Comment on "Distinction of Electron Dispersion in Time-Resolved Photoemission Spectroscopy"

In a recent paper [Phys. Rev. Lett. 125, 043201 (2020)] (Ref.1) Liao et al. propose a theory of the interferometric photoemission delay based on the concepts of the photoelectron phase and photoelectron effective mass. The present comment discusses the applicability and limitations of the proposed approach based on an ab initio analysis supported by vast literature. Two central assumptions of the paper are questioned, namely that the photoelectron can be characterized by a phase (have a well-defined phase velocity), and that it can always be ascribed an effective mass Theories based on these concepts are concluded to be inapplicable to real solids, which is illustrated by the example of the system addressed in Ref. 1. That the basic assumptions of the theory are never fulfilled in nature discredits the underlying idea of the "time-domain interferometric solid-state energy-momentum-dispersion imaging method" suggested in Ref. 1. Apart from providing a necessary caution to experimentalists, the present comment also gives an insight into the photoelectron wave function and points out problems and pitfalls inherent in modeling real crystals.