Körber Thomas, Stäglich Robert, Gainaru Catalin, Böhmer Roland, Rössler Ernst A
Anorganische Chemie III and Nordbayerisches NMR Zentrum, Universität Bayreuth, 95440 Bayreuth, Germany.
Fakultät Physik, Technische Universität Dortmund, 44221 Dortmund, Germany.
J Chem Phys. 2020 Sep 28;153(12):124510. doi: 10.1063/5.0022155.
Relaxation spectra of molecular glass formers devoid of secondary relaxation maxima, as measured by dielectric spectroscopy (DS), nuclear magnetic resonance (NMR) relaxometry, photon correlation spectroscopy (PCS), and Fabry-Perot interferometry, are quantitatively compared in terms of the Kohlrausch stretching parameter β. For a reliable estimate of β, the excess wing contribution has to be included in the spectral analysis. The relaxation stretching probed by PCS and NMR varies only weakly among the liquids (β = 0.58 ± 0.06). It is similar to that found in DS, provided that the liquid is sufficiently nonpolar (relaxation strength Δε≲6). For larger strengths, larger β (narrowed relaxation spectra) are found when compared to those reported from NMR and PCS. Frequency-temperature superposition (FTS) holds for PCS and NMR. This is demonstrated by data scaling and, for the few glass formers for which results are available, by the equivalence of the susceptibilities χ ωτ∝χ τ∝χ ω, i.e., measuring at a constant frequency is equivalent to measuring at a constant temperature or constant correlation time. In this context, a plot of the spin-lattice relaxation rate R(T) as a function of the spin-spin relaxation rate R(T) is suggested to reveal the stretching parameter without the need to perform frequency-dependent investigations. Dielectrically, we identify a trend of increasing deviations from FTS with increasing Δε. Depending on the technique and glass former, the relative relaxation strength of the excess wing varies, whereas its exponent appears to be method independent for a given substance. For polar liquids, we discuss possible reasons for the discrepancy between the results from PCS and NMR as compared to those from DS.
通过介电谱（DS）、核磁共振（NMR）弛豫测量法、光子相关光谱法（PCS）和法布里-珀罗干涉测量法测得的，没有二级弛豫最大值的分子玻璃形成体的弛豫光谱，根据科尔劳施拉伸参数β进行了定量比较。为了可靠地估计β，在光谱分析中必须包括过量翼部贡献。PCS和NMR探测到的弛豫拉伸在这些液体中变化很小（β = 0.58 ± 0.06）。如果液体足够非极性（弛豫强度Δε≲6），则与DS中发现的情况相似。对于更大的强度，与NMR和PCS报道的结果相比，会发现更大的β（弛豫光谱变窄）。频率-温度叠加（FTS）适用于PCS和NMR。这通过数据缩放得到证明，并且对于少数有结果可用的玻璃形成体，通过磁化率χ ωτ∝χ τ∝χ ω的等价性得到证明，即在恒定频率下测量等同于在恒定温度或恒定相关时间下测量。在这种情况下，建议绘制自旋-晶格弛豫率R(T)作为自旋-自旋弛豫率R(T)的函数图，以揭示拉伸参数，而无需进行频率相关的研究。在介电方面，我们发现随着Δε的增加，与FTS的偏差有增加的趋势。根据技术和玻璃形成体的不同，过量翼部的相对弛豫强度会有所变化，而对于给定物质，其指数似乎与方法无关。对于极性液体，我们讨论了PCS和NMR的结果与DS的结果之间存在差异的可能原因。
