Pulsed Laser Depletion Spectroscopy of Ultracold NaCs Molecules

We have labeled several deeply bound vibrational quanta in the $X^1\Sigma ^+$ electronic state in a sample of ultracold NaCs ($\sim $200 $\mu $K). The molecules are prepared from Magneto-Optical Traps (MOTs) via photoassociation with a laser field detuned from the Cs $6^2S_{1/2} -6^2P_{3/2} $ transition. We illuminate the sample with a tunable depletion pulse from a pulsed dye laser $\sim $100 $\mu $s before a Resonance Enhanced Multi-Photon Ionization (REMPI) detection pulse. By leaving the REMPI pulse frequency fixed, and scanning the depletion pulse frequency, we observe the vibrational progression of the excited electronic state from a single ground vibrational level. This technique allows us to scan large portions of the spectrum ($\sim $30 nm for a single laser dye) very quickly and with great efficiency, though with lower resolution ($\sim $1 cm$^{-1})$ than the CW depletion method. With this technique we have also observed the dissociation limit of NaCs above the Cs $6^2P_{3/2} $asymptote, which allows an independent verification for binding energies. An analysis of these spectra indicates the presence of $X^1\Sigma ^+$ molecules in the $\nu $ = 4, 5, 6, 9, 10, 11, 19 vibrational states.