Report to the 1999 General Assembly for 1996-99
The mandate of C15 broadly covers the properties of charged and neutral atoms and molecules, and the dynamics of these entities, including interactions with electromagnetic radiation. In 1996, the name of the Commission was changed to include "Optical Physics" in recognition of the important role played by lasers and other optical devices in the study of atoms and molecules. The revised mandate makes explicit reference to "the application of optical and laser techniques to the manipulation of atomic and molecular species," but it does not include the development of optical devices themselves. The latter is covered by the mandate of the International Commission on Optics (ICO). A member of the ICO serves as an Associate Member on C15 in order to ensure good communication. There is also an exchange of Associate Members with C17 (Quantum Electronics) and C12 (Nuclear Physics).
Commission C15 has traditionally supported two major conferences held in alternate years: the International Conference on Atomic Physics (ICAP), and the International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC). In addition, one or two other topical conferences are supported each year. The members of C15 actively promote the role of IUPAP to future conference organizers and encourage the submission of applications for support.
Commission C15 meets every other year in conjunction with the ICAP meeting. In alternate years, business is carried out by e-mail. Over the past three-year period, C15 met in Amsterdam on August 6, 1996, and in Windsor, Canada on August 4, 1998. In addition to discussions of recommendations for conference support, nominees for the new officers and membership of C15 were extensively discussed at the Windsor meeting. As a general comment, C15 feels that the revised structure of IUPAP has done a great deal of good in involving the Commission Chairs more directly with the activities of IUPAP.
The meetings sponsored by C15 were as follows:
New Developments in the Field
The first realizations of Bose-Einstein Condensation (BEC) were achieved in 1995, using magnetically trapped, laser-cooled Rb, Na and Li atoms. This has ushered in a new era of intense and strongly expanding research activities. One important new result concerns the demonstration that the condensate is coherent and remains so when released from the trap. As a consequence, there exists the possibility of constructing an "atom laser". Evidence for the gain mechanism has already been obtained. An atom laser would revolutionize atom optics and atom interferometry, as well as other possible applications.
As one fascinating example, it has recently been demonstrated that, under the influence of gravity, the atoms in a BEC trapped in an optical lattice can tunnel from one region of potential to another, while maintaining their coherence with the condensate as a whole. The effect, which is analogous to the Josephson effect in superconductors, produces a pulsed atom laser containing "droplets" of coherent atoms.
The elusive goal of observing BEC in hydrogen was finally achieved in 1998, after many years of experimental research. The difficulty is that hydrogen atoms interact quite weakly, and the lasers needed to control and manipulate hydrogen atoms had not been developed. The demonstration of BEC is here particularly important because the atomic properties of H can be accurately calculated.
In recognition of their contributions to this field, the Royal Swedish Academy of Sciences awarded the 1997 Nobel Prize in Physics to Steven Chu, Claude Cohen-Tannoudji and William C. Phillips "for the development of methods to cool and trap atoms with laser light.
High-precision measurements and calculations contain to play a most important role in contemporary atomic physics. For instance, using advanced laser techniques, the frequency of the 1s - 2s transition in hydrogen has been determined with a precision of only 3 parts in 1013. This sets a new record for the most accurate measurement of any frequency in the visible or ultraviolet region of the spectrum. In addition to the scientific value (most accurate values of the Rydberg constant and 1s Lamb shift in hydrogen) high-precision measurements of optical frequencies should result in better atomic clocks. Similar advances in both theory and experiment have been made in spectroscopic studies of helium and other three-body systems. On the theoretical side, the nonrelativistic part of the problem is now solved for all practical purposes, and interest is shifting to the first high-precision tests of quantum electrodynamic effects in systems more complicated than hydrogen.
For well over 20 years much atomic physics work has been directed toward tests of the Weinberg-Salam Standard Model. Many laser-based experiments have thus been undertaken to detect the very minute parity non-conservation (PNC) effects, usually in heavy atoms. By combining the PNC data with results of sophisticated atomic structure calculations, the so-called weak charge Qw can be determined. In 1997 an atomic-beam-laser experiment for Cs the PNC amplitude was determined with precision of 0.035%, about 3 times better than the best previous values. Besides confirming the Standard Model, evidence was also provided for the existence of the nuclear anapole moment. The latter is theoretically predicted as arising during the interaction of an electron with the nucleus by the exchange of a virtual photon which interacts with the parity-violating components of the nuclear wave function. The anapole moment thus shows up in parity-violating atomic transitions.
About 50 years ago, H. Casimir predicted the existence of an attractive force between two parallel, uncharged metal plates, separated by vacuum. While many calculations and experiments have provided support to the existence of the Casimir effect (a consequence of QED), a direct observation was first reported in 1997.
Storage and cooler rings for ions have made a wealth of new precision measurements in atomic and molecular physics possible. The currently operating five storage rings for atomic and molecular physics experiments provide unique opportunities for new measurements through their combination of the properties of ion accelerators and ion traps. Recent results include accurate determinations of Lamb shifts for H-like and He-like ions of uranium, U91+ and U90+, respectively. (Such ions have also been produced in Electron-Beam Ion Traps, EBIT). The results are in agreement with QED calculations, thereby providing important confirmation of QED effects in the presence of strong Coulomb fields. In the field of molecular physics a wide range of processes which occur for example in the atmospheres of the Earth and other planets, or in interstellar space, have been studied under very controlled conditions in the laboratory.
The corresponding electron storage rings predate the ionic ones by many years, and the synchrotron radiation they produce has long been a primary research tool in chemistry and biology, as well as physics. Indeed, the first atomic physics experiments of this kind were carried out in the early 1970's. Recently, a number of powerful new synchrotron radiation facilities have been constructed in various countries around the world, opening the way to a range of interesting atomic and molecular physics experiments. For example, the triple ionization of atomic Li by a single photon has recently been demonstrated. These facilities will play an increasingly important role as a standard research tool for applications such as chemical analysis, and structural determinations of biological molecules.
Highly charged ions can carry a large amount of potential energy, which greatly exceeds the kinetic energy of the ions (which emerge from powerful ion sources or ion traps) and make slow collisions with solid surfaces. This may lead to "hollow atoms" (dynamically neutral atoms with all or nearly all electrons in outer shells, while inner shells remain empty). A number of fundamental atomic processes can be probed with hollow atoms, and they can also be used to create nanoscale structures on surfaces.
Using laser techniques, an excited atomic state, with a record lifetime of about 10 y has been observed and measured. This highly metastable state in the Yb ion decays by exotic electric octupole (E3) transitions.
Gordon Drake, Chairman A36@uwindsor.ca