Wätzel J, Murray A.J., Berakdar J.

A photoelectron forced to pass through two atomic energy levels before receding from the residual ion shows interference fringes in its angular distribution as a manifestation of a two-slit-type interference experiment in wave-vector space. This scenario was experimentally realized by irradiating a rubidium atom by two low-intensity continuous-wave lasers [Pursehouse et al., Phys. Rev. Lett. 122, 053204 (2019)]. In a one-photon process, the first laser excites the 5p level whereas the second uncorrelated photon elevates the excited population to the continuum. This same continuum state can also be reached when the second laser excites the 6p state, and the first photon then triggers the ionization. As the two lasers are weak and their relative phases uncorrelated, the coherence needed for generating the interference stems from the atom itself. Increasing the intensity or shortening the laser pulses enhances the probability that two photons from both lasers act at the same time, and hence, the coherence properties of the applied lasers are expected to affect the interference fringes. Here, this aspect is investigated in detail, and it is shown how tuning the temporal shapes of the laser pulses allows for tracing the time dependence of the interference fringes. We also study the influence of applying a third laser field with a random amplitude, resulting in a random fluctuation of one of the ionization amplitudes and discuss how the interference fringes are affected.

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