The rate of photon absorption is $$ n = \int B_{12}I(\nu)\phi(\nu) d\nu\ ,$$per atom, where $I(\nu)$ is the specific intensity, $B_{12}$ is the absorption Einstein coefficient (as defined by this equation and assuming stimulated emission is negligible) and $\phi(\nu)$ is a profile function, centred on the transition frequency, that must be integrated over, and which encapsulates any broadening processes.
Multiplying the integrand by an extra $h\nu/c$ will give the rate of change of momentum, or the force, per atom (assuming that spontaneous emission is isotropic). Multiplying by the number of atoms per unit volume gives the force on that volume.
This assumes that the gas is optically thin (i.e. the probability of a photon being absorbed at line centre is considerably less than 1). If it is optically thick in the line then a more complicated radiative transfer calculation will be needed and the force will be reduced.
