  As the simplest nitroaromatic compound, nitrobenzene is an interesting model
system to explore the rich photochemistry of nitroaromatic compounds. Previous
measurements of nitrobenzene's photochemical dynamics have probed structural
and electronic properties, which, at times, paint a convoluted and sometimes
contradictory description of the photochemical landscape. A sub-picosecond
structural probe can complement previous electronic measurements and aid in
determining the photochemical dynamics with less ambiguity. We investigate the
ultrafast dynamics of nitrobenzene triggered by photoexcitation at 267 nm
employing megaelectronvolt ultrafast electron diffraction with femtosecond time
resolution. We measure the first 5 ps of dynamics and, by comparing our
measured results to simulation, we unambiguously distinguish the lowest singlet
and triplet electronic states. We observe ground state recovery within 160 +/-
60 fs through internal conversions and without signal corresponding to
photofragmentation. Our lack of dissociation signal within the first 5 ps
indicates that previously observed photofragmenation reactions take place in
the vibrationally "hot" ground state on timescales considerably beyond 5 ps.
