The energy spectrum of the cosmic microwave background (CMB) provides a powerful tool for constraining standard and non-standard physics in the primordial Universe. Previous studies mainly highlight spectral distortions (mu-, y- and r-type) created by episodes of early energy release; however, several processes also lead to copious photon production, which requires a different treatment. Here, we carry out a first detailed study for the evolution of distortions caused by photon injection at different energies in the CMB bands. We provide detailed analytical and numerical calculations illustrating the rich phenomenology of the associated distortion signals. We show that photon injection at very high and very low frequencies creates distortions that are similar to those from pure energy release. In the mu-era (z>3x10^5), a positive or negative chemical potential can be formed, depending on the balance between added photon energy and number. At lower redshifts (z<3x10^5), partial information about the photon injection process (i.e., injection time and energy) can still be recovered, with the distortion being found in a partially comptonized state. We briefly discuss current and future constraints on scenarios with photon production. We also argue that more detailed calculations for different scenarios with photon injection may be required to assess in which regimes these can be distinguished from pure energy release processes.
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