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European Quantum Electronics Conference (EQEC)

Photoassociation of ultracold metastable helium atoms

Jérémie Léonard, Allard P. Mosk, M. Walhout, M. Tobias, C. S. Unnikrishnan, Michèle Leduc, Claude N. Cohen-Tannoudji

Published Jan 1, 2003• 0 citations
Publisher

Abstract

Photoassociation of metastable He (2/sup 3/S/sub 1/) atoms at ultra low temperatures close to the BEC phase transition is studied, which was achieved previously in an Ioffe-Pritchard magnetic trap and detected optically through absorption imaging of the optical density profile. In the present experiment, the cloud is brought close to the BEC transition by evaporation and then illuminated by laser pulse tunable around the three (2/sup 3/S/sub 1/-2/sup 3/P/sub 0,1,2/) resonance lines at 1083 nm. A subsequent optical density measurement is used to detect any increases in the trap loss rate that may have been caused by the pulse. The experiment is particularly interested in detecting losses due to the production of ultracold molecules by photoassociation (PA), and in finding several weakly bound S-P states that have not previously been detected.

DOI
10.1109/eqec.2003.1314135

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Conference on Lasers and Electro-Optics•Jan 1, 2000•0 citations
Photoassociation of metastable He (2/sup 3/S/sub 1/) atoms at ultra low temperatures close to the BEC phase transition is studied, which was achieved previously in an Ioffe-Pritchard magnetic trap and detected optically through absorption imaging of the optical density profile. In the present experiment, the cloud is brought close to the BEC transition by evaporation and then illuminated by laser pulse tunable around the three (2/sup 3/S/sub 1/-2/sup 3/P/sub 0,1,2/) resonance lines at 1083 nm. A subsequent optical density measurement is used to detect any increases in the trap loss rate that may have been caused by the pulse. The experiment is particularly interested in detecting losses due to the production of ultracold molecules by photoassociation (PA), and in finding several weakly bound S-P states that have not previously been detected.
The paper reports on a new measurement method of the s-wave scattering length of metastable helium atoms by photoassociation (PA) technique. Lightshifts of the PA resonance lines are measured, which results from the coupling of a bound state in the excited state potential and the continuum of scattering states above the ground state potential. Ultracold molecules are formed by photoassociation from spin-polarized metastable helium atoms, which are trapped in a magnetic trap and cooled down to a temperature of 16 /spl mu/K by evaporative cooling technique. The PA laser is red detuned to the atomic D0 line (2/sup 3/S-2/sup 3/P/sub 0/) and scanned around the bound state levels in the purely long-range 0/sub 0//sup +/ potential to provide precise PA spectra.
We produce giant, purely long-range helium dimers by photoassociation of metastable helium atoms in a magnetically trapped, ultracold cloud. The photoassociation laser is detuned close to the atomic 2(3)S1-2(3)P0 line and produces strong heating of the sample when resonant with molecular bound states. The temperature of the cloud serves as an indicator of the molecular spectrum. We report good agreement between our spectroscopic measurements and our calculations of the five bound states belonging to a 0(+)(u) purely long-range potential well. These previously unobserved states have classical inner turning points of about 150a(0) and outer turning points as large as 1150a(0).
This thesis describes the results of several experiments that studied the photoassociation of an ultracold atomic Rb gas. In the first experiment, we produced ultracold diatomic molecules from an atomic gas via single-color photoassociation. The molecules were detected with resonance-enhanced multiphoton ionization. Trapping of these molecules in a quadrupole magnetic trap, with lifetimes up to 20 seconds, was also demonstrated. In addition, the rate constant for inelastic collisions between the trapped molecules and atoms was determined from measurements of the atomic density dependence of the decay rate of the trapped molecules. In another experiment, stimulated Raman photoassociation of Rb atoms in a Mott insulator state was studied. A Bose-Einstein condensate (BEC) of 87Rb atoms was loaded into a three-dimensional optical lattice formed by the interference pattern of three orthogonal standing wave laser fields. This system constitutes a very good realization of the Bose-Hubbard model, which predicts a quantum phase transition between a superfluid state and a Mott insulator state at a particular lattice height. A time-of-flight imaging method was used to study the state of the atomic gas, and the quantum phase transition was observed at the predicted lattice height. The signature of the phase transition was the disappearance and reappearance of peaks in the image that arose from the interference of atoms originating from different lattice sites. Two coherent laser fields were applied to the gas in its Mott insulating state, and tuned close to a Raman photoassociation resonance, and this resulted in an observable loss of atoms due to the formation of molecules. This transition exhibited a double-peaked spectrum, with one of the peaks arising from photoassociation of atoms in sites containing only two atoms, and the other from sites containing three atoms. Also, the loss of atoms vs. the duration of the Raman photoassociation period was studied, with the lasers tuned to the peak corresponding to two atoms per site. It was found that a central core of the gas, containing about 40 percent of the atoms, exhibited a coherent oscillation between an atomic and molecular quantum gas.
We report a new method to detect photoassociation (PA) resonance signals through the collective dipole oscillations produced by a momentum transfer from the PA excitation lasers to an ultracold atomic cloud confined by a magnetic trap. Using the amplitudes of these oscillations for several molecular excited states, we derive a precise value for the s -wave scattering length between the ultracold atoms. This new and simple method provides results in excellent agreement with previous spectroscopic PA measurements based on atom losses or temperature rise. We present a simple model interpreting the momentum and energy transferred from the PA process to trap confined ultracold atoms.
A magneto-optical trap for He(2{sup 3}S) metastable atoms has been constructed, utilizing superconducting magnetic gradient coils and a Ti:Sapphire ring laser for pumping the helium 2{sup 3}S-2{sup 3}P transition. The He(2{sup 3}S) atoms are produced by a weak rf discharge in helium gas at a temperature of 1.4K. The discharge products flow through a small orifice into the trap cell, where a fraction of the He(2{sup 3}S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experiments suggest that {approximately}10{sup 6} atoms are trapped in a small volume at {approximately}1 mK, with a trap lifetime of 10-100 msec limited by resonantly-enhanced He(2{sup 3}S)-He(2{sup 3}P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be held at ultra-low temperature in near-perfect vacuum, in a dc magnetic trap. Measurements of decay times of the trapped atoms should yield rate coefficients in the quantum regime for He(2{sup 3}S)-He(2{sup 3}P) and He(2{sup 3}S)-He(2{sup 3}S) Penning reactions, and perhaps the He(2{sup 3}S) radiative lifetime.