A combined multi-reference pump-probe simulation method with application to XUV signatures of ultrafast methyl iodide photodissociation

Abstract

UV pump-XUV/X-ray probe measurements have been successfully applied in the study of photo-induced chemical reactions. Although rich element-specific e lectronic s tructure i nformation i s a ccessible w ithin X UV/X-ray (inner-shell) a bsorption s pectra, i t c an b e difficult to interpret the chemistry directly from the spectrum without supporting theoretical simulations. A multireference method to completely simulate UV pump-XUV/X-ray probe measurement has been developed and applied to study the methyl iodide photodissociation process. Multireference, fewest-switches surface hopping (FSSH) trajectories were used to explore the coupled electronic and ionic dynamics upon photoexcitation of methyl iodide. Interpretation of previous measurements is provided by associated multireference, restricted active space, inner-shell spectral simulations. This combination of multireference FSSH trajectories and XUV spectra provides an interpretation of transient features appearing in previous measurements within the first 100 fs after photoexcitation and validates the significant branching ratio in the final excited-state population. This methodology should prove useful for interpretation of the increasing number of inner-shell probe studies of molecular excited states or for directing new experiments toward interesting regions of the potential energy landscape.

Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy

“…Simulated XUV transients representing the A-band dissociation of CH 3 I are directly obtained from ref. 37 . To facilitate comparisons to the experiments, the simulations are modified to reflect partial (I:I* = 2:1) and nearly-complete (I:I* = 13:1) electronic state changes upon passage through the conical intersection.…”

“…While i- C 3 H 7 I appears to represent an intermediate case in which the wave packet bifurcates between the 1 Q 1 and 3 Q 0 states in a ~2:1 ratio, t- C 4 H 9 I appears to represent a case of nearly-complete transfer to the 1 Q 1 state. Owing to the importance of alkyl iodides as a benchmark system, direct observation of the conical intersection gating the formation of photoproducts is a very appealing target for both experiment and theory 35 – 37 . Although a number of time-resolved experiments on the dynamics of alkyl iodide photodissociation in the A -band have been reported using femtosecond XUV transient absorption and Coulomb explosion imaging 35 , 36 , 38 , 39 , limitations in the temporal resolution of previously-reported experiments have precluded a direct observation of passage through the conical intersection.…”

Ultrafast strong-field dissociation of vinyl bromide: An attosecond transient absorption spectroscopy and non-adiabatic molecular dynamics study

“…The algorithm for the calculation of the XAS is mainly based on the works of Josefsson and co-workers 26 and Wang, Odelius, and Prendergast. 42 Here, both the valence space of vinyl bromide and the excitation processes of the core electrons are described in the same theoretical framework of the restricted active space self-consistent field (RASSCF) method. 85,86 RASSCF is an extension to the general CASSCF approach, where the active space is further partitioned into three subspaces RAS1, RAS2, and RAS3, with additional constraints applied to their occupation.…”

“…This was already mentioned by Wang, Odelius, and Prendergast. 42 Following their proposed procedure, we applied the original correction to the overall Hamiltonian as introduced in Ref. 104 .…”

Mapping Wave Packet Bifurcation at a Conical Intersection in CH3I by Attosecond XUV Transient Absorption Spectroscopy

“…Fig.3ashows an expanded view of the experimental transient in the 44.0-49.5 eV spectral region. Signatures of dynamics launched by single-photon excitation can be identified through comparisons to a simulated transient representing CH 3 I photodissociation in the A-band plotted in Fig.3b 20. In particular, signatures of conical intersection and dissociation dynamics in the A-band in the experimental transient can be identified and distinguished from overlapping spectral…”