Abstract: We briefly introduce R2G2, an R CRAN package to visualize spatially explicit biological data within the Google Earth interface. Our package combines a collection of basic graph-editing features, including automated placement of dots, segments, polygons, images (including graphs produced with R), along with several complex three-dimensional (3D) representations such as phylogenies, histograms and pie charts. We briefly present some example data sets and show the immediate benefits in communication gained from using the Google Earth interface to visually explore biological results. The package is distributed with detailed help pages providing examples and annotated source scripts with the hope that users will have an easy time using and further developing this package. R2G2 is distributed on http://cran.r-project.org/web/packages.
Abstract: We describe a model of inelastic and elastic collisional dynamics of atoms in an optical dipole trap that utilizes numerical evaluation of statistical mechanical quantities and numerical solution of equations for the evolution of number and temperature of trapped atoms. It can be used for traps that possess little spatial symmetry and when the ratio of trap depth to sample temperature is relatively small. We compare simulation results with experiments on 88Sr and 84Sr, which have well-characterized collisional properties.
Abstract: We report Bose-Einstein condensation of 88Sr, which has a small, negative s-wave scattering length (a88=-2a0). We overcome the poor evaporative cooling characteristics of this isotope by sympathetic cooling with 87Sr atoms. 87Sr is effective in this role despite the fact that it is a fermion because of the large ground-state degeneracy arising from a nuclear spin of I=9/2, which reduces the impact of Pauli blocking of collisions. We observe a limited number of atoms in the condensate (Nmax≈104) that is consistent with the value of a88 and the optical dipole trap parameters.
Abstract: We report quantum degeneracy in a gas of ultracold fermionic 87Sr atoms. By evaporatively cooling a mixture of spin states in an optical dipole trap for 10.5 s, we obtain samples well into the degenerate regime with T/TF=0.26-0.06+0.05. The main signature of degeneracy is a change in the momentum distribution as measured by time-of-flight imaging, and we also observe a decrease in evaporation efficiency below T/TF∌0.5.
Abstract: We report Bose-Einstein condensation of 84Sr in an optical dipole trap. Efficient laser cooling on the narrow intercombination line and an ideal s-wave scattering length allow the creation of large condensates (N0 3×105) 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.
Abstract: We describe repumping and spectroscopy of laser-cooled strontium (Sr) atoms using the (5s5p)3P2-(5s4d)3D2 transition. Atom number in a magneto-optical trap is enhanced by driving this transition because Sr atoms that have decayed into the (5s5p)3P2 dark state are repumped back into the (5s2)1S0 ground state. Spectroscopy of 84Sr, 86Sr, 87Sr and 88Sr improves the value of the (5s5p)3P2-(5s4d)3D2 transition frequency and determines the isotope shifts for the transition accurately enough to guide laser-cooling experiments with less abundant isotopes.
Abstract: We report measurement of the inelastic and elastic collision rates for 88Sr atoms in the (5s5p)3P0 state in a crossed-beam optical dipole trap. Since the (5s5p)3P0 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 (5s5p)3P2 state. Large loss-rate coefficients for both states indicate that evaporative cooling toward quantum degeneracy in these systems is unlikely to be successful.
Abstract: We present results from two-photon photoassociative spectroscopy of the least-bound vibrational level of the X 1 state of the 88Sr2 dimer. Measurement of the binding energy allows us to determine the s-wave scattering length a88=−1.4(6)a0. For the intermediate state, we use a bound level on the metastable 1S0-3P1 potential, which provides large Franck-Condon transition factors and narrow one-photon photoassociative lines that are advantageous for observing quantum-optical effects such as Autler-Townes resonance splittings.
Abstract: A study of ion equilibration in annular regions of ultracold strontium plasmas is reported. Plasmas are formed by photoionizing laser-cooled atoms with a pulsed dye laser. The experimental probe is spatially-resolved absorption spectroscopy using the 2S1/2-2P<sub>1/2 transition of the Sr+ ion. The kinetic energy of the ions is calculated from the Doppler broadening of the spectrum, and it displays clear oscillations during the first microsecond after plasma formation. The oscillations, which are a characteristic of strong coulomb coupling, are fit with a simple phenomenological model incorporating damping and density variation in the plasma.
Abstract: Absorption imaging and spectroscopy can probe the dynamics of an ultracold neutral plasma during the first few microseconds after its creation. Quantitative analysis of the data, however, is complicated by the inhomogeneous density distribution, expansion of the plasma and possible lack of global thermal equilibrium for the ions. In this paper, we describe methods for addressing these issues. Using simple assumptions about the underlying temperature distribution and ion motion, the Doppler-broadened absorption spectrum obtained from plasma images can be related to the average temperature in the plasma.
Abstract: We report photoassociative spectroscopy of 88Sr2 in a magneto-optical trap operating on the 1S03P1 intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600–2400 MHz from the 1S01P1 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 1P1 atomic lifetime (= 5.22$\pm$0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.
Abstract: We report optical absorption imaging of ultracold neutral plasmas. Imaging allows direct observation of the ion density profile and expansion of the plasma. The frequency dependence of the plasma’s optical depth gives the ion absorption spectrum, which is broadened by the ion motion. We use the spectral width to monitor ion equilibration in the first 250 ns after plasma formation. On a microsecond time scale, we observe the radial acceleration of ions resulting from pressure exerted by the trapped electron gas.
Abstract: We report the use of photoassociative spectroscopy to determine the ground-state s-wave scattering lengths for the main bosonic isotopes of strontium, 86Sr and 88Sr. Photoassociative transitions are driven with a laser red detuned by up to 1400 GHz from the 1S0-1P1 atomic resonance at 461 nm. A minimum in the transition amplitude for 86Sr at -494$\pm$5 GHz allows us to determine the scattering lengths 610a0<a86<2300a0 for 86Sr and a much smaller value of -1a0<a88<13a0 for 88Sr.
Abstract: 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.
Abstract: 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.
Abstract: This work describes intensity saturation of photoassociative transitions of 86 Sr at the quantum mechanical unitarity limit. The saturation behavior, which results in a roll-over of the photoassociation rate for intensities greater than the saturation inten- sity, features interesting physics. Unlike other photoassociation spectroscopy (PAS) experiments, photoassociation occurs in a magneto-optical trap operating on the nar- row dipole-forbidden transition at 689 nm. A laser red-detuned from the principal atomic transition at 461 nm by as much as 1300 GHz induces the photoassociation of ground state atoms to excited molecular states. Our previous studies [1] suggest that some of the PAS transitions for 86 Sr are sensitive to the intensity of the laser. This work delves more deeply into these high intensity PAS effects, a sub ject that has not, to our knowledge, been studied experimentally for alkaline-earth atoms.
Abstract: This thesis describes trapping and evaporative cooling of ultracold 87Sr and 88Sr mixtures in an optical dipole trap to produce the first Bose-Einstein condensate (BEC) of 88Sr. Furthermore, this work presents thermalization studies that characterize the scattering properties of these ultracold strontium samples. Such ultracold atomic gases have become an important area of research because of their potential for improv- ing optical frequency standards and for realizing quantum computation using neutral atoms. A BEC of 88Sr is particularly interesting because the small value of its back- ground s-wave scattering length may enable the use of optical Feshbach resonances to create two-dimensional solitons. However, the small scattering length for 88Sr also hinders efficient evaporative cooling in the optical dipole trap, a necessary step to producing a BEC. Experiments with other ultracold gases have successfully over- come this hurdle by mixing in a second atomic species which, by introducing stronger interactions with the weakly interacting species, enables evaporation to colder tem- peratures via sympathetic cooling. For this work, we use 87Sr to sympathetically cool 88Sr during forced evaporation to quantum degeneracy. Previous experiments in the Killian Lab characterized 88Sr in detail. Here, I emphasize the new or improved aspects that have allowed trapping and cooling of the mixtures of 87Sr and 88Sr: trapping of 87Sr by itself, spectroscopic measurements of all the stable strontium isotopes to guide the trapping of isotopic mixtures, imaging of both 87Sr and 88Sr, and the various trade-offs necessary to simultaneously trap 87Sr and 88Sr. Finally, I discuss how the thermalization studies of the scattering properties of the isotopes guide the forced evaporation of mixed isotope samples. These efforts result in the production of the BEC of 88Sr, but they also point the way to future studies of fermionic quantum degeneracy in 87Sr and to the rich physics of mixed species ultracold atomic systems.
