Observation of a Singlet—Triplet Transition in Gas Phase Pyridine by Ion and Electron Impact

The excited states of pyridine have been investigated in the gas phase at low pressure using the techniques of ion‐ and electron‐impact energy‐loss spectroscopy. In addition to known singlet—singlet transitions and optically allowed Rydberg transitions above 4.3 eV, a transition peaking in intensity at 4.1 eV has been observed. This transition is induced by impact of He⁺ at several kiloelectron volts, but not by proton impact. Using electrons with incident energy of 13.0 eV, the 4.1 eV peak is also observed in the large‐angle scattered electron spectrum but not in the small‐angle spectrum. These observations confirm the assignment of this energy loss process to a singlet—triplet transition. The onset of the transition to the triplet state is estimated to be $\text{3.8 ± 0.1\hspace{0.17em}}\mathrm{eV}$ above the ground state of pyridine.