We report optical absorption imaging of ultracold neutral strontium plasmas. The ion absorption spectrum determined from the images is Doppler broadened and thus provides a quantitative measure of the ion kinetic energy. For the particular plasma conditions studied, ions heat rapidly as they equilibrate during the first 250 ns after plasma formation. Equilibration leaves ions on the border between the weakly coupled gaseous and strongly coupled liquid states. On a longer time scale of microseconds, pressure exerted by the trapped electron gas accelerates the ions radially.

We study equilibration of strongly coupled ions in an ultracold neutral plasma produced by photoionizing laser-cooled and trapped atoms. By varying the electron temperature, we show that electron screening modifies the equilibrium ion temperature. Even with few electrons in a Debye sphere, the screening is well described by a model using a Yukawa ion-ion potential. We also observe damped oscillations of the ion kinetic energy that are a unique feature of equilibration of a strongly coupled plasma. DOI: 10.1103 There has been significant theoretical study of the equilibration of strongly coupled plasmas [6 -12], especially in the context of plasmas produced with high-intensity lasers. In addition to generating fundamental interest, this problem challenges computational resources and techniques. Experimental results have been lacking, however, because of the fast time scales involved and limited diagnostics.Ultracold neutral plasmas [13], produced by photoionizing clouds of laser-cooled and trapped atoms, are ideal for experimental studies. The equilibration of the plasma is relatively slow ( 100 ns) due to lower plasma density. Ultracold neutral plasmas also offer a high level of control and diagnostics. By varying laser intensities and wavelengths, it is possible to accurately set the initial density and energy of the system. Optical imaging [14] provides an in situ probe of plasma properties with excellent spatial, temporal, and spectral resolution.In this Letter, we explore ion equilibration during the first microsecond after the plasma is created. The density sets the time and the energy scale for equilibration, but electron screening effects are evident. Even when the number of electrons per Debye sphere is small, the equilibration temperature of the ions agrees with a model [15] that uses a Yukawa ion-ion potential.We also observed oscillations of the ion kinetic energy. For many years, this phenomenon has been the subject of intense study through analytic calculations [7] and simulations [6,[8][9][10][11][12]] of one-component strongly coupled plasmas, but it has not previously been observed experimentally. The oscillations and their damping reflect universal dynamics of a Coulomb system with spatial correlations.Details on laser cooling, plasma formation, and imaging are given in [14,16]. The experiment starts with strontium atoms that are cooled and trapped in a magneto-optical trap (MOT). The neutral atom cloud is characterized by a temperature of about 10 mK, 2 10 8 atoms, and a Gaussian density distribution. We vary the atom density by changing the MOT parameters, or by turning the MOT off and releasing the atoms in a ballistic expansion. Up to 30% of the neutral atoms are then ionized with one photon from the cooling laser and one photon from a pulsed dye laser. The ion density distribution equals the atom distribution at the time of photoionization and is given by n i r n 0i exp ÿr 2 =2 2 , with from 0.6 to 1 mm and n 0i from 2 10 9 to 1:4 10 10 cm ÿ3 . The electron density, n e r , closely follows ...

We present results from two-photon photoassociative spectroscopy of the least-bound vibrational level of the

We report the magnetic trapping of metastable 3 P 2 atomic strontium. Atoms are cooled in a magneto-optical trap ͑MOT͒ operating on the dipole-allowed 1 S 0 -1 P 1 transition at 461 nm. Decay via 1 P 1 → 1 D 2 → 3 P 2 continuously loads a magnetic trap formed by the quadrupole magnetic field of the MOT. Over 10 8 atoms at a density of 8ϫ10 9 cm Ϫ3 and temperature of 1 mK are trapped. The atom temperature is significantly lower than what would be expected from the kinetic and potential energies of atoms as they are transferred from the MOT. This suggests the occurrence of thermalization and evaporative cooling in the magnetic trap. Laser-cooled alkaline-earth-metal atoms offer many possibilities for practical applications and fundamental studies. The two valence electrons in these systems give rise to triplet and singlet levels connected by narrow intercombination lines that are utilized for optical frequency standards ͓1͔.Laser cooling on such a transition in strontium may lead to a fast and efficient route to all-optical quantum degeneracy ͓2,3͔, and there are abundant bosonic and fermionic isotopes for use in this pursuit. The lack of hyperfine structure in the bosonic isotopes and the closed electronic shell in the ground states make alkaline-earth-metal atoms appealing testing grounds for cold-collision theories ͓4 -6͔, and collisions between metastable alkaline-earth-metal atoms is a relatively new and unexplored area for research ͓7͔.In this paper we characterize a technique that should benefit all these experiments-the continuous loading of metastable 3 P 2 atomic strontium ( 88 Sr) from a magneto-optical trap ͑MOT͒ into a purely magnetic trap. This idea was discussed in a recent theoretical study of alkaline-earth-metal atoms and ytterbium ͓8͔. Katori et al. ͓9͔ and Loftus et al. ͓10͔ have also reported observing this phenomenon in their strontium laser-cooling experiments. Continuous loading of a magnetic trap from a MOT was recently described for chromium atoms ͓11͔.This scheme should allow for collection of large numbers of atoms at high density since atoms are shelved in a dark state and are less susceptible to light-assisted collisional loss mechanisms ͓4,6,12͔. It is an ideal starting place for many experiments such as sub-Doppler laser cooling on a transition from the metastable state, as has been done with calcium ͓13͔, production of ultracold Rydberg gases ͓14͔ or plasmas ͓15͔, and evaporative cooling to quantum degeneracy. Optical frequency standards based on laser-cooled alkaline-earthmetal atoms, which are currently limited by high sample temperatures ͓1͔, may benefit from the ability to trap larger numbers of atoms and evaporatively cool them in a magnetic trap.We will first describe the operation of the Sr MOT and how this loads the magnetic trap with 3 P 2 atoms. Then we will characterize the loading and decay rates of atoms in the magnetic trap. Finally, we will present measurements of the 3 P 2 sample temperature.Sr atoms are loaded from a Zeeman-slowed atomic beam ͓16͔ and cooled and tr...

We report photoassociative spectroscopy of 88 Sr2 in a magneto-optical trap operating on the 1 S0 → 3 P1 intercombination line at 689 nm. Photoassociative transitions are driven with a laser red-detuned by 600-2400 MHz from the 1 S0 → 1 P1 atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the 1 P1 atomic lifetime (τ = 5.22 ± 0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.

We report Bose-Einstein condensation of 84 Sr in an optical dipole trap. Efficient laser cooling on the narrow intercombination line and an ideal s-wave scattering length allow creation of large condensates (N0 ∼ 3 × 10 5 ) even though the natural abundance of this isotope is only 0.6%. Condensation is heralded by the emergence of a low-velocity component in time-of-flight images.

We report the use of photoassociative spectroscopy to determine the ground-state s-wave scattering lengths for the main bosonic isotopes of strontium, 86 Sr and 88 Sr. Photoassociative transitions are driven with a laser red detuned by up to 1400 GHz from the 1 S 0 -1 P 1 atomic resonance at 461 nm. A minimum in the transition amplitude for 86 Sr at ÿ494 5 GHz allows us to determine the scattering lengths 610a 0 < a 86 < 2300a 0 for 86 Sr and a much smaller value of ÿ1a 0 < a 88 < 13a 0 for 88 Sr. DOI: 10.1103/PhysRevLett.95.223002 PACS numbers: 32.80.Pj Photoassociative spectroscopy (PAS) of ultracold gases, in which a laser field resonantly excites colliding atoms to rovibrational states of excited molecular potentials, is a powerful probe of atomic cold collisions [1]. Transition frequencies have been used to obtain dispersion coefficients of molecular potentials, which yield the most accurate value of the atomic excited-state lifetime [2 -5]. Transition amplitudes are related to the wave function for colliding ground-state atoms [6,7] and can be used to determine the ground-state s-wave scattering length [8][9][10][11][12][13].The s-wave scattering length is a crucial parameter for determining the efficiency of evaporative cooling and the stability of a Bose-Einstein condensate (BEC). It also sets the scale for collisional frequency shifts, which can limit the accuracy and stability of atomic frequency standards.The cold collision properties of alkaline-earth atoms such as strontium, calcium, and magnesium, and atoms with similar electronic structure, such as ytterbium, are currently the focus of intense study. These atoms possess narrow optical resonances that have great potential for optical frequency standards [14 -18]. Laser cooling on narrow transitions [19] is an efficient route to high phasespace density [20,21], and a BEC was recently produced with ytterbium [22]. Fundamental interest in alkaline-earth atoms is also high because their simple molecular potentials allow accurate tests of cold collision theory [23][24][25].PAS of calcium [12] and ytterbium [13] was recently used to determine s-wave scattering lengths of these atoms. This Letter reports the use of PAS to determine the groundstate s-wave scattering lengths for the main bosonic isotopes of strontium, 86 Sr and 88 Sr, which have relative abundances of 10% and 83%, respectively. We find a huge scattering length for 86 Sr of 610a 0 < a 86 < 2300a 0 . Appreciable uncertainty comes from the value of C 6 for the ground-state potential. In contrast, for 88 Sr we find ÿ1a 0 < a 88 < 13a 0 . Recently posted [5] PAS results for 88 Sr yielded an improved measurement of the 5s5p 1 P 1 atomic lifetime ( 5:263 0:004 ns), which we use in our analysis. Disagreement between our reported value of a 88 and results of Ref.[5] will be discussed below.For PAS of strontium, atoms are initially trapped in a magneto-optical trap (MOT) operating on the 461 nm 1 S 0 -1 P 1 transition, as described in Refs. [4,26]. We are able to produce pure samples of each iso...

We report measurement of the inelastic and elastic collision rates for 88 Sr atoms in the ͑5s5p͒ 3 P 0 state in a crossed-beam optical dipole trap. Since the ͑5s5p͒ 3 P 0 state is the lowest level of the triplet manifold, large loss rates indicate the importance of principle-quantum-number-changing collisions at short range. We also provide an estimate of the collisional loss rates for the ͑5s5p͒ 3 P 2 state. Large loss-rate coefficients for both states indicate that evaporative cooling toward quantum degeneracy in these systems is unlikely to be successful. DOI: 10.1103/PhysRevA.79.060702 PACS number͑s͒: 34.50.Cx Metastable 3 P J states of alkaline-earth-metal atoms and atoms with similar electronic structure ͑Fig. 1͒ display vastly different optical and ultracold collisional properties compared to states found in alkali-metal atoms more commonly used in ultracold atomic physics experiments. The extremely long-lived 3 P 0 states in Sr and Yb serve as the upper levels in neutral-atom optical frequency standards ͓1͔. The weakly allowed 1 S 0 − 3 P 1 intercombination transition serves as the basis for powerful laser-cooling techniques ͓2͔ and may enable useful optical tuning of the ground-state scattering length ͓3͔. 3 P 2 atoms interact through long-range anisotropic interactions ͓4,5͔ that allow magnetic tuning of the interactions and cause rapid inelastic collisional losses ͓6-8͔. 3 P J states of alkaline-earth-metal atoms have also been proposed for lattice-based quantum computing ͓9,10͔.Here, we report measurement of the inelastic and elastic collision rates for 88 Sr atoms in the ͑5s5p͒ 3 P 0 state in a crossed-beam optical dipole trap. The measurement of the ultracold collisional properties of the 3 P 0 state is of great interest because of its role in optical clocks. We also report an estimate of the collisional loss rates for the 88 Sr ͑5s5p͒ 3 P 2 state. Large loss-rate coefficients for both states make efficient evaporative cooling in the ultracold regime unlikely.Early laser-cooling experiments with Sr ͓11͔ and Ca ͓12͔ showed that it is straightforward to magnetically trap metastable 3 P 2 atoms through natural decay in a magneto-optical trap. This generated interest in the possibility of achieving quantum degeneracy in this state and motivated calculations that found novel collisional properties of the metastable 3 P J levels ͓4͔. Magnetic dipole-dipole and electric quadrupolequadrupole interactions between 3 P 2 atoms produce anisotropic long-range potentials with bound states and collisional rates that can be tuned with magnetic field. Reference ͓5͔ showed that s-wave colliding states can be coupled to much higher partial waves of outgoing channels of other magnetic sublevels even if the initial state is maximally spin polarized. This leads to two-body inelastic loss rates in magnetically trapped samples ͓6͔ that are on the order of elastic collision rates, making efficient evaporative cooling toward quantum degeneracy of 3 P 2 atoms in a magnetic trap unlikely. This was confirmed in experiment...