Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706.
Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706
Proc Natl Acad Sci U S A. 2022 Feb 8;119(6). doi: 10.1073/pnas.2117398119.
Two-dimensional (2D) optical spectroscopy contains cross-peaks that are helpful features for determining molecular structure and monitoring energy transfer, but they can be difficult to resolve from the much more intense diagonal peaks. Transient absorption (TA) spectra contain transitions similar to cross-peaks in 2D spectroscopy, but in most cases they are obscured by the bleach and stimulated emission peaks. We report a polarization scheme, <0°,0°,+θ(t),-θ(t)>, that can be easily implemented in the pump-probe beam geometry, used most frequently in 2D and TA spectroscopy. This scheme removes the diagonal peaks in 2D spectroscopies and the intense bleach/stimulated emission peaks in TA spectroscopies, thereby resolving the cross-peak features. At zero pump-probe delay, θ = 60° destructively interferes two Feynman paths, eliminating all signals generated by field interactions with four parallel transition dipoles, and the intense diagonal and bleach/stimulated emission peaks. At later delay times, θ(t) is adjusted to compensate for anisotropy caused by rotational diffusion. When implemented with TA spectroscopy or microscopy, the pump-probe spectrum is dominated by the cross-peak features. The local oscillator is also attenuated, which enhances the signal two times. This overlooked polarization scheme reduces spectral congestion by eliminating diagonal peaks in 2D spectra and enables TA spectroscopy to measure similar information given by cross-peaks in 2D spectroscopy.
二维（2D）光学光谱包含有助于确定分子结构和监测能量转移的交叉峰，但它们很难从强度更高的对角峰中分辨出来。瞬态吸收（TA）光谱包含与 2D 光谱中的交叉峰相似的跃迁，但在大多数情况下，它们被漂白和受激辐射峰所掩盖。我们报告了一种偏振方案，<0°，0°，+θ(t)，-θ(t)>，它可以很容易地在泵浦-探测光束几何结构中实现，这是 2D 和 TA 光谱学中最常用的方案。该方案消除了 2D 光谱学中的对角峰和 TA 光谱学中的强漂白/受激辐射峰，从而解析出交叉峰特征。在零泵浦-探测延迟时，θ=60°会破坏两个费曼路径，消除所有由与四个平行跃迁偶极子的场相互作用产生的信号，以及强的对角和漂白/受激辐射峰。在稍后的延迟时间，θ(t)会被调整以补偿由旋转扩散引起的各向异性。当与 TA 光谱学或显微镜一起实现时，泵浦-探测光谱主要由交叉峰特征主导。本振也被衰减，这将信号增强了两倍。这种被忽视的偏振方案通过消除 2D 光谱中的对角峰来减少光谱拥挤，并使 TA 光谱学能够测量类似于 2D 光谱学中交叉峰提供的信息。
