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One System, Multiple Approaches
Published in Thomas C. Weinacht, Brett J. Pearson, Time-Resolved Spectroscopy, 2018
Thomas C. Weinacht, Brett J. Pearson
Comparing the X-ray diffraction and spectroscopic (esp. TRIS) results provides interesting differences in the measured timescales. TRIS measured a timescale of approximately 50 fs for the wave packet to go from the FC region to the first PES intersection, with a total time of 140 fs required to relax back down to the HT (or CHD) ground state. These times are different from the 80 fs measured for structural changes using diffraction. At least in part, the different timescales for the two measurements reflect the fact that structural and chemical/spectroscopic changes are not necessarily synchronous. For instance, near a conical intersection, the electronic character of a state can change quite dramatically over a short timescale without much change in molecular structure. Conversely, as a molecule dissociates on a single electronic state, the character may not change very much while the structure changes significantly. Thus, it can be very useful to have multiple measurement approaches to characterize dynamics that involve both structural and chemical/spectroscopic changes. Together, the diffraction and spectroscopic measurements in CHD provide a more complete picture of the isomerization dynamics, with each highlighting different aspects of the reaction.
Photodissociation dynamics of CO-forming channel of methyl formate at 193 nm: a computational study
Published in Molecular Physics, 2022
Po-Yu Tsai, Federico Palazzetti
Recently, the authors performed direct dynamics simulations starting from all the nominal saddle points, which lead to the primary CO products [15]. The features of product state distributions from each saddle point provide information to identify the measured CO-forming mechanism in experiments [4,6,9]. Further theoretical investigations were focused on a deeper characterisation of the dynamics by the application of the generalised multi-centre impulsive model (GMCIM) [16], which provides an explanation about the product state distribution in sudden approximation. It was concluded, however, that both the direct dynamics simulations and GMCIM calculations that initiate the dynamics events from the saddle points with the accomplished intramolecular vibrational redistribution (IVR), failed to provide a unified explanation [15] to both the kinetic energy and rovibrational energy distributions of CO measured in VMI [4] and infrared emission experiments [6,9]. Nowadays, the CO-forming mechanism at 248 nm is considered to be an unconventional mechanism [5,7,10,12,13,16] involving multiple state dynamics via nonadiabatic transition at S1/S0 conical intersection [11,12], a pathway that can bypass all the saddle points. The relation between the roaming-like features in product state distribution and the dissociation mechanism involving nonadiabatic transitions through the conical intersection have been investigated in previous studies [3–7,10].