  We study gravitational absorption effects using effective on-shell scattering
amplitudes. We develop an in-in probability-based framework involving plane-
and partial-wave coherent states for the incoming wave to describe the
interaction of the wave with a black hole or another compact object. We connect
this framework to a simplified single-quantum analysis. The basic ingredients
are mass-changing three-point amplitudes, which model the leading absorption
effects and a spectral-density function of the black hole. As an application,
we consider a non-spinning black hole that may start spinning as a consequence
of the dynamics. The corresponding amplitudes are found to correspond to
covariant spin-weighted spherical harmonics, the properties of which we
formulate and make use of. We perform a matching calculation to
general-relativity results at the cross-section level and derive the effective
absorptive three-point couplings. They are found to behave as ${\cal
O}(G_\text{Newton}^{s+1})$, where $s$ is the spin of the outgoing massive
state.
