Water is the most important substance on the planet. Its unique and versatile hydrogen bond network underlies many of the processes responsible for life as we know it. Yet, and despite centuries of study, vital questions regarding the intrinsic nature of water remain unanswered. One route to this knowledge is via the development of a "universal first principles" water model. Such models are currently under development via collaborations between our group and theoreticians, based on high precision terahertz spectra and detailed calculations of water clusters. The bend and stretching vibrations of the hydrogen bonds, with the associated quantum tunneling motions, provide a very sensitive measure of the water potential surface. 250. Keutsch, F. N., and Saykally, R. J., “Water Clusters: Untangling the mysteries of the liquid, one molecule at a time”, PNAS 98, 10533-10540 (2001).
Previously, our water cluster spectra (up to hexamers) were measured using terahertz (Thz) sources with sufficiently high power for high precision spectroscopy, viz. CO2 laser-pumped FIR lasers. Over the past decade, technological improvements in terahertz radiation sources have pushed the limits of what is possible. 121. Blake, G. A., Laughlin, K. B., Cohen, R. C., Busarow, K. L., Gwo, D. -H., Schmuttenmaer, C. A., Steyert, D. W., and Saykally, R. J., “The Berkeley Tunable Far Infrared Laser Spectrometers,” Rev. Sci. Inst. 62, 1701 (1991).
Solid-state Schottky diode frequency multipliers currently produce output powers on the order of 10 μW over a broad range of frequencies (i.e. 150 GHz tunability for a given multiplier array). In ongoing collaborations with Prof. Peter Siegel (CalTech/JPL), we have performed spectroscopy studies of water clusters using advanced Thz source technology suitable for this purpose. We have also established collaborations with Dr. Alan Lee and his company LongWave Phontonics, using arrays of quantum cascade lasers (QCLs). These devices, while well-established in the mid-IR, are still experimental in the THz region, but provide much higher power (tens of milliwatts) with continuous tunability and operate at higher frequencies. These two technologies are thus complementary. 395. Lin, W., Steyert, D. W., Hlavacek, N. C., Mukhopadhyay, A., Page, R. H., Siegel, P. H., Saykally, R. J. "Terahertz Vibration-Rotation-Tunneling Spectroscopy of the Propane-Water Dimer: the ortho--state of a 20 cm-1 Torsion" Chem. Phys. Lett., 614, 167-171 (2014). 405. Cole, W. T. S., Hlavacek, N. C., Lee, A. W. M., Kao, T.-Y., Hu, Q., Reno, J. L., Saykally, R. J., "A Terahertz VRT Spectrometer Employing Quantum Cascade Lasers" Chem. Phys. Lett., 638, 144-148 (2015).
There two directions to our current water cluster spectroscopy experiments:
1) Extending measurements to larger clusters, seeking to refine descriptions of hydrogen bond cooperativity; 411. Cole, W. T. S., Farrell, J. D., Wales, D. J., Saykally, R. J. "Structure and torsional dynamics of the water octamer from THz laser spectroscopy near 215 μm" Science, 352, 1194-1197 (2016) (SI).
2) Measuring key hydrogen bond vibrations that have not previously been accessible, e.g. out of plane librations; 406. Cole, W. T. S., Fellers, R. S., Viant, M. R., Leforestier, C., Saykally, R. J. "Far-Infrared VRT Spectroscopy of the Water Dimer: Characterization of the 20 μm Out-of Plane Librational Vibration" J. Chem. Phys., 143, 154306 (2015).