Figure shows the cryo-SEVI spectrum of OHˉ without (black) and with (red) IR pre-excitation. Both spectra are fully rotationally resolved and assigned; selected transitions are indicated in Figures and. The two most intense IR-off transitions, the R3(0) and R1(0) transitions, originate from the (v=0,J=0) state of the anion. The relative intensities of these peaks compared to those originating from higher lying anion rotational states yields a rotational temperature of 20 K. The red trace shows the IR cryo-SEVI spectrum where the IR laser is tuned to the R(0) transition of the anion at 3591 cm -1 , exciting anions from the (0,0) level to (1,1) level. One observes significant depletion of the R3(0) and R1(0) transitions, while five new transitions originating from the (1,1) anion appear that are assigned in Figure . These are "sequence band" transitions between the v=1 anion and neutral levels and are interspersed with the transitions originating from the anion v=0 level. One can also obtain a "vibrational action spectrum" by scanning the IR laser and following the change in intensity of one of the red IR-on features, as was done for the R3(1) peak at 14799 cm -1 . While the idea of IR cryo-SEVI seems simple in principle, the spectra in Figure illustrate that at least for OHˉ, very cold ions and high photoelectron kinetic energy resolution are required. The red peaks resulting from vibrationally excited anions all lie within a few cm -1 of the nearest ground state feature and would thus be obscured if the resolution were sub-optimal or if the anions were warmer; the latter scenario would lead to more rotational features in the IR-off spectrum that overlap spectrally with the IR-on peaks. In addition, the fraction of vibrationally excited molecules would be reduced when higher rotational levels of the vibrational ground state are populated, as those require slightly different IR excitation frequencies.