  Depopulation of the Mn$^{2+}$ state in ZnO:Mn upon illumination, monitored by
quenching of the Mn$^{2+}$ EPR signal intensity, was observed at temperatures
below 80~K. Mn$^{2+}$ photoquenching is shown to result from the Mn$^{2+}$
$\to$ Mn$^{3+}$ ionization transition, promoting one electron to the conduction
band. Temperature dependence of this process indicates the existence of an
energy barrier for electron recapture of the order of 1~meV. GGA$+U$
calculations show that after ionization of Mn$^{2+}$ a moderate breathing
lattice relaxation in the 3+ charge state occurs, which increases energies of
$d$(Mn) levels. At its equilibrium atomic configuration, Mn$^{3+}$ is
metastable since the direct capture of photo-electron is not possible. The
metastability is mainly driven by the strong intra-shell Coulomb repulsion
between $d$(Mn) electrons. Both the estimated barrier for electron capture and
the photoionization energy are in good agreement with the experimental values.
