Redshift of the Heα emission line of He-like ions under a plasma environment

By carefully following the spatial and temporal criteria of the Debye-Hiickel (DH) approximation, we present a detailed theoretical study on the redshifts of the spectroscopically isolated He(alpha)lines corresponding to the 1s2p(1)P -> 1s(2 1)S emission from two-electron ions embedded in external dense plasma. We first focus our study on the ratio R = delta omega(alpha)/omega(0) between the redshift delta omega(alpha)due to the external plasma environment and the energy omega(0)of the He-alpha line in the absence of the plasma. Interestingly, the result of our calculation shows that this ratio R turns out to vary as a nearly universal function of a reduced Debye length lambda(D)(Z) = (Z - 1)D. Since the ratio R dictates the necessary energy resolution for a quantitative measurement of the redshifts and, at the same time, the Debye length D is linked directly to the plasma density and temperature, the dependence of R on D should help to facilitate the potential experimental efforts for a quantitative measurement of the redshifts for the He-alpha line of the two-electron ions. In addition, our study has led to a nearly constant redshift delta omega(alpha) at a given D for all He-like ions with Z between 5 and 18 based on our recent critical assessment of the applicability of the DH approximation to atomic transitions. These two general features, if confirmed by observation, would offer a viable and easy alternative in the diagnostic efforts of the dense plasma.