  Electron paramagnetic resonance (EPR) can provide unique insight into the
chemical structure and magnetic properties of dopants in oxide and
semiconducting materials that are of interest for applications in electronics,
catalysis, and quantum sensing. Here, we demonstrate that EPR in combination
with scanning tunneling microscopy (STM) allows for probing the bonding and
charge state of alkali metal atoms on an ultrathin magnesium oxide layer on a
Ag substrate. We observe a magnetic moment of $1\mu_\mathrm{B}$ for Li$_2$,
LiNa, and Na$_2$ dimers corresponding to spin radicals with a charge state of
$+1e$. Single alkali atoms have the same charge state and no magnetic moment.
The ionization of the adsorbates is attributed to charge transfer through the
oxide to the metal substrate. Our work highlights the potential of EPR-STM to
provide insight into dopant atoms that are relevant for the control of the
electrical properties of surfaces and nanodevices.
