Let There Be Light

by Lynn Yarris
(Excerpt from the full article.)

Although the ALS is designed for peak production of soft x-ray and VUV light, it still produces photons below those energies on the spectrum. For example, Beamline 1.4, which operates off a bend-magnet and serves three experimental endstations, generates beams of infrared (IR) light at far higher brightness than conventional IR sources such as Globars (TM), which are silicon-carbide filaments that radiate IR light when heated. The ability to focus IR light down to a 10 micron spot size opens up new possibilities for environmental studies and other areas of research.

A molecule, because of the non-stop motion of its atoms, vibrates at a characteristic frequency within the IR spectrum that can be used to identify its type, much like a fingerprint can identify a person. When a molecule is struck by an IR photon that matches its vibrational frequency, it will resonate. This resonance can be detected through a variety of spectroscopic techniques and used to not only "fingerprint" the molecule, but to also measure and manipulate it.

Working with infrared microscopy at an ALS experimental station, physicist Michael Martin (center), manager of Beamline 1.4, helped two Bay Area science teachers develop a classroom study unit as part of an education partnership program held at Berkeley Lab.

Michael Martin, manager of Beamline 1.4 says, "We can shine our (IR) beam on an unknown sample and determine its chemical composition. With our fast time resolutions (a spectral analysis can be recorded every five nanoseconds), we can then observe changes in this chemical composition while they are taking place and see which molecular species go away as they turn into something else."

Although the bend magnet for beamline 1.4 probably has the poorest focus in the ALS for XUV research, Martin says it is "just fine" for IR spectroscopy. As a result, ALS light that might otherwise have been discarded is being put to good use by research teams such as the one led by Donald Sparks, who chairs the University of Delaware's Department of Plant and Soil Sciences. Sparks and his team are using IR photons to do molecular-scale studies of reactions between chemicals that occur naturally in a soil and those that are introduced by human activity such as metals, industrial chemicals, and pesticides.

In addition to environmental studies, IR photons at the ALS are also being used in studies aimed at discovering new types of materials; measuring energy bandgaps in novel semiconductors and high-T_c superconductors; investigating sub-monolayers of atoms adsorbed onto surfaces during interface reactions and corrosion; and probing the reactive transients, the molecules that form temporarily during the intermediate stages of a chemical reaction and often actually determine the reaction's outcome. Martin, who is a solid-state physicist, has worked with Wayne McKinney, the principle designer of Beamline 1.4, to ensure that the IR experimental endstations are second to none in user friendliness.

"The goal is for users to be able to come in, click a button, and get their data without having to understand how the beamline system works," Martin says. "This is how you reach out to users who want to come here to do science and not to learn beam physics."