X-ray scattering is one of the principal tools used for modern structure determination in chemistry and biology. It is conventional wisdom that hydrogen atoms remain "invisible" to traditional x-ray and electron diffraction techniques. As a result, x-ray studies of many important chemical and biological systems yield only partial structure functions. Experimenters often have to resort to supplementary measurements with other techniques that are more hydrogen sensitive to obtain information that can then be melded with their x-ray data to provide a complete structure picture. For example, the atom-pair-correlation function for liquid water can be obtained only by the synthesis of x-ray and neutron diffraction measurements. Clearly, techniques that directly probe hydrogen atoms in covalent and intermolecular hydrogen bonds could significantly enhance the scope of structure determination in aqueous solutions and biological systems.

Schematic depiction of photoelectron wave backscattered from neighboring atoms.

Mapping the Invisible

From the depths of the ocean to the surface of Venus, we have learned how to map unseen territory with tools such as radar and sonar, which use waves of reflected light or sound to "see" features of a hidden landscape. But what kind of waves could be used to explore the "landscape" inside a molecule? It turns out that an electron ejected from an atom (e.g., by soft x rays) behaves like a wave propagating outward that can be backscattered by nearby atoms. The backscattered portions of the electron wave interfere with the outgoing portions, and the effects of this interference appear as slight oscillations (fine structure) in the x-ray absorption spectrum. Through careful analysis of the extended x-ray absorbtion fine structure (EXAFS) of a substance, scientists can deduce things such as the type, number, and distance of atoms in the vicinity of the absorbing atom. In this work, researchers have applied this technique to gaseous H2O—water vapor—and were able to identify the faint EXAFS signature of hydrogen, which, despite its importance in nature and industry, has otherwise proven "invisible" to traditional probes of molecular structure.

EXAFS refers to small-amplitude oscillations in the x-ray absorption coefficient that can extend hundreds of electron volts above a core-level absorption edge. These oscillations arise from final-state interference effects of backscattered photoelectrons from neighboring atoms. In contrast, an isolated atom exhibits a smooth and essentially structureless absorption background corresponding to the photoionization process induced by a core level excitation. The well-known utility of EXAFS as a structural probe arises from the ability to selectively excite individual atomic species, thereby allowing the local environment (e.g., solvent cage) around a selected absorbing atom to be directly characterized. Since hydrogen possesses a single core electron, the cross section for scattering a photoelectron for a neighboring atom is small, and inter- and intramolecular hydrogen is generally thought to be undetectable in state-of-the-art experiments.

 

LEFT: Raw x-ray absorption spectra of 20-mTorr water vapor recorded at low (1-eV) resolution and high (0.1-eV) resolution (inset).

RIGHT: Normalized EXAFS oscillations for hydrogen in water vapor. The solid curve is the experimental data, and the dashed curve is calculated for isolated water molecules. The inset shows the fast Fourier transform of the experiment data and shows a single scattering distance arising from the covalent O–H bond in water vapor.

In the reported measurements, however, the researchers have now demonstrated that EXAFS experiments on water vapor at the oxygen K edge, conducted on ALS Beamline 9.3.2, have the ability to detect scattering from hydrogen atoms. Experiments were conducted in a typical gas cell backfilled with about 20 mTorr of water vapor. Argon-filled cells were used as control samples. The measured EXAFS oscillations from hydrogen in water vapor are shown along with the calculated values of the oscillations for isolated water molecules obtained with multiple-scattering theory and the FEFF8 ab intio electronic structure code. There is remarkable agreement between experiment and theory, which conclusively demonstrates a single scattering distance arising from the covalent O–H bond in water vapor. In reanalysis of an earlier experiment by others, the researchers used the measured O–H scattering phase shift to demonstrate that O–H bonds can be quantitatively measured in more complex liquids like water.

This work has illustrated the general utility of hydrogen EXAFS as a new structural tool in the elucidation of hydrogen bonds in complex condensed-phase samples. Advances in synchrotron x-ray spectroscopy, coupled with existing computational techniques, imply that observation of hydrogen EXAFS is likely to become routine. The phase-shift function determined in this work can be used to extract quantitative O–H bond distances in other systems, thus becoming a useful complement to existing structural methods in chemistry, biology, and materials science.

Research conducted by K.R. Wilson and R.J. Saykally (University of California, Berkeley); J.G. Tobin (Lawrence Livermore National Laboratory); and A.L. Ankudinov and J.J. Rehr (University of Washington).

Research funding: National Science Foundation. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: K.R.Wilson, J. G. Tobin, A.L. Ankudinov, J.J. Rehr, and R.J. Saykally, "Extended x-ray absorption fine structure from hydrogen atoms in water," Phys. Rev. Lett. 85(20), 4289 (2000).

ALSNews Vol. 183, August 29, 2001