  Future high spectral resolution telescopes will enable us to place tight
constraints on turbulence in the intra-cluster medium through the line widths
of strong emission lines. At the same time, these bright lines are the most
prone to be optically thick. This requires us to separate the effects of
resonant scattering from turbulence, both of which could broaden the lines. How
this can be achieved has yet not been quantitatively addressed. In this paper,
we propose a flexible new parametrization for the line profile, which allows
these effects to be distinguished. The model has only 3 free parameters, which
we calibrate with Monte-Carlo radiative transfer simulations. We provide
fitting functions and tables that allow the results of these calculations to be
easily incorporated into a fast spectral fitting package. In a mock spectral
fit, we explicitly show that this parameterization allows us to correctly
estimate the turbulent amplitude and metallicity of a cluster such as Perseus,
which would otherwise give significantly biased results. We also show how the
physical origin of the line shape can be understood analytically.
