Pump–probe experiments with subfemtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of subfemtosecond pulse pairs from a two-colour X-ray free-electron laser. By measuring the delay between the two pulses with an angular streaking diagnostic, we characterize the group velocity of the X-ray free-electron laser and show control of the pulse delay down to 270 as. We confirm the application of this technique to a pump–probe measurement in core-ionized para-aminophenol. These results reveal the ability to perform pump–probe experiments with subfemtosecond resolution and atomic site specificity.

亚飞秒分辨率的泵浦探针实验是理解量子系统中电子动力学的关键。在这里，我们演示了双色 X 射线自由电子激光器亚飞秒脉冲对的生成和控制。通过使用角条纹诊断测量两个脉冲之间的延迟，我们表征了 X 射线自由电子激光器的群速度，并显示了对低至 270 as 的脉冲延迟的控制。我们确认了该技术在核心电离对氨基苯酚的泵探针测量中的应用。这些结果揭示了以亚飞秒分辨率和原子位点特异性进行泵浦探针实验的能力。