Modeling M-shell x-ray emissions of xenon in intense laser-produced xenon cluster plasmas

When femto-second laser pulses with intensities in excess of 10¹⁹ W/cm² interact with clusters of Xe atoms, a complicated propagation and ionization dynamics takes place. For example, highly amplified line emissions have been observed at and around 2.86 Å in plasmas that were created by this dynamics. One possibility for producing such amplifications that involves the Ni-like ionization stage of xenon has recently been investigated and shown to produced gain coefficients comparable to those seen experimentally under a specific set of assumptions. The ionization dynamics by which this amplification is achieved is yet to be fully understood and correlated to measured x-ray outputs from these experiments. Xe M-shell ions emit x rays into three well separated x-ray wavelength regions. Emissions that fill n = 2 hole states lie in the 2.5-3.1 Å region, n = 4 to n = 3 transitions lie in the 9-20 Å region, and n = 4 to n = 4 transitions lie in the 100-130 Å region. All have been observed, and in a transition at 99.8 Å, a Ni-like amplified emission was observed. One goal of the work described in this talk is to correlate the three emissions occurring in the Ni-like ionization stage of xenon to the time dependent evolution of the Xe plasma. In this work, we study Xe´s ionization dynamics both from the points of view of molecular dynamics calculations (MD) and of hot-spot (HS) rate equation cluster calculations. A second goal of this work is to compare and to correlate these two sets of calculations. From the MD calculations, electron distribution functions, heating rates, the cluster expansion dynamics, free-free x-ray energy losses, and approximate cluster ionization rates are calculated that are inclusive of tunneling ionization. From the HS calculations, x-ray loss rates, x-ray spectra, heating rates, and ionization rates are calculated. Outputs like expansion rates from MD calculations will be used as inputs - o the HS calculations. Similarly, outputs like bound-bound and bound-free x-ray loss rates from HS calculations will be used as inputs to the MD calculations. The MD and HS dynamics surrounding the Ni-like ionization stage are studied and benchmarked against each other and against measured xray spectra.