Hot Ground State Cooling Following Ultrafast Photoisomerization: Time-Resolved Infrared Spectroscopy

The ultrafast photophysics of many isomerizing molecules involves subpicosecond formation of a twisted hot ground state, which transfers energy to the environment through vibrational relaxation (cooling) over several picoseconds. In time-resolved infrared (TR-IR) spectroscopy, hot ground state transients show frequency shifts and band reshapings, which cannot be described through kinetic models that assume static spectral functions. We report a simple anharmonic cascade framework, which uses a single adjustable parameter associated with scaling the probability of vibrational energy transfer to the environment, for describing hot ground state cooling (HGSC) in TR-IR spectroscopy. The model is demonstrated against measurements on the cyan fluorescent protein chromophore. To best describe HGSC band shape evolution, the model utilizes ab initio data on anharmonic vibrational structure and nonadiabatic molecular dynamics trajectories of S₁→ S₀ internal conversion for realistic vibration occupation numbers of the nascent hot ground state. The modeling framework is readily extended to include mode-specific rates for intermolecular energy transfer and can be applied to any ultrafast isomerizing molecule for which anharmonic vibrational properties can be computed.