MICRO-XANES - CHEMICAL CONTRAST IN THE SCANNING-TRANSMISSION X-RAY MICROSCOPE

The scanning transmission X-ray microscope on the X1A undulator beam line, at the National Synchrotron Light Source, has been used for imaging various biological as well as polymer samples at 55 nm Rayleigh resolution. The microscope is operated mainly in direct imaging mode, where X-rays of fixed energy are diffractively focussed to a microprobe and the sample is scanned in two dimensions. However, by varying the X-ray energy while keeping the beam focussed to one spot on the sample we can also determine the localized chemical composition of the sample from the carbon X-ray absorption near edge spectra in an area smaller than 0.2 mum by 0.2 mum. The spatial distribution of the chemical constituents can be obtained by imaging at absorption maxima of specific chemical bonds, while retaining the 0.05 mum spatial resolution of the microscope. The chemical sensitivity of X-ray-absorption near-edge spectroscopy provides a powerful contrast mechanism for imaging organic systems. The well-known differences in energy among pi* resonances of different chemical bonds have been used to distinguish different polymer phases and have been applied to biological systems.