Simulating Molecular Single Vibronic Level Fluorescence Spectra with Ab Initio Hagedorn Wavepacket Dynamics.

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Quantum Dynamics

Background:

• Simulating molecular fluorescence spectra is crucial for understanding photophysical processes.
• Accurate prediction of single vibronic level (SVL) spectra requires methods that account for vibrational complexities.
• Existing methods may struggle with arbitrary initial vibrational states or complex molecular systems.

Purpose of the Study:

• To develop and present a practical, ab initio time-dependent method for efficient simulation of SVL fluorescence spectra.
• To apply this method to polyatomic molecules, starting from arbitrary initial vibrational levels.
• To validate the method's accuracy against experimental data and previously reported simulations.

Main Methods:

• Utilized Hagedorn wavepackets for efficient simulation of molecular dynamics.
• Employed wavepacket dynamics on a 66-dimensional harmonic potential energy surface.
• Constructed the potential energy surface using density functional theory (DFT) calculations.
• Incorporated mode distortion and mode mixing (Duschinsky rotation) within the harmonic approximation.

Main Results:

• Successfully computed SVL fluorescence spectra for anthracene from multiply excited vibrational levels.
• Reproduced previously reported simulation results for singly excited levels (e.g., 12¹, 1̅1̅¹).
• Achieved good agreement between computed spectra and experimental data for various initial states.
• Demonstrated that all spectra can be obtained from a single wavepacket trajectory, enhancing computational efficiency.

Conclusions:

• The Hagedorn wavepacket method provides an efficient and accurate approach for simulating SVL fluorescence spectra.
• The method effectively handles vibrational complexities like mode distortion and mixing.
• This technique offers a practical tool for studying photophysics in polyatomic molecules from diverse initial vibrational conditions.