The technique of grazing incidence x-ray absorption spectroscopy is finding growing applications in a number of different scientific disciplines. Important applications include dilute species in thin films, oriented membranes, molecules sorbed to surfaces and ions adsorbed on wet surfaces. For a dilute species on a surface, enhancement in the fluorescence signal is obtained not only by spreading the beam over a larger area but also by exploiting the total reflection of x-rays from the substrate, which significantly increases the signal. In order to achieve this, the sample is held at a grazing angle to the beam, i.e., below the critical angle of the substrate. By using the polarized nature of the beam and by changing the orientation of the sample, information regarding molecular orientations within the specimen can also be obtained. Due to the growing demand for the availability of such an instrument, SSRL has designed and built a grazing incidence x-ray absorption spectroscopy (GIXAS) instrument, which has been successfully commissioned and used during the 1995 run.
The SSRL GIXAS apparatus is designed to be flexible, not only in the nature of the sample which can be studied, but also in its exploitation of the polarized nature of the synchrotron x-ray beam. Thus, an innovative design of a large pivoting cradle, with the beam travelling on axis through the bearing mounts, enables the sample, collimating slits and upstream ion chamber to be rotated in steps between 0 and 90° around the axis of the beam. This allows the examination of polarization at different angles with respect to the sample surface, with minimal realignment required between measurements. The system is constructed with several O-ring seals, allowing an isolated helium path for low-energy spectroscopy studies. Additionally, a cryogenic sample holder attachment is planned for the future.
One of the aims of the GIXAS apparatus was to have all axes under computer control. The motivation for this was twofold: swift realignment of the instrument upon installation at a new beam line and precise and rapid alignment of a new specimen after a sample change; both goals were met. There are a total of 22 stepper motors in the system, including an optical rail alignment, motorized slits collimating in both vertical and horizontal directions, and various axes controlling the position and angle of the sample, including the grazing angle. All of these are manipulated using a single Advanced Control Systems controller/driver system under computer control from the standard SSRL XAS Collect software. Thus, motors or combinations of motors can readily be scanned while monitoring the response of a detector and alignment is not only straightforward but also reproducible.
During 1995, the apparatus was employed in two user experiments, one using GIXAS to look at the nature of transition metal dopants in thin films of titania (Russell Howe, U. New South Wales) and the other using Standing Waves to look at monolayers of biological membranes (Jim Penner-Hahn, U. Michigan). Both user groups reported that the new apparatus worked well for their experiments.
The GIXAS conceptual design was by Ingrid Pickering and mechanical engineering and design were by Robert Mayer. Bill Butler and Scott Jansson carried out mechanical fabrication and assembly while Luc Lessard and Jeff Maggard did electrical assembly. Martin George programmed the software driver to enable use of the ACS controller with his XAS Collect software. This development was supported primarily by DOE Office of Basic Energy Sciences. The GIXAS apparatus will be available to general users during 1996. Anyone interested in requesting it for their experiment should contact Ingrid Pickering