title: Vibrational dephasing in ionic liquids as a signature of hydrogen bonding.
abstract: Understanding both structure and dynamics is crucial for producing tailor-made ionic liquids (ILs). We studied the vibrational and structural dynamics of medium versus weakly hydrogen-bonded CH groups of the imidazolium ring in ILs of the type [1-alkyl-3-methylimidazolium][bis(trifluoromethanesulfonyl)imide] ([Cn mim][NTf2 ]), with n=1, 2, and 8, by time-resolved coherent anti-Stokes Raman scattering (CARS) and quantum-classical hybrid (QCH) simulations. From the time series of the CARS spectra, dephasing times were extracted by modeling the full nonlinear response. From the QCH calculations, pure dephasing times were obtained by analyzing the distribution of transition frequencies. Experiments and calculations reveal larger dephasing rates for the vibrational stretching modes of C(2)H compared with the more weakly hydrogen-bonded C(4,5)H. This finding can be understood in terms of different H-bonding motifs and the fast interconversion between them. Differences in population relaxation rates are attributed to Fermi resonance interactions.
authors.affiliation: Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock (Germany).
authors.forename: Susana
authors.initials: S
authors.lastname: Chatzipapadopoulos
authors.affiliation: Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock (Germany).
authors.forename: Tobias
authors.initials: T
authors.lastname: Zentel
authors.affiliation: Institute of Chemistry, University of Rostock, Dr.-Lorenz-Weg 1, 18059 Rostock, (Germany). ralf.ludwig@uni-rostock.de.|Leibniz-Institute of Catalysis, University of Rostock, A.-Einstein-Str. 29a, 18059 Rostock (Germany). ralf.ludwig@uni-rostock.de.
authors.forename: Ralf
authors.initials: R
authors.lastname: Ludwig
authors.affiliation: Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock (Germany).
authors.forename: Matthias
authors.initials: M
authors.lastname: Lütgens
authors.affiliation: Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock (Germany). stefan.lochbrunner@uni-rostock.de.
authors.forename: Stefan
authors.initials: S
authors.lastname: Lochbrunner
authors.affiliation: Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock (Germany). oliverkuehn@uni-rostock.de.
authors.forename: Oliver
authors.initials: O
authors.lastname: Kühn
keywords: coherent anti-Stokes Raman scattering; hydrogen bonding; ionic liquids; quantum chemical calculations; vibrational dephasing