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. The interface represents a unique coordination environment with properties distinct from those of the bulk. In the image charge picture, derived from early surface tension measurements, ions have traditionally been considered to be depleted from the liquid/vapor interface. However, recent works have contradicted this simple picture, finding some ions exhibit enhanced surface concentration, with their respective surface affinities often following the well-known Hofmeister series. Of particular importance is that of the air-water interface as it is involved in many atmospheric reactions. However, it is 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 aqueous interfaces. Under the electric dipole approximation, due to necessary symmetry constraints, these techniques are interface specific and thus give no signal from bulk.
Extending the multiplex homodyne broadband electronic sum frequency generation (ESFG) technique in the visible wavelengths developed by Tahara et al. to the deep ultraviolet (below 250 nm), we recently developed broadband deep-UV electronic sum frequency generation (DUV-ESFG) spectroscopy. By using a white light continuum as one of the input pulses, we can obtain a broadband interfacial electronic spectrum in a single laser shot, allowing us to analyze number, shape, and position of various resonances. Recently, using our broadband DUV-ESFG setup, the CTTS spectrum of iodide at the air-water interface was measured (right). Compared to the bulk, we observed several key differences including a redshift, linewidth narrowing, and differences in the relative intensities of the J=3/2 and J=1/2 bands. Our current work focuses on measuring the spectra of other biologically and/or atmospherically relevant anions, expanding the detection range, and improving the S/N of the system.
To achieve a better understanding of selective ion adsorption, we have developed deep-UV resonant electronic second harmonic generation, which exploits the resonant charge-transfer-to-solvent transitions to directly probe the ion at the interface. Using this technique, we have measured the thermodynamics of ions at the interface and found that, in agreement with predictions, certain ions such as iodide and thiocyanate are indeed enhanced at the air/water interface. Collaborating with theorists, it was determined the driving force for this surface activity to be the energetic gains of moving under-coordinated water from the interface back into the bulk. In the future we hope to expand on these measurements, studying the thermodynamics of ion adsorption at water/metal and other more complex interfaces, with the end goal of re-writing the textbook on how ions behave at the interface.
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2. Yamaguchi, S. & Tahara, T. Precise electronic χ(2) spectra of molecules adsorbed at an interface measured by multiplex sum frequency generation. J. Phys. Chem. B 108, 19079–19082 (2004).
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