Cluster chemistry--the study of interactions among closely packed atoms or molecules ranging from 2 to 1000 in number--is gathering public as well as scientific interest. One of the most exciting objects of study is the cluster of carbon atoms known as buckyball. Also intriguing to scientists are chemical reactions in a cluster because they may imitate the processes of catalysis, materials synthesis, and atmospheric chemistry.

Clusters have been called the fifth state of matter, after solids, liquids, gases, and plasmas. The unique properties of clusters fall between those of single atoms and molecules and those of bulk solids. John Miller and Howard Carman, both of ORNL's Health Sciences Research Division, have applied novel laser-based techniques to the study of molecular clusters.

One property of clusters has made it possible for Miller to identify a new form of "laser snow." Laser snow was first observed and named in 1975 by William Happer of Columbia University in New York City. He witnessed the precipitation of white particles after exciting a gaseous mixture of cesium and hydrogen with a laser beam. The excited cesium atoms collided and reacted with hydrogen molecules, producing a cesium hydride precipitate. Laser snow was also detected in experiments with sulfur hexafluoride, thiophenol, and carbon disulfide.

At an April 1994 meeting of the American Physical Society, Miller reported the detection of a different form of laser snow by using a laser beam to ionize one molecule in a cluster of carbon disulfide molecules. This time, the proximity of the molecules to each other in the cluster, rather than collisions between them, was responsible for the new form of laser snow. Another difference between this experiment and the earlier laser snow studies is that the products of the chemical reaction are ions that can be easily detected by mass spectrometry.

Miller, who collaborated with University of Tennessee (UT) professor Charles Feigerle and graduate student Sunil Desai on this work, says, "Although visible particles were not observed in our low-pressure experiment, the mass spectrum revealed the presence of sulfur and carbon sulfide polymers. These are the same species observed in earlier laser snow experiments with high-pressure gases."

In the ORNL-UT experiment, the laser beam ionized a carbon disulfide (CS2) molecule, breaking it into an S+ ion and a CS molecule and freeing an electron. The S+ ion reacted with a nearby CS2 molecule, forming an S2+ ion. This ion then grabs a sulfur atom from another neighboring carbon disulfide molecule, and so on. In this snowball effect, a sulfur polymer as large as S12+ can be formed. Some carbon sulfide ions also latched onto neighboring carbon disulfide molecules, forming polymers as large as (CS)4+. A mass spectrometer detected the presence and masses of the sulfur and carbon sulfide ions. It also detected intermediate species that provide clues to how these polymers are formed.

Miller says that the cluster mechanism eliminates the need for collisions to induce chemical reactions. "When one member of a cluster of CS2 molecules is excited, it finds its next-door neighbor to be a suitable reaction partner and no collision is needed. Any extra energy from the reaction can be used to eject weakly bound molecules from the cluster. The initially ionized molecule 'eats' its way through the cluster like a molecular 'Pac Man,' creating bigger and bigger polymers with each bite."

Miller and Desai use a high-power picosecond laser to generate positive ions of molecular clusters. They have also studied nitric oxide clusters mixed with other important species found in the atmosphere.

Carman's research focuses on producing negative ions of clusters that are then detected in a mass spectrometer. In his technique, developed with C. E. Klots and Robert Compton of the same division, several tunable lasers are used to excite the outer electrons of alkali atoms (e.g., cesium). Each excited electron is very far from the atomic nucleus and thus looks like a "free electron" to any nearby particle. If the cluster under study encounters such an electron, it may grab it to form a negatively charged cluster ion, which is then detected. In this way, Carman has produced negative cluster ions of carbon (C)n-, cousins of the famous fullerenes that include buckyballs. He has also produced negative cluster ions of nitric oxide, a species in polluted atmospheres.

It is now possible to generate in a molecular beam apparatus microscopic clusters containing nitric oxides, sulfur oxides, and chlorine-containing molecules attached to a small number of water molecules. The concentrations, sizes, and temperatures of these clusters can be experimentally varied. These "binary" clusters can serve as models for the study of atmospheric processes in water droplets or ice crystals.

Recently, it has been observed that chlorine-nitrate reactions on ice crystals in polar stratospheric clouds convert inert chlorine into photochemically active chlorine (Cl2). Active chlorine is thought to be primarily responsible for destruction of the earth's protective ozone layer. The energy of sunlight converts the active chlorine to products that stimulate ozone-destroying chain reactions.

Miller and Carman propose to perfect the generation in the laboratory of these binary clusters. They hope to study the photochemistry of clusters of chlorine-containing compounds on ice crystals to identify the products that lead to ozone losses. They also wish to explore binary clusters to gain knowledge about stratospheric sulfate and nitrate aerosols. The payoff could be better understanding of the atmosphere through cluster chemistry.


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