  The Keldysh theory of photoionization for solids is generalized to atomically
thin two-dimensional semiconductors. We derive a closed-form formula and its
asymptotic forms for a two-band model with a Kane dispersion. These formulas
exhibit characteristically different behaviors from their bulk counterparts
which are attributed to the scaling of the 2D density of states. We validate
our formulas by comparing them to recent strong-field ionization experiments in
monolayer MoS2 with good agreement. Our work is expected to find a wide range
of applications in intense light - 2D material interaction.
