We examine the possibility for a diagnostic of energetic particle beams based on passive
spectroscopy, with a focus on the atomic lines observed in the edge region of tokamaks.
Our investigation employs a quasi-linear model for the collective electric field generated
by the plasma-beam instability. If the beam is sufficiently energetic, the electric
field can be comparable to the thermal Holtsmark microfield, and the corresponding
Stark effect on atomic energy levels can be observable on spectra. We investigate
this issue and perform new hydrogen line shape calculations. The applicability of
the model to the diagnostics of runaway electrons is discussed.