Models of picosecond-scale mobilities in glassy and polycrystalline quinoline

In this paper we further explore the applicability of a vibrational T2 process based on the extended-exponential modulation model [Rothschild, Perrot, and Guillaume, J. Chem. Phys. 87, 7293 (1987)] to Raman correlation data of concentrated aqueous solutions of LiSCN and KSCN [Katō, Mol. Phys. 48, 1119 (1983); Katō and Takenaka, Mol. Phys. 46, 257 (1982)]. In general, the values of dispersion parameter α in the modulation function exp[−(t/τ)α], obtained from the fit of the theory to the isotropic correlation data of the CN oscillator, predict the prevalence of interrelated, collective dynamic processes in the medium that are the cause of the instantaneous oscillator transition frequency shifts (motional narrowing). In particular we predict, from the observed concentration dependence of α, strong short-time (fraction to several ps) cation–water–anion interactions that, in the more concentrated LiSCN–H2O systems at 303 K, are above a site percolation threshold with a value of α∼0.3 (close to that found in glasses). The expectation value of t, 〈t〉=τΓ(1+1/α), becomes critical near a concentration of 5 mol/ℓ and shows a pronounced Vogel–Fulcher-type temperature dependence (T0=250 K) in the 10 mol/ℓ LiSCN–H2O system over a range 0.45–76 ps. However, since α approaches its limiting value=1 at the highest temperature reported (353 K), the large-cluster cation–water–anion distributions in LiSCN–H2O must be rather tenuous. In contrast, the characteristics of α and of 〈t〉 for the KSCN–H2O systems agree with the relatively weak cation–water forces; the (inverse) concentration dependence of α is linear, its temperature dependence is flat, and the Vogel–Fulcher-type temperature behavior of 〈t〉 for the 10 mol/ℓ solution stretches merely from 0.7 to 1.4 ps.

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Picosecond dynamics and molecular aggregation from vibrational dephasing in the fluid phases of some 4-n-alkyl-4′-cyanobiphenyl liquid crystals

Rothschild

Perrot

Zen

1991

The Journal of Chemical Physics

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We describe the temperature dependence of the inhomogeneously broadened CN Raman profile I(ω) at ωc ∼2230 cm−1 of the title compounds (n=1, 6, 8, 9, 11, 12) in their isotropic liquid phase and solutions (CHCl3, CCl4) by simulating the oscillator amplitude correlation function by a vibrational equilibrium renewal process in terms of random fluctuations of the oscillator transition frequency ω(t)=ωc +ω1(t) about its central value ωc. To this effect, the autocorrelation function of the frequency shift ω1(t) is expressed as a probability density function (PDF) F̂(t) of recurrence times of the stochastic motional narrowing events in the local environment of the CN oscillators. System-related physical meaning and satisfactory data fit is obtained if F̂(t) is understood as an expansion in terms of parallel, independent exponential relaxation processes with characteristic times τ that are distributed by a PDF ρα(τ)=〈τ〉h(τ)/τ, where α is the dispersion parameter of the extended exponential and 〈τ〉 the expectation of τ. Width and ranges of h(τ) show strong molecule–molecule clustering, possibly indicating a trend with alkyl chain length. At temperatures just above the mesophase–liquid-phase transition, the range of the prevalent relaxation times τ in the local environment of the CN oscillators is of the order of 1–4 ps. Only at temperatures near 570 K or by high dilution in the solvents are the inter- and intracluster forces sufficiently diminished to approximate those of ordinary fluids. We consider our method to give a realistic description of the dynamics of types of macroscopically isotropic fluids where, nevertheless, the shape, size, and polarity of their molecules lead to a degree of aggregation that weakens the identity and the influence of constituent members. The temporary structure of the macroscopically isotropic fluids in the liquid-crystal systems is best understood by admitting a significant presence of randomly distributed local regions of dynamic nematicity, causing temperature-dependent relaxation pathways over 10–50 Å distances.

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Models of picosecond-scale mobilities in glassy and polycrystalline quinoline

Cited by 3 publications

References 16 publications

Picosecond dynamics in amorphous systems with local order

Picosecond dynamics in amorphous systems with local order

Site percolation and Vogel–Fulcher behavior on picosecond time scales in concentrated electrolytes: Raman spectra of aqueous solutions of LiSCN and KSCN

Picosecond dynamics and molecular aggregation from vibrational dephasing in the fluid phases of some 4-n-alkyl-4′-cyanobiphenyl liquid crystals

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