Dynamic Optical Spectroscopy and Pyrometry under Optical and X-ray Laser Heating at European XFEL

Experiments accessing extreme conditions at X-ray free electron lasers (XFELs) involve rapidly evolving conditions of temperature. Here we report time-resolved, direct measurements of temperature using spectral streaked optical pyrometry (SOP) of X-ray and optical laser-heated states at the High Energy Density (HED) instrument of the European XFEL. This collection of typical experiments, coupled with numerical models, outlines the reliability, precision, and meaning of time dependent temperature measurements using optical
emission at XFEL sources. Dynamic temperatures above 1500 K are measured continuously from spectrallyand temporally-resolved thermal emission at 450 – 850 nm, with time resolution down to 10 – 100 ns for 1– 200 µs streak camera windows, including in single sweep mode. Targets include zero-pressure foils freestanding in air and in vacuo, and high-pressure samples compressed in diamond anvil cell multi-layer targets. Radiation sources used in the presented examples are 20-femtosecond hard X-ray laser pulses at 17.8 keV, in
single pulses or 2.26 MHz pulse trains of up to 30 pulses, and 250-nanosecond infrared laser single pulses. A range of further possibilities for optical measurements of visible light in X-ray laser experiments using streak
optical spectroscopy are also explored, including for study of X-ray induced optical fluorescence, which often appears as background in thermal radiation measurements. We establish several scenarios where combined
emissions from multiple sources are observed and discuss their interpretation. Challenges posed by using X-ray lasers as non-invasive probes of sample state are addressed.