  We analyze the valley selection rules for optical transitions from impurity
states to the conduction band in two-dimensional Dirac materials, taking a
monolayer of MoS2 as an example. We employ the analytical model of a shallow
impurity potential which localizes electrons described by a spinor wave
function, and, first, find the system eigenstates taking into account the
presence of two valleys in the Brillouin zone. Then, we find the spectrum of
the absorbance and calculate the photon-drag electric current due to the
impurity-band transitions, drawing the general conclusions regarding the valley
optical selection rules for the impurity-band optical transitions in gapped
Dirac materials.
