  In a large class of scalar-tensor theories that are potential candidates for
dark energy, a nonminimal coupling between the scalar and the photon is
possible. The presence of such an interaction grants us the exciting prospect
of directly observing dark sector phenomenology in the electromagnetic
spectrum. This paper investigates the behavior of one-electron atoms in this
class of modified gravity models, exploring their viability as probes of
deviations from general relativity in both laboratory and astrophysical
settings. Building heavily on earlier studies, our main contribution is
threefold: A thorough analysis finds additional fine-structure corrections
previously unaccounted for, which now predict a contribution to the Lamb shift
that is larger by nearly 4 orders of magnitude. In addition, they also predict
a scalar-mediated photon-photon interaction, which now constrains the scalar's
coupling to the photon independently of the matter coupling. This was not
previously possible with atomic precision tests. Our updated constraints are
$\log_{10}\beta_m \lesssim 13.4$ and $\log_{10}\beta_\gamma \lesssim 19.0$ for
the matter and photon coupling, respectively, although these remain
uncompetitive with bounds from other experiments. Second, we include the
effects of the nuclear magnetic moment, allowing for the study of hyperfine
structure and the 21 cm line, which hitherto have been unexplored in this
context. Finally, we also examine how a background scalar leads to equivalence
principle violations.
