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Abstract

The Heidelberg electrostatic Cryogenic Storage Ring (CSR) [32] provides an environment for storing ion beams up to hours under interstellar conditions. A low radiative background and a residual particle density of < 140 cm^-3 is achieved by cooling the experimental chambers of the CSR down to 6 K by liquid helium. In such an environment internally excited infrared-active molecules cool to their lowest rovibrational levels by emitting photons. The radiative cooling of a rotationally hot OH^- beam at 60 keV is probed by near-threshold photodetachment. Thereby the radiative lifetimes of the lowest rotational states (193(3)_stat(3) )_sys)s for J = 1) are measured directly by the exponential decays and the rotational transition dipole moment is determined. Calculations [84, 90] for the transition dipole moment in the vibrational ground state deviate about (7 +/- 1)% from the experimental value of dv=0 = 0:971(10) D. The correlation between the rotational energy levels and their corresponding radiative lifetimes is used to reveal the individual level populations and their relative photodetachment cross sections. In the steady state an internally cold OH^- beam is reached with more than 90% of the population in the rovibrational ground state, corresponding to an effective internal temperature of 15 K.