Theoretical modelling of ultrafast photodynamics
This thesis presents detailed electronic structure calculations and mixed quantum-classical dynamics simulations of the photodynamics of two small polyatomic molecules using "on-the-fly" surface-hopping. Most of the emphasis in this work is on CS2, which upon absorption of a UV photon undergoes a complex photodissociation process propagating across the potential energy surfaces of multiple singlet and triplet electronic states, under the influence of both nonadiabatic and spin-orbit coupling. Backed by extensive CASSCF and post-CASSCF electronic structure calculations, excitation to the 11B2 state is considered as a first exploration of the dynamics over the first picosecond, accounting for the lowest-lying four singlet and four triplet states. Following this, dynamics occurring after excitation to the 21B2 state, which is the state typically excited in time-resolved experimental studies of this system, are simulated. The additional computational complexity (with dynamics evolving on 19 interacting singlet and triplet states) and the limitations of "on-the-fly" techniques for a simulation of this size is discussed. This motivates initial steps towards generating full-dimensional grid-based surfaces for CS2 on which dynamics could later be simulated. These studies reinforce the importance of spin-orbit coupling in the dynamics and shine a light on the competitive nature of the singlet and triplet dissociation channels. Secondly, the short-time dynamics of trimethylamine are simulated, also using surface-hopping. Two sets of simulations are compared with regard to their description of the main dynamical features of the system, including dissociation of a methyl fragment and the extensive interplay between the low-lying 3pxyz and 3s Rydberg states, behaviour characteristic of tertiary substituted aliphatic amine systems. It is concluded that the sixth singlet state (3d) plays a significant role in the dissociation mechanism. The calculations and simulations here demonstrate the increasing utility of the conceptually intuitive surface-hopping approach in studying two contrasting classes of photochemical reactions, namely over-the-barrier photodissociation in CS2 and the photodynamics of low-lying Rydberg states in trimethylamine. In both cases, a comparison is made with complementary time-resolved experimental work by collaborators, articulating the need for experiment and theory to work together to provide a complete description of these fundamental chemical processes.