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1. Energy relaxation of N2O in gaseous, supercritical, and liquid xenon and SF6

   https://doi.org/10.1063/5.0235760  

   Töpfer, Kai; Erramilli, Shyamsunder; Ziegler, Lawrence D; Meuwly, Markus (November 2024, The Journal of Chemical Physics)

   Rotational and vibrational energy relaxation (RER and VER) of N2O embedded in xenon and SF6 environments ranging from the gas phase to the liquid, including the supercritical regime, is studied at a molecular level. Calibrated intermolecular interactions from high-level electronic structure calculations, validated against experiments for the pure solvents, were used to carry out classical molecular dynamics simulations corresponding to experimental state points for near-critical isotherms. The computed RER rates in low-density solvents of krotXe=(3.67±0.25)×1010 s−1 M−1 and krotSF6=(1.25±0.12)×1011 s−1 M−1 compare well with the rates determined by the analysis of two-dimensional infrared experiments. Simulations find that an isolated binary collision description is successful up to solvent concentrations of ∼4 M. For higher densities, including the supercritical regime, the simulations do not correctly describe RER, probably due to the neglect of solvent–solute coupling in the analysis of the rotational motion. For VER, the near-quantitative agreement between simulations and pump–probe experiments captures the solvent density-dependent trends. 

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2. Ultrafast 2DIR comparison of rotational energy transfer, isolated binary collision breakdown, and near critical fluctuations in Xe and SF ₆ solutions

   https://doi.org/10.1063/5.0118395  

   Rotondaro, Matthew C.; Jain, Arkash; Erramilli, Shyamsunder; Ziegler, Lawrence D. (November 2022, The Journal of Chemical Physics)

   The density dependence of rotational and vibrational energy relaxation (RER and VER) of the N 2 O ν 3 asymmetric stretch in dense gas and supercritical Xe and SF 6 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. N 2 O RER is ∼30% more efficient in SF 6 than in Xe due to additional relaxation pathways in SF 6 and electronic factor differences. 2DIR analysis revealed that N 2 O RER exhibits a critical slowing effect in SF 6 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 N 2 O 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 N 2 O from these near critical Xe density fluctuations. The N 2 O ν 3 VER density dependence in SF 6 is different than that seen for RER, indicating a different coupling to the near critical environment than RER. N 2 O ν 3 VER is only about ∼7 times slower than RER in SF 6 . 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 6 that is not possible for Xe. 

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3. Ultrafast solvation dynamics of nitrous oxide in alkylmethylimidazolium ionic liquids

   https://doi.org/10.1063/5.0325484  

   Parrack, Tyler A; Garrett-Roe, Sean (May 2026, The Journal of Chemical Physics)

   Nitrous oxide is a potent greenhouse gas and the leading ozone depleting pollutant. 1-alkyl-3-methylimidazolium bistriflimide based ionic liquids are effective at trapping N2O as well as CO2. Despite being similar to CO2 in many ways, N2O has a permanent dipole moment that allows it to dissolve in both nonpolar and polar solvents. Fourier transform infrared spectroscopy, polarization-controlled two-dimensional infrared spectroscopy, and ultrafast pump–probe spectroscopy of the asymmetric stretch (ν3) band of N2O characterize its solvation structure and dynamics in a series of alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids ([CnC1Im][Tf2N], n = 2–12). The asymmetric stretch redshifts by 4 cm−1 as the imidazolium alkyl chain increases from 2 to 12 carbons. Vibrational relaxation occurs on a 71–94 ps timescale, decreasing with chain length. Vibrational energy redistribution rates are independent of chain length (14 ps). Rotational correlation times increase with chain length from 20 to 37 ps. Reorientational dynamics are modeled as wobbling in a cone and compared to hydrodynamic and quasihydrodynamic models of rotational diffusion. The polarization-weighted frequency fluctuation correlation functions (PW-FFCFs) decay due to both reorientation-induced spectral diffusion and structural spectral diffusion (SSD). The PW-FFCF is modeled as wobbling in a cone with a first-order Stark effect. The experiments support the solvation of N2O in the charge enhanced domains of the ionic liquids, similar to CO2. 

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4. Lipid Landscapes: Vibrational Spectroscopy for Decoding Membrane Complexity

   https://doi.org/10.1146/annurev-physchem-090722-010230  

   Chen, Xiaobing; Al-Mualem, Ziareena A; Baiz, Carlos R (June 2024, Annual Review of Physical Chemistry)

   Cell membranes are incredibly complex environments containing hundreds of components. Despite substantial advances in the past decade, fundamental questions related to lipid-lipid interactions and heterogeneity persist. This review explores the complexity of lipid membranes, showcasing recent advances in vibrational spectroscopy to characterize the structure, dynamics, and interactions at the membrane interface. We include an overview of modern techniques such as surface-enhanced infrared spectroscopy as a steady-state technique with single-bilayer sensitivity, two-dimensional sum-frequency generation spectroscopy, and two-dimensional infrared spectroscopy to measure time-evolving structures and dynamics with femtosecond time resolution. Furthermore, we discuss the potential of multiscale molecular dynamics (MD) simulations, focusing on recently developed simulation algorithms, which have emerged as a powerful approach to interpret complex spectra. We highlight the ongoing challenges in studying heterogeneous environments in multicomponent membranes via current vibrational spectroscopic techniques and MD simulations. Overall, this review provides an up-to-date comprehensive overview of the powerful combination of vibrational spectroscopy and simulations, which has great potential to illuminate lipid-lipid, lipid-protein, and lipid-water interactions in the intricate conformational landscape of cell membranes. 

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5. The vibrational relaxation of a charged solute probes the vibrational density of states at oxide/water interfaces

   https://doi.org/10.1063/5.0317085  

   Mandal, Bijoya; Zou, Yunqian; Dadashi, Somaiyeh; DelloStritto, Mark J; Remsing, Richard C; Piontek, Stefan M; Li, Hao; Klein, Michael L; Borguet, Eric (April 2026, The Journal of Chemical Physics)

   The vibrational lifetime of solute molecules is predicted to be slower at interfaces; however, ultrafast measurements show that this behavior can vary dramatically depending on interfacial structure and vibrational coupling pathways. Surprisingly, the nitrile stretch of SCN− exhibits unexpectedly rapid vibrational energy relaxation at aqueous (D2O) alumina interfaces, while there is no appreciable difference in relaxation in bulk and interfacial H2O. For interfacial D2O, the CN stretch lifetime is nearly three times shorter than in bulk D2O (T1 ∼ 22 ps). Ab initio simulations reveal an increased vibrational density of states (VDOS) at the low frequency OD stretch region compared to bulk D2O, enhancing overlap between SCN− and D2O vibrational modes. Additional factors—including stronger transition dipole–transition dipole coupling arising from reduced dielectric screening and increased orientational ordering of interfacial molecules—further accelerate vibrational relaxation at the interface. To directly probe how interfacial VDOS varies with surface structure, we employed the CN stretch lifetime of SCN− as a reporter of the O–D VDOS at two model alumina surfaces: Al2O3(0001)/D2O and Al2O3(112¯0)/D2O. IR pump–vibrational sum frequency generation (vSFG) probe measurements show a shorter vibrational lifetime at the Al2O3(0001)/D2O interface (7.1 ps) compared to the Al2O3(112¯0)/D2O interface (8.7 ps). Molecular dynamics simulations support these findings, showing that the low-frequency O–D stretch VDOS at the Al2O3(0001)/D2O interface is approximately ∼1.2 times higher than at the Al2O3(112¯0)/D2O surface. The higher VDOS provides more available accepting states for vibrational energy transfer, thus shortening the vibrational lifetime. Together, these results demonstrate that vibrational lifetimes of interfacial solutes provide a powerful experimental probe of interfacial VDOS and solute–solvent coupling. This approach offers new insight into vibrational relaxation pathways and the microscopic origins of energy dissipation in the bulk and at interfaces. 

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