Heating and ionization are among the most fundamental processes in ultra-short, relativistic laser–solid interactions. However, capturing their spatiotemporal evolution experimentally is challenging due to the inherently transient and non-local thermodynamic equilibrium (NLTE) nature. Here, time-resolved resonant X-ray emission spectroscopy, in conjunction with simultaneous X-ray absorption imaging, is employed to investigate such complex dynamics in a thin copper wire driven by an optical high-intensity laser pulse, with sub-picosecond temporal resolution. The diagnostics leverage the high brightness and narrow spectral bandwidth of an X-ray free-electron laser (XFEL), to selectively excite resonant transitions of highly charged ions within the hot dense plasma generated by the optical laser. The measurements reveal a distinct rise-and-fall temporal evolution of the resonant X-ray emission yield—and consequently the selected ion population—over a 10 ps timescale, accompanied by an inversely correlated x-ray transmission. In addition, off-resonance emissions with comparable yields on both sides of the XFEL photon energy are clearly observed, indicating balanced ionization and recombination rates. Furthermore, experimental results are compared with comprehensive simulations using atomic collisional-radiative models, particle-in-cell (PIC), and magneto-hydrodynamics (MHD) codes to elucidate the underlying physics. Multi-scale simulations reveal extreme sensitivity of basic plasma parameters with widely used models, such as temperature and ionization depth, which are able to be constrained by incorporating a detailed accounting of laser spatial profiles, and pre-plasma conditions, and NLTE collisional processes. These results provide new insights into heating and ionization dynamics in hot, dense matter and are applicable to the high-energy-density science and inertial fusion energy research, both as an experimental platform for accessing theoretically challenging conditions and as a benchmark for improving models of high-power laser–plasma interactions.
 

ultra-short, relativistic laser–solid interactions,X-ray free-electron laser,Heating and ionization,inertial fusion energy research