We present the first results from studies of liquid water microjets by soft X-ray absorption spectroscopy. Near the oxygen K-edge (∼530 eV) a fine-structure pattern very similar to that found for gaseous water monomers is observed when the surface-selective total ion yield (TIY) is measured, but a broadened and blue-shifted spectrum emerges when detecting the bulk-sensitive total electron yield (TEY). TIY EXAFS measurements produce a nearest neighbor O-O distance for surface molecules (3.00 Å) slightly longer than that of the isolated water dimer (2.98 Å), whereas the O-O distance extracted from TEY EXAFS corresponds to that accepted for bulk water (2.85 Å). Together, these results evidence an equilibrium liquid water surface dominated by water molecules interacting weakly at longer distances than in the bulk, thus supporting predictions from computer simulations.

We present a combined experimental/computational study of the near-edge x-ray absorption fine structure of the liquid water surface which indicates that molecules with acceptor-only hydrogen bonding configurations constitute an important and previously unidentified component of the liquid/vapour interface. A detailed microscopic picture of the liquid water surface underlies many important phenomena, ranging from the terrestrial CO 2 and H 2 O cycles to surface wetting and ice formation. Surface hydrogen bond configurations, which determine important interfacial properties, e.g. surface tension and interfacial mobility, remain incompletely characterized. Molecular dynamics (MD) simulations of water molecules residing in the ∼5 Å [1] liquid-vapour interface have revealed an overall relaxation of bulk properties (e.g. dipole moment, geometry, diffusion constant, density) toward gas-phase values, precipitated by the disintegration of the three-dimensional liquid hydrogen bond network [2-5]. Sum frequency generation (SFG) studies have provided the first important experimental insights into the molecular details of the liquid water surface. Shen and co-workers [6] report a vibrational band slightly red-shifted from the gas-phase symmetric stretch of water vapour consistent with a free O-H oscillator. They further concluded that >20% of the surface molecules are oriented with one free O-H bond extending out of the surface by about 38 • .

Articles you may be interested inThe liquid surface of chiral ionic liquids as seen from molecular dynamics simulations combined with intrinsic analysis J. Chem. Phys. 139, 214701 (2013); 10.1063/1.4833335 Local vibrational properties of GaAs studied by extended X-ray absorption fine structure A nonadditive methanol force field: Bulk liquid and liquid-vapor interfacial properties via molecular dynamics simulations using a fluctuating charge model J. Chem. Phys. 122, 024508 (2005); 10.1063/1.1827604Thioethanol on Cu(111) at room temperature: A near edge x-ray absorption fine structure and x-ray photoelectron spectroscopy study X-ray absorption spectroscopy is a powerful probe of local electronic structure in disordered media. By employing extended x-ray absorption fine structure spectroscopy of liquid microjets, the intermolecular O-O distance has been observed to undergo a 5.9% expansion at the liquid water interface, in contrast to liquid methanol for which there is a 4.6% surface contraction. Despite the similar properties of liquid water and methanol ͑e.g., abnormal heats of vaporization, boiling points, dipole moments, etc.͒, this result implies dramatic differences in the surface hydrogen bond structure, which is evidenced by the difference in surface tension of these liquids. This result is consistent with surface vibrational spectroscopy, which indicates both stronger hydrogen bonding and polar ordering at the methanol surface as a consequence of ''hydrophobic packing'' of the methyl group.

Soft x-ray absorption spectroscopy is a powerful probe of surface electronic and geometric structure in metals, semiconductors, and thin films. Because these techniques generally require ultrahigh vacuum, corresponding studies of volatile liquid surfaces have hitherto been precluded. We describe the design and implementation of an x-ray experiment based on the use of liquid microjets, permitting the study of volatile liquid surfaces under quasi-equilibrium conditions by synchrotron-based spectroscopy. The liquid microjet temperatures are also characterized by Raman spectroscopy, which connects our structural studies with those conducted on liquid samples under equilibrium conditions. In recent experiments, we have observed and quantified the intermolecular surface relaxation of liquid water and methanol and have identified a large population of ''acceptor-only'' molecules at the liquid water interface.

We have measured the X-ray absorption (XA) spectrum of liquid (298 K) methanol at the oxygen and carbon K edges. The 4a 1 orbital at the O K edge exhibits a pronounced sensitivity to the formation of intermolecular hydrogen bonds, with significant differences observed between the vapor and bulk spectra, whereas the C K edge reveals only subtle corresponding spectral changes. Comparison with DFT computed spectra of model methanol clusters indicates that the bulk liquid comprises long chains (n > 6) and rings of hydrogen-bonded monomers.

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