Tracking the ultrafast XUV optical properties of x-ray free-electron-laser heated matter with high-order harmonics

Gareth O. Williams (1, 2) , S. Künzel (3, 2) , S. Daboussi (4) , B. Iwan (5, 6) , A. Gonzalez (7, 8) , W. Boutu (8, 7) , V. Hilbert (6) , U. Zastrau (9) , H. Lee (10) , B. Nagler (11) , E. Granados (11) , E. Galtier (11) , P. Heimann (12) , B. Barbrel (13) , G. Dovillaire (14) , R. Lee (15) , J. Dunn (15) , V. Recoules (16) , C. Blancard (16) , P. Renaudin (16) , A. de La Varga (17) , P. Velarde (17) , P. Audebert (13) , H. Merdji (8, 7) , Ph. Zeitoun (4) , M. Fajardo (18)

1 GoLP/Instituto de Plasmas e Fusão Nuclear-Laboratório Associado
2 IST - Instituto Superior Técnico, Universidade Técnica de Lisboa
3 Inst Super Tecn, GoLP Inst Plasmas & Fusao Nucl
4 LOA - Laboratoire d'optique appliquée
5 Institute of optics and quantum electronics
6 Friedrich-Schiller-Universität = Friedrich Schiller University Jena [Jena, Germany]
7 LIDyl - Laboratoire Interactions, Dynamiques et Lasers (ex SPAM)
8 ATTO - Attophysique
9 XFEL - European XFEL GmbH
10 CWR - Centre for Water Research
11 SLAC - SLAC National Accelerator Laboratory
12 LBNL - Lawrence Berkeley National Laboratory [Berkeley]
13 LULI - Laboratoire pour l'utilisation des lasers intenses
14 Imagine Optic
15 LLNL - Lawrence Livermore National Laboratory
16 DAM/DIF - DAM Île-de-France
17 Instituto de Fusión Nuclear
18 GoLP/Instituto Superior Tecnico

We present measurements of photon absorption by free electrons as a solid is transformed to plasma. A femtosecond x-ray free-electron laser is used to heat a solid, which separates the electron and ion heating time scales. The changes in absorption are measured with an independent probe pulse created through high-order-harmonic generation. We find an increase in electron temperature to have a relatively small impact on absorption, contrary to several predictions, whereas ion heating increases absorption. We compare the data to current theoretical and numerical approaches and find that a smoother electronic structure yields a better fit to the data, suggestive of a temperature-dependent electronic structure in warm dense matter.

Domains: Theoretical and/or physical chemistry; Mechanics of materials [physics.class-ph]

Gareth O. Williams, S. Künzel, S. Daboussi, B. Iwan, A. Gonzalez, et al.. Tracking the ultrafast XUV optical properties of x-ray free-electron-laser heated matter with high-order harmonics. Physical Review A : Atomic, molecular, and optical physics [1990-2015], 2018, 97 (2), ⟨10.1103/PhysRevA.97.023414⟩. ⟨cea-01741518⟩