FIELD-INDUCED STRUCTURAL CHANGES IN ADSORBED LAYERS OF POLAR MOLECULES STUDIED BY PHOTON-STIMULATED DESORPTION

Photon-stimulated desorption with tunable synchrotron radiation was used to study the influence of a high electric field (≈10 V/nm) on the structure of adsorbed layers of the polar molecules water, ammonia and methanol. It is found that at 80 K, the field induces the growth of water whiskers at the surface. These whiskers are stable only at low temperature : at ≈120 K the structure changes into one from which no photo-desorption is found. This is interpreted as a transition to an ordered structure similar to that of ordinary ice. This structure is probably (100) oriented to the surface, so that the individual dipole moments in the water molecules are all aligned parallel to the external field