The femtochemistry of nitrobenzene following excitation at 240 nm.

Although the photochemistry of nitrobenzene has been extensively studied, the assignment of fragmentation channels and their specific dynamics remains challenging. Here the photochemistry of nitrobenzene following 240 nm excitation into its S₄ excited singlet state is investigated by femtosecond laser-induced ionization using an intense 800 nm pulse, coupled with time-resolved Coulomb explosion imaging and covariance mapping. We assign photochemical channels by observing correlations between the molecular fragment ions of the associated product pairs, enabling the time-resolved dynamics of channels leading to NO, NO₂, and C₆H₅NO to be fully characterized. NO is produced via two distinct pathways, leading to translationally cold and hot photofragments with risetimes of  ~ 8 ps and  ~ 14 ps, respectively. NO₂ photofragments are characterised by a bimodal risetime of  ~ 8 ps and  ≳ 2 ns, and can be detected within the first picosecond following ultra-violet photon absorption. C₆H₅NO is formed with a risetime of 17 ps. Kinetic energy disposals determined for the three chemical channels agree well with previous work. The techniques employed offer new opportunities to study the time-resolved photochemistry of relatively complex molecules in the gas phase.