|Isotope and Temperature Effects in Liquid Water Probed by Soft X Rays|
The geometric structure of liquid water has been investigated in detail by many techniques, but many details are still under debate, such as the actual number of hydrogen bonds (at a given time) between the various water molecules. Even less is known about the electronic structure. Since it is the intermittent bonding between water molecules that gives liquid water its peculiar characteristics, the electronic structure plays a crucial role in understanding the properties of the liquid state. Consequently, information essential for insight into chemical and biological processes in aqueous environments is lacking. To address this need, researchers from Germany and the U.S. have used soft x-ray spectroscopy at the ALS to gain detailed insight into the electronic structure of liquid water. Their spectra show a strong isotope and a weak temperature effect, and, for the first time, a splitting of the primary emission line in x-ray emission spectra. By making use of the internal "femtosecond clock" of the core-hole lifetime, a detailed picture of the electronic structure can be painted that involves fast dissociation processes of the probed water molecules.
In the past, the investigation of liquids has been mostly limited to experiments focusing on structural information. In contrast, the study of the electronic structure of liquids is a technical challenge that only began to be met in 2002 with the first publications about high-resolution x-ray spectroscopy of water. Recently, the German-American team used soft x-ray emission (XES) and absorption spectroscopy (XAS) to successfully probe the electronic structure of liquids. These element-specific techniques probe occupied and unoccupied electronic states, respectively, and thereby provide deep insight into the local chemical environment of the selected atomic species. The experiments were performed at ALS Beamline 8.0.1 using a custom-designed liquid-flow-through cell with a 100-nm-thick silicon nitride membrane.
The team found that their high-resolution x-ray absorption and emission spectra of normal water (H2O) and deuterated water (D2O) exhibited a strong isotope effect (i.e., a variation between the H2O and D2O spectra). Furthermore, the XES spectra of both showed a splitting of the 1b1 emission line, a weak temperature effect, and a pronounced dependence on the excitation energy. The latter can be best seen in a novel resonant-XES (RIXS) map representation.
The XES spectra can be well described as a superposition of two independent components, but an explanation based solely on the existence of two distinctly different hydrogen-bond configurations in the water is contradicted by the direction and the strength of the observed isotope effect. Instead, comparison of the XES spectra of liquid H2O and D2O, gas-phase water, ice, and aqueous NaOH and NaOD solutions suggests that an ultrafast dissociation of the water molecules is induced by the x-ray excitation and plays an important role for the spectral shape. In this interpretation, the high-energy component of 1b1 is ascribed to intact water molecules, while the low-energy component is related to dissociated water molecules (i.e., one proton is removed). The temperature-dependence is interpreted such that local hydrogen bonds promote the dissociation process. If the temperature is increased, more hydrogen bonds are broken, and consequently the team observed that the fraction of water molecules dissociated by the x-ray beam decreased with increasing temperature.
Thus, additional pieces can be contributed to the liquid-water puzzle: dynamics play an important role in understanding not only the ground-state properties but also the interaction of water with soft x rays. Comparing normal and deuterated water allows us to gain insight into the femtosecond dissociation dynamics of water molecules in a liquid environment, where the involvement of a specific H (or D) atom in a bond promotes the dissociation of the corresponding water molecule. Further experiments will be needed to fully unravel the local geometric and electronic structure of hydrogen-bonded water networks.
Research conducted by O. Fuchs, M. Blum, M. Weigand, F. Maier, and E. Umbach (University of Würzburg, Germany); L. Weinhardt, M. Bär, and C. Heske (University of Nevada, Las Vegas); M. Zharnikov, Y. Zubavichus, and M. Grunze (University of Heidelberg, Germany); W. Yang and J.D. Denlinger (ALS).
Research funding: German Federal Ministry of Education and Research. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.
Publications about this research: O. Fuchs, M. Zharnikov, L. Weinhardt, M. Blum, M. Weigand, Y. Zubavichus, M. Bär, F. Maier, J.D. Denlinger, C. Heske, M. Grunze, and E. Umbach, "Isotope and temperature effects in liquid water probed by x-ray absorption and resonant x-ray emission spectroscopy," Phys. Rev. Lett. 100, 027801 (2008) and Phys. Rev. Lett. 100, 249802 (2008).