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

Correspondence between electronic structure calculations and simulations: nonadiabatic dynamics in CS2

The choice of ab initio electronic structure method is an important factor in determining the fidelity of nonadiabatic dynamics simulations. We present an in-depth comparison of two simulations of photodissociation in the CS2 molecule following excitation to the 1 1^B_2 state. The simulations account for nonadiabatic and spin-orbit coupling, and are performed using the SHARC surface-hopping approach combined with state-averaged SA8-CASSCF(8,6)/SVP and SA8-CASSCF(10,8)/SVP {\it{ab initio}} calculations, with additional reference calculations at the MRCI(14,10)/aug-cc-pvTZ level. The relative performance and veracity of the simulations can be assessed by inspection of the potential energy curves along specific coordinates. The simulations demonstrate direct competition between internal conversion and intersystem crossing, with strong correlation between molecular geometry, electronic state density, and dynamics

Mapping the Complete Reaction Path of a Complex Photochemical Reaction

We probe the dynamics of dissociating CS2 molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilises the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms

Determining Orientations of Optical Transition Dipole Moments Using Ultrafast X-ray Scattering

Identification of the initially prepared, optically active state remains a challenging problem in many studies of ultrafast photoinduced processes. We show that the initially excited electronic state can be determined using the anisotropic component of ultrafast time-resolved X-ray scattering signals. The concept is demonstrated using the time-dependent X-ray scattering of <i>N</i>-methyl morpholine in the gas phase upon excitation by a 200 nm linearly polarized optical pulse. Analysis of the angular dependence of the scattering signal near time zero renders the orientation of the transition dipole moment in the molecular frame and identifies the initially excited state as the 3p_<i>z</i> Rydberg state, thus bypassing the need for further experimental studies to determine the starting point of the photoinduced dynamics and clarifying inconsistent computational results