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European Quantum Electronics Conference (EQEC)

A new measurement of s-wave scattering length of metastable helium atoms by photoassociative spectroscopy

Jaewan Kim, Steven Moal, M. Portier, Michèle Leduc, Claude N. Cohen-Tannoudji, Olivier Dulieu

Published Jan 18, 2006• 0 citations
Publisher

Abstract

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.

DOI
10.1109/eqec.2005.1567403

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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 observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped 4 He* atoms in the metastable 2 3 S 1 state. A pair of ultracold spin-polarized 2 3 S 1 helium atoms is excited into a molecular bound state in the purely long-range 0 u + potential connected to the 2 3 S 1 -2 3 P 0 asymptote. The shift arises from the optical coupling of the molecular excited bound state with the scattering states and the bound states of two colliding 2 3 S 1 atoms. We measure the frequency-shifts for several ro-vibrational levels in the 0 u + potential and find a linear dependence on the photoassociation laser intensity. Comparison with a theoretical analysis provides a good indication for the s -wave scattering length a of the quintet ( 5 Σ g + ) potential, a = 7.2 ± 0.6 nm, which is significantly lower than most previous results obtained by non-spectroscopic methods.
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.
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.
We present an exhaustive analysis of the light-induced frequency shifts of the photoassociation lines of ultracold metastable 4 He* atoms in a magnetic trap. The measurements of the shifts of several vibrational levels bound in the purely long-range J = 1, 0 u + potential linked to the 2 3 S 1 –2 3 P 0 asymptote were reported in a previous paper (Kim et al 2005 Europhys. Lett. 72 548), and are analysed here. The simplicity of this system makes it very appropriate for a detailed study. Indeed, the purely long-range character of the excited potential allows one to calculate exact excited molecular wavefunctions and to use asymptotic expansions at large internuclear distances of the ground-state wavefunctions appearing in Franck–Condon-type integrals. Moreover, the number of collisional channels to be considered is strongly reduced by the absence of hyperfine structure for 4 He* and the use of polarized ultracold atoms and polarized light. This allows us to derive semi-analytical expressions for the shifts showing explicitly their linear dependences on the s-wave scattering length a of spin polarized metastable 4 He* atoms. This explains how it is possible to derive the measurement of a from these shifts.
We present a new measurement of the sss-wave scattering length aaa of spin-polarized helium atoms in the 2 3S12\text{ }^{3}S_{1}2 3S1​ metastable state. Using two-photon photoassociation spectroscopy and dark resonances, we measure the energy ${E}_{v=14}=\ensuremath{-}91.35\ifmmode\pm\else\textpm\fi{}0.06\text{ }\text{ }\mathrm{MHz}$ of the least-bound state v=14v=14v=14 in the interaction potential of the two atoms. We deduce a value of $a=7.512\ifmmode\pm\else\textpm\fi{}0.005\text{ }\text{ }\mathrm{nm}$, which is at least 100 times more precise than the best previous determinations and is in disagreement with some of them. This experiment also demonstrates the possibility to create exotic molecules binding two metastable atoms with a lifetime of the order of $1\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$.