Frequency shifts of photoassociative spectra of ultracold metastable helium atoms: A new measurement of the s -wave scattering length

Abstract

Abstract. -We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped 4 He * atoms in the metastable 2 3 S1 state. A pair of ultracold spinpolarized 2 3 S1 helium atoms is excited into a molecular bound state in the purely long range 0 + u potential connected to the 2 3 S1 −2 3 P0 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 S1 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.Introduction. -The present article describes a photoassociation (PA) experiment with ultracold helium atoms in the metastable (τ ∼ 8000 s) 2 3 S 1 state. Since the development of laser cooling techniques which provide sub-millikelvin ultracold atomic samples, the photoassociation of such atoms has been of much interest and provides detailed information on the inter-atomic interaction and collisional properties [1]. The measurement of the PA spectra allows very precise measurement of the s-wave scattering length [2-5], a crucial parameter for understanding the collisional properties of the ultracold atoms or molecules and the dynamic behavior of the condensate.One of the interesting phenomena in PA is the light induced frequency shift of the PA spectra. As demonstrated in Ref. [6,7] for Na and Ref. [8,9] for Li, the light shift of the PA spectra is clearly visible at moderate laser intensity for the ultracold samples. The observed shifts are described by a theoretical calculation based on the theory developed by Bohn and Julienne [10] or Simoni et al. [11]. The laser light couples the excited molecular bound state with the continuum of scattering states and the ground molecular bound states, which results in the shift of the PA resonance curve when the laser frequency is swept over the resonance. The dependance of the shift on a has two origins. First, in the limit of large and positive scattering length a, the energy of the least bound-state E LBS in the ground state potential

Laser-intensity dependence of photoassociation in ultracold metastable helium

“…We conclude by noting that our theoretical findings are consistent with the limited experimental data available [17]. Our values for the shift ratios s 1 (v = 1)/s 1 (v = 0) and s 1 (v = 2)/s 1 (v = 0) of 1.81 and 4.57 for σ − polarization lie within the experimental values of 1.71 ± 0.14 and 4.20 ± 0.48 respectively, and our v = 0 widths of 3.24 MHz at 9 mW/cm 2 and 11.9 MHz at 5 W/cm 2 are comparable to the experimental values estimated from Fig.…”

Control of laser-induced frequency shift in ultracold cesium molecules by an external magnetic field

“…LIFS has to be considered not only for the accurate measurement of PA spectroscopy [3] but also the formation of ultracold ground molecules [4]. The LIFS linear relation coefficient of laser intensity, i.e., the slope of LIFS, provides a versatile tool for measuring the s-wave scattering length [5], investigating anomalous quantum correlation [6], and manipulating the individual hyperfine states in ultracold molecular ions [7]. LIFS is also relevant to proposed schemes for utilizing PA to alter the interactions between ultracold atoms [8][9][10], which has been the key to many breakthroughs [11].…”

“…LIFS has been experimentally studied by several groups dealing with ultracold helium molecules 4 He 2 [5,12], and ultracold alkali molecule samples such as 7 Li 2 [10,13], Na 2 [14], Rb 2 [15], and Cs 2 [16]. Several theoretical models have also been proposed for LIFS [8,10,15], among which a calculation based on the multichannel scattering theory is typically developed [17].…”

Long-range interactions between aHe(2S3)atom and a

“…Recently, there has been considerable interest in the study of helium dimers and ultracold helium collisions associated with metastable helium atoms [1,2,3,4,5,6,7,8,9,10,11].…”

“…Molecular lines to the red of the D 2 atomic transition in helium dimers associated with the He(2 3 S)-He(2 3 P 2 ) asymptote are of interest for control of the scattering length using an "optical Feshbach resonance" [4,6]. Published data are scarce on the long-range part of the He(2 3 S)-He(2 3 P ) potential energies, which determine the energy level structures of the ultralong-range dimers formed in photoassociation of ultracold metastable helium atoms.…”