From 14-23 November 99, I participated in the commissioning of the new liquid surface spectrometer and Langmuir trough at Beamline 9-ID of the Advanced Photon Source. At various times, the commissioning also involved Ben Ocko and Elaine Dimasi (Brookhaven National Lab), my groupmates J. Kent Blasie, Songyan Zheng and Andrey Tronin (U. Pennsylvania), Jarek Majewski (Los Alamos National Lab) and Eric Sirota (Exxon), as well as the staff of CMC-CAT, Thomas Gog, Arun Bommannavar and Chitra Venkataraman.
We collected reflectivity from the free surface of the pure water subphase held within the Langmuir trough. Reflectivity from monolayers will have to wait until the next run. Without a block to put into the subphase to help damp out oscillations in the water surface, it was very difficult to measure the reflectivity signal without a lot of noise, but we did succeed in measuring reflectivity from the clean water surface out to 1.0 Å-1, as shown in the plot below. The blue curve is the Fresnel function, the red curve shows the expected behavior for an interface of width ~2.5 Å. We obtain this value for the interfacial width from the inset, which shows the natural log of the normalized reflectivity plotted versus qz2. The slope of the line is the square of the width of the interface.
We also collected GID data from a monolayer of arachidic acid on a pure water subphase compressed to a surface pressure of 15 mN/m. (The movement of the sample two-theta arm does not induce oscillations in the surface of the monolayer, so we could perform these measurements without a block in the subphase.) We measured GID signals in excess of 104 counts/sec. (In the plot below, we counted for 2 sec at each point.)
Using a different sample chamber and scattering from the free surface
of eicosane, we demonstrated the feasibility of a resonant x-ray reflectivity
measurment. The spectrometer required virtually no realignment for energy
changes of ± 100 eV, the typical energy range required for a resonance
experiment. Even when we changed from 8.15 keV to the iron K-edge,
7.112 keV, the monochromator afforded us sufficient freedom to steer the
beam through the entrance slits of the spectrometer, and we could observe
the reflection of the beam from the liquid surface without further
realignment of the spectrometer. On our next run, we hope to apply this
unique capability of the CMC-CAT LSS to determine the position of metal
atoms within the profile structure of Langmuir films of synthetic peptides
that bind prosthetic groups (see my homepage for more information about
these peptides).