Pump-probe simulation methods with application to XUV signatures of ultrafast photodissociation in alkyl halides

Wednesday, August 28, 2019 - 3:00pm to 4:00pm

Speaker:  Han Wang, LBNL

Program Description:

UV pump-XUV probe measurements have been successfully applied in the study of photo-induced chemical reactions. Although rich element-specific electronic structure information is accessible within XUV (inner-shell) absorption spectra, it can be difficult to interpret the chemistry directly from the spectrum without supporting theoretical simulations. Linear-response time-dependent density functional theory (LR-TDDFT) based fewest-switches surface hopping (FSSH) calculations have been applied to study the photo-induced C-Br bond breaking process of CHBr3. The maximum overlap method (MOM) was applied to simulate the transient XUV absorption for individual snapshots from excited-state trajectories and reveals atomic level details of the bond breaking event evident in measurements. Due to strong spin-orbit coupling and the multi-reference character of the excited states heavier halide molecules, such as methyl iodide, we developed and applied a multi-reference method to completely simulate UV pump-XUV probe measurements to study photodissociation in this molecule. Multi-reference FSSH trajectories were used to explore the coupled electronic and ionic dynamics upon photoexcitation. Interpretation of previous measurements is provided by associated multi-reference, restricted active space, inner-shell spectral simulations. This combination of multi-reference FSSH trajectories and XUV spectra, provides an interpretation of transient features appearing within the first 100 fs and validates the significant branching ratio between difference spin-orbit split final states in the photodissociated excited-state population. These methodologies should prove useful for interpretation of the increasing number of inner-shell probe studies of molecular excited states or for directing new experiments towards interesting regions of their potential energy landscapes.   

 

Pump-probe simulation methods with application to XUV signatures of ultrafast photodissociation in alkyl halides
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