Speed of sound in methane under conditions of planetary interiors
Thomas G. White, Hannah Poole, E. E. McBride, M. Oliver, Adrien Descamps, L. B. Fletcher, William A. Angermeier, Cameron H. Allen, Karen Appel, F. P. Condamine, C. B. Curry, Francesco Dallari, S. Funk, Eric Galtier, E. J. Gamboa, M. Gauthier, P. Graham, Sebastian Goede, Daniel Haden, Jongjin B. Kim, Hae Ja Lee, Benjamin K. Ofori-Okai, Scott Richardson, Alex Rigby, Christopher Schoenwaelder, Peihao Sun, Bastian Witte, T. Tschentscher, U. Zastrau, Bob Nagler, J. B. Hastings, G. Monaco, D. O. Gericke, S. H. Glenzer, G. Gregori
Abstract
We present direct observations of acoustic waves in warm dense matter. We analyze wave-number- and energy-resolved x-ray spectra taken from warm dense methane created by laser heating a cryogenic liquid jet. X-ray diffraction and inelastic free-electron scattering yield sample conditions of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>0.3</a:mn><a:mo>±</a:mo><a:mn>0.1</a:mn></a:mrow></a:math> eV and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>0.8</b:mn><b:mo>±</b:mo><b:mn>0.1</b:mn></b:mrow></b:math> g/<c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msup><c:mi mathvariant="normal">cm</c:mi><c:mrow><c:mo>−</c:mo><c:mn>3</c:mn></c:mrow></c:msup></c:math>, corresponding to a pressure of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mo>∼</e:mo><e:mn>13</e:mn></e:math> GPa. Inelastic x-ray scattering was used to observe the collective oscillations of the ions. With a highly improved energy resolution of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mo>∼</f:mo><f:mn>50</f:mn></f:math> meV, we could clearly distinguish the Brillouin peaks from the quasielastic Rayleigh feature. Data at different wave numbers were utilized to derive a sound speed of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mn>5.9</g:mn><g:mo>±</g:mo><g:mn>0.5</g:mn></g:mrow></g:math> km/s, marking a high-temperature data point for methane and demonstrating consistency with Birch's law in this parameter regime. Published by the American Physical Society 2024