  Photodetachment thermometry on a beam of OH$^-$ in a cryogenic storage ring
cooled to below 10 K is carried out using two-dimensional, frequency and time
dependent photodetachment spectroscopy over 20 minutes of ion storage. In
equilibrium with the low-level blackbody field, we find an effective radiative
temperature near 15 K with about 90% of all ions in the rotational ground
state. We measure the J = 1 natural lifetime (about 193 s) and determine the
OH$^-$ rotational transition dipole moment with 1.5% uncertainty. We also
measure rotationally dependent relative near-threshold photodetachment cross
sections for photodetachment thermometry.
