Analysis of the S1 ← S0 and D0 ← S1 Band Systems in the Picolines by Means of Resonance Enhanced Multi Photon Ionization (REMPI) and Mass Analyzed Threshold Ionization (MATI) Spectroscopy

Background: The vibronic structures of the first electronically excited state (S1) and ionic ground state (D0) of 2-, 3- and 4-picoline were investigated by means of resonance enhanced multi photon ionization (REMPI) and mass analyzed threshold ionization (MATI) spectroscopy. Objective: The experimental results were supported by geometry optimizations and frequency analyses at the (time-dependent) density functional theory ((TD)DFT) level of theory. Furthermore, the (TD)DFT results were used to calculate the vibrational intensities in the REMPI and MATI spectra by a multidimensional Franck-Condon approach. Method: The experimental data and theoretical predictions matched well for 2- and 4-picoline whereas deviations were observed in the case of 3-picoline. The latter was attributed to a vibronic coupling of the S1 state (1π*←n) to a higher lying 1π*←π state, which is likely to occur for all picolines with 3-picoline exhibiting the strongest coupling. The S1 excitation energies of 2-, 3- and 4-picoline were determined to be 34766 ± 3 cm-1 (4.3104 ± 0.0004 eV), 34660 ± 3 cm-1 (4.2973 ± 0.0004 eV) and 35177 ± 4 cm-1 (4.3614 ± 0.0005 eV) respectively. Results: They are in excellent agreement with the literature. The MATI spectra yielded an adiabatic ionization energy of 72371 ± 5 cm-1 (8.9729 ± 0.0006 eV), 72613 ± 5 cm-1 (9.0029 ± 0.0006 eV) and 72932 ± 4 cm-1 (9.0424 ± 0.0005 eV) for 2-, 3- and 4-picoline respectively. Conclusions: These are the most accurate values reported to this day. In addition, the geometry and vibronic structure of the first electronically excited state and ionic ground state have been determined.