Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA.
Department of Chemistry, University of Kansas, Lawrence, Kansas 66045-7582, USA.
J Chem Phys. 2017 May 7;146(17):174102. doi: 10.1063/1.4982045.
Two-photon absorption (2PA) spectra of several prototypical molecules (ethylene, toluene, trans- and cis-stilbene, and phenanthrene) are computed using the equation-of-motion coupled-cluster method with single and double substitutions. The states giving rise to the largest 2PA cross sections are analyzed in terms of their orbital character and symmetry-based selection rules. The brightest 2PA transitions correspond to Rydberg-like states from fully symmetric irreducible representations. Symmetry selection rules dictate that totally symmetric transitions typically have the largest 2PA cross sections for an orientationally averaged sample when there is no resonance enhancement via one-photon accessible intermediate states. Transition dipole arguments suggest that the strongest transitions also involve the most delocalized orbitals, including Rydberg states, for which the relative transition intensities can be rationalized in terms of atomic selection rules. Analysis of the 2PA transitions provides a foundation for predicting relative 2PA cross sections of conjugated molecules based on simple symmetry and molecular orbital arguments.
采用含单双取代的含时微扰耦合簇方法计算了几种典型分子（乙烯、甲苯、反式和顺式二苯乙烯、菲）的双光子吸收（2PA）谱。根据轨道特征和基于对称性的选择定则，分析了产生最大 2PA 截面的态。最亮的 2PA 跃迁对应于完全对称的不可约表示的类里德堡态。对称性选择定则规定，当没有通过单光子可及的中间态进行共振增强时，对于各向同性平均样品，完全对称跃迁通常具有最大的 2PA 截面。跃迁偶极子参数表明，最强的跃迁也涉及最离域的轨道，包括里德堡态，对于这些轨道，相对跃迁强度可以根据原子选择定则来合理化。对 2PA 跃迁的分析为基于简单的对称性和分子轨道论点预测共轭分子的相对 2PA 截面提供了基础。
