  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.
