The absolute photodetachment cross section of OH − anions at a rotational and translational temperature of 170 K is determined by measuring the detachment-induced decay rate of the anions in a multipole radio-frequency ion trap. In comparison with previous results, the obtained cross section shows the importance of the initial rotational state distribution. Using a tomography scan of the photodetachment laser through the trapped ion cloud, the derived cross section is model-independent and thus features a small systematic uncertainty. The tomography also yields the column density of the OH − anions in the 22-pole ion trap in good agreement with the expected trapping potential of a large field free region bound by steep potential walls.PACS numbers: 33.80. Eh,33.80.Ps,33.70.Ca,95.30.Ky Photodetachment of the excess electron from a negative ion represents a fundamental light-matter interaction process that reveals detailed information on atomic and molecular structure and dynamics. The additional electron is only bound by virtue of electron-electron interactions and its binding energy often depends sensitively on electron correlations in the entangled multi-electron wavefunction. Spectroscopic information, obtained from photoelectron energy measurements, is used to study the electronic, vibrational and rotational Eigenstate spectrum of anionic and neutral molecules. This also yields insight in transition states structures of reactive collisions of neutral molecules [1]. In addition, ultrafast timeresolved studies of wavepacket or electron rearrangement dynamics inside clusters have employed anion photoelectron spectroscopy [2]. Multiphoton electron detachment in short laser pulses provides information on the electronatom interaction in strong laser fields [3]. Cross sections for photodetachment reveal complementary information to photoelectron spectra and serve to challenge calculations for bound-free transition matrix elements [4].The most detailed photodetachment cross section studies on a molecular anion have been carried out for the hydroxyl anion OH − . Much of the work on OH − has focused on relative cross sections. Near threshold, comparison of the relative cross section as a function of electron energy with Wigner threshold laws allows for precise tests of the long-range electron-neutral interactions. Specifically, the subtle coupling of the two lowest Λ-doublet states has been observed, which leads to a long-range electron-dipole interaction [5,6]. Also, relative cross section measurements for transitions of specific rotational states have been carried out for OH − [7,8]. These results indicate deviations of the photodetachment cross section from s-wave electron detachment calculations. Interpretation, however, is obscured by the incomplete knowledge of the OH − rotational population. Even for the OH − anion only two absolute cross section measurements have been performed to date [9,10], which disagree with each other and deviate from the calculated one by almost an order of magnitude [4].An important ...