Electronic Structure of Liquid Alkanes: A Representative Case of Liquid Hexanes and Cyclohexane Studied Using Polarization-Dependent Two-Photon Absorption Spectroscopy
Electronic Structure of Liquid Alkanes: A Representative Case of Liquid Hexanes and Cyclohexane Studied Using Polarization-Dependent Two-Photon Absorption Spectroscopy
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液体烷烃的电子结构：使用偏振相关双光子吸收光谱研究液体己烷和环己烷的代表案例
DOI：
10.1021/acs.jpca.1c06230
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Bradforth, Stephen E.
中科院分区：
文献类型：
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作者：
Bhattacharyya, Dhritiman;Zhang, Yuyuan;Elles, Christopher G.;Bradforth, Stephen E.
液体环己烷和己烷的双光子吸收 (2PA) 光谱报告能量范围为 6.4–8.5 eV (177–145 nm)，提供有关它们在散装液体中电子结构的详细信息。使用宽带泵浦-探针方式，我们通过同时吸收白光连续谱 (1.8-3.9 eV) 的 266 nm (4.6 eV) 泵浦光子和一个紫外-可见光探针光子来测量连续 2PA 光谱。通过单取代和双取代的运动方程耦合团簇法 (EOM-CCSD) 计算孤立气相分子的理论单光子吸收 (1PA) 和 2PA 截面，以证实实验光谱的分配，自然跃迁轨道 (NTO) 分析提供了跃迁中参与轨道的可视化。我们的分析表明，与涉及其他里德伯轨道的溶剂化跃迁相比，涉及电子提升至 3s 里德伯轨道的最低激发能的溶剂化跃迁发生更大程度的蓝移（环己烷为~0.55 eV，己烷为~0.18 eV），这与在水和醇中观察到的行为相似。所有其他转变在溶剂化时经历可忽略不计的（环己烷）或约 0.15-0.2 eV（己烷）的轻微红移。在这两种烷烃中，光谱完全由里德堡跃迁主导：1PA 和 2PA 光谱中最强烈的谱带是由于电子分别被激发到里德堡“p”和“d”型轨道，尽管终止于 3s 里德堡的一个跃迁具有显着的 2PA 强度。这项工作表明，烷烃中的气相电子跃迁性质在溶剂化时没有显着改变。此外，使用孤立分子框架的电子结构计算似乎为光谱分配的半定量图片提供了合理的起点，并且还可以分析液相吸收光谱的溶剂化变色位移。
Two-photon absorption (2PA) spectra of liquid cyclohexane and hexanes are reported for the energy range 6.4–8.5 eV (177–145 nm), providing detailed information about their electronic structures in bulk liquid. Using a broadband pump–probe fashion, we measured the continuous 2PA spectra by simultaneous absorption of a 266 nm (4.6 eV) pump photon and one UV–vis probe photon from the white-light continuum (1.8–3.9 eV). Theoretical one-photon absorption (1PA) and 2PA cross sections of isolated gas phase molecules are computed by the equation of motion coupled-cluster method with single and double substitutions (EOM-CCSD) to substantiate the assignment of the experimental spectra, and the natural transition orbital (NTO) analysis provides visualization of the participating orbitals in a transition. Our analysis suggests that upon solvation transitions at the lowest excitation energy involving promotion of electron to the 3s Rydberg orbitals are blue-shifted (∼0.55 eV for cyclohexane and ∼0.18 eV for hexanes) to a greater extent as compared to those involving other Rydberg orbitals, which is similar to the behavior observed for water and alcohols. All other transitions experience negligible (cyclohexane) or minor red-shift by ∼0.15-0.2 eV (hexane) upon solvation. In both alkanes, the spectra are entirely dominated by Rydberg transitions: the most intense bands in 1PA and 2PA spectra are due to the excitation of electrons to the Rydberg “p” and “d” type orbitals, respectively, although one transition terminating in the 3s Rydberg has significant 2PA strength. This work demonstrates that the gas phase electronic transition properties in alkanes are not significantly altered upon solvation. In addition, electronic structure calculations using an isolated-molecule framework appear to provide a reasonable starting point for a semiquantitative picture for spectral assignment and also to analyze the solvatochromic shifts for liquid phase absorption spectra.