Clayton Simien

Electron-temperature evolution in expanding ultracold neutral plasmas.

We have used the free expansion of ultracold neutral plasmas as a time-resolved probe of electron temperature. A combination of experimental measurements of the ion expansion velocity and numerical simulations characterize the crossover from an elastic-collision regime at low initial Gamma(e), which is dominated by adiabatic cooling of the electrons, to the regime of high Gamma(e) in which inelastic processes drastically heat the electrons. We identify the time scales and relative contributions of various processes, and we experimentally show the importance of radiative decay and disorder-induced electron heating for the first time in ultracold neutral plasmas.

Experimental realization of an exact solution to the Vlasov equations for an expanding plasma.

We study the expansion of ultracold neutral plasmas in the regime in which inelastic collisions are negligible. The plasma expands due to the thermal pressure of the electrons, and for an initial spherically symmetric Gaussian density profile, the expansion is self-similar. Measurements of the plasma size and ion kinetic energy using fluorescence imaging and spectroscopy show that the expansion follows an analytic solution of the Vlasov equations for an adiabatically expanding plasma.

Magnetic trapping of metastable 3P2 atomic strontium

We report the magnetic trapping of metastable 3 P 2 atomic strontium. Atoms are cooled in a magneto-optical trap (MOT) 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 8 × 10 9 cm - 3 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.

Absorption imaging and spectroscopy of ultracold neutral plasmas

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.

Quantum interference and light polarization effects in unresolvable atomic lines: Application to a precise measurement of the6,7LiD2lines

We characterize the effect of quantum interference on the line shapes and measured line positions in atomic spectra. These effects, which occur when the excited state splittings are of order the excited state line widths, represent an overlooked but significant systematic effect. We show that excited state interference gives rise to non-Lorenztian line shapes that depend on excitation polarization, and we present expressions for the corrected line shapes. We present spectra of 6,7 Li D lines taken at multiple excitation laser polarizations and show that failure to account for interference changes the inferred line strengths and shifts the line centers by as much as 1 MHz. Using the correct lineshape, we determine absolute optical transition frequencies with an uncertainty of <= 25kHz and provide an improved determination of the difference in mean square nuclear charge radii between 6 Li and 7 Li. This analysis should be important for a number of high resolution spectral measurements that include partially resolvable atomic lines.

Progress at NIST in measuring the D-lines of Li isotopes using an optical frequency synthesizerThis paper was presented at the International Conference on Precision Physics of Simple Atomic Systems, held at École de Physique, les Houches, France, 30 May – 4 June, 2010.

Precise spectroscopic experiments with light atoms can provide information about nuclear properties that are very difficult to obtain in electron scattering experiments. For example, relative nuclear radii of low-Z isotopes can be determined accurately from isotope shifts. Theory has attained sufficient accuracy to study exotic, short-lived halo nuclei by interpreting precise spectroscopic measurements. However, serious inconsistencies remain in the measured isotope shifts for the D1 and D2 lines of the stable isotopes (6Li and 7Li). The latest experiments, within the last decade, are in strong disagreement with each other and with theory. We report on the progress of a new experiment at the National Institute of Standards and Technology (NIST) to measure these lithium D lines using an optical frequency comb. A preliminary result for the splitting isotope shift (SIS) is presented.

Optical Probes of Ultracold Neutral Plasmas

We describe the optical diagnostics used to study ultracold neutral plasmas. Imaging and spectroscopy based on both ion absorption and fluorescence provide accurate measurements of ion kinetic energy, plasma size, and the number of ions in the plasma. Absorption measurements yield lower signal‐to‐noise ratios because they are highly sensitive to laser intensity fluctuations, but the resulting measurement of the number of ions requires no external calibration. Fluorescence measurements of ion number must be calibrated with absorption measurements, but the measurements are less sensitive to technical noise sources. Spatially resolved fluorescence measurements also have the advantage of separating ion kinetic energy due to expansion from thermal kinetic energy.

Ultracold neutral plasmas

Ultracold neutral plasmas occupy an exotic regime of plasma physics in which electrons form a swarming, neutralizing background for ions that sluggishly move in a correlated manner. Strong interactions between the charged particles give rise to surprising dynamics such as oscillations of the average kinetic energy during equilibration and extremely fast recombination. Such phenomena offer stimulating and challenging problems for computational scientists, and the physics can be applied to other environments, such as the interior of gas giant planets and plasmas created by short-pulse laser irradiation of solid, liquid, and cluster targets.

Absorption imaging of ultracold neutral plasmas

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 plasmas 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.

Optically Imaging an Ultracold Strontium Plasma

Ultracold neutral plasmas are formed by photoionizing laser‐cooled atoms near the ionization threshold. Through the application of atomic physics techniques and diagnostics, these experiments stretch the boundaries of traditional neutral plasma physics. The electron temperature in these plasmas ranges from 1–1000 K and the ion temperature is around 1 K. The density can be as high as 1010 cm−3. Fundamental interest stems from the possibility of creating strongly‐coupled plasmas, but recombination, collective modes, and thermalization in these systems have also been studied. Optical absorption images of a strontium plasma, using the Sr+ 2S1/2 → 2P1/2 transition at 422 nm, depict the density profile of the plasma, and probe kinetics on a 50 ns time‐scale. The Doppler‐broadened ion absorption spectrum measures the ion velocity distribution, which gives an accurate measure of the ion dynamics in the first microsecond after photoionization.