P. Jungwirth, D. J. Tobias, J. Phys. Chem. B 105, 10468 (2001)
Interfacial chemistry can be found in myriad environments of scientific significance including biological membranes, ocean and atmospheric chemistry, and electrochemistry. In fact, molecular behavior at the interface can be drastically different than in the bulk. Of particular import is that of the air-water interface as it is involved in many atmospheric reactions. However, it is quite difficult to probe the interface, because the interfacial signal tends to be overwhelmed by the bulk signal, and thus not readily discernible. Fortunately with the advent of ultrafast laser spectroscopy, we are now able to employ second order nonlinear optical techniques, Sum Frequency Generation (SFG) and Second Harmonic Generation (SHG), to probe ions and molecules at the air-water interface. Due to necessary symmetry constraints, these techniques are interface specific and thus give no signal from bulk.
Currently, we are developing a technique which will provide heterodyne detected ultra-broadband UV-SFG spectra. Because we are using ultra-broadband pulses, we are able to obtain a full spectrum in a single laser shot, allowing us to analyze number, shape, and position of various resonances. This technique can not only determine the concentration of ions and molecules at the air-water interface, but their orientation as well, thus providing brand new information about ions and molecules at the air water interface.
Yamaguchi, S.; Tahara, T. Heterodyne-Detected Electronic Sum Frequency Generation: 'Up' vs 'Down' Alignment of Interfacial Molecules. J. Chem. Phys. 2008, 129.
Nihonyanagi, S.; Singh, P.C.; Yamaguchi, S.; Tahara, T. Ultrafast Vibrational Dynamics of a Charged Aqueous Interface by Femtosecond Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation. Bull. Chem. Soc. Jpn. 2012, 85, 758-760.