  One of the most promising electromagnetic signatures of compact object
mergers are kilonovae: approximately isotropic radioactively-powered transients
that peak days to weeks post-merger. Key uncertainties in modeling kilonovae
include the emission profiles of the radioactive decay products---non-thermal
beta- and alpha-particles, fission fragments, and gamma-rays---and the
efficiency with which they deposit their energy in the ejecta. The total
radioactive energy and the efficiency of its thermalization sets the luminosity
budget and is therefore necessary for predicting kilonova light curves. We
outline the uncertainties in r-process decay, describe the physical processes
by which the energy of the decay products is absorbed in the ejecta, and
present time-dependent thermalization efficiencies for each particle type. We
determine the net heating efficiency and explore its dependence on r-process
yields---in particular, the production of translead nuclei that undergo
alpha-decay---and on the ejecta's mass, velocity, composition, and magnetic
field configuration. We incorporate our results into new time-dependent,
multi-wavelength radiation transport simulations, and calculate updated
predictions of kilonova light curves. Thermalization has a substantial effect
on kilonova photometry, reducing the luminosity by a factor of roughly 2 at
peak, and by an order of magnitude or more at later times (15 days or more
after explosion). We present simple analytic fits to time-dependent net
thermalization efficiencies, which can easily be used to improve light curve
models. We briefly revisit the putative kilonova that accompanied gamma ray
burst 130603B, and offer new estimates of the mass ejected in that event. We
find that later-time kilonova light curves can be significantly impacted by
alpha-decay from translead isotopes; data at these times may therefore be
diagnostic of ejecta abundances.
