February 2, 2005, Berkeley, CA--A team of scientists used a first-of-its-kind spectroscopy system at the U.S. Department of Energy's Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory to obtain the first direct observations of negatively charged ions accumulating on the surfaces of salt solutions. Their study was published in the January 28 issue of the journal Science.
Scientists have had to rely on theory and indirect observations to explain how anions in salt grains rearrange themselves when the salt liquefies after absorbing water vapor from the surrounding air. Molecular dynamic simulations indicate that the anions migrate and become enriched at the droplet's surface. Other techniques allow scientists to observe the indirect, telltale signs of this change. But until recently, there was no way of directly observing and quantifying this phenomenon.
Enter the high-pressure photoelectron spectroscopy system, a second-generation instrument that began operating at one of the ALS's beamlines (11.0.2) last year. (A first-generation system was developed in 2000). Like all photoelectron spectroscopy systems, it identifies elements by detecting their unique spectral signals. But unlike others, it can do this under similar pressures and humidities faced by everyday phenomena. The system has already elucidated the century-old puzzle of how a liquid-like film exists on the surface of ice close to its melting point. It has also shed light on how the rhodium metal surface adsorbs and removes carbon monoxide and nitrogen oxide gases in car exhaust converters.
The ALS is one of only two places in the world where photoelectron spectroscopy can be performed on surfaces at pressures above 1 torr. The only other such instrument is at a synchrotron in Berlin, Germany, which also uses the design developed at the ALS by Miquel Salmeron, a physicist with Berkeley Lab's Materials Sciences Division. All other photoelectron spectroscopy systems require a dry, high-vacuum environment.
"A lot of information is lost once a compound is placed in a vacuum, so we developed an instrument to circumvent this problem. In order to understand real-world chemical processes, we need to analyze them as they occur in the real world," said Salmeron, who collaborated with John Hemminger of the University of California at Irvine on the salt solution study. UC Irvine's Sutapa Ghosal, and Berkeley Lab's Hendrik Bluhm, Eleonore Hebenstreit, Guido Ketteler, Bongjin Simon Mun, Frank Ogletree, and Felix Requejo also contributed to the research.
Their work paves the way for a better understanding of the role played by tiny airborne salt particles, emitted into the air by crashing waves and sea spray, in a slew of atmospheric and environmental processes, such as the destruction of the ozone layer. "This is important because many atmospheric phenomena are driven by reactions with the surface of sea salt aerosols," Salmeron said. "We found that anions concentrate at the surface of salt grains as they dissolve in water."