High resolution measurement of the K alpha spectrum of Fe xxv-xviii: New spectral diagnostics of nonequilibrium astrophysical plasmas

We present laboratory measurements of high-resolution spectra of iron I(a emission under transient ionization conditions similar to those that are believed to exist in stellar flares and young supernova remnants. Taking advantage of our high spectral resolution (lambda/Delta lambda greater than or equal to 2000, we identify a number of transitions that can serve as diagnostics of ionizing plasmas. By varying the excitation energy in the experiments, we constrain the effects of the electron distribution on these diagnostic lines. Using our measured line ratios, we deduce values for the ionization time, eta= N(e)t, in the plasma, which agree with the actual values to similar to 20% accuracy. This result gives us confidence to our ability to derive similar constraints on astrophysical plasmas from remote X-ray spectroscopic observations.