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Physical Review A

Observation of cold Rb 2 molecules trapped in an optical dipole trap using a laser-pulse-train technique

Carlos Renato Menegatti, Bruno Marangoni, Luís Gustavo Marcassa

Published Nov 9, 2011• 7 citations
Publisher

Abstract

In this work, we have developed and characterized a laser-pulse-train technique to observe cold Rb${}_{2}$ molecules trapped in an optical dipole trap. The molecules are produced in a magneto-optical trap, and then loaded into a crossed optical dipole trap. The time evolution of the molecular population is obtained by applying a laser pulse train, which photoionizes the ground-state molecules through intermediate molecular bands. Our results show that this technique allows us to obtain a faster data acquisition rate of the time evolution of the molecule population than other techniques.

DOI
10.1103/physreva.84.053405

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In this work, we have developed and characterized a laser-pulse-train technique to observe cold Rb2{}_{2}2​ molecules trapped in an optical dipole trap. The molecules are produced in a magneto-optical trap, and then loaded into a crossed optical dipole trap. The time evolution of the molecular population is obtained by applying a laser pulse train, which photoionizes the ground-state molecules through intermediate molecular bands. Our results show that this technique allows us to obtain a faster data acquisition rate of the time evolution of the molecule population than other techniques.
We have developed an approach to continuously load ultracold ^{85}Rb_{2} vibrational ground-state molecules into a crossed optical dipole trap from a magneto-optical trap. The technique relies on a single high-power light beam with a broad spectrum superimposed onto a narrow peak at an energy of about 9400 cm^{-1}. This single laser source performs all the required steps: the short-range photoassociation creating ground-state molecules after radiative emission, the cooling of the molecular vibrational population down to the lowest vibrational level v_{X}=0, and the optical trapping of these molecules. Furthermore, we probe by depletion spectroscopy and determine that 75% of the v_{X}=0 ground-state molecules are in the three lowest rotational levels J_{X}=0, 1, 2. The lifetime of the ultracold molecules in the optical dipole trap is limited to about 70 ms by off-resonant light scattering. The proposed technique opens perspectives for the formation of new molecular species in the ultracold domain, which are not yet accessible by well-established approaches.
We report the first observation of optically trapped cold neutral molecules. Cesium dimers in the elect ronic ground state are produced directly in a magneto-optical trap and transferred to a dipole trap formed at the focus of a CO2{\mathrm{CO}}_{2}CO2​ laser beam ( $\ensuremath{\lambda}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10.6\ensuremath{\mu}\mathrm{m}$). These neutral molecules were detected using photoionization and time-of-flight spectroscopy. Initial experiments indicate a cold molecule trap lifetime on the order of half a second.
In this paper, we report observation of an enhancement of the ground state molecule Rb 2 formation in a Rb magneto-optical trap due to a photoassociation laser. Such an enhancement effect is due to flux enhancement of atomic pairs at short internuclear distances. Our experimental observations consist in the measurement of the molecular formation rate constant due to a probe laser as a function of the trap laser intensity. The results are compared with a simple semi-classical model, showing good agreement. We conclude that the production of cold Rb 2 may be enhanced using appropriate laser parameters; this may be useful for future experiments involving production and trapping of ultracold ground state molecules.
In this paper, we report the measurement of molecular formation rate constant of Rb2{\mathrm{Rb}}_{2}Rb2​ directly in a magneto-optical trap (MOT). The ground state molecules are detected by two-photon ionization, resonantly enhanced through the intermediate ${a}^{3}{\ensuremath{\Sigma}}_{u}^{+}\ensuremath{\rightarrow}{2}^{3}{\ensuremath{\Pi}}_{g}$ molecular band. We have measured the rate constant as a function of atomic density to conclude that the molecules Rb2{\mathrm{Rb}}_{2}Rb2​ are formed in the MOT by a photoassociation process caused by the trapping laser beams. We also measured the rate constant as a function of the trapping laser intensity. The results here presented are of importance to future experiments involving trapping of cold molecules.
The authors have used laser cooling and trapping techniques to investigate photoionization and cold collisions. With laser-trapped Rb, they have measured the photoionization cross section from the first excited (5P) level by observing the photoionization-induced loss rate of neutral atoms from the trap. This technique has the advantage that it directly measures the photoionization rate per atom. Knowing the ionizing laser intensity and the excited-state fraction, the measured loss rate gives the absolute cross section. Using this technique, the Rb 5P photoionization cross section at {approximately}400 nm has been determined with an uncertainty of 9%. The authors are currently attempting to extend this method to the 5D level. Using time-ordered pulses of diode-laser light (similar to the STIRAP technique), they have performed very efficient two-photon excitation of trapped Rb atoms to 5D. Finally, they will present results from a recent collaboration which combines measurements form conventional molecular spectroscopy (single photon and double resonance) with photoassociation collisions of ultracold Na atoms to yield a precise ({le}1 ppm) value for the dissociation energy of the X {Sigma}{sub g}{sub +} ground state of the Na{sub 2} molecule.
We report on the observation of translationally cold Cs2 molecules at temperatures in the 300–70 μK range, produced by the photoassociation of cold Cs atoms in a vapor-cell magneto-optical trap. The detection method is based on pulsed-laser photoionization of Cs2 molecules into Cs2+ ions, selectively detected through a time-of-flight mass spectrometer. This technique, together with the usual trap-loss method, enables us to perform the spectroscopy of the long-range attractive molecular states below the 6s 2S1/2+6p 2P3/2 dissociation limit.