Research

Our research endeavors focus on the detailed interrogation and manipulation of reaction dynamics at the molecular and atomic level. Current thrusts include the spectroscopic characterization of bimolecular interactions, coherent control of chemical dynamics, and the categorization of quantum confinement effects in semiconductor nanostructures. The experiments utilize an array of tools, including nanosecond and state-of-the-art femtosecond lasers, ultrashort pulse shaping, mass spectrometry and ion imaging, absorption and fluorescence spectroscopy, as well as confocal microscopy.

Bimolecular Interactions and Reaction Dynamics. Frequency and time-resolved laser spectroscopy and time-of-flight ion imaging methods are implemented to accurately characterize inter-molecular potential energy surfaces and the dynamics that occur on these surfaces. Two moieties are first stabilized in a weakly bound complex by cooling the species in a supersonic expansion. By cooling the complexes to specific temperatures, we are able to stabilize the complexes with preferred orientations between the constituents. The He···ICl(X,v=0) complex, for instance, is found to have a T-shaped orientation at T~5 K and at lower temperatures, T~0.5 K, the complexes have preferred linear geometries. These complexes serve as launching pads for investigating the photo-induced dynamics that occur from these initial orientations.

Coherent Control of Chemical Dynamics. Ultrashort laser pulses are used to initiate and monitor the dynamics of molecules that can follow competing pathways. Furthermore, the properties of the excitation pulse are manipulated to quantum mechanically control the yields of the different product channels. The coherent control of biomolecular reactions is also being pursued. Two reactants are stabilized in a non-reactive complex. A laser promotes the reactants above the barrier for reaction. The probability for reaction is then controlled by steering the reactants to specific intermolecular orientations and energies.

Quantum Confinement Effects. A number of spectroscopic techniques are utilized to determine how shape affects the optical properties of semiconductor quantum nanostructures. Specifically, we, in collaboration with the group of Professor Buhro, are investigating the dependence of band gap energies on the diameter of semiconductor nanowires. These nanowires are ideal for studying the two-dimensional quantum confinement of excitons since they can be synthesized with diameters as small as 3.5 nm and lengths on the order of microns. We are now working towards using a confocal microscope coupled with ultrafast lasers to directly measure the excitonic dynamics within individual nanowires.

Selected Publications

• J.J. Glennon, R. Tang, W.E. Buhro*, and R.A. Loomis*, "Semiconductor nanowires exhibit delocalized photoluminescence intensity fluctuations," submitted to Science.
• D.S. Boucher and R.A. Loomis*, "Stabilization of different conformers of weakly bound complexes to access varying excited-state intermolecular dynamics," submitted to Adv. Chem. Phys.
• S.A. Ray, A.B. McCoy*, J.J. Glennon, J.P. Darr, E.J. Fesser, J.R. Lancaster, and R.A. Loomis*, "Experimental and theoretical investigations of the He•••I2 rovibronic spectra in the I2 B–X, 20–0 region," J. Chem. Phys., 125, 164314 (2006).
• J.P. Darr and R.A. Loomis*, "Low-energy, collision-induced vibrational energy relaxation within a heavy molecule-light atom supersonic expansion: ICl/He," Phys. Chem. Chem. Phys., 7, 3323 (2005).
• D.S. Boucher, J.P. Darr, M.D. Bradke, R.A. Loomis*, and A.B. McCoy*, "A combined experimental/ theoretical investigation of the He + ICl interactions: Determination of the binding energies of the T-shaped and linear He•••I35Cl(X,v"=0) isomers ," Phys. Chem. Chem. Phys., 6, 5275 (2004).
• W.E. Buhro*, H. Yu, J. Li, R.A. Loomis, P.C. Gibbons, and L.-W. Wang, "Cadmium selenide quantum wires and the transition from 3D to 2D confinement," J. Am. Chem. Soc., 125, 16168 (2003).