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Laser Cooling, Electron Collisions Highlight 1995 DAMOP Meeting

Laser cooling of neutral atoms, recent advances in electron collision physics, and the use of atomic, molecular and optical physics as a tool were among the highlights of the 1995 annual meeting of the APS Division of Atomic, Molecular and Optical Physics (DAMOP), held 17-19 May at the University of Toronto in Ontario, Canada, in conjunction with the Canadian Association of Physicists.

Atom Optics and Atom Interferometers. David Pritchard of MIT kicked off the Monday morning plenary session by describing recent experimental demonstrations of optical elements for atoms, including mirrors, lenses and diffraction gratings made from both light and matter. Using these tools, his team has constructed a three-grating atom interferometer in which the atom wave is sent on both sides of a stretched metal foil before recombining. An electric field was placed on one side of the foil, shifting the interference pattern and allowing Pritchard and his colleagues to measure the electric polarizability of sodium with unprecedented accuracy. Atom interferometers can also be applied to fundamental tests of quantum mechanics.

The session also featured an invited lecture by the recipient of the 1995 APS I.I. Rabi Prize, Randall G. Hulet (Rice University). He reported on prospects for observing quantum statistical effects in laser cooled lithium and was officially honored during Thursday evening's banquet.

Laser Cooling of Neutral Atoms. Researchers at a number of institutions are employing variations of evaporative cooling techniques to study neutral atoms. Cooling neutral atoms to submicrokelvin temperatures enables scientists to study novel phenomena at long wavelengths, most notably quantum degeneracy effects. During a Wednesday morning session, Wolfgang Ketterle of MIT described how he used rf induced evaporation to reduce the temperature of magnetically trapped sodium atoms by a factor of 12 and increase the phase space density by more than two orders of magnitude. According to Ketterle, the current limitation of the cooling process is non-adiabatic spinflip transitions in the center of the trap. He expects further progress to be possible after transferring the atoms into a trap without a zero magnetic field in the center.

E.A. Cornell (University of Colorado/JILA) achieved similar results with the invention of the TOP magnetic trap, which combines the tight confinement of a quadrupole trap with the long lifetime of a Ioffe trap. Because there is no lower limit to the temperature of atoms confined in a TOP trap, he believes that further advances in evaporative cooling may lead to Bose condensation. M.A. Kasevich (Stanford University) confined laser cooled sodium atoms in an optical dipole force trap formed from the intersection of two far-detuned laser beams, then evaporatively cooled them by reducing the trap depth.

Using AMO Physics as a Tool. Atomic, molecular and optical physics is emerging as an important tool in a number of areas. For instance, a new technique based on the optogalvanic effect has been developed for the measurement of stable isotope ratios. According to D.E. Murnick (Rutgers University), who spoke during a Thursday morning session cosponsored by the APS Committee on Applications of Physics, the technique uses the specificity of laser resonance spectroscopy to achieve the sensitivity and accuracy typical of sophisticated isotope ratio mass spectrometers. Carbon and oxygen isotopic analysis is an important tool in geology, environmental science, biology and medicine, and the technique is already being applied to analyze exhaled human breath as a diagnostic for gastrointestinal infection.

Atomic collisions are playing an increasingly important role in fusion research, according to F.W. Meyer (Oak Ridge National Laboratory), who described a new approach to provide adequate power and particle exhaust for the International Thermonuclear Experimental Reactor (ITER) presently under engineering design. The approach uses a poloidal divertor chamber into which the hot edge plasma can escape, and where cooling and pumping can occur with minimum disruption of the core fusion plasma. Because of its high density conditions and open magnetic field lines, atomic collisions and surface interactions play significant roles in reducing power-loading to manageable levels, and in defining the behavior, parameters and composition of the divertor plasma. Because the divertor plasma will be cold, in contrast to the past emphasis on hot core plasma contamination, the focus will shift to much lower energy atomic collisions, to lower ionization stages, and to molecular species, said Meyer.

Advances in Electron Collision Physics. Scientists at the University of Wisconsin are using atom traps to measure electron-atom cross sections, according to Chun C. Lin, who reported on his group's experiments during a Thursday afternoon session. Electron beam collisions with Rb atoms in a magneto-optical trap cause some atoms to escape through the recoil. Total scattering cross sections are measured by leaving the trap off for several milliseconds after the electron beam pulse, allowing the slowest recoiled atoms sufficient time to exit the trap volume. Lin said that it is also possible to determine the ionization cross sections by using a very short electron-beam pulse, them immediately turning the trap back on to recapture the non-ionized atoms and allow only the ions to escape.

Ultra Fast and High Field Atomic Physics. Using direct x-ray imaging of Xe(M) emission in stable self-trapped channels, scientists at the University of Illinois at Chicago have experimentally demonstrated the first combined expression of two complex nonlinear processes: the multiphoton production of x rays from clusters, and high-intensity modes of channeled propagation in plasmas. Unifying these phenomena enables researchers to produce, apply and control power densities at ultrahigh (>10^19 W/cm3) levels, which is essential for achieving the efficient amplification of x-rays.

According to C.K. Rhodes, who spoke during a Friday afternoon session, the harmonious use of these nonlinear phenomena is expected to lead to an advanced generation of extra-ordinarily bright x-ray sources in the multi-kilovolt region. The goal is to attain levels sufficient for biological holographic imaging capable of providing a high resolution visualization of the molecular anatomy of cells, tissues and organisms in the natural state. In addition, the work provides confirming evidence for the action of a superstrong coherent multi-electron intense field interaction in the x-ray generation from the clusters. It also furnishes new information on the dynamics of the radial intensity distributions associated with the channeled propagation.

Evolution of Magnetic Moments in Clusters. Understanding field-induced and temperature-induced moment fluctuations in clusters is an essential step toward connecting magnetism in atoms with that of clusters and solids, according to L.A. Bloomfield (University of Virginia, Charlottesville), who spoke during a Thursday afternoon session. It is well-known that when a magnetic atom in a grounded state enters a magnetic field, its energy level shifts, which can cause the atom to deflect during its flight through a gradient magnetic field. But studies of cluster magnetism must take into account crossings between nearby levels.

"The energy levels in clusters of two to 20 atoms are very closely spaced and strongly coupled, so that small adiabatic changes in the magnetic field may transfer a cluster from one state to another," said Bloomfield. "In particular, these field changes can alter a cluster's total magnetic moment or the projection of that moment on the field gradient." Hot clusters have more closely spaced levels than cold clusters and are thus more susceptible to the fluctuations. Temperature can also produce spontaneous moment fluctuations in the clusters by exchanging angular momentum between magnetic and non-magnetic component

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Editor: Barrett H. Ripin