Speaker
Description
With the advent of coherent light sources featuring attosecond resolution and sub-femtosecond UV/VIS pulses generated via high-order harmonic generation, observing pure, laser-induced electronic coherences before nuclear response has become possible.\textsuperscript{1,2} Accurately tracking these nonlinear dynamics and non-adiabatic transitions requires computational methods capable of providing a full quantum mechanical description of vibronic wave packets. While full quantum mechanical approaches, e.g., the Multi-Configuration Time-Dependent Hartree (MCTDH) method, provide exact solutions to the time-dependent Schr\"odinger equation, they face exponential scaling constraints in higher dimensions, becoming computationally prohibitive already for a few-center molecular targets. Conversely, classical trajectory-based approaches such as Trajectory Surface Hopping, while demonstrating wide applicability to compute excitation probabilities, tracking the appearance of non-adiabatic couplings or determining branching ratios, lack essential relative phase information to fully characterize the molecular wave packets. Critically, they rely on ad hoc decoherence corrections, whereas Multiple Spawning approaches smoothly split wave functions using adaptive Gaussian trajectories to preserve phase-sensitive observables while offering scalable, on-the-fly quantum calculations.\textsuperscript{3--5} To bridge methodological gaps in tracking these dynamics, the Coherent external Field Full Multiple Spawning approach incorporating First Order Corrections (XFFMS-SPA1) releases the Independent First Generation Approximation to start with a set of fully coupled initial trajectories and adds first-order saddle-point corrections to the potential energy evaluation. Applied to the benchmark LiH molecule, this method accurately reproduced complex time-dependent wave packet evolutions, population dynamics, and wave packet overlaps/revivals using a significantly low number of initial trajectories, matching MCTDH accuracy while successfully modeling non-perturbative phenomena like two-photon Rabi oscillations.\textsuperscript{6} Looking forward, this efficient framework holds strong potential for designing, simulating, and steering ultrafast, phase-controlled photochemical reaction pathways in small-to-medium molecules using tailored, few-femtosecond laser pulses. We here now present our ongoing progress on novel applications for multi-state excited states dynamics in triatomic systems. We explore the sub-fs UV/VIS induced dynamics in CO$_2$ molecule, that presents degeneracies in its molecular landscape and for which scarce reliable data is available, even for single-state scenarios.
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