Simulating closed- and open-quantum photoinduced electron dynamics for time-resolved NEXAFS.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Near edge x-ray absorption fine structure (NEXAFS) spectroscopy provides insights into electronic structure.
• Ultrafast internal conversion is a critical process in photochemistry and photophysics.
• Accurate theoretical modeling of NEXAFS spectra is essential for understanding molecular dynamics.

Purpose of the Study:

• To develop a real-time method for computing NEXAFS spectra based on the time-dependent Schrödinger equation.
• To apply this method to study ultrafast internal conversion in gas-phase thymine.
• To validate the method by reproducing experimental NEXAFS spectra and tracking population transfer.

Main Methods:

• Propagation of the time-dependent Schrödinger equation in the electronic state space.
• Calculation of transition dipole moments using linear-response time-dependent density functional theory (LR-TDDFT).
• Implementation compatible with singly excited states and generalizable to correlated wavefunction methods.
• Application of the stochastic Schrödinger equation for time-resolved NEXAFS signal generation.

Main Results:

• Computed NEXAFS O K-edge spectra for thymine from ground, S2, and S1 states.
• Reproduced experimental spectra, identifying a peak at 526.5 eV associated with the S1 state.
• Successfully modeled the S2 → S1 population transfer using a 60 fs decay time, capturing time-resolved spectral changes.
• Computed ground- and excited-state NEXAFS spectra for azobenzene isomers.

Conclusions:

• The developed real-time NEXAFS method accurately describes molecular dynamics, including ultrafast internal conversion.
• The method provides a powerful tool for investigating transient electronic states and spectral evolution.
• This approach can be extended to study complex photochemical processes in various molecular systems.