  This work describes the decay mechanism of photoexcited thiourea, both in gas
phase and in solution, from the information inferred from the topography of the
excited and ground state potential energy surfaces and mixed singlet/triplet
quantum classical molecular dynamics simulations. Our gas phase results reveal
T1/S0 intersystem crossing as the dominant (49%) intrinsic decay channel to the
ground state, which reaches a population of 0.28 at the final time of our
simulations (10 ps). Population of the T1, would occur after internal
conversion to the S1 from the spectroscopic S2 electronic state, followed by
S1->T2 intersystem crossing and T2->T1 internal conversion processes. Minor
decay channels occurring exclusively along the singlet manifold, i.e. S2->S0
(33%) and S1->S0 (18%), were also observed to play a role in the relaxation of
photoexcited thiourea in the gas phase. The explicit incorporation of
water-thiourea interactions in our simulations was found to provoke a very
significant delay in the decay to the ground state of the system, with no
transitions to the S0 being registered during the first 10 ps of our
simulations. Intermolecular vibrational energy redistribution and explicit
hydrogen bond interaction established between water molecules and the NH2 group
of thiourea were found to structurally or energetically hamper the access to
the intersystem crossing or internal conversion funnels with the ground state.
