title: Experimental measurements of the collisional absorption of XUV radiation in warm dense aluminium.
abstract: The collisional (or free-free) absorption of soft x rays in warm dense aluminium remains an unsolved problem. Competing descriptions of the process exist, two of which we compare to our experimental data here. One of these is based on a weak scattering model, another uses a corrected classical approach. These two models show distinctly different behaviors with temperature. Here we describe experimental evidence for the absorption of 26-eV photons in solid density warm aluminium (T_{e}≈1 eV). Radiative x-ray heating from palladium-coated CH foils was used to create the warm dense aluminium samples and a laser-driven high-harmonic beam from an argon gas jet provided the probe. The results indicate little or no change in absorption upon heating. This behavior is in agreement with the prediction of the corrected classical approach, although there is not agreement in absolute absorption value. Verifying the correct absorption mechanism is decisive in providing a better understanding of the complex behavior of the warm dense state.
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: B
authors.initials: B
authors.lastname: Kettle
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: T
authors.initials: T
authors.lastname: Dzelzainis
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: S
authors.initials: S
authors.lastname: White
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: L
authors.initials: L
authors.lastname: Li
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: B
authors.initials: B
authors.lastname: Dromey
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: M
authors.initials: M
authors.lastname: Zepf
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: C L S
authors.initials: CL
authors.lastname: Lewis
authors.affiliation: Group of Lasers and Plasmas, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal.
authors.forename: G
authors.initials: G
authors.lastname: Williams
authors.affiliation: Group of Lasers and Plasmas, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal.
authors.forename: S
authors.initials: S
authors.lastname: Künzel
authors.affiliation: Group of Lasers and Plasmas, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal.
authors.forename: M
authors.initials: M
authors.lastname: Fajardo
authors.affiliation: Laboratoire d'Optique Appliquée, ENSTA ParisTech, CNRS, Ecole Polytechnique, Université Paris-Saclay, 828 Boulevard des Marchaux, 91762 Palaiseau Cedex, France.
authors.forename: H
authors.initials: H
authors.lastname: Dacasa
authors.affiliation: Laboratoire d'Optique Appliquée, ENSTA ParisTech, CNRS, Ecole Polytechnique, Université Paris-Saclay, 828 Boulevard des Marchaux, 91762 Palaiseau Cedex, France.
authors.forename: Ph
authors.initials: P
authors.lastname: Zeitoun
authors.affiliation: Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom.
authors.forename: A
authors.initials: A
authors.lastname: Rigby
authors.affiliation: Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom.
authors.forename: G
authors.initials: G
authors.lastname: Gregori
authors.affiliation: Central Laser Facility, Harwell, Oxfordshire OX11 0QX, United Kingdom.
authors.forename: C
authors.initials: C
authors.lastname: Spindloe
authors.affiliation: Central Laser Facility, Harwell, Oxfordshire OX11 0QX, United Kingdom.
authors.forename: R
authors.initials: R
authors.lastname: Heathcote
authors.affiliation: Centre for Plasma Physics, School of Mathematics and Physics, Queen's University Belfast, University Road, Belfast BT7 1NN, United Kingdom.
authors.forename: D
authors.initials: D
authors.lastname: Riley