Ultrafast 2DIR comparison of rotational energy transfer, isolated binary collision breakdown, and near critical...

The density dependence of rotational and vibrational energy relaxation (RER and VER) of the N₂O ν₃ asymmetric stretch in dense gas and supercritical Xe and SF₆ 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. N₂O RER is ∼30% more efficient in SF₆ than in Xe due to additional relaxation pathways in SF₆ and electronic factor differences. 2DIR analysis revealed that N₂O RER exhibits a critical slowing effect in SF₆ 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 N₂O 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 N₂O from these near critical Xe density fluctuations. The N₂O ν₃ VER density dependence in SF₆ is different than that seen for RER, indicating a different coupling to the near critical environment than RER. N₂O ν₃ VER is only about ∼7 times slower than RER in SF₆. 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 SF₆ that is not possible for Xe.