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arXiv:2010.04068 · Physical Review A

Detection of NaRb Feshbach molecules by photodissociation

Fan Jia, Zhichao Guo, Lintao Li, Dajun Wang

Published Oct 28, 2020• 7 citations
arXivHTMLPDF

Abstract

We demonstrate detection of NaRb Feshbach molecules at high magnetic field by combining molecular photodissociation and absorption imaging of the photofragments. The photodissociation process is carried out via a spectroscopically selected hyperfine Zeeman level correlated with the Na ($3{P}_{3/2}$) $+$ Rb ($5{S}_{1/2}$) asymptote which, following spontaneous emission and optical pumping, leads to ground-state atoms in a single level with near unity probability. Subsequent to the dissociation, the number of molecules is obtained by detecting the resultant $^{23}\mathrm{Na}$ and $^{87}\mathrm{Rb}$ atoms. We have also studied the heating effect caused by the photodissociation process and optimized the detection protocol for extracting the temperature of the molecular cloud. This method enables the $in\phantom{\rule{0.16em}{0ex}}situ$ detection of fast timescale collision dynamics between NaRb Feshbach molecules and will be a valuable capability in studying few-body physics involving molecules.

DOI
10.1103/physreva.102.043327

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We demonstrate detection of NaRb Feshbach molecules at high magnetic field by combining molecular photodissociation and absorption imaging of the photofragments. The photodissociation process is carried out via a spectroscopically selected hyperfine Zeeman level correlated with the Na (3P3/23{P}_{3/2}3P3/2​) +++ Rb (5S1/25{S}_{1/2}5S1/2​) asymptote which, following spontaneous emission and optical pumping, leads to ground-state atoms in a single level with near unity probability. Subsequent to the dissociation, the number of molecules is obtained by detecting the resultant 23Na^{23}\mathrm{Na}23Na and 87Rb^{87}\mathrm{Rb}87Rb atoms. We have also studied the heating effect caused by the photodissociation process and optimized the detection protocol for extracting the temperature of the molecular cloud. This method enables the insituin\phantom{\rule{0.16em}{0ex}}situinsitu detection of fast timescale collision dynamics between NaRb Feshbach molecules and will be a valuable capability in studying few-body physics involving molecules.
We report the creation of ultracold bosonic 23 Na 87 Rb Feshbach molecules via magneto-association. By ramping the magnetic field across an interspecies Feshbach resonance (FR), at least 4000 molecules can be produced out of the near degenerate ultracold mixture. Fast loss due to inelastic atom–molecule collisions is observed, which limits the pure molecule number, after residual atoms removal, to 1700. The pure molecule sample can live for 21.8(8) ms in the optical trap, long enough for future molecular spectroscopy studies toward coherently transferring to the singlet ro-vibrational ground state, where these molecules are stable against chemical reaction and have a permanent electric dipole moment of 3.3 Debye. We have also measured the Feshbach molecule's binding energy near the FR by the oscillating magnetic field method and found these molecules have a large closed-channel fraction.
We have successfully prepared an optically trapped ultracold mixture of 23{}^{23}23Na and 87{}^{87}87Rb atoms and studied their interspecies Feshbach resonances. Using two different spin combinations, several sss- and ppp-wave resonances are identified by observing high inelastic losses and temperature increases for both species near resonant magnetic field values. The two sss-wave resonances observed below 500 G between atoms in their lowest-energy levels are possible candidates for Feshbach molecule association. Our results are well characterized by a coupled-channel model which also refines the ground-state interaction potentials between 23{}^{23}23Na and 87{}^{87}87Rb. This work opens up the prospect for preparing ultracold ensembles of ground-state bosonic NaRb molecules which are chemically stable and can provide strong dipolar interactions.
Utilizing single-photon photoassociation, we have achieved ultracold rubidium molecules with a high number density that provides a new efficient approach toward molecular quantum degeneracy. A new detection mechanism for ultracold molecules utilizing inelastic atom–molecule collision is demonstrated. The resonant coupling effect on the formation of the X 1 Σ + g ground state 85 Rb 2 allows for a sufficient number of more deeply bound ultracold molecules, which induced an additional trap loss and heating of the co-existing atoms owing to the inelastic atom–molecule collision. Therefore, after the photoassociation process, the ultracold molecules can be investigated using the absorption image of the ultracold rubidium atoms mixed with the molecules in a crossed optical dipole trap. The existence of the ultracold molecules was then verified, and the amount of accumulated molecules was measured. This method detects the final produced ultracold molecules, and hence is distinct from the conventional trap loss experiment, which is used to study the association resonance. It is composed of measurements of the time evolution of an atomic cloud and a decay model, by which the number density of the ultracold 85 Rb 2 molecules in the optical trap was estimated to be >5.2 × 10 11 cm −3 .
We report a high-resolution spectroscopic investigation of the long-range states of the 23Na87Rb^{23}\mathrm{Na}^{87}\mathrm{Rb}23Na87Rb molecule near its Na(32S1/2)+Rb(52P3/2)\mathrm{Na}(3{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2})+\mathrm{Rb}(5{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{3/2})Na(32S1/2​)+Rb(52P3/2​) asymptote. This study was performed with weakly bound ultracold molecules produced via magnetoassociation with an interspecies Feshbach resonance. We observed several regular vibrational series, which are assigned to the five attractive long-range states correlated with this asymptote. The vibrational levels of two of these states have sharp but complex structures due to hyperfine and Zeeman interactions. For the other states we observed significant linewidth broadenings due to strong predissociation caused by spin-orbit couplings with states correlated to the lower Na(32S1/2)+Rb(52P1/2)\mathrm{Na}(3{\phantom{\rule{0.16em}{0ex}}}^{2}{S}_{1/2})+\mathrm{Rb}(5{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{1/2})Na(32S1/2​)+Rb(52P1/2​) asymptote. The long-range C6{C}_{6}C6​ van der Waals coefficients extracted from our spectrum are in good agreement with theoretical values.
We report the successful production of an ultracold sample of absolute ground-state ^{23}Na^{87}Rb molecules. Starting from weakly bound Feshbach molecules formed via magnetoassociation, the lowest rovibrational and hyperfine level of the electronic ground state is populated following a high-efficiency and high-resolution two-photon Raman process. The high-purity absolute ground-state samples have up to 8000 molecules and densities of over 10^{11} cm^{-3}. By measuring the Stark shifts induced by external electric fields, we determined the permanent electric dipole moment of the absolute ground-state ^{23}Na^{87}Rb and demonstrated the capability of inducing an effective dipole moment over 1 D. Bimolecular reaction between ground-state ^{23}Na^{87}Rb molecules is endothermic, but we still observed a rather fast decay of the molecular sample. Our results pave the way toward investigation of ultracold molecular collisions in a fully controlled manner and possibly to quantum gases of ultracold bosonic molecules with strong dipolar interactions.
By applying an external magnetic field, we have determined the population of the magnetic sublevels of photodissociated atoms from the intensities of atomic fluorescence lines split by the Zeeman effect. A preferential population of the magnetic sublevels of the atomic fragment Na(3 2P3/2), which arises from the direct photodissociation through the D1Π(continuum) state of the NaK molecule, has been observed. The results are explained by assuming the separability of electronic and nuclear motion during the dissociation. The observed results could be rationalized by the sum of the photodissociation amplitudes caused by excitation of the P, Q and R branches. The population of the magnetic sublevels of Rb(5 2P3/2) atoms produced by predissociation of the C1Πu state of the Rb2 molecule by the repulsive molecular state c3Σ+u has been determined from the Zeeman spectra of the emission Rb(5 2P3/2→ 5 2Sm′1/2). The observed results are interpreted by including the spin-orbit coupling between an excited level of the C1Πu state and levels of the c3Σ+u state and the effect of the external magnetic field.