Abstract Details
Time Resolved X-ray Absorption Spectroscopy of Warm Dense Carbon
Author: Byoung-ick Cho
Requested Type: Poster Only
Submitted: 2009-04-21 11:46:43
Co-authors: P. A. Heimann, J. Feng, K. Engelhorn, C. Weber, H. J. Lee, R. W. Falcone
Contact Info:
Advanced Light Source
Lawrence Berkeley National Lab
Berkeley, CA 94720
USA
Abstract Text:
Although solid carbon is one of the most extensive studied materials, its properties in the warm dense regime are not well known. Especially non-equilibrium warm dense carbon is of special interest relevant to fundamental significance in the phase transitions and energy relaxation processes, and practical importance at the limit of current technologies such as inertial confinement fusion.
Investigating electronic structure would provide a key understanding of warm dense matter. X-ray absorption spectroscopy is a powerful technique for this purpose. However the static method is not compatible with rapidly varying transient state of warm dense matter. Therefore we use time resolved x-ray absorption spectroscopy to study the bonding properties of carbon at a few eV temperature and near solid density.
The experiment was performed at beam line 6.0.2 at Advanced Light Source (ALS). An intense 150 fs, 800 nm laser pulse isochorically heated amorphous carbon and diamond films, creating warm dense carbon for a short time before the materials expand significantly. Heated carbon foils were probed using broadband 70 ps soft x-rays of synchrotron radiation. Laser and x-ray pulses were both spatially and temporally overlapped on the target. Transmitted x-rays were dispersed with a spectrometer with 250 line/mm grating and a spectrum near carbon K-edge (270 eV ~ 320 eV) was detected with an ultrafast x-ray streak camera.
The near edge structure of the carbon K-edge shows π * and σ* anti-bonding features indicating coordination number and strength of interaction between atoms. The time resolved x-ray absorption spectra show the increase of π* and the decrease of σ* resonances with 2 ps time resolution. Evolutions of both resonances have two different time scales which may be related to the time scales of energy relaxation and expansion of super heated solid. Detailed analysis of the spectra and the associated changes in electronic properties of warm dense carbon will be presented.
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