Research in the Allen group focuses on molecular structure, molecular orientation, chemical reaction mechanisms, and reaction kinetics at gas / liquid, gas / solid, and liquid / solid interfaces. Physical and chemical phenomena that exist and occur at interfaces are critically important to many applications, e.g. heterogeneous environmental/atmospheric processes, materials (surface properties thereof), and biological membranes. In the Allen lab, state-of-the-art nonlinear optical technologies are utilized to conduct interface-selective studies. We develop and utilize vibrational broad bandwidth sum frequency generation (BBSFG) and scanning SFG technology. BBSFG arises from the mixing of two laser pulses, a 800 nm picosecond kHz pulse and a tunable infrared 85 femtosecond kHz pulse, temporally and spatially to produce the sum of the two input frequencies. The intensity of the coherent SFG response is dependent on relative molecular orientation, vibrational resonance with the surface molecules, and the noncentrosymmetric nature of the interface of interest. We detect and resolve the wavelength of the SFG photons (the BBSFG spectrum) by dispersing the component wavelengths and collect with CCDs. Students in the Allen Lab design and conduct experiments to elucidate interfacial structure at the molecular-level.
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