Kevin LehmannKevin K. Lehmann


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.

Recent Publications:

Helium Nanodroplets:

  1. C. Callegari, A. Conjusteau, I. Reinhard, K.K. Lehmann, G. Scoles, and F. Dalfovo, Superfluid hydrodynamic model for the enhanced moments of inertia of molecules in Liquid 4He, Phys. Rev. Lett. 83, 5058-5061 (1999); 84, 1848 (2000).
  2. G. Scoles and K.K. Lehmann, Nanomatrices are cool, Science 287, 2429-2430 (2000).
  3. J. Reho, U. Merker, M.R. Radcliff, K.K. Lehmann, and G. Scoles, Spectroscopy of Mg Atoms solvated in helium nanodroplets, J. Chem. Phys. 112, 8409-8416 (2000).
  4. J. Reho, U. Merker, M.R. Radcliff, K.K. Lehmann, and G. Scoles, Spectroscopy and Dynamics of Al atoms solvated in superfluid helium nanodroplets, J. Phys. Chem. A104, 3620-3626 (2000).
  5. C. Callegari, I. Reinhard, K.K. Lehmann, G. Scoles, K. Nauta, and R.E. Miller, Finite-Size Effects and Molecular Relaxation in Superfluid Helium Nanodroplets: Microwave-Infrared Double Resonance Spectroscopy of Cyanaocetylene, J. Chem. Phys. 113, 4636-4646 (2000).
  6. A. Conjusteau, C. Callegari, I. Reinhard, K.K. Lehmann, and G. Scoles, Microwave Spectra of HCN and DCN in 4 He nanodroplets: A test of adiabatic following, J. Chem. Phys. 113, 4840-4843 (2000).
  7. J. Reho, J. Higgins, C. Callegari, K.K. Lehmann, and G. Scoles, Alkali-helium exciplex formation on the surface of helium nanodroplets. I: Dispersed emission spectroscopy, J. Chem. Phys. 113, 9686-9693 (2000).
  8. J. Reho, J. Higgins, K.K. Lehmann, and G. Scoles, Alkali-helium exciplex formation on the surface of helium nanodroplets. II: A time-resolved study. J. Chem. Phys. 113, 9694-9701 (2000).
  9. A. Conjusteau, C. Callegari, I. Reinhard, K.K. Lehmann, and G. Scoles, First Overtone Spectroscopy of acetylenic Stretches in 4He Clusters, J. Chem. Phys. 113, 10535-10550 (2000).
  10. K.K. Lehmann, Buoyancy Corrections for the Potential of an Impurity in a 4He nanodroplet, Molecular Physics, 98, 1991-1993 (2000).
  11. K.K. Lehmann, Rotation in liquid 4He: Lessons from a highly simplified model, J. Chem. Phys. 114, 4643-4648 (2001).
  12. J. H. Reho, J. P. Higgins, and K. K. Lehmann, Dynamics of the 13Πg State of K2 on Helium Nanodroplets, Royal Society of Chemistry, Faraday Discussions (Cluster Dynamics). 118, 33-42 (2001).
  13. Carlo Callegari, Kevin K. Lehmann, Roman Schmied, and Giacinto Scoles, Helium nanodroplet isolation ro-vibrational spectroscopy: methods and recent results, J. Chem. Phys. 115, 10090-10110 (2001).
  14. J. H. Reho, J. P. Higgins, M. Nooijen, K. K. Lehmann, G. Scoles, and M. Gutowski, Photoinduced nonadiabatic dynamics in quartet Na3 and K3 formed using Helium nanodroplet isolation., J. Chem. Phys. 115, 10265-10274 (2001).
  15. Kevin K. Lehmann, Calculation of Hydrodynamic Mass for Atomic Impurities in Helium, Phys. Rev. Lett. 88, 145301-145304 (2002).
  16. Kevin K. Lehmann and Carlo Callegari, Quantum Hydrodynamic Model for the enhanced moments of Inertia of molecules in Helium Nanodroplets: Application to SF6, J. Chem. Phys., 117, 1595-1603 (2002). Also reprinted in Virtual Journal of Nanoscale Science & Technology 6, issue 4 (July 22, 2002).
  17. Kevin K. Lehmann, Microcanonical Thermodynamics Properties of Helium Nanodroplets, Journal of Chemical Physics 119, 3336-3342 (2003).
  18. Kevin K. Lehmann and Roman Schmied, Energetics and possible formation and decay mechanisms of Vortices in helium nanodroplets, Phys. Rev. B. 68, 224520 (2003).
  19. Kevin K. Lehmann, Bias in the temperature of a helium nanodroplet measured by an embedded rotor, J. Chem. Phys. 120, 513-515 (2004).
  20. Pierre Çarçabal, Roman Schmied, Kevin K. Lehmann and Giacinto Scoles, Helium nanodroplet isolation spectroscopy of perylene and its complexes with molecular oxygen, J. Chem. Phys. 120, 6792-6793 (2004).
  21. Kevin K. Lehmann and Adriaan Dokter, Evaporative cooling of helium nanodroplets with angular momentum conservation, Physical Review Letters 92, 173401 (2004).
  22. Roman Schmied, Pierre Çarçabal, Adriaan Dokter, Kevin K. Lehmann, and Giacinto Scoles, UV spectra of benzene isotopomers and dimers in helium nanodroplets, J. Chem. Phys. 121, 2701-2710 (2004).

CRDS related papers:

  1. C.R. Bucher, K.K. Lehmann, D.F. Plusquellic, and G.T. Fraser, Doppler-free nonlinear Absorption in ethylene by use of cw cavity ring down spectroscopy, Applied Optics, 39, 3154-3164 (2000).
  2. Wen-Bin Yan, John Dudek, Kevin K. Lehmann, and Paul Rabinowitz, A fast innovative infrared analyzer for monitoring ultra-trace moisture in semiconductor gases, Tec. Pap. ISA 416, 247-256 (2001).
  3. John B. Dudek, Peter B. Tarsa, Armando Velasquez, Mark Wladyslawski, Paul Rabinowitz, and Kevin K. Lehmann, Trace moisture detection using continuous-wave cavity ring-down spectroscopy. Analytical Chemistry 75, 4599-4609 (2003).
  4. Peter B. Tarsa, Paul Rabinowitz, and Kevin K. Lehmann, Evanescent field absorption in a passive optical fiber using continuous wave cavity ring-down spectroscopy, Chemical Physics Letters 383, 297-303 (2004).
  5. Peter B. Tarsa, Diane M. Brzozowski, Paul Rabinowitz, and Kevin K. Lehmann, Cavity Ring-Down Strain Gauge, Optics Letters 29, 1339-1341 (2004).
  6. Peter B. Tarsa, Aislyn D. Wist, Paul Rabinowitz, and Kevin K. Lehmann, Single cell detection by cavity ring-down spectroscopy, Applied Physics. 85, 4523 (2004).

Highly excited vibrational states papers:

  1. H.K. Srivastava, A. Conjusteau, H. Mabuchi, A. Callegari, K.K. Lehmann, and G. Scoles, Rovibrational spectroscopy of the v=6 manifold in 12C2H2 and 13C2H2, J. Chem. Phys. 113, 7376-7383 (2000).
  2. A. Callegari, U. Merker, P. Engels, H.K. Srivastava, K.K. Lehmann, and G. Scoles, Intramolecular vibrational redistribution in aromatic molecules I: Eigenstate resolved CH stretch first overtone spectra of benzene, J. Chem. Phys. 113, 10583-10596 (2000); 114, 3344 (2001).
  3. H. K. Srivastava, A. Conjusteau, H. Mabuchi, A. Callegari, K.K. Lehmann, and G. Scoles, A sub-Doppler resolution double resonance molecular beam infrared spectrometer operating at chemically relevant energies (similar to 2 eV), Review of Scientific Instruments 71, 4032-4038 (2000).
  4. A. Callegari, R. Pearman, S. Choi, P. Engels, H. K. Srivastava, M. Gruebele, K.K. Lehmann, and G. Scoles, Intramolecular Vibrational Relaxation in aromatic molecules II: An experimental and computational study of pyrrole and triazine, Molecular Physics 101, 551-568 (2003).
  5. Raul Z. Martinez, Stuart Carter, and Kevin K. Lehmann, Spectroscopy of highly excited vibrational states of HCN in its ground electronic state, J. Chem. Phys. 120, 691-703 (2004).