When heavy negative particles (
, 
, 
, etc.)
stop in matter, they usually form exotic atoms in highly excited states
with principal quantum number
where 
is the
reduced mass of the exotic atom and 
is the electron mass.
The exotic-atom formation is followed by an atomic cascade consisting
of multistep transitions to lower atomic states.
For hadronic atoms,
the atomic cascade is a complete life
history because the hadrons get absorbed by the nuclei with high
probability before reaching the ground state.
Muonic atoms (where the absorption is weak) de-excite to the ground
state and engage in various reactions (muon catalyzed fusion, muon
transfer, molecular formation) with initial conditions determined by the
atomic cascade.
In both cases, the atomic cascade can reveal important information
about the properties of exotic atoms and reactions with atoms in excited
states.
This paper, supplementing the earlier reviews
[1,2,3,4], focuses on the recent progress in
theoretical studies of the atomic cascade in light muonic and pionic
atoms.