Quantum mechanics

Thermal equilibrium

Chemical physics

Social psychology

Redistribution (election)

Atomic physics

Excitation

Molecule

Picosecond

Internal energy

Dissipation

Vibrational energy relaxation

Thermodynamics

Laser

Physics

Degrees of freedom (physics and chemistry)

Organic chemistry

Psychology

Politics

Optics

Political science

Molecular physics

Relaxation (psychology)

Molecular dynamics

Computational chemistry

Chemistry

Molecular vibration

Non-equilibrium thermodynamics

Chemical Science

In some molecular systems, such as nucleobases, polyenes or the active ingredients of sunscreens, substantial amounts of photo-excitation energy are dissipated on a sub-picosecond time scale, raising questions such as: where does this energy go or among which degrees of freedom it is being distributed at such early times? Here we use transient absorption spectroscopy to track excitation energy dispersing from the optically accessible vibronic subsystem into the remaining vibrational subsystem of the solute and solvent. Monitoring the flow of energy during vibrational redistribution enables quantification of local molecular heating. Subsequent heat dissipation away from the solute molecule is characterized by classical thermodynamics and molecular dynamics simulations. Hence, we present a holistic approach that tracks the internal temperature and vibronic distribution from the act of photo-excitation to the restoration of the global equilibrium. Within this framework internal vibrational redistribution and vibrational cooling are emergent phenomena. We demonstrate the validity of the framework by examining a highly controversial example, carotenoids. We show that correctly accounting for the local temperature unambiguously explains their energetically and temporally congested spectral dynamics without the ad hoc postulation of additional 'dark' states. An immediate further application of this approach would be to monitor the excitation and thermal dynamics of pigment-protein systems.

The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating