  Spectral distortions (SDs) of the cosmic microwave background (CMB) provide a
powerful tool for studying particle physics. Here we compute the distortion
signals from decaying particles that convert directly into photons at different
epochs during cosmic history, focusing on injection energies
$E_\mathrm{inj}\lesssim 20\,\mathrm{keV}$. We deliver a comprehensive library
of SD solutions that can be used to study a wide range of particle physics
scenarios. We use {\tt CosmoTherm} to compute the SD signals, including effects
on the ionization history and opacities of the Universe. We also consider the
effect of blackbody-induced stimulated decay, which can modify the injection
history significantly. Then, we use data from COBE/FIRAS and EDGES to constrain
the properties of the decaying particles. We explore scenarios where these
provide a dark matter (DM) candidate or constitute only a small fraction of DM.
We complement the SD constraints with CMB anisotropy constraints, highlighting
new effects from injections at very-low photon energies ($h\nu\lesssim
10^{-4}\,\mathrm{eV}$). Our model-independent constraints exhibit rich
structures in the lifetime-energy domain, covering injection energies
$E_\mathrm{inj}\simeq 10^{-10}\mathrm{eV}-10\mathrm{keV}$ and lifetimes
$\tau_X\simeq 10^5\,\mathrm{s}-10^{33}\mathrm{s}$. We discuss the constraints
on axions and axion-like particles that convert directly into two photons,
revising existing SD constraints in the literature. Our limits are competitive
with other constraints for axion masses $m_a c^2\gtrsim 27\,\mathrm{eV}$ and we
find that simple estimates based on the overall energetics are generally
inaccurate. Future CMB spectrometers could significantly improve the obtained
constraints, thus providing an important complementary probe of early-universe
particle physics.
