X-Rays with the Gas Phase

The X-rays enter the analysis chamber typically through a thin window made of 50-100nm thick membranes of Al, Si, or SijN4. Such windows have high transmission (e.g., the transmission of the 100 nm Al window varies from 70% to 90% for photon energies between 400 and 1000 eV) and prevent gases from entering the X-ray source (synchrotron beamline or anode), which needs to be operated in vacuum. The window must be placed as close as possible to the sample to minimize the absorption of X-rays by the gas phase inside the reaction chamber. 

This is especially important for X-ray absorption spectroscopy experiments, where absorption of the X-rays by gas molecules leads to modulations of the incident photon flux at the sample surface and is often difficult to account for. Typical working distances between the X-ray window and the sample are in the centimeter range. 

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