We report the design and performance of a diffractometer which is used on the 53 pole wiggler beamline at the Photon Factory for grazing incidence xhyphen;ray diffraction study of monolayers spread on the surface of water. It is a simple twohyphen;circle (ohgr;hyphen;2thgr;) diffractometer. A Langmuir trough is mounted on a vertical translation stage on the ohgr; axis. A onehyphen;dimensional position sensitive detector is vertically mounted on the 2thgr; arm with a 50hyphen;mmhyphen;high Soller collimator (0.1deg; resolution) in front of it. With this position sensitive detector, the intensity distribution along the Bragg rod is simultaneously recorded during the 2thgr; scan. A saggitally focusing Si(111) doublehyphen;crystal monochromator is used to monochromatize and horizontally focus the xhyphen;ray beam. Xhyphen;ray beams are then reflected downward and vertically focused by a bent mirror. The grazing incidence angle of the xhyphen;ray beam to the surface of water is also controlled by adjusting this mirror. The instrumental angular resolution in the horizontal plane was measured to be 0.17deg;. With this diffractometer, we studied the structure of arachidic acid monolayers spread on surfaces of (i) pure water, (ii) 10minus;4M Cd2+solution, and (iii) 10minus;4M cyanine dye solution. By analyzing the xhyphen;ray intensity distribution along the Bragg rod using a simple model proposed by Kjaeretal. lsqb;K. Kjaer, J. Alshyphen;Nielsen, C. A. Helm, P. Tippmanhyphen;Krayer, and H. Mouml;hwald, J. Phys. Chem.93, 3200 (1989)rsqb; we obtained the following conclusions. In the case on pure water, molecules tilt toward the nearest neighbors at low surface pressures and stand upright at higher pressures. In the presence of Cd2+, molecules stand upright even at a very low pressure. In the presence of cyanine dye in the water, molecules tilt toward the next nearest neighbors at low pressures and stand upright at higher pressures.
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