  We report a theoretical investigation of photoionization by a pair of
coherent, ultrashort fundamental and second-harmonic extreme-ultraviolet
pulses, where the photon energies are selected to yield the same photoelectron
energy for ionization of two different sub-shells. This choice implies that the
fundamental energy is equal to the difference in energy of the ionic states,
and they are therefore coupled by the fundamental photon. By deriving
analytical expressions using the essential-states approach, we show that this
Rabi coupling creates coherence between the two photoelectron wave packets,
which would otherwise be incoherent. We analyze how the coupling is affected by
the parameters such as relative phase, pulse width, delay between the two
pulses, the Rabi coupling strength, and photoelectron energy. Our discussion
mostly considers Ne 2p and 2s photoionization, but it is generally valid for
many other quantum systems where photoionization from two different shells is
observed.
