  A method is presented to monitor the internal energy distribution of cluster
anions via delayed electron detachment by pulsed photoexcitation and
demonstrated on Co$_4{}^-$ in an electrostatic ion beam trap. In cryogenic
operation, we calibrate the detachment delay to internal energy. By laser
frequency scans, at room temperature, we reconstruct the time-dependent
internal energy distribution of the clusters. The mean energies of ensembles
from a cold and a hot ion source both approach thermal equilibrium. Our data
yield a radiative emission law and the absorptivity of the cluster for thermal
radiation.
