Laser method identifies, counts toxic molecules

A spectroscopy technique that offers advances in detection of toxic chemicals and counting of molecules has been demonstrated by a National Institute of Standards and Technology (NIST) scientist and collaborators. Described in the Feb. 8 issue of the Journal of Chemical Physics, the NIST-patented technique may be useful for development of miniaturized chemical sensors, as well as for fundamental surface science studies.

The technique (a variation on cavity ring-down spectroscopy) relies on laser light reflecting and circulating inside a prism-like optical resonator. The time it takes the light to diminish (or ring down) changes depending on whether specific chemicals are present near the resonator and on how much light they absorb. This information can be used to identify and quantify specific molecules.

The technique can detect small amounts (100 parts per million) of trichloroethylene, a toxic commercial solvent that is prevalent but difficult to locate in the environment. The sensitivity is equivalent to the best of other published optical methods that could be used outside a laboratory. A highly selective coating is expected to enhance performance further.

The technique also was used to determine the number of molecules per unit area on a surface (“absolute coverage”) without the need for ultrahigh vacuum experimental conditions, which are typically required for such measurements. Hence, the new approach enables quantitative studies of real-world surface processes, such as catalytic reactions. Absolute coverage measurements are useful in surface science, providing key information about surface reactions or structures for many applications, such as improving solar cell efficiency.

The research was performed with collaborators from Eindhoven University of Technology in the Netherlands and the University of Maryland; partial funding was provided by the U.S. Department of Energy, Environmental Management Science Program.