Mode-Specific versus Local Heating Effects in Infrared-Laser-Driven Reactions

Using an infrared laser to control a molecule's reactivity by targeting specific vibrational modes has long been of interest to chemists. Rapid intramolecular vibrational energy redistribution (IVR) in molecules poses significant challenges to achieving this, as it quickly transfers the pumped energy to other molecular degrees of freedom (τ_IVR ∼ 1 ps). With recent advances in femtosecond pulsed laser capabilities, however, infrared-laser-driven vibrationally assisted reactivity is worth revisiting. In this theoretical work, we quantify the contributions of both mode-specific assistance and laser-induced heating to reaction rate enhancements of laser-driven molecules. Notably, reactions with lower activation barriers exhibit smaller relative rate enhancements. Furthermore, local heating contributions dominate for low-barrier reactions while the vibrationally assisted component is more prominent for high-barrier reactions (precise statements of what low- and high- barriers depend on thermal diffusivity of the solvent). We obtain approximate bounds for these rate enhancements. While pulsed laser driving yields rate enhancements several orders of magnitude greater than continuous-wave driving for the same absorbed power, the overall increments remain modest under typical experimental conditions, except for low-frequency modes, where they can be substantial.