Modeling irreversible molecular internal conversion using the time-dependent variational approach with sD₂ ansatz.

Area of Science:

• Quantum dynamics
• Chemical physics
• Spectroscopy

Background:

• Non-linear system-bath coupling significantly influences molecular dynamics.
• Understanding internal conversion is key to controlling photochemical processes.

Purpose of the Study:

• To investigate the impact of non-linear coupling between system and bath vibrational modes on internal conversion dynamics.
• To develop and apply a new theoretical approach accounting for entangled electron-vibrational states.

Main Methods:

• Utilized the Dirac-Frenkel variational approach with a novel sD2 ansatz.
• Expanded bath quantum harmonic oscillator states into superpositions of quantum coherent states.
• Employed a non-adiabatically coupled three-level model.

Main Results:

• Demonstrated that efficient irreversible internal conversion occurs when system vibrational levels resonate with a lower electronic state.
• Showed that quadratic vibrational-bath coupling is responsible for this efficient conversion.
• Highlighted the necessity of a non-Gaussian bath wavepacket representation.

Conclusions:

• Non-linear system-bath coupling, particularly quadratic coupling, plays a critical role in dictating internal conversion efficiency.
• The developed theoretical framework accurately captures complex electron-vibrational dynamics.
• Quadratic couplings lead to characteristic broadening and squeezing of bath wavepackets.