cw depletion scans showing rotational population of photoassociated $X{}^1{\Sigma }^{+}(v^{\prime \prime }=0)$ molecules. $\left(2\right)0^{+}(v^{\prime }=22)\leftarrow X{}^1{\Sigma }^{+}(v^{\prime \prime }=0)$ rotational lines are labeled with calculations from Ref. [27]. The calculated line positions are shifted overall by 4.45 GHz to match the observed spectra. We set the PA frequency to $B{}^1\Pi (v^{\prime }=17,J^{\prime }=2,1)$ in (a) and (b), and $B{}^1\Pi (v^{\prime }=13,J^{\prime }=2,1)$ in (c) and (d).