Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids

Area of Science:

• Physical Chemistry
• Molecular Dynamics
• Thermodynamics

Background:

• Understanding energy dissipation in liquids is crucial for chemical reaction dynamics.
• Molecular relaxation processes govern how excited molecules return to equilibrium.

Purpose of the Study:

• To investigate excess energy flow pathways during rotational and translational relaxation in four neat liquids.
• To compare energy transfer dynamics in polar, hydrogen-bonded liquids versus nonpolar liquids.
• To assess the validity of linear response theory in these nonequilibrium processes.

Main Methods:

• Classical molecular dynamics simulations.
• Energy flux analysis.
• Study of four neat liquids: H2O, MeOH, CCl4, and CH4.

Main Results:

• Ultrafast relaxation observed for both rotational and translational excitation in all liquids.
• Significantly faster energy flow in polar, hydrogen-bonded liquids (H2O, MeOH) compared to nonpolar liquids (CCl4, CH4).
• Rotational excitation in H-bonded liquids primarily transfers energy to librations; nonpolar liquids show balanced transfer to translational and rotational motions.
• Translational excitation predominantly transfers energy to translational motions across all studied liquids.
• Energy flow is highly localized, with >70% transferring to the first solvent shell.
• Linear response theory validity is solvent-dependent, deviating most for rotational excitation in nonpolar liquids.

Conclusions:

• Liquid properties (polarity, hydrogen bonding) significantly influence ultrafast energy relaxation pathways.
• Energy dissipation mechanisms differ based on excitation type and solvent characteristics.
• Linear response theory is not universally applicable to these nonequilibrium relaxation phenomena.