Ultrafast nonadiabatic dynamics probed by nitrogen K-edge absorption spectroscopy

Area of Science:

• Quantum dynamics simulations
• Ultrafast spectroscopy
• Theoretical chemistry

Background:

• X-ray Free Electron Lasers (X-FELs) enable femtosecond timescale structural dynamics probing.
• Interpreting X-FEL data requires advanced theoretical methods for ultrafast regimes.
• Time-dependent treatments beyond the Born-Oppenheimer approximation are crucial.

Purpose of the Study:

• To perform quantum dynamics simulations of ultrafast X-ray Absorption Near-Edge Structure (XANES) spectra.
• To investigate photoexcited pyrazine dynamics involving coupled electronic states.
• To evaluate computationally efficient methods for simulating excited-state XANES spectra.

Main Methods:

• Time-evolving Gaussian basis functions (GBFs) for quantum dynamics.
• Multi-configurational second order perturbation theory restricted active space (RASPT2) method.
• Maximum Overlap Method (MOM) combined with time-dependent Density Functional Theory (TDDFT).

Main Results:

• MOM/TDDFT qualitatively reproduces RASPT2 simulation features with significantly lower computational cost.
• Conical intersections require multi-configurational treatments for accurate simulation.
• A small number of GBFs effectively describes the spectroscopic observable for nuclear dynamics.

Conclusions:

• Computationally efficient methods like MOM/TDDFT are valuable for interpreting ultrafast XANES spectra.
• Multi-configurational approaches are necessary for capturing complex dynamics like conical intersections.
• This work offers efficient computational strategies for analyzing larger systems in ultrafast spectroscopy.