45697-AC6
Water Structure Around Hydrophobic Solutes by Femtosecond 2D-Vibrational Spectroscopy
2D-NMR has become a powerful tool because of its ability to measure nonbonded molecular contacts. However, it is limited to long-lived contacts. 2D-IR is a new and rapidly developing field endeavoring to overcoming this lifetime limitation by detecting couplings between non-bonded vibrators. Even these methods have problems with the problem that is the focus of this proposal: detecting the spectrum, and thereby the structure, of water in contact with a hydrophobic solute. The intense IR absorption of water precludes 2D-IR experiments on dilute aqueous solutions. Another major technical advance is needed. A mixed IR-Raman 2D spectroscopy named doubly vibrationally enhanced (DOVE) spectroscopy has been recently developed by Wright. DOVE spectroscopy could substitute a Raman transition of the water vibration for the overly strong IR transition. To date, this experiment has been implemented with long pulses (ns or ps), with consequent reductions in sensitivity and complications in interpretation. As a result, only intramolecular coupling have been measured. This project is extending DOVE spectroscopy to use femtosecond pulses. The gain in sensitivity is needed to detect the intermolecular coupling between a solute and its solvation shell of water. The first phase of this project was to solve a general problem—how to measure well resolved Raman spectra with femtosecond pulses. The conventional view has been that this problem is unsolvable due to the fact that femtosecond pulses have broad (300 cm-1) spectra. We solved this problem theoretically and demonstrated experimental success in fs-CARS experiments by introducing the concept of simultaneous detection in both time and frequency domains. These concepts provide an essential foundation of extending DOVE experiments to femtosecond pulses. The second phase of the project was to design and construct a new experimental set-up to generate the required pulses and combine them at the sample with the correct timing, polarization and angles. Pulses at three distinctly different frequencies are required. The first is in the vibrational IR and excites the solute vibration. The second is in the near IR and acts on a solute-solvent combination band to transfer coherence to the water. The third pulse is in the visible does CARS scattering from the water. Because these pulses are in disparate frequency ranges, the creation, characterization and detection of each pulse and the manipulation of the pulses through a common set of optics presents a significant challenge. This challenge has been met, and we can successfully overlap all the required pulses to the sample while retaining good pulse properties. At this time, we are stalled at the point of detecting the desired signal from mixing of all three pulses. However, our characterization studies have identified three areas that need improvement. First, with the large phase-matching angles required, femtosecond pulses show substantial tilt at the sample. Gratings will be added to the set-up to compensate for this problem. Second, a more sensitive photon-counting detector will replace the current diode array. Finally, our laser power is substantially degraded by aging of the pump diodes. Funds have been found to replace these diodes in the near future. All these improvements and the first successful demonstration of fs-DOVE detection of solute-water couplings are scheduled for a one-year extension of the project period.
