XANES and EXAFS in Cu-Ti and Ni-Zr glasses

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements have been done at the K-edge of Cu in Cu-Ti glasses and on the K-edges of Ni and Zr in Ni-Zr glasses using a synchrotron radiation source. The results are discussed in terms of the shape shift and intensity of the absorption edge as well as the principal absorption maximum. The values of bondlength calculated by the one-electron multiple scattering XANES theory as well as the graphical analysis EXAFS technique show good agreement.     

Dynamic optical property changes: implications for reflectance feedback control of photocoagulation.

During laser treatment, coagulation affects the optical properties of the tissue. In particular, the formation of a white lesion significantly increases the scattering coefficient. This change in the optical properties in turn affects the laser light distribution in the tissue. The white lesion formed during photocoagulation of the retina has a dynamic effect upon reflection and fluence rate. This problem has been simulated on a model medium consisting of a thin absorbing layer covered with a 1 cm thick layer of albumin. The albumin layer is subdivided into coagulated (white) and uncoagulated (clear) layers. The optical properties of each layer have been determined and these values have been used to model light distribution in the medium. One-dimensional adding-doubling and three-dimensional Monte Carlo methods have provided light distributions in the medium for varying thicknesses of the coagulated albumin. Computed fluence reaching the absorbing layer decreased in the presence of a 275 microns or thicker coagulated layer. The coagulated layer attenuates light because it is highly scattering; however, this scattering also leads to a sub-surface peak in fluence rate at a level higher than the incident fluence. The latter effect outweighed the former for coagulated layer thicknesses less than 275 microns. Computed reflectance of argon laser light from a semi-infinite coagulated region initially increased linearly as a function of thickness. As the coagulation thickness increased beyond 4-5 optical depths, the reflectance approached a constant value, R infinity, at 9 optical depths (2 mm). Experimentally measured total reflectance is shown to be an inadequate indicator of the thickness of a lesion (finite coagulated volume); however, central reflectance from a lesion measured with a CCD camera confirmed the computed trends. These results provide a theoretical foundation for control of lesion thickness using reflectance images.     

Preliminary results on reflectance feedback control ofphotocoagulation in vivo

The size of therapeutic laser-induced retinal lesions is critical for effective treatment and minimal complications. Due to tissue variability, the size of a lesion that results from a given set of laser irradiation parameters cannot be predicted. Real time feedback control of lesion size is implemented based on two-dimensional reflectance images acquired during irradiation. Preliminary results of feedback controlled lesions formed in pigmented rabbits demonstrate an ability to produce uniform lesions despite variations in tissue absorption or changes in laser power     

XANES and EXAFS study of some metal-metalloid glasses

XANES and EXAFS techniques are proving very popular in the study of local environment in disordered systems. Results of such studies in a large number of metal (Fe, Co, Ni, etc)-metalloid (B, Si, C, etc) glasses are reported. Experiments were done with synchrotron radiation as well as an x-ray tube. The values of bond lengths and co-ordination numbers computed from one-electron single scattering Fourier transform method turn out substantially smaller. The values of bondlength determined from the other EXAFS calculation method and the multiple-scattering computation scheme show good agreement. Importance of choice of suitable reference materials for analysis of data is emphasized. The experimental work was done at EXAFS 5.1 Station at Daresbury Laboratory, U.K.