For many years scientists have tried to understand the unique properties of ice in terms of the behavior of the molecules in the topmost layer. However, despite extensive studies the exact structure and dynamical motion of the individual water molecules at the ice surface have remained elusive. An international team of physicists (J. Braun, A. Glebov, A. P. Graham, A. Menzel) in the group of Peter Toennies at the Max Planck Institute for Fluid Dynamics in Göttingen have used the scattering of very low-energy He atoms for the successful analysis of the structural arrangement of water molecules on the ice surface and have also gained direct information on their vibrational motion. The results of these experiments, published in the March 23 issue of the Physical Review Letters [80, 2638 (1998)], indicate that the molecules are surprisingly mobile which explains many peculiarities in the interactions of ice with its environment.
Why do solid ice crystals have a melted surface-layer at temperatures far below the bulk melting point 0°C, that allows us to ski, skate and slide so easily; Why do two pieces of ice, when put together, adhere and become one; Why are different molecules in the earth stratosphere easily trapped on the surface of ice particles, where they can react, with consequences such as depletion of the ozone layer? This wide variety of intriguing questions have made ice one of the most frequently studied materials. However, until now, no definite answers to these and many other questions have been forthcoming since all of the attempts to gain information on the microscopic structure of a single crystal ice surface have failed. Very recently, even the powerful method of electron diffraction, routinely used in surface structure analysis, failed to provide any clear evidence on the structural arrangement of the topmost layer of ice. The group of scientists from the Lawrence Berkeley National Laboratory, Free University in Amsterdam, and the University of Pierre and Marie Curie in Paris [Surface Science 381, 190 (1997); also see report of Charles Seife in Science 274, 2012 (1996)] have suggested, on the basis of theoretical simulations, that the uppermost water molecules vibrate so strongly that a coherent diffraction pattern cannot be observed.
In the attempt to resolve this problem the researchers in Göttingen have employed low-energy helium atom scattering. This technique has the advantage of being completely nondestructive and exclusively sensitive to the topmost layer of crystals. Since the (111) surface of platinum has nearly the same lattice spacing as ice, it was used as a template on which single crystal ice films of 10-100 nm thickness were grown. Only after cooling the surface to 30 K was it possible to observe a sharp intense series of diffraction peaks. These not only provide information on the lattice spacing and arrangement of the first layer molecules but also indicate at least a partial alignment of the hydrogen atoms (ferroelectric ordering) at the surface.
A further advantage of the He atom scattering technique is that with the same equipment high-resolution time-of-flight energy loss and gain spectra can be measured. These spectra provide information on the frequencies and wave-lengths of the collective vibrations (phonons) at the surface. As the crystal was again cooled down to 30 K, a very intense inelastic peak emerged from a strong multiphonon background. This intense inelastic peak was simulated with a theoretical model which allowed its assignment to a special very large amplitude in-plane shearing motion of the surface molecules. At higher temperatures, this motion becomes increasingly enhanced leading to a high density "phonon bath" and ultimately individual molecules will break away from their original sites. This explains the liquid-like topmost layer as well as the difficulties experienced in the electron diffraction experiments.
This vibrational disorder at the ice surface also explains why two pieces of ice fuse when pressed together. The H2O molecules at the ice crystal surface form hydrogen bonds with those of another ice surface when two crystals are brought in contact, thus, increasing their coordination. This results in a stiffening of the soft surface vibrations, making the interface solid. In addition, the high rate of accommodation of molecules on the surface of ice particles in the stratosphere can also be understood in terms of the facile energy transfer of the molecules with the phonon bath available at the surface. The situation is rather similar to the ping-pong ball dropped onto a concrete floor covered with a soft rubber carpet. Without the carpet the ball would bounce back while the soft rubber overlayer allows the ball to lose all its translational energy permitting it to be accommodated on the surface. Many of the other fascinating properties of ice can also be explained in terms of the enhanced vibrations of water molecules at the surface.
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