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Physical Review A

Ultracold-molecule production by laser-cooled atom photoassociation

Y. B. Band, Paul S. Julienne

Published Jun 1, 1995• 135 citations
Publisher

Abstract

We propose a method for copiously producing ultracold ground electronic-state molecules by two-photon absorption to a Rydberg molecular level (that spontaneously radiates to the ground molecular state) during collisions of ultracold atoms in a laser trap. Using the ${\mathrm{Na}}_{2}$ molecule as an example, the sequential two-photon absorption is via an intermediate high-lying vibrational level of the excited ${1}_{\mathit{g}}$ state and results in the formation of a given vibrational level of a $^{1}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}$ Rydberg molecular state. Realistic calculations demonstrating the validity of the method are presented.

DOI
10.1103/physreva.51.r4317

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Summary form only given. We report results from an efficient new method for producing translationally cold (T<1 mK) potassium molecules in the ground X/sup 1//spl Sigma//sub g//sup +/ electronic state. We use the two-photon R-transfer scheme conceptually similar to a proposal by Band and Julienne (1995). First, ultracold potassium atoms in a MOT are photoassociated at long range to the 1/sub g/ molecular electronic state, which correlates to the 1/sup 1//spl Pi//sub g/ state at short range. The resulting molecules are then excited with a second laser to the 5/sup 1//spl Pi//sub u/ or 6/sup 1//spl Pi//sub u/ Rydberg states. These Rydberg states have favorable Franck-Condon factors for spontaneous decay into low vibrational levels of the ground state. The molecules are detected by use of two-color resonant ionization with pulsed lasers. By scanning one of the detection lasers, we observe transitions from several vibrational levels of the X state. The largest observed molecule formation rate in a single vibrational level is about 10/sup 5/ molecules/s for /spl nu/=25. Molecules in the lowest vibrational levels, including /spl nu/=0, are also observed with slightly lower rates of production. The translational temperature of the ground-state molecules is measured by analyzing their ballistic decay from the detection region. It is approximately 500 /spl mu/K, roughly the same as the estimated temperature of potassium atoms in the MOT.

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Journal of Physics B Atomic Molecular and Optical Physics•Sep 25, 2006•52 citations
We propose a method for copiously producing ultracold ground electronic-state molecules by two-photon absorption to a Rydberg molecular level (that spontaneously radiates to the ground molecular state) during collisions of ultracold atoms in a laser trap. Using the Na2{\mathrm{Na}}_{2}Na2​ molecule as an example, the sequential two-photon absorption is via an intermediate high-lying vibrational level of the excited 1g{1}_{\mathit{g}}1g​ state and results in the formation of a given vibrational level of a 1\ensuremathΠu^{1}\mathrm{\ensuremath{\Pi}}_{\mathit{u}}1\ensuremathΠu​ Rydberg molecular state. Realistic calculations demonstrating the validity of the method are presented.
We have developed a two-step " R-transfer" method that efficiently produces translationally ultracold potassium molecules in the X (1)Sigma(+)(g) electronic ground state. Laser-cooled atoms are initially photoassociated at large internuclear separation R to form molecules in high vibrational levels of the 1 (1)Pi(g) state, which are in turn excited by an additional laser to shorter-range Rydberg states such as 5 (1)Pi(u) and 6 (1)Pi(u). Subsequent radiative decay produces ground-state molecules at rates up to 10(5) molecules/second per vibrational level.
We investigate the feasibility of forming ultracold LiH from a mixture of the ultracold atomic gases, by using B 1 Π as an intermediate state in the photoassociation process. Using accurate molecular potential energy curves and dipole transition moments, we calculate and compare two possible schemes to populate vibrational levels of the ground electronic state, X 1 Σ + : (1) two-photon stimulated radiative association and (2) excitation to bound levels of the B 1 Π state, followed by spontaneous emission to the X 1 Σ + state. With laser intensities and atomic densities that are easily attainable experimentally, we find that significant quantities of molecules can be formed in various v , J levels of the electronic ground state. We examine the spontaneous emission cascade which takes place from the upper vibrational levels on a time scale of milliseconds. We discuss the issue of back-stimulation for the two-photon process and ways to mitigate it. Because photon emission in the cascade process does not contribute to trap loss, a sizable population of molecules in v = 0 can be achieved.
In the first part of this dissertation we develop and successfully demonstrate two methods, based on photoassociation of ultracold potassium atoms, for producing ultracold K2 molecules, with sub mK temperatures, in deeply bound vibrational levels of the ground XS+g 1 state. The principal objective has been to develop an “ultracold molecule factory” that can be used to support experiments on spectroscopy, collisions, and chemistry of ultracold molecules. ^ In the first experiment the ground-state molecules are produced by the radiative decay of excited, n≅191 AS+u 1 state molecules, photoassociated from ultracold potassium atoms in a vapor-cell MOT. The decay is primarily into the vibrational continuum, but a small fraction decays into bound levels of the X state near n = 36. The experimentally observed molecule production rate is about 103 per second. ^ In our second experiment, we use two-step laser excitation to effect an “R-transfer” from long-range to intermediate internuclear separations, greatly increasing the efficiency for radiative production of low- n molecules in the X state. Molecules are formed in the Pg1 state by photoassociation, then excited by a second laser to the 51Pu or 61Pu Rydberg states, in selected vibrational levels from n = 13–18. Finally, radiative decay produces very large numbers of X state molecules with n≅25 and significant numbers of molecules in very low vibrational levels, including n = 0. We have observed production rates up to about 105 molecules/second per vibrational level. ^ In both experiments the ground-state molecules are detected with a sensitive and selective resonant two-color photoionization with nanosecond pulses. ^ In the second part of this dissertation we demonstrate a novel method for high sensitivity absorption measurements, that combines a phase modulated, high-resolution pulsed laser with an actively stabilized optical cavity to achieve a sensitivity of ∼10−9 per cm. This is comparable to the best existing methods for pulsed absorption measurements, but unlike most other methods, it can also be readily extended to the UV region and to Doppler-free spectroscopy. ^
In recent years several groups of researchers have succeeded in making samples of cold molecules, at temperatures below 1 K, and of ultracold molecules, at temperatures below 1 mK. We concentrate on the formation of Cs2 dimers. Starting with laser-cooled atoms, photoassociation forms molecules in an excited electronic state. Various mechanisms are presented for stabilizing these molecules by spontaneous emission into a bound level of the ground electronic state. The design of new formation schemes requires precise knowledge of molecular potential curves that is well beyond the accuracy of ab initio calculations. We show that analytical long-range potentials can be fitted on photoassociation spectra and yield, through the determination of the C3 long-range coefficient, a value of the radiative lifetime of the 6P3/2 cesium level with an unprecedented accuracy. For short-range potentials it is possible to avoid explicit calculations by using asymptotic methods that generalize to long-range molecules the quantum-defect concepts developed for Rydberg atoms. Considering two coupled channels, we show how generalized Lu–Fano plots can extract precise information from experimental spectra.
Summary form only given. We report results from an efficient new method for producing translationally cold (T<1 mK) potassium molecules in the ground X/sup 1//spl Sigma//sub g//sup +/ electronic state. We use the two-photon R-transfer scheme conceptually similar to a proposal by Band and Julienne (1995). First, ultracold potassium atoms in a MOT are photoassociated at long range to the 1/sub g/ molecular electronic state, which correlates to the 1/sup 1//spl Pi//sub g/ state at short range. The resulting molecules are then excited with a second laser to the 5/sup 1//spl Pi//sub u/ or 6/sup 1//spl Pi//sub u/ Rydberg states. These Rydberg states have favorable Franck-Condon factors for spontaneous decay into low vibrational levels of the ground state. The molecules are detected by use of two-color resonant ionization with pulsed lasers. By scanning one of the detection lasers, we observe transitions from several vibrational levels of the X state. The largest observed molecule formation rate in a single vibrational level is about 10/sup 5/ molecules/s for /spl nu/=25. Molecules in the lowest vibrational levels, including /spl nu/=0, are also observed with slightly lower rates of production. The translational temperature of the ground-state molecules is measured by analyzing their ballistic decay from the detection region. It is approximately 500 /spl mu/K, roughly the same as the estimated temperature of potassium atoms in the MOT.
We form ultracold ground-state Rb 2 molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0 u + state, which decays radiatively into high vibrational levels of the X 1 Σ g + state. Sensitive and vibrationally state-selective detection is achieved by means of resonantly-enhanced two-photon ionization with a pulsed laser. Frequency scans of the detection laser reveal a long vibrational progression to a previously unobserved electronic excited state, which we identify as the 2 1 Σ u + state. Most of its vibrational spectrum is in excellent agreement with predictions based on ab initio potentials, although the lowest vibrational levels exhibit strong perturbative mixing with the triplet 2 3 Π u state. The detection method reported here, with minor variations, should be effective for the entire potential well of the X state. In this work no transitions are observed from vibrational levels above v = 118, but this turns out to be a limitation not of the detection method but rather of the photoassociative formation scheme, due to re-excitation of the highest- v levels by the same photoassociation laser that produces them.