Simulation of Impulsive Vibrational Spectroscopy.

Area of Science:

• Computational Chemistry
• Photochemistry
• Molecular Biophysics

Background:

• Ultrafast energy dynamics in DNA/RNA nucleobases are crucial for understanding UV damage and repair.
• Electronic excitation initiates complex molecular motions and energy dissipation pathways.

Purpose of the Study:

• To investigate the subpicosecond energy distribution in DNA/RNA nucleobases following UV excitation.
• To elucidate the role of pump pulse intensity on molecular dynamics and vibrational mode activation.

Main Methods:

• Fully atomistic electron-nuclear real-time propagation protocol.
• Computation of impulsive vibrational spectroscopy.
• Ehrenfest approximation for mean potential energy surface calculations.

Main Results:

• UV excitation creates a coherent superposition of ground and excited electronic states.
• Low pump fluency correlates molecular displacements with ground-state normal modes.
• High pump fluency shifts dynamics towards excited-state normal modes.
• Excited-state forces drive nuclear motion, distributing energy and potentially initiating nonradiative deactivation.

Conclusions:

• Pump pulse intensity critically controls energy flow and vibrational coupling in nucleobases.
• Excited-state forces are key drivers of ultrafast nonradiative deactivation.
• Understanding these initial dynamics is vital for photoprotection mechanisms.