The ion distribution in an electrolyte solution in contact with a charged polymerized phospholipid membrane was directly measured with long-period x-ray standing waves. The 27-angstrom-thick lipid monolayer was supported on a tungsten/silicon mirror. X-ray standing waves were generated above the mirror surface by total external reflection of a 9.8-kiloelectron volt x-ray beam from a synchrotron undulator. The membrane surface, which contained negatively charged phosphate headgroups, was bathed in a dilute ZnCl2 solution. The concentration of Zn2+ in the condensed layer at the membrane surface and the Zn2+ distribution in the diffuse layer were measured as a function of headgroup charge. The Debye length of the diffuse layer varied between 3 and 58 angstroms. The results qualitatively agree with the Gouy-Chapman-Stern model.

Structural information on an atomic scale has been obtained for a Langmuir-Blodgett (LB) trilayer system by means of long-period x-ray standing waves. The LB trilayer of zinc and cadmium arachidate was deposited on a layered synthetic microstructure (LSM) consisting of 200 tungsten/silicon layer pairs with a 25 A period. A 30 A thermally induced inward collapse of the zinc atom layer that was initially located in the LB trilayer at 53 A above the LSM surface has been observed. The mean position and width of the zinc atom layer was determined with a precision of +/- 0.3 A.

No abstract

was taken at the same time. The OWG electrode had sheet resistance of ca. 500 fi/cm1 2 and donor density of ca. 102O/cm3 ([SbCl3] in the spray solution = 2.5 wt %); the optical path length was 1 cm.

We present details of the theory and experimental observation of dynamical diffraction of x rays at grazing angle from crystal planes normal to a surface. We are able to associate different features of the specularly reAected and diffracted-reAected beam cruxes with the contributions from the a and P branches of the dispersion surfaces. The theory predicts surface propagation modes to which internal and external beams can couple only through the diffraction process. An experiment is described in which the specularly reAected and rejected-diffracted beams were simultaneously observed for 8-keV x rays incident on germanium. The agreement with first-order theory is good, but we observe systematic deviations. We present calculations that illustrate how eigenstates of thewave fields, which are x-ray standing waves with nodal planes normal to the surface of the crystal, can be used to obtain atomic registration at a surface or interface.