It is remarkable that time delay is an experimentally measurable quantity, but time itself is not. Time delay in quantum collisions and in photoionisation/photodetachment of atomic and molecular systems is reviewed in this paper. Wigner–Eisenbud formalism of time delay in quantum collision of a wavepacket with a target is discussed. Its equivalence with Smith's formalism of time delay, based on an independent basis for time delay in terms of excess particle density in the collision zone, is demonstrated. Similarity and difference between quantum collision of an electron with a positive atomic/molecular ion and photoionisation/photodetachment of a neutral atom/molecule are discussed, and the underlying quantum dynamics involving the time-reversal symmetry between solutions with outgoing and ingoing wave boundary conditions is pointed out to interpret photoionisation/photodetachment as half-scattering. This relationship is subsequently taken advantage to extend the formalism of Wigner–Eisenbud–Smith time delay in photoionisation/photodetachment. The measurability of time delay is accounted for in terms of a self-adjoint quantum operator that characterises it, even if there is no such operator for time itself. A few illustrative examples of theoretical and experimental studies of time delay are given to indicate outstanding advances made in this field in the last two decades.

中文翻译：

---

原子和分子碰撞和光电离/光分离的时间延迟

值得注意的是，时间延迟是一个实验可测量的量，但时间本身不是。本文综述了量子碰撞以及原子和分子系统的光电离/光分离中的时间延迟。讨论了波包与目标的量子碰撞时延的 Wigner-Eisenbud 形式主义。证明了它与 Smith 的时间延迟形式主义的等效性，基于碰撞区中过量粒子密度的时间延迟的独立基础。讨论了电子与正原子/分子离子的量子碰撞与中性原子/分子的光电离/光分离之间的异同，并且指出了涉及具有出射波边界条件和入射波边界条件的解之间的时间反转对称性的潜在量子动力学，将光电离/光分离解释为半散射。随后利用这种关系扩展了 Wigner-Eisenbud-Smith 光离/光分离时间延迟的形式。时间延迟的可测量性是根据表征它的自伴随量子算符来考虑的，即使时间本身没有这样的算符。给出了一些关于时间延迟的理论和实验研究的说明性例子，以表明过去二十年在该领域取得的突出进展。随后利用这种关系扩展了 Wigner-Eisenbud-Smith 光离/光分离时间延迟的形式。时间延迟的可测量性是根据表征它的自伴随量子算符来考虑的，即使时间本身没有这样的算符。给出了一些关于时间延迟的理论和实验研究的说明性例子，以表明过去二十年在该领域取得的突出进展。随后利用这种关系扩展了 Wigner-Eisenbud-Smith 光离/光分离时间延迟的形式。时间延迟的可测量性是根据表征它的自伴随量子算符来考虑的，即使时间本身没有这样的算符。给出了一些关于时间延迟的理论和实验研究的说明性例子，以表明过去二十年在该领域取得的突出进展。