PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 766 February 21, 2006 by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi
THE AMERICAN INSTITUTE OF PHYSICS (AIP) will observe its 75th anniversary this year. AIP was established in New York City in 1931 to help facilitate publishing and other services for five scientific organizations: the American Physical Society (APS), the Optical Society of America (OSA), the Acoustical Society of America (ASA), the Society of Rheology (SOR), and the American Association of Physics Teachers (AAPT). Later five more Member Societies were added: the American Crystallographic Association (ACA), the American Astronomical Society (AAS), the American Association of Physicists in Medicine (AAPM), AVS: Science & Technology of Materials, Interfaces, and Processing, and the American Geophysical Union (AGU). Today, AIP is one of the largest physics journal publishers in the world, and the non-overlapping membership of its 10 member societies numbers more than 100,000 (general AIP website: http://www.aip.org/index.html ). Physics News Update, the weekly summary of physics research you are reading at this moment, is prepared the AIP Media and Government Relations (MGR) division, operating out of AIP's headquarters in College Park, MD, just outside Washington, DC (associated websites are www.aip.org/pnu and www.aip.org/news/links.html ). To mark AIP's 75th anniversary, we plan to run a series of occasional comparisons between noteworthy physics topics from 1931 and 2006. Herewith the first of these: PHASE CONTRAST IMAGING WITH AN ELECTRON MICROSCOPE. Physicists in Germany have taken a crucial step towards achieving sharper images of biological samples and other "weak-contrast" objects. Typically microscope images of samples made of low-weight elements like hydrogen, carbon, nitrogen, and oxygen, are characterized by poor contrast. In the new approach, contrast will be improved for a transmission electron microscope (TEM) by imposing a large relative phase shift to the electron waves scattered from samples. The use of a beam of electrons as an illumination source for microscopy was pioneered in the early 1930s by Ernst Ruska, who won a Nobel Prize for the effort half a century later. Since then, electron microscopes have been a workhorse for imaging small things, often with a spatial resolution superior to that available with light microscopes. Nevertheless, even electron microscopes have resolution problems. In a TEM device most of the electrons pass through the thin electron-transparent sample without scattering. Scattering of electron waves, when it does happen, occurs not because of absorption---the amplitude of the electron beam is largely undiminished---but through the shifting of the electron phase. Scattered and unscattered waves are focused and recombine downstream of the sample in a recording medium, typically a charged coupled device (CCD). Unfortunately, in weak phase objects the phase shifting is slight, resulting in poor contrast. What scientists at the University of Karlsruhe and the Max-Planck Institute for Biophysics in Frankfurt have done to remedy this situation is to interpose a special free-suspended micro-scaled electrostatic lens beyond the sample; this electrostatic lens has the effect of shifting the phase of the unscattered waves by a further 90 degrees but leaving the scattered waves unshifted (see figure at http://www.aip.org/png/2006/249.htm ). This dramatically improves the contrast in the resultant images. This electrostatic lens is called a Boersch phase plate in honor of Hans Boersch, who proposed the technique in 1947. It has not been achieved until now because of its demanding size specifications. (Schultheiss et al., Review of Scientific Instruments, March 2006; contact Fabian Perez-Willard, [EMAIL PROTECTED]; website, http://www.lem.uni-karlsruhe.de/ ) MOLECULES GET MORE CLASSICAL at high pressures. That is, a new study of molecules being squeezed in a diamond anvil cell shows that as the pressure goes up, the force between atoms in a diatomic molecule behaves more and more like the classic Hooke's law, according to which the force between two objects connected by an elastic spring is proportional to the contraction or extension of the spring. Two scientists at the Carnegie Institution of Washington, and Lawrence Livermore National Laboratory, Alexander Goncharov and Jonathan Crowhurst, have loaded several species of molecule, such as H2, D2, and N2, into their cell and then observed what happened at high temperature and high pressure. By varying these two parameters the molecular sample can often be transformed from a fluid into a crystal or back again, or the molecules themselves might even be broken apart. The researchers (contact Goncharov at [EMAIL PROTECTED]) first heated the samples using a near-infrared laser and then probed the various excited vibrational quantum states using the technique of Raman spectroscopy. By carefully noting the frequency and linewidths of these stretching modes, they could deduce the energetics of the binding between the atoms even as the molecule was being subject to the extreme conditions. The findings, such as the realization that the binding becomes more like a classical harmonic oscillator at high pressure, should aid in such pursuits as the quest to observe metallic hydrogen. (Physical Review Letters, 10 February 2006) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. For that reason, you are free to post it, if you like, where others can read it, providing only that you credit AIP. Physics News Update appears approximately once a week. AUTO-SUBSCRIPTION OR DELETION: By using the expression "subscribe physnews" in your e-mail message, you will have automatically added the address from which your message was sent to the distribution list for Physics News Update. If you use the "signoff physnews" expression in your e-mail message, the address in your message header will be deleted from the distribution list. Please send your message to: [EMAIL PROTECTED] (Leave the "Subject:" line blank.)