  We investigate the discrete orbital angular momentum (OAM) of photoelectrons
freed in strongfield ionization. We use these `twisted' electrons to provide an
alternative interpretation on existing experimental work of vortex
interferences caused by strong field ionization mediated by two counterrotating
circularly polarized pulses separated by a delay. Using the strong field
approximation, we derive an interference condition for the vortices. In
computations for a neon target we find very good agreement of the vortex
condition with photoelectron momentum distributions computed with the strong
field approximation, as well as with the time-dependent methods Qprop and
R-Matrix. For each of these approaches we examine the OAM of the
photoelectrons, finding a small number of vortex states localized in separate
energy regions. We demonstrate that the vortices arise from the interference of
pairs of twisted electron states. The OAM of each twisted electron state can be
directly related to the number of arms of the spiral in that region. We gain
further understanding by recreating the vortices with pairs of twisted
electrons and use this to determine a semiclassical relation for the OAM. A
discussion is included on measuring the OAM in strong field ionization directly
or by employing specific laser pulse schemes as well as utilizing the OAM in
time-resolved imaging of photo-induced dynamics.
