  Ionization with ultrashort pulses in the extreme ultraviolet (XUV) regime can
be used to prepare an ion in a superposition of spin--orbit substates. In this
work, we study the coherence properties of such a superposition, created by
ionizing xenon atoms using two phase-locked XUV pulses at different
frequencies. In general, if the duration of the driving pulse exceeds the
quantum beat period, dephasing will occur. If however, the frequency difference
of the two pulses matches the spin--orbit splitting, the coherence can be
efficiently increased and dephasing does not occur.
