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Vibrational Sum Frequency Generation Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces
The Journal of Physical Chemistry Letters ( IF 4.8 ) Pub Date : 2019-08-23 00:00:00 , DOI: 10.1021/acs.jpclett.9b01487 Dhritiman Bhattacharyya 1 , Angelo Montenegro 1 , Noah T. Plymale 2 , Chayan Dutta 1 , Nathan S. Lewis 2 , Alexander V. Benderskii 1
The Journal of Physical Chemistry Letters ( IF 4.8 ) Pub Date : 2019-08-23 00:00:00 , DOI: 10.1021/acs.jpclett.9b01487 Dhritiman Bhattacharyya 1 , Angelo Montenegro 1 , Noah T. Plymale 2 , Chayan Dutta 1 , Nathan S. Lewis 2 , Alexander V. Benderskii 1
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The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hindered at room temperature and proceeds via 120° jumps between three isoenergetic minima in registry with the crystalline Si substrate. We have used line-shape analysis of polarization-selected vibrational sum frequency generation spectroscopy to determine the rotational relaxation rate of the surface methyl groups and have measured the temperature dependence of the relaxation rate between 20 and 120 °C. By fitting the measured rate to an Arrhenius dependence, we extracted an activation energy (the rotational barrier) of 830 ± 360 cm–1 and an attempt frequency of (2.9 ± 4.2) × 1013 s–1 for the methyl rotation process. Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation.
中文翻译：
振动总和频率产生光谱法测量甲基封端的硅（111）表面上的甲基的旋转势垒
甲基封端的Si（111）表面具有3倍的面内对称性，其中甲基的取向垂直于基材。甲基的螺旋桨状旋转在室温下受阻，并通过三个等能量极小值之间的120°跃迁进行，与晶体Si衬底一致。我们使用极化选择的振动总和频率产生光谱的线形分析来确定表面甲基的旋转弛豫率，并测量了弛豫率在20到120°C之间的温度依赖性。通过将测得的速率与Arrhenius依赖关系拟合，我们提取了830±360 cm –1的激活能量（旋转势垒）和（2.9±4.2）×10 13 s的尝试频率–1用于甲基旋转过程。与3倍余弦电势中的甲基谐波频率的比较表明，受阻旋转是通过单个甲基的不相关跳跃而不是齿轮状协调旋转而发生的。
更新日期：2019-08-23
中文翻译：
振动总和频率产生光谱法测量甲基封端的硅（111）表面上的甲基的旋转势垒
甲基封端的Si（111）表面具有3倍的面内对称性，其中甲基的取向垂直于基材。甲基的螺旋桨状旋转在室温下受阻，并通过三个等能量极小值之间的120°跃迁进行，与晶体Si衬底一致。我们使用极化选择的振动总和频率产生光谱的线形分析来确定表面甲基的旋转弛豫率，并测量了弛豫率在20到120°C之间的温度依赖性。通过将测得的速率与Arrhenius依赖关系拟合，我们提取了830±360 cm –1的激活能量（旋转势垒）和（2.9±4.2）×10 13 s的尝试频率–1用于甲基旋转过程。与3倍余弦电势中的甲基谐波频率的比较表明，受阻旋转是通过单个甲基的不相关跳跃而不是齿轮状协调旋转而发生的。
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