  The Gamma Factory (GF) initiative aims at the construction of a unique
experimental tool exploiting resonant interaction of light with
ultra-relativistic partially stripped ions (PSI) stored in circular
accelerators at CERN. Resonant excitation of high-energy electronic transitions
in the ions is achieved through Doppler-boosting (by twice the Lorentz factor;
from hundred to several thousand times) of light energy. In order to
efficiently excite the ions, and hence generate intense beams of
scattered/fluorescent photons, a detailed knowledge of the ions' electronic
energy structure and the dynamics of optical excitation is required.
Spectroscopic properties of PSI selected for the GF operation, as well as their
optical excitation schemes, are investigated. Two regimes of the ion--light
interaction are identified, leading to different dynamics of the excitation.
The efficiency of the ion--light interaction, as well as the number of photons
emitted from a single ion bunch, are estimated, both analytically and
numerically, for three ions considered for the GF, i.e.~Li-like
${}^{208}_{\phantom{0}82}$Pb$^{79+}$, Li-like ${}^{40}_{20}$Ca$^{17+}$, and
H-like ${}^{208}_{\phantom{0}82}$Pb$^{81+}$.
