  Nonequilibrium calculations in the presence of an electric field are usually
performed in a gauge, and need to be transformed to reveal the gauge-invariant
observables. In this work, we discuss the issue of gauge invariance in the
context of time-resolved angle-resolved pump/probe photoemission. If the probe
is applied while the pump is still on, one must ensure that the calculations of
the observed photocurrent are gauge invariant. We also discuss the requirement
of the photoemission signal to be positive and the relationship of this
constraint to gauge invariance. We end by discussing some technical details
related to the perturbative derivation of the photoemission spectra, which
involve processes where the pump pulse photoexcites electrons due to
nonequilibrium effects.
