Two new soft X-ray scanning transmission microscopes located at the Advanced Light Source (ALS) have been designed, built and commissioned. Interferometer control implemented in both microscopes allows the precise measurement of the transverse position of the zone plate relative to the sample. Long-term positional stability and compensation for transverse displacement during translations of the zone plate have been achieved. The interferometer also provides low-distortion orthogonal x, y imaging. Two different control systems have been developed: a digital control system using standard VXI components at beamline 7.0, and a custom feedback system based on PC AT boards at beamline 5.3.2. Both microscopes are diffraction limited with the resolution set by the quality of the zone plates. Periodic features with 30 nm half period can be resolved with a zone plate that has a 40 nm outermost zone width. One microscope is operating at an undulator beamline (7.0), while the other is operating at a novel dedicated bending-magnet beamline (5.3.2), which is designed speci®cally to illuminate the microscope. The undulator beamline provides count rates of the order of tens of MHz at highenergy resolution with photon energies of up to about 1000 eV. Although the brightness of a bending-magnet source is about four orders of magnitude smaller than that of an undulator source, photon statistics limited operation with intensities in excess of 3 MHz has been achieved at high energy resolution and high spatial resolution. The design and performance of these microscopes are described.

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The C 1s and O 1s X-ray absorption spectra of poly(ethylene terephthalate) (PET) have been recorded using transmission, fluorescence, and electron yield detection. The corresponding electron energy loss spectra (EELS) have been recorded in a scanning transmission electron microscope. These results are compared to the C 1s and O 1s spectra of gas phase 1,4-dimethyl terephthalate (the monomer of PET) recorded using EELS. The comparison of monomer and polymer materials in different phases and with different techniques has aided the understanding of the relative strengths and limitations of each technique as well as assisting the spectral interpretation. Good agreement is found in the overall shape and the energies of the spectral features. Relatively minor differences in intensities can be understood in terms of the properties of the individual spectroscopic techniques. The critical dose for radiation damage by 100 keV electrons incident on PET at 100 K is found to be (1.45 ( 0.15) × 10 3 eV nm -3 . In contrast, the critical dose for radiation damage by 302 eV X-rays incident on PET at 300 K is (1.2 ( 0.6) × 10 4 eV nm -3 . A figure of merit involving the product of critical energy dose and spectral efficiency (as expressed by the appropriate G value) is developed. This indicates that, for near-edge studies involving a 20 eV spectral width, there is ∼500-fold advantage of X-ray absorption studies on room temperature PET relative to electron energy loss studies of cooled PET.