We have been working on the development of a technique for determining the static polarizabilities of refractory metal atoms. The technique involves producing a dense, cold atomic beam (n {approx equal}10{sup 11} {minus} 10{sup 12} atoms/cm{sup 3}, T {perpendicular} {approx equal} 5 mK) that is illuminated for a short time by a standing wave formed by a Nd:YAG laser. This technique would have an impact on laser isotope separation, ion mobility physics, and theoretical atomic physics. The interaction between the atoms and the laser light results in velocity kicks to the atoms along the two directions defined by the photons in the laser standing wave. The velocity kicks force the atoms to form into planes of alternating high and low density downstream. By Bragg scattering near-resonant light off these planes the static polarizability can be determined. The velocity kicks will also cause the atomic density distribution further downstream to take on a bimodal appearance. A measurement of the separation of the peaks of this distribution can be used to determine the polarizability. Here we report our progress on this technique and the present status of the experiments. A new method for producing focused beams of neutrals, ions, and clusters usingmore » two-stage laser ablation has been developed as a result of our experiments. 7 refs., 12 figs.« less
We have been working on the development of a technique for determining the static polarizabilities of refractory metal atoms. The technique involves producing a dense, cold atomic beam (n {approx equal}10{sup 11} {minus} 10{sup 12} atoms/cm{sup 3}, T {perpendicular} {approx equal} 5 mK) that is illuminated for a short time by a standing wave formed by a Nd:YAG laser. This technique would have an impact on laser isotope separation, ion mobility physics, and theoretical atomic physics. The interaction between the atoms and the laser light results in velocity kicks to the atoms along the two directions defined by the photons in the laser standing wave. The velocity kicks force the atoms to form into planes of alternating high and low density downstream. By Bragg scattering near-resonant light off these planes the static polarizability can be determined. The velocity kicks will also cause the atomic density distribution further downstream to take on a bimodal appearance. A measurement of the separation of the peaks of this distribution can be used to determine the polarizability. Here we report our progress on this technique and the present status of the experiments. A new method for producing focused beams of neutrals, ions, and clusters usingmore » two-stage laser ablation has been developed as a result of our experiments. 7 refs., 12 figs.« less
A light-force technique for measuring polarizabilities is being used to measure the polarizability of the uranium atom. The technique uses a pulsed Nd:YAG laser to produce an intense, standing-wave field that provides velocity kicks to atoms that traverse the field region. These velocity kicks result in Doppler-shifted absorption by the atoms. The change in absorption of resonant light, provided by a tunable diode laser is measured. This measurement and a measurement of the standing-wave laser fluence allow determination of the polarizability. We plan to report the results of our measurements, along with a new scheme for generating a dense beam of uranium atoms.
The theory of a method to measure electric-dipole polarizabilities is presented. The method uses two-stage laser ablation to produce a pulsed beam of atoms from a solid target. A pair of slits makes the velocity distribution of the atomic beam nonuniform in a way that is well characterized. The light-force technique uses the polarization forces experienced by an atom in the intense, inhomogeneous electromagnetic field of a standing-wave laser to change the velocity distribution of the atomic beam. The large forces cause measurable Doppler shifts in the resonant frequency of the atoms. These frequency shifts change the amount of absorption of resonant light, yielding information about the change in the velocity distribution of the atoms. The detailed shape of the final absorption distribution is polarizability dependent. In the classical picture of the light force, the standing-wave electric field induces a time-varying dipole moment in each atom. Each atom then experiences a Lorentz force due to coherent interaction of the oscillating dipole moment with the time-varying magnetic field. The quantum-mechanical picture corresponds to the Kapitza-Dirac effect for atoms: an atom absorbs a photon from one of the two beams which form the standing wave, and is then stimulated to emit this photon by the other, counterpropagating beam. This paper provides a classical treatment of the light force experienced by an atom in a standing-wave light field; the polarizability is treated quantum mechanically. The theory presented here can be applied to atoms with a scalar polarizability, such as rubidium, and to atoms with significant tensor components, such as uranium. Experimental results from application of the light-force technique to a measurement of the polarizability of rubidium are also presented.
We have developed a light-force technique for measuring polarizabilities. This new method uses the polarizaton forces experienced by an atom in the intense, inhomogeneous electric field of a standing-wave laser to deduce the polarizability. Using a Nd:YAG laser at ${\ensuremath{\lambda}}_{\mathrm{SW}}$=1.064 \ensuremath{\mu}m for the light force, we measured the polarizability of rubidium at this wavelength to be \ensuremath{\alpha}(${\mathrm{\ensuremath{\omega}}}_{\mathrm{SW}}$)=114\ifmmode\pm\else\textpm\fi{}9 A˚3 (\ensuremath{\sigma}). Conversion of this ac polarizability to a static value using a semiempirical formula gives a value of ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{dc}}$=52 A˚3, within 10% of the best published experimental results.
The interaction of a beam of two level atoms with a standing wave laser field has been the subject of extensive investigation in recent times. A standing wave perpendicular to an atomic beam allows the manipulation of the transverse velocity distribution. The interaction of an atomic beam with a strong standing wave laser field has been systematically investigated by both theoretical simulation and experiment in this thesis. We have developed a numerical model based on a continued fraction solution for the light pressure forces to simulate the near perpendicular interaction of an atomic beam with a strong standing wave laser field. The reshaping of the transverse spatial profile of an atomic beam by a perpendicular, near resonant, standing wave laser field is investigated for red detunings using a longitudinal velocity selective detection system. The experimental dependence of the reshaped profile on the longitudinal and transverse atomic velocities agrees well with a continued fraction solution for the light force. In particular, a regime is found where channelling of the atoms near the antinodes of the standing wave produces a central peak in the spatial distribution of the atomic beam in the far field which have also been observed elsewhere.
The static dipole polarizability of helium atoms in the metastable 23S1 and 21S0 states has been determined by measuring the deflection of a beam of excited helium atoms that is caused by an inhomogeneous electric field of known properties. The necessary velocity distribution information is obtained from time-of-flight measurements, and a resonance quenching technique made it possible to distinguish the singlet and triplet components of the beam. The results, $\ensuremath{\alpha}(2^{3}S_{1})=(44.6\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ cm3 and $\ensuremath{\alpha}(2^{1}S_{0})=(108\ifmmode\pm\else\textpm\fi{}13)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ cm3, agree with the theoretical value that has been used to calibrate earlier polarizability measurements of the alkali metals and the heavier noble gases.
The development of atom trapping and cooling techniques has been a boon for the field of low temperature plasma physics. By forming plasmas through photoionization (PI) of cold atoms, researchers are able to create cold, dense plasmas that often lie in the strong-coupling regime. This is especially remarkable considering that many naturally-occurring plasmas in the strong-coupling regime are in inaccessible environments (e.g. the insides of stars and gas planets), whereas these laboratory-based plasmas prepared in cold-atom systems are relatively easy to produce and analyze with standard AMO techniques. In this thesis, I present my work from several projects on photoionization, plasma formation and dynamics, and plasma trap development. I first discuss an experiment that I spearheaded on expansion dynamics in cold, non-neutral plasmas. In this experiment, cold plasmas are formed by photoionizing rubidium atoms in a MOT, and the resulting plasma ions are allowed to expand for a variable wait time before being imaged with a single-ion imaging system. This ion imaging system, along with the rest of the experimental setup, was originally used to study Rydberg atoms. I have adapted the setup for plasmas by installing a new laser and changing the experimental timings. I report on plasma features seen in this experimental setup, such as shock shells and time-evolved ion pair correlations. I also present findings from a computational study of wavelength-dependent PI of Rydberg atoms, namely Rydberg-state rubidium and cesium. The main goal of the study is to find PI minima, wavelengths where the atom’s PI cross section decreases dramatically. These minima can be caused by Cooper minima or shape resonances, which each have their own telltale signs. This computational study paves the way for an informed experimental measurement of the locations of some of the identified PI minima, and comparing their experimental locations with the computational results. This is, in part, interesting because the latter depend on whether length or velocity gauge is used. An experimental measurement of PI cross sections could be a good way to explore the significance of gauge dependence in l-dependent potentials. I conclude with a discussion of a project centered on creating a small MOT with diverging cooling beams inside of a metal enclosure. This project was a collaboration with researchers at Eastern Michigan University. The idea to make a MOT with these design features was born out of plans for cryostatic plasma experiments. Now, it has potential applications in plasma experiments, precision measurements, and quantum information. Design-wise, the diverging beams are formed by sending narrow laser beams into 1.5 mm ball lenses, which cause the beams to diverge rapidly. Six ball lenses, one for each cooling beam, are mounted on a metal structure called the ball lens optical box (BLOB), which encases the trapped atom cloud. This enclosure provides immediate benefits for plasma experiments, as it offers Faraday shielding from stray electric fields. The first build successfully produced and maintained a MOT, making it the first known case of a MOT made with diverging beams from ball lenses. No further experiments are planned for this first build, but the concept of a ball lens MOT will certainly be implemented into future experimental efforts. Subsequent designs may include more design features, such as interior electrodes for custom field configurations.