There are two nonqnivalent sites of yttrium in h lattice of YzSiO5. Both of them may be occupied SubstiNtionally by &+. resulting in a laser active material. We measured absorption spenra of YzSiOs:Elt at several low temperatures and we studied its emission specr" at 4.2 K following selective excilatioo of individual Stark components of the "113,2 +" 115p transition by the light of a lunable mlour centre laser. The results of lhese two experiments allow classification of the absorption and emission lines into two independent systems, thus yielding two energy level schemes 1 and 2. Moreover, simple crystal field calculations slrongly suggest from which YzSiOs cryslallographic sile spectrum 1 arises, and from which one spectrum 2. suggests a saturable absorption behaviour of the Yso:E?+ sample. It seem therefore of interest to h o w accurately the crystal field sublevels of the 4113,z and 'I15p states, between which the laser emission takes place, as well as the sublevels of higher states of erbium, terminals of excited state absorptions from 4113/2, which might interfere in the operation of the YSO:E?+ laser.According to Maksmov et nl (1970), YSO is a monoclinic crystal (space group C&) with two non-equivalent yttrium sites, both with the lowest (Cl) point symmetry. Table 1 lists for each of them the coordinates of Y3+ and of the nearest ions; the notation 'site a', 'site b' defined by this table will be used throughout the present paper. E?+ ions
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