  The Stark coefficient of the Er 3+ ion in Y 2 SiO 5 is determined for
transition). The stark photon echo beating method (SPEB) was used: the
pseudo-Stark frequency shift was measured from the period of beats of the
temporal form of the photon echo signal. Light beatings were caused by the use
of a weak electric field pulse that splits the frequencies of optical
transitions of nonequivalent subgroups of echo-active Er 3+ ions in Y 2 SiO5
