For the first time, vibrational spectra of the pyridine cation in the electronic ground state have been measured via several intermediate states (00, 16b20, 16b40, 6a10, 6b1, 16a10, 10a10 and 121) by Mass-Analyzed Threshold Ionization (MATI) spectroscopy. From the MATI spectra, the adiabatic ionization energy of pyridine has been determined to be 74 185 ± 6 cm−1 (9.1978 ± 0.0008 eV). Several vibronic modes in the ionic ground state could be assigned for the first time. An intensity gain of vibrations having b1 symmetry could be observed by activating the ion ground state. Also, a breakdown of the “Δν = 0 propensity rule” for the excitation via the 16b2 and 16b4 states of the first excited states are displayed in the recorded spectra. In conjunction with ab initio calculations these observations can be explained by a strong geometrical distortion along the 16b vibration in the first excited state, leading to a “boat distortion”.
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