Ultrafast internal conversion in ethylene. I. The excited state lifetime

Area of Science:

• Physical Chemistry
• Quantum Dynamics
• Molecular Spectroscopy

Background:

• Non-adiabatic dynamics in molecules are crucial for understanding photochemical reactions.
• The excited electronic state lifetime of ethylene (C2H4) has been a subject of debate between theoretical predictions and experimental observations.
• Previous theoretical models failed to accurately reproduce experimental excited state lifetimes.

Purpose of the Study:

• To investigate the non-adiabatic dynamics of ethylene upon π → π∗ excitation.
• To resolve the long-standing discrepancy between theoretical and experimental lifetimes of ethylene's excited state.
• To elucidate the factors contributing to the observed photoion yield decay.

Main Methods:

• Combined theoretical and experimental approach.
• Femtosecond time-resolved photoelectron spectroscopy (TRPES) simulation using ab initio multiple spawning with multi-state second order perturbation theory.
• Pump-probe experiments utilizing femtosecond vacuum ultraviolet pulses (hν = 7.7 eV).

Main Results:

• Excellent agreement achieved between simulated TRPES and experimental photoion signals.
• The study explains the previously observed discrepancy in excited state lifetimes.
• Energetic factors were identified as the primary contributors to the fast decay of photoion yield, with electronic factors also playing a role.

Conclusions:

• The combined theoretical and experimental approach successfully reconciles differing results on ethylene's excited state lifetime.
• The findings provide a clearer understanding of the non-adiabatic dynamics governing ethylene's excited state.
• Energetic factors significantly influence the decay pathways of the photoion yield in ethylene.