Scientists map how intense X-rays reshape matter between plasma and solid
Researchers used extreme X-ray pulses to observe and control how copper transitions into warm dense matter—a state relevant to nuclear fusion, planetary cores, and astrophysics. The findings reveal new mechanisms for manipulating X-ray beams themselves, potentially enabling more precise tools for materials processing and energy research.
Originaltitel: Transient absorption of warm dense matter created by an X-ray free-electron laser
<p>Warm dense matter is at the boundary between a plasma and a condensed phase and plays a role in astrophysics, planetary science and inertial confinement fusion research. However, its electronic structure and ionic structure upon irradiation with strong laser pulses remain poorly understood. Here, we use an intense and ultrafast X-ray free-electron laser pulse to simultaneously create and characterize warm dense copper using <em>L</em>-edge X-ray absorption spectroscopy over a large irradiation intensity range. Below a pulse intensity of 10<sup>15</sup> W cm<sup>−2</sup>, an absorption peak below the <em>L</em> edge appears, originating from transient depletion of the 3<em>d</em> band. This peak shifts to lower energy with increasing intensity, indicating the movement of the 3<em>d</em> band upon strong X-ray excitation. At higher intensities, substantial ionization and collisions lead to the transition from reverse saturable absorption to saturable absorption of the X-ray free-electron laser pulse, two nonlinear effects that hold promise for X-ray pulse-shaping. We employ theoretical calculations that combine a model based on kinetic Boltzmann equations with finite-temperature real-space density-functional theory to interpret these observations. The results can be used to benchmark non-equilibrium models of electronic structure in warm dense matter.</p>