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This work presents first insights into the dynamics of free-surface release clouds from dynamically compressed polystyrene and pyrolytic graphite at pressures up to 200 GPa, where they transform into diamond or lonsdaleite, respectively. These ejecta clouds are released into either vacuum or various types of catcher systems, and are monitored with high-speed recordings (frame rates up to 10 MHz). Molecular dynamics simulations are used to give insights to the rate of diamond preservation throughout the free expansion and the catcher impact process, highlighting the challenges of diamond retrieval. Raman spectroscopy data show graphitic signatures on a catcher plate confirming that the shock-compressed PS is transformed. First electron microscopy analyses of solid catcher plates yield an outstanding number of different spherical-like objects in the size range between ten(s) up to hundreds of nanometres, which are one type of two potential diamond candidates identified. The origin of some objects can unambiguously be assigned, while the history of others remains speculative.

High energy density physics 32, 63 - 69 (2019). doi:10.1016/j.hedp.2019.05.002, We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) X-ray free electron laser (XFEL) facility, using an X-ray pump, X-ray probe scheme to observe ultrafast changes in the structure of heated graphite. The 9.8 keV XFEL beam was focused to give an intensity on the order of ∼ 10$^{19}$ W/cm$^2$, and the evolution of the diffraction pattern observed up to delays of 300 fs. The interplanar diffraction peaks weaken significantly within 10s of femtoseconds, but in-plane diffraction orders i.e. those with Miller Index $(hk0)$, persist up to 300 fs, with the observed signal increasing. We interpret this as nonthermal damage through the breaking of interplanar bonds, which at longer timescales leads to ablation by removal of intact graphite sheets. Post-experiment examination of the graphite samples shows damage which is comparable in size to the range of the excited photoelectrons. These results highlight the challenges of accurately modelling X-ray driven heating, as it becomes a routine approach to generating high energy density states., Published by Elsevier, Amsterdam [u.a.]

The effects of hydrocarbon dissociation and subsequent diamond precipitation on the internal structure and evolution of icy giant planets like Neptune and Uranus have been discussed for more than three decades. Inside these celestial bodies, gravity compresses mixtures of light elements to densities of several grams per cubic centimeter while the temperature reaches thousands of Kelvins resulting in thermal energies on the order of chemical bonding and above. Under these conditions, simple hydrocarbons like methane, which are highly abundant in the atmospheres of these planets, are believed to undergo structural transitions that release molecular hydrogen from deeper layers and may lead to compact stratified cores. Indeed, the isentropes of Uranus and Neptune intersect temperature-pressure conditions where first polymerization occurs, and then, in deeper layers, a phase separation into diamond and hydrogen may be possible. Here we show experimental evidence for this phase separation process obtained by in situ X-ray diffraction from polystyrene samples dynamically compressed to 150GPa and 5000 K, which resembles the environment ~10,000 km below the surfaces of Neptune and Uranus. Our findings demonstrate the necessity of high pressures for initiating carbon-hydrogen demixing and imply that diamond precipitation may require ~10x higher pressures than previously suggested by experiments investigating non-isolated hydrocarbons. Besides underlining the general importance of chemical processes inside giant planets, these results will inform evolutionary models of Uranus and Neptune, where carbon-hydrogen demixing can be a significant source for the convection necessary to explain their unusual magnetic fields. Additionally, our experiment demonstrates an alternative path for producing nanodiamonds for scientific and industrial applications that may be superior to current methods using oxygen-deficient explosives.

We present the study of optical and spectral properties of radiation-induced stable point defects, known as color centers (CCs), in lithium fluoride (LiF) for the detection of 10 keV XFEL beam at Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan. A thick LiF crystal was irradiated in four spots with 10 keV XFEL beam (pulse duration = 10 fs) with different number of accumulated shots. After irradiation the colored-LiF spots were characterized with an optical microscope in fluorescence mode and their photoluminescence intensity and spectra were analyzed.