High resolution photoelectron spectroscopy: The vibrational spectrum of the 2-aminopyridine cation

We describe a new time-of-flight photoelectron spectrometer that combines molecular beam techniques with two-photon ionization by a high repetition rate laser. The instrument routinely achieves a resolution of 5 meV. In a first application we studied the vibrational spectrum of the 2-aminopyridine ion. By tuning the laser to various vibrational states of the intermediate S1 resonance we were able to assign the vibrational frequencies of modes 6a, 12, I2 and 1. Other vibrational lines could be assigned by comparison with similar molecules. The ionization potential was found to be 8.099±0.003 eV, which differs from the literature value by 0.124 eV. The discrepancy might be explained by a fast intersystem crossing to a nearby triplet state.