  A fully microscopic model of the doping-dependent exciton and trion line
widths in the absorption spectra of monolayer transition metal dichalcogenides
in the low temperature and low doping regime is explored. The approach is based
on perturbation theory and avoids the use of phenomenological parameters. In
the low-doping regime, we find that the trion line width is relatively
insensitive to doping levels while the exciton line width increases
monotonically with doping. On the other hand, we argue that the trion line
width shows a somewhat stronger temperature dependence. The magnitudes of the
line widths are likely to be masked by phonon scattering for $T \geq 20$ K in
encapsulated samples in the low doping regime. We discuss the breakdown of
perturbation theory, which should occur at relatively low doping levels and low
temperatures. Our work also paves the way towards understanding a variety of
related scattering processes, including impact ionization and Auger scattering
in clean 2D samples.
