Photoelectric charge from metallic filters: An online XUV pulse energy diagnostics

Extreme ultraviolet (XUV) radiation is a tool of choice for studying ultrafast processes and atomic physics. Most experiments employing sources of XUV radiation, such as high harmonic generation (HHG) or x-ray lasers, benefit from knowing the number of photons delivered to target in every single shot, because of the possible shot-to-shot pulse fluctuation of the sources. Nonetheless, many setups lack noninvasive XUV pulse energy diagnostics, hindering the simultaneous measurement of pulse energy and utilization of the XUV beam for applications. We present an online XUV pulse energy monitoring method based on the detection of photoelectric charge from thin metallic foil filters transmitting the XUV beam, which can be easily implemented at every pulsed XUV source that includes a high-pass filter system in the form of metallic filters. Consequently, implementation of our method is as straightforward as connecting the filter to an oscilloscope. In the paper, we describe all the physics aspects of such measurement and show the dependence of measured photoelectric charge on the incident pulse energy. To prove the versatility of our approach, we performed this measurement on two different high-flux HHG beamlines, taking consecutive shots at 1 kHz.