ESCAPE TIME OF HELIUM-LIKE-ALPHA RESONANCE-LINE PHOTONS EMITTED FROM OPTICALLY THICK PLASMAS

The heliumlike alpha resonance line is one of the strongest and most frequently observed transitions in plasma spectroscopy. In optically thick plasmas of moderate density, photons emitted in this line typically undergo many absorptions and reemissions before escaping. In the present investigation the time required for this process to occur in aluminum plasma is calculated by numerical solution of the time-dependent equation of radiative transfer, in which the photon transit time between interactions is taken into account. An analytic model is also developed in which the collisional quenching probability is parametrized in terms of an equivalent two-level atom. This model facilitates interpretation of the numerical results and permits economical estimates of the escape time for different plasma elements and conditions. One practical implication of these results is that a subpicosecond x-ray pulse from a femtosecond laser-produced high-density plasma can be broadened by multiple scattering if it propagates into a moderate-density preformed plasma.