Fourth-order perturbative model for photoinduced internal conversion processes.

Area of Science:

• Photochemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Internal conversion (IC) rapidly converts electronic excitation energy to heat in molecular systems.
• Traditional kinetic theories struggle with IC due to simultaneous electronic and nuclear relaxation.
• Understanding IC is vital for biological and synthetic molecular functionalities.

Purpose of the Study:

• To develop a novel perturbative fourth-order phenomenological model for photoinduced IC.
• To incorporate finite laser bandwidths and nonequilibrium nuclear motions into IC modeling.
• To provide a framework for analyzing IC dynamics without requiring first-principles computational expertise.

Main Methods:

• A perturbative fourth-order phenomenological model was developed.
• The model incorporates effects of finite laser bandwidths and nonequilibrium nuclear motions.
• Parameters are obtainable through standard spectroscopic measurements.

Main Results:

• The model was applied to internal conversion preceding electrocyclic ring-opening in α-terpinene.
• The rate of wavepacket approach to excited-state degeneracy dictates population decay profile shape (Gaussian vs. exponential).
• Wavepacket dynamics, promoting mode displacement, and thermal fluctuations influence IC sensitivity but not the core physical picture.

Conclusions:

• A new model simplifies the study of photoinduced internal conversion.
• The wavepacket's approach to degeneracy is the key factor in population decay dynamics.
• A wavepacket representation can visualize correlations between population transfer and energy dissipation.