  Tunneling across superconducting junctions proceeds by a rich variety of
processes, which transfer single electrons, Cooper pairs, or even larger
numbers of electrons by multiple Andreev reflections. Photon-assisted tunneling
combined with the venerable Tien-Gordon model has long been a powerful tool to
identify tunneling processes between superconductors. Here, we probe
superconducting tunnel junctions including an impurity-induced Yu-Shiba-Rusinov
(YSR) state by exposing a scanning tunneling microscope with a superconducting
tip to microwave radiation. We find that a simple Tien-Gordon description
describes tunneling of single electrons and Cooper pairs into the bare
substrate, but breaks down for tunneling via YSR states by resonant Andreev
reflections. We develop an improved theoretical description which is in
excellent agreement with the data. Our results establish photon-assisted
tunneling as a powerful tool to analyze tunneling processes at the atomic scale
which should be particularly informative for unconventional and topological
superconductors.
