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arXiv:cond-mat/0304446 · Physical Review Letters

Giant Helium Dimers Produced by Photoassociation of Ultracold Metastable Atoms

Jérémie Léonard, M. Walhout, Allard P. Mosk, T. Müller, Michèle Leduc, Claude N. Cohen-Tannoudji

Published Aug 15, 2003• 67 citations
arXivHTMLPDF

Abstract

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).

DOI
10.1103/physrevlett.91.073203

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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).
We perform a theoretical analysis to interpret the spectra of purely long-range helium dimers produced by photoassociation (PA) in an ultracold gas of metastable helium atoms. The experimental spectrum obtained with the PA laser tuned closed to the $2\phantom{\rule{0.3em}{0ex}}^{3}S_{1}\ensuremath{\leftrightarrow}2\phantom{\rule{0.3em}{0ex}}^{3}P_{0}$ atomic line has been reported in a previous paper. Here, we first focus on the corrections to be applied to the measured resonance frequencies in order to infer the molecular binding energies. We then present a calculation of the vibrational spectra for the purely long-range molecular states, using adiabatic potentials obtained from perturbation theory. With retardation effects taken into account, the agreement between experimental and theoretical determinations of the spectrum for the 0u+{0}_{u}^{+}0u+​ purely long-range potential well is very good. The results yield a determination of the lifetime of the 23P2\phantom{\rule{0.3em}{0ex}}^{3}P23P atomic state.
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.
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.
A theoretical analysis of photoassociation in ultracold metastable helium has been undertaken for the 2(^3)S1 + 2(^3)S1 to 2(^3)S1 + 2(^3)P transition. A full multichannel calculation is presented for two aspects of the system, a) the accurate calculation of energies for the bound vibrational levels of the dimer; and b) the non-perturbative calculation of the photoassociation line shapes allowing for abitrary laser intensity and detuning. A strong emphasis is placed on the accuracy of the calculations, and the elimination of approximations that have been used in previous investigations. The results indicate that full multichannel calculations are required rather than approximate single-channel calculations in some important situations, and are especially useful for making assignments between experiment and theory. Bound levels have been previously measured from the photoassociation of cold metastable thermal gases to energies close to the three asymptotes j = 0, 1, 2 of the 2(^3)S1 → 2(^3)Pj atomic transition. Previous analysis of these measurements was made using either approximate accumulated phase techniques based on long-range single-channel potentials, or using multichannel calculations and long-range potentials. Because accurate short-range electronic potentials have been calculated only recently, this thesis presents the first multichannel calculation for the levels near the j = 1, 2 asymptotes. The new short-range potentials had previously been used in an approximate single-channel calculation, but this type of calculation ignores the effects of non-adiabatic and Coriolis couplings. Presented here are the binding energies calculated from a fully multichannel technique which includes these couplings, and some differences of up to 10% from the single-channel calculations are observed. The detailed knowledge of the bound wave functions is then exploited to determine a set of observability criteria that produce a near unique assignment of theoretical levels to experimental observations. From these unique assignments, a 1% correction to the short-range electronic potentials has been implemented that significantly improves the agreement between theory and experiment. Experimental measurements have previously been made of line shifts in the photoassociation spectra of vibrational levels in the 0+u long-range potential that asymptotes to 2(^3)S1 + 2(^3)P0 and their variation with laser intensity. These results were previously analysed by performing an approximate second-order perturbative calculation of the line shift that is valid only for low laser intensities. In this thesis a completely non-perturbative calculation of the spectra and line shapes for abitrary laser intensity is presented for photoassociation to the levels probed in experiment. The techniques of dressed state formalism and a modified radiative coupling are employed to properly treat the laser coupling for large interatomic separations. These calculations produce resonance profiles superimposed on a very significant background, a feature that is not seen in photoassociation calculations of other atomic species. From the calculated line shapes, the line shifts and line widths were determined for varying intensity and a small nonlinear intensity-dependence of up to 2.5% was found for the line shift. The effects of invoking the dipole approximation for the laser coupling for the long-range molecular system are analysed by solving the equations using the complete laser coupling that does not assume the dipole approximation. Important differences are obtained between the profiles with and without the dipole approximation, and the photoassociation resonances are found to be more discernible at larger intensities when the complete laser coupling is used.
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.