Photoelectron spectrum of the pyridyl radical.

We report the photoelectron spectrum of the pyridyl radical (C 5 H 4 N), a species of interest in astrochemistry and combustion. The radicals were produced via hydrogen abstraction in a fluorine discharge and ionized with synchrotron radiation. Mass-selected slow photoelectron spectra of the products were obtained from photoelectron-photoion coincidence spectra. A Franck-Condon simulation based on computed geometries and vibrational frequencies identified contributions of the o - and p -pyridyl radicals. For the o -isomer an adiabatic ionisation energy of 7.70 eV was obtained, in excellent agreement with a computed value of 7.72 eV. The spectrum of o -pyridyl is characterized by a long progression in an in-plane bending mode and the N-C stretch that contains the radical site.