Absorption Spectra of Erbium Tricyclopentadienide at Liquid‐Helium Temperature

The absorption spectra of Er(C₅H₅)₃ in 2‐methyl tetrahydrofuran have been studied at room temperature, liquid‐nitrogen, and liquid‐helium temperature. Hypersensitive line groups have been found at ∼19 000 and ∼26 000 cm⁻¹. Large splittings of $\mathrm{J\hspace{0.17em}=\hspace{0.17em}}\frac{3}{2}$ levels have also been observed. A correlation between hypersensitivity and large second‐degree terms of the crystalline potential is suggested. Although some line groups in Er(C₅H₅)₃ showed unusually large splittings, no drastic changes in the usual energy‐level pattern for ${\text{4f}}^{11}$ were found in this volatile, organometallic compound of erbium.