Formation of Ultracold Polar Molecules in the Rovibrational Ground State

Ultracold LiCs molecules in the absolute ground state $X{}^1\Sigma ^{+}$, $v^{\prime \prime }=0$, $J^{\prime \prime }=0$ are formed via a single photoassociation step starting from laser-cooled atoms. The selective production of $v^{\prime \prime }=0$, $J^{\prime \prime }=2$ molecules with a 50-fold higher rate is also demonstrated. The rotational and vibrational state of the ground state molecules is determined in a setup combining depletion spectroscopy with resonant-enhanced multiphoton ionization time-of-flight spectroscopy. Using the determined production rate of up to $5\times {10}^3\mathrm{\text{molecules}}/\mathrm{s}$, we describe a simple scheme which can provide large samples of externally and internally cold dipolar molecules.

Figure 1

Sketch of the excitation and detection scheme. (a) photoassociation by Ti:Sa laser at 946 nm, (b) spontaneous decay into deeply bound ground state molecules, (c) two-photon ionization with resonant intermediate state. For depletion spectroscopy: (d) excitation of ground state molecules, (e) redistribution of ground state population; potentials curves from Ref. 19, 20, 32.

Figure 2

The $v^{\prime }=4$, $J^{\prime }=1$ (left trace) and $J^{\prime }=2$ (right trace) PA resonances in the $B{}^1\Pi$ state. The $J^{\prime }=1$ trace has been enlarged by a factor of 5 for better visibility.

Figure 3

Resonant-enhanced multiphoton ionization of ground state molecules produced by photoassociation via $B{}^1\Pi$, $v^{\prime }=4$, $J^{\prime }=2$. In the upper part, calculated line positions between all vibrational levels in the $X{}^1\Sigma ^{+}$ and in the $B{}^1\Pi$ state are marked. The star (*) marks the resonance used in the depletion spectroscopy. At the moment, we do not have a full assignment of all observed resonances.

Figure 4

Depletion laser scan with the PA laser resonant to the $B{}^1\Pi$, $v^{\prime }=4$, $J^{\prime }=2$ level (a) and to the $v^{\prime }=4$, $J^{\prime }=1$ level (b). In the upper part of the graphs, calculated wavelengths for transitions from $X{}^1\Sigma ^{+}$, $v^{\prime \prime }=0$, $J^{\prime \prime }$ to $B{}^1\Pi$, $v^{\prime }=12$, $J^{\prime }$ (labeled $J^{\prime \prime }\mathrm{\text{−}}{J}^{\prime }$) are marked. The dotted and dash-dotted gray lines in (b) are fitted spectra (see text for details) assuming PA via the ($-$) and ($+$) parity component of $J^{\prime }=1$, respectively, offset for visibility. The dashed (red) line is a fit combining both parities. The background at large formation rates as in (a) is dominated by ${\mathrm{LiCs}}^{+}$ ions from off-resonant excitation of other vibrational ground state levels. For small formation rates as in (b), spurious detection of fast ${\mathrm{Cs}}^{+}$ ions constitutes the main background [16].