Cross peaks on 2D optical spectra are indicative of interactions between molecular excitonic states. Currently, the two conventional assignments of cross peaks are direct coupling and population transfer between excitonic states. Here, we show that there is another possible source of cross peaks. We theoretically demonstrate that for a model comprising two nondirectly interacting excitons or two-level systems (TLSs), cross peaks can arise if there is a complex-valued or quantum frequency-gap cross-correlation function between the two TLSs. Considering only real-valued or classical cross-correlation functions will result in no cross peaks. We derive and validate the mathematical expressions describing such cross peaks. We then simulate the 2D electronic spectra of an example model system comprising nondirectly interacting TLSs whose quantum cross-correlation functions arise from coupling to a common overdamped Brownian oscillator mode. We show that there are clear observational differences between such quantum correlation cross peaks with conventional direct coupling and population transfer cross peaks.

中文翻译：

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量子互相关函数产生的二维光学光谱上的交叉峰


2D 光学光谱上的交叉峰表示分子激子状态之间的相互作用。目前，交叉峰的两个常规分配是激子态之间的直接耦合和种群转移。在这里，我们展示了交叉峰还有另一个可能的来源。我们从理论上证明，对于包含两个非直接相互作用的激子或两能级系统 （TLS） 的模型，如果两个 TLS 之间存在复值或量子频率-间隙互相关函数，则可能会出现交叉峰值。仅考虑实值或经典互相关函数将导致没有交叉峰值。我们推导并验证了描述这种交叉峰的数学表达式。然后，我们模拟了一个示例模型系统的二维电子光谱，该系统包含非直接相互作用的 TLS，其量子互相关函数是由于耦合到常见的过阻尼布朗振荡器模式而产生的。我们表明，这种量子相关交叉峰与传统直接耦合和种群转移交叉峰之间存在明显的观测差异。