Lawrence D. Ziegler

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.