WELCOME!

Located on the first floor of the chemistry building, Dr. Lehmann arrived in Charlottesville in the Fall of 2005. Having said goodbye to his Princeton lab, he has begun building a new lab group at the University of Virginia Department of Chemistry.

RESEARCH DESCRIPTION

Ultrasensitive Spectroscopy

There are many problems of both fundamental and of practical importance that require measurement of extremely low concentrations of certain impurities. Molecular Spectroscopy provides one approach that excels in the high specificity provided by the detailed structure in the spectrum, particularly for molecules in the gas phase. Lehmann’s group has been working on the development of new trace sensors, largely based upon the method of cavity ring-down spectroscopy (CRDS). In CRDS, one forms a stable optical cavity using mirrors with reflectivity > 99.99% and observes absorption of a sample contained inside the cavity by an increase in the rate of decay of light that is trapped between the mirrors. Sample absorption as low as 1 part in 109 per pass of the cell can be measured in this way. The Lehmann group pioneered the use of low cost and rugged diode lasers developed for the telecom industry in CRDS and has demonstrated detection of a number of small molecules, such as H₂O, NH₃, and CH₄ at levels below one part per billion in a sample gas. Tiger Optics, Inc. is now selling instruments based upon this work to several industries. Recently, we have developed a new, fiber optic version of CRDS and have demonstrated that this could be used to detect a single cell that sticks to the surface of an optical fiber. We are presently working on an instrument to detect the CO₂ given off by a single cell as a monitor of the cell’s rate of respiration and thus energy usage. This should allow us to follow energy usage as a cell grows, is challenged by toxins or harsh conditions, or germinates from a spore. We have plans to develop a new broad bandwidth version of CRDS that will allow multiple chemical species to be monitored simultaneously, such as with an FTIR, but with much higher sensitivity. Breath analysis for medical diagnosis is an important potential application of CRDS that we would like to explore.

Spectroscopy in Superfluid Helium

Research in the Lehmann group has long used laser spectroscopy and theoretical modeling to study molecular dynamics – studying chemical reactions at their most fundamental level. In recent years, this line of work has focused on the spectroscopy of atoms and molecules dissolved in nanodroplets of superfluid helium. Helium Nanodroplet Isolation (HENDI) combines many of the most attractive features of both high resolution, molecular beam spectroscopy and more traditional rare gas matrix spectroscopy. The droplets cool any solvated molecule down to a temperature of only 0.38 K but remain liquid, which allows molecules to move and rotate nearly freely with relaxation times three to four orders of magnitude longer than in traditional liquids. This allows for the study of the interaction of molecules with a unique solvent. Fundamental questions are yet unresolved, such as how the molecules come into equilibrium with the superfluid and why quantized vortices (which are common in build liquid helium) have not been observed in the droplets. The droplets allow the production of new chemical species and new isomers of known compounds.

We are working on the spectroscopy of free radicals in helium. Traditional wisdom is that the reaction of two free radicals can occur without a barrier, but high level ab initio calculations suggest that in many such reactions (such as O₂ + O -> O₃), small entrance channel barriers exist and these are believed to play an important role in the rates of three body recombination; a process that produces O₃ in the atmosphere. It should be possible to quench entrance channel complexes and study their properties using HENDI. We are also attempting to study small para-hydrogen clusters, which are also predicted to form a superfluid. We plan to build an instrument to study ions in helium droplets, which will allow use of mass selected droplets. This should allow, among other experiments, the measurement of binding energies of atomic and molecular cluster ions by the measurement of the number of helium atoms evaporated from the droplet after molecule formation.