飞秒时间分辨拉曼光谱用于研究β-胡萝卜素单重激发态内转换和振动弛豫过程

Ultrafast internal conversion and vibrational relaxation in singlet excited-state all-trans-β-carotene as revealed by femtosecond time-resolved stimulated Raman spectroscopy

搭建了飞秒时间分辨受激拉曼光谱(FSRS)装置,并用于研究全反式β-胡萝卜素单重电子激发态超快内转换和振动弛豫过程.基于三脉冲"抽运-探测"方案搭建的时间分辨受激拉曼光谱装置同时实现了150fs的时间分辨率和23.7cm-1的光谱分辨率,光谱检测范围为300—4000cm-1.对全反式β-胡萝卜素电子激发态的飞秒时间分辨拉曼光谱研究表明,β-胡萝卜素被激发到S2态后,经由寿命约为0.3ps的中间态SX态实现向S1态的内转换(S2→SX→S1),S1态继而发生分子内和环境冷却引起的振动弛豫过程,时间尺度分别为0.3—0.6ps和～11ps,其中"热"S1态的寿命和S1→S0内转换的时间尺度相当(～11ps);S0态的分子内振动弛豫和"热"S0态振动弛豫过程的时间尺度分别为～0.3ps和～15ps.上述β-胡萝卜素的电子激发态内转换和振动弛豫特性有助于理解它在光合作用体系中的辅助捕光功能.

A femtosecond time-resolved stimulated Raman scattering （FSRS） apparatus was built, and was used to study the uhrafast internal conversion and vibrational relaxation processes between the singlet excited states of all-trans-β-carotene in solution. The FSRS apparatus, constructed on the basis of a three-pulse pump-probe scheme, achieved a time resolution of 150 fs, a spectral resolution of 23.7cm^-1, and a detection spectral window of 300-4000cm^-1 The FSRS results are interpreted in terms of a serial internal-conversion scheme between the low-lying singlet excited states, i.e. S2→Sx→S1→S0. Spectral dynamics analysis and kinetics analysis proved that the intra-molecular vibrational relaxation in the St state proceeds on a time scale of 0.3-0.6ps, while the ＇hot＇ SI state decays with a time constant similar to the lifetime of this state （-11ps）. Furthermore, the time scale of intra-molecular vibrational relaxation and vibrational cooling processes in the So state are shown to be -0.3 and -15ps, respectively. These results are useful for a deeper insight into the light-harvesting function of carotenoids in photosynthetic organisms.