Ultrafast 2DIR comparison of rotational energy transfer, isolated binary collision breakdown, and near critical fluctuations in Xe and SF solutions.

The density dependence of rotational and vibrational energy relaxation (RER and VER) of the NO ν asymmetric stretch in dense gas and supercritical Xe and SF solutions for near critical isotherms is measured by ultrafast 2DIR and infrared pump-probe spectroscopy. 2DIR analysis provides precise measurements of RER at all gas and supercritical solvent densities. An isolated binary collision (IBC) model is sufficient to describe RER for solvent densities ≤ ∼4M where rotational equilibrium is re-established in ∼1.5-2.5 collisions. NO RER is ∼30% more efficient in SF than in Xe due to additional relaxation pathways in SF and electronic factor differences. 2DIR analysis revealed that NO RER exhibits a critical slowing effect in SF at near critical density (ρ* ∼ 0.8) where the IBC model breaks down. This is attributable to the coupling of critical long-range density fluctuations to the local NO free rotor environment. No such RER critical slowing is observed in Xe because IBC break down occurs much further from the Xe critical point. Many body interactions effectively shield NO from these near critical Xe density fluctuations. The NO ν VER density dependence in SF is different than that seen for RER, indicating a different coupling to the near critical environment than RER. NO ν VER is only about ∼7 times slower than RER in SF. In contrast, almost no VER decay is observed in Xe over 200 ps. This VER solvent difference is due to a vibrationally resonant energy transfer pathway in SF that is not possible for Xe.