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首页> 外文期刊>Zeitschrift fur Naturforschung. A, A journal of physical sciences >Anharmonic Librations and the Temperature Dependence of NQR Frequencies
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Anharmonic Librations and the Temperature Dependence of NQR Frequencies

机译：非谐振动和NQR频率的温度依赖性

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The temperature dependence of NQR frequencies of nuclei in molecules undergoing quasi-harmonic rotational oscillations is calculated. Following the model proposed by Bayer, the instantaneous orientation of the electric field gradient principal axes is assumed to change from its average orientation as a result of rotational oscillations, while the EFG magnitude remains constant. The eigenstates and energy eigenvalues of a rotator in a potential of the form V(θ) = V_2θ~2 — V_4θ~4 are calculated by standard perturbation methods, followed by the evaluation of statistical averages of square angular amplitudes. For molecular crystals with more than one molecule in a unit cell the angular displacements are expressed in terms of normal coordinates, and the same procedure is followed. The final expression for the change in the NQR frequency consists of two terms, the first one being related to the harmonic part of the potential and the second one to the quartic term. At the absolute zero it yields a nonzero contribution to < θ~2 >, and for high temperatures it reproduces the nonlinear character of ν_Q (T) observed experimentally. The model has been tested with measurements on solid Cl_2 performed by Nakamura and Chihara, combined with libration mode frequencies measured in Raman spectroscopy by Cahill and Leroi and analysed according to our anharmonic libration picture. The fitting of our equation to experimental data supports Cahill and Leroi's hypothesis about the existence of an undetected libration mode in Cl_2 Raman spectra of frequency near 60 cm~(-1).
机译：计算了经历准谐波旋转振荡的分子中核的NQR频率的温度依赖性。根据Bayer提出的模型，假定电场梯度主轴的瞬时方向由于旋转振荡而从其平均方向发生变化，而EFG幅度保持恒定。通过标准扰动方法，计算出形式为V（θ）=V_2θ〜2_V_4θ〜4的势能的旋转器的本征态和能量本征值，然后对方角幅度的统计平均值进行评估。对于在一个晶胞中具有一个以上分子的分子晶体，其角位移以法向坐标表示，并遵循相同的步骤。 NQR频率变化的最终表达式包含两个术语，第一个与电位的谐波部分相关，第二个与四次项相关。在绝对值为零时，它对<θ〜2>产生非零贡献，而在高温下，它再现了实验观察到的ν_Q（T）的非线性特征。该模型已通过Nakamura和Chihara对固体Cl_2的测量进行了测试，并结合了Cahill和Leroi在拉曼光谱中测量的游离模式频率，并根据我们的非谐游离图像进行了分析。我们的方程对实验数据的拟合支持了Cahill和Leroi的假设，即在频率为60 cm〜（-1）的Cl_2拉曼光谱中存在未检测到的释放模式。

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