Investigation of multiple electronic excited state relaxation pathways following 200 nm photolysis of gas-phase imidazole

Imidazole acts as a subunit in the DNA base adenine and the amino acid histidine-both important biomolecules which display low fluorescence quantum yields following UV excitation. The low fluorescence quantum yields are attributed to competing non-radiative excited state relaxation pathways that operate on ultrafast timescales. Imidazole is investigated here as a model compound due to its accessibility to high level ab initio calculations and time-resolved gas-phase spectroscopic techniques. Recent non-adiabatic dynamics simulations have identified three non-radiative relaxation mechanisms which are active following 6.0-6.2 eV excitation. Presented herein is a comprehensive investigation of each mechanism using a combination of femtosecond time-resolved ion yield and total kinetic energy release spectroscopies to monitor the formation of associated photoproducts. Relaxation along the (1)πσ state constitutes the predominant deactivation pathway. Timescales for NH-dissociation are extracted and distinguished from alternative H-atom sources based on their kinetic energy distributions. Larger photoproducts are observed to a lesser extent and attributed to ring fragmentation following NH-puckering and CN-stretching relaxation paths.