We present a novel technique to monitor dynamics in interfacial systems through temporal correlations in x-ray photoelectron spectroscopy (XPS) signals. To date, the vast majority of time-resolved x-ray spectroscopy techniques rely on pump–probe schemes, in which the sample is excited out of equilibrium by a pump pulse, and the subsequent dynamics are monitored by probe pulses arriving at a series of well-defined delays relative to the excitation. By definition, this approach is restricted to processes that can either directly or indirectly be initiated by light. It cannot access spontaneous dynamics or the microscopic fluctuations of ensembles in chemical or thermal equilibrium. Enabling this capability requires measurements to be performed in real (laboratory) time with high temporal resolution and, ultimately, without the need for a well-defined trigger event. The time-correlation XPS technique presented here is a first step toward this goal. The correlation-based technique is implemented by extending an existing optical-laser pump/multiple x-ray probe setup by the capability to record the kinetic energy and absolute time of arrival of every detected photoelectron. The method is benchmarked by monitoring energy-dependent, periodic signal modulations in a prototypical time-resolved XPS experiment on photoinduced surface-photovoltage dynamics in silicon, using both conventional pump–probe data acquisition, and the new technique based on laboratory time. The two measurements lead to the same result. The findings provide a critical milestone toward the overarching goal of studying equilibrium dynamics at surfaces and interfaces through time correlation-based XPS measurements.

中文翻译：

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通过 X 射线光电子能谱中的时间相关性揭示实时界面电子动力学

我们提出了一种通过 X 射线光电子能谱 (XPS) 信号中的时间相关性来监测界面系统动力学的新技术。迄今为止，绝大多数时间分辨 X 射线光谱技术依赖于泵浦-探针方案，其中样品被泵浦脉冲激发出平衡，随后的动力学由到达一系列相对于激发的明确定义的延迟。根据定义，这种方法仅限于可以直接或间接由光启动的过程。它无法获得自发动力学或化学或热平衡中集合的微观波动。启用此功能需要在实时（实验室）时间内以高时间分辨率进行测量，最终，无需明确定义的触发事件。这里介绍的时间相关 XPS 技术是实现这一目标的第一步。这种基于相关性的技术是通过扩展现有的光学激光泵/多 X 射线探测器设置来实现的，该设置能够记录每个检测到的光电子的动能和绝对到达时间。该方法通过在硅中光致表面光电压动力学的原型时间分辨 XPS 实验中监测能量相关的周期性信号调制来进行基准测试，使用传统的泵-探针数据采集和基于实验室时间的新技术。两次测量导致相同的结果。