  Integrating nanoscale opto-electronic functions is vital for applications
such as optical emitters, detectors, and quantum information. Lanthanide atoms
show great potential in this endeavor due to their intrinsic transitions. Here,
we investigate Er adatoms on Si(100)-2x1 at 9K using a scanning tunneling
microscope (STM) coupled to a tunable laser. Er adatoms display two main
adsorption configurations that are optically excited between 800 nm and 1200 nm
while the STM reads the resulting photocurrents. Our spectroscopic method
reveals that various photocurrent signals stem from the bare silicon surface or
Er adatoms. Additional photocurrent peaks appear as the signature of the Er
adatoms relaxation, triggering efficient dissociation of nearby trapped
excitons. Calculations using the density functional theory with spin-orbit
coupling correction highlight the origin of the observed photocurrent peaks as
specific 4f->4f or 4f->5d transitions. This spectroscopic technique can pave
the way to an optoelectronic analysis of atomic and molecular assemblies by
offering unique insight into their intrinsic quantum properties.
