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

The scanning transmission X-ray microscope on the X1A undulator beam l ine, at the National Synchrotron Light Source, has been used for imagi ng various biological as well as polymer samples at 55 nm Rayleigh res olution. The microscope is operated mainly in direct imaging mode, whe re X-rays of fixed energy are diffractively focussed to a microprobe a nd 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 w e 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 cons tituents can be obtained by imaging at absorption maxima of specific c hemical bonds, while retaining the 0.05 mum spatial resolution of the microscope. The chemical sensitivity of X-ray-absorption near-edge spe ctroscopy provides a powerful contrast mechanism for imaging organic s ystems. The well-known differences in energy among pi resonances of d ifferent chemical bonds have been used to distinguish different polyme r phases and have been applied to biological systems.