  The electronic structure changes of SmS surfaces under potassium (K) doping
are elucidated using synchrotron-based core-level photoelectron spectroscopy
and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and
ARPES spectra indicate that the Sm mean valence of the surface increased from
the nearly divalent to trivalent states, with increasing K deposition.
Carrier-induced valence transition (CIVT) from Sm$^{2+}$ to Sm$^{3+}$ exhibits
a behavior opposite to that under conventional electron doping. Excess
electrons are trapped by isolated excitons, which is inconsistent with the
phase transition from the black insulator with Sm$^{2+}$ to the gold metal with
Sm$^{3+}$ under pressure. This CIVT helps to clarify the pressure-induced
black-to-golden phase transition in this material, which originates from the
Mott transition of excitons.
