Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, São Paulo, Brazil.
J Phys Chem A. 2012 Jul 5;116(26):7033-8. doi: 10.1021/jp303789s. Epub 2012 Jun 25.
In this report, we investigate the influence of temperature on the two-photon absorption (2PA) spectrum of all-trans-β-carotene using the femtosecond white-light-continuum Z-scan technique. We observed that the 2PA cross-section decreases quadratically with the temperature. Such effect was modeled using a three-energy-level diagram within the sum-over-essential states approach, assuming temperature dependencies to the transition dipole moment and refractive index of the solvent. The results show that the transition dipole moments from ground to excited state and between the excited states, which governed the two-photon matrix element, have distinct behaviors with the temperature. The first one presents a quadratic dependence, while the second exhibits a linear dependence. Such effects were attributed mainly to the trans→cis thermal interconversion process, which decreases the effective conjugation length, contributing to diminishing the transition dipole moments and, consequently, the 2PA cross-section.
在本报告中，我们使用飞秒白光连续 Z 扫描技术研究了温度对全反式-β-胡萝卜素的双光子吸收（2PA）光谱的影响。我们观察到 2PA 截面随温度呈二次方减小。这种效应是通过在总和重要状态方法内使用三能级图模型来模拟的，假设对跃迁偶极矩和溶剂折射率的温度依赖性。结果表明，从基态到激发态以及激发态之间的跃迁偶极矩，控制着双光子矩阵元，其随温度呈现出不同的行为。前者呈二次方关系，后者呈线性关系。这些效应主要归因于反式→顺式热互变过程，它缩短了有效共轭长度，导致跃迁偶极矩减小，从而使 2PA 截面减小。
