Mapping the Complete Reaction Path of a Complex Photochemical Reaction

Abstract

We probe the dynamics of dissociating CS_{2} 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 a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the 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.

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

“…10. The 2 eV probe is chosen to demonstrate the utility of a highly selective probe, the 6 eV probe represents to the maximum energy attainable from conventional non-linear optics, while 9.2 eV and 15 eV probes are within the range covered by femtosecond filamentation sources, high-harmonic generation, or seeded freeelectron laser sources [80][81][82][83] and approach energies sufficiently high to be considered universal photoionisation probes. The spectra shown in Fig.…”

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

“…2. The (8,6) active space in simulation A contains eight electrons distributed among six orbitals, and expands to a (10,8) active space in simulation B. Both active spaces contain the degenerate sulfur lone pair highest occupied molecular orbitals (HOMOs), the s bonding molecular orbitals (MO) and the p* lowest unoccupied molecular orbital (LUMO) pair, labelled d-i in Fig.…”

“…The (10,8) active space features an additional electron pair in the second-highest occupied MO (HOMOÀ1), and a s* antibonding virtual MO (c and j). In total the (8,6) active space comprises 345 determinants (225 singlet, 120 triplet) and (10,8) 5096 determinants (3136 singlet, 1960 triplet), illustrating the factorial scaling of computational cost with the size of the active space in CASSCF calculations. It is therefore important to minimise the active space to keep computations tractable, while balancing the quality of the outcomes.…”

“…1,2 The initial stages of photochemical dynamics occur on ultrashort timescales and generally involve rapid electronic relaxation via internal conversion mediated by nonadiabatic couplings 3,4 or intersystem crossing by spin-orbit couplings. 5,6 Over the past decade, remarkable progress has been made in experimental techniques capable of following the dynamics, including ultrafast spectroscopy [7][8][9][10][11][12] and diffraction. [13][14][15][16][17][18][19] Mechanistic interpretation of the experimental observations often involves comparison to simulations, making it possible to pull out features not immediately obvious from experiments alone.…”

“…Important experimental studies of the CS 2 molecule in the time-domain include seminal molecular-alignment and UV photoelectron imaging work, 11,12 and extensive studies using photoelectron spectroscopy. 10,[45][46][47] We simulate the photodissociation dynamics of CS 2 with the surface-hopping SHARC method, 36,48,49 which is based on Tully's semiclassical fewest switches algorithm 50 and accounts for both nonadiabatic couplings (internal conversion) and spin-orbit coupling (intersystem crossing). This process is shown schematically in Fig.…”