Department of Chemical Physics, Lund University, Box 124, SE-21000, Lund, Sweden.
J Phys Chem B. 2009 Dec 24;113(51):16409-19. doi: 10.1021/jp906604j.
Two-dimensional electronic spectroscopy (2D) has been applied to beta-carotene in solution to shine new light on the ultrafast energy dissipation network in carotenoids. The ability of 2D to relieve spectral congestion provides new experimental grounds for resolving the rise of the excited state absorption signal between 18,000 and 19,000 cm(-1). In this spectral region, the pump-probe signals from ground state bleach and stimulated emission overlap strongly. Combined modeling of the time-evolution of 2D spectra as well as comparison to published pump-probe data allow us to draw conclusions on both the electronic structure of beta-carotene as well as the spectral densities giving rise to the observed optical lineshapes. To account for the experimental observations on all time scales, we need to include a transition in the visible spectral range from the first optically allowed excited state (S(2)-->S(n2)). We present data from frequency resolved transient grating and pump-probe experiments confirming the importance of this transition. Furthermore, we investigate the role and nature of the S* state, controversially debated in numerous previous studies. On the basis of the analysis of Feynman diagrams, we show that the properties of S*-related signals in chi(3) techniques like pump-probe and 2D can only be accounted for if S* is an excited electronic state. Against this background, we discuss a new interpretation of pump-deplete-probe and intensity-dependent pump-probe experiments.
二维电子光谱学（2D）已被应用于溶液中的β-胡萝卜素，以揭示类胡萝卜素中超快能量耗散网络的新情况。2D 缓解光谱拥挤的能力为解决 18,000 至 19,000 cm^(-1) 之间激发态吸收信号的上升提供了新的实验依据。在这个光谱区域，来自基态漂白和受激辐射的泵浦探测信号强烈重叠。对二维光谱的时间演化的组合建模以及与已发表的泵浦探测数据的比较，使我们能够得出关于β-胡萝卜素的电子结构以及导致观察到的光学线形状的光谱密度的结论。为了在所有时间尺度上解释实验观察结果，我们需要包括来自第一可见光谱范围的跃迁，即从第一光学允许的激发态（S(2)-->S(n2)）。我们提供了来自频率分辨瞬态光栅和泵浦探测实验的数据，证实了这一跃迁的重要性。此外，我们研究了 S态的作用和性质，这在许多先前的研究中存在争议。基于费曼图的分析，我们表明，在像泵浦探测和 2D 这样的 chi(3)技术中，与 S-相关的信号的性质只能在 S*是一个激发电子态的情况下得到解释。在此背景下，我们讨论了对泵浦耗散探测和强度相关泵浦探测实验的新解释。
