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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 of x-ray and optical laser-heated states at the High Energy Density 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 spectrally- and temporally-resolved thermal emission at 450–850 nm, with time resolution down to 10–100 ns for 1–200 μs streak camera windows, using single shot and integrated modes. Targets include zero-pressure foils free-standing in air and in vacuo, and high-pressure samples compressed in diamond anvil cell multi-layer targets. Radiation sources used are 20-fs 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-ns 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 the 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 the sample state are addressed.
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Spectral Pyrometry for Practical Temperature Measurement in the TEM
Recent work in ultra-high temperature in situ electron microscopy has presented the need for accurate, contact-free temperature determination at the microscale. Optical measurement based on thermal radiation (pyrometry) is an attractive solution but can be difficult to perform correctly due to effects, such as emissivity and optical transmission, that must be accounted for. Here, we present a practical guide to calibrating and using a spectral pyrometry system, including example code, using a Czerny-Turner spectrometer attached to a transmission electron microscope. Calibration can be accomplished using a thermocouple or commercial heated sample holder, after which arbitrary samples can be reliably measured for temperatures above ∼600∘C. An accuracy of 2% can be expected with the possibility of sub-second temporal resolution and sub-Kelvin temperature resolution. We then demonstrate this capability in conjunction with traditional microscopic techniques, such as diffraction-based strain measurement for thermal expansion coefficient, or live-video sintering evolution.
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- PAR ID:
- 10594206
- Publisher / Repository:
- Oxford Academic (Microscopy Society of America)
- Date Published:
- Journal Name:
- Microscopy and Microanalysis
- Volume:
- 31
- Issue:
- 1
- ISSN:
- 1431-9276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/mam/ozae114
