Rich physics emerges as the result of interplay between discrete and continuous quantum states. A process of reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) reveals a similar phenomenon. In RABBITT, primary XUV driven ionization is aided by secondary IR photon absorption or emission. The latter processes involve transitions between continuous states. We demonstrate that when RABBITT crosses the ionization threshold and proceeds via a discrete bound state, its phase makes a sudden jump which can be related to the phase of the continuous transitions above the threshold. Up to now, this phase remained undetermined experimentally and could only be estimated theoretically. The under-threshold RABBITT provides alternative means of assessing this quantity.

中文翻译：

---

跨电离阈值的RABBITT相变

离散和连续量子态之间相互作用的结果是出现了丰富的物理学。通过双光子跃迁的干扰（RABBITT）重建阿秒跳动的过程揭示了类似的现象。在RABBITT中，主要的XUV驱动的电离作用是通过辅助IR光子的吸收或发射来实现的。后面的过程涉及连续状态之间的转换。我们证明，当RABBITT越过电离阈值并通过离散的束缚态进行时，其相位会突然跳变，这可能与阈值以上连续跃迁的相位有关。到目前为止，该阶段在实验上尚未确定，只能从理论上进行估计。阈值以下的RABBITT提供了评估此数量的替代方法。