Absorption and natural circular dichroism (NCD) spectra of ErAl₃(BO₃)₄ single crystal were measured at 90 K in the range of 10000 – 28200 cm^-1. The spectra were decomposed on the Lorentz shape components and the natural optical activities (NOA) of f-f transitions between Stark components of the ground and excited multiplets were found. The NCD spectra permitted us to find out existence of two non equivalent positions of Er³⁺ ion in one of its excited states, which are due to the local decrease of the crystal symmetry in this state. Very large NOA of a vibronic line was revealed. This phenomenon was accounted for basing on the new quantum mechanical formula for the NOA of electron transitions. The principle difference of the NOA properties of electric dipole allowed and parity forbidden transitions is discussed.

中文翻译：

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ErAl ₃（BO ₃）₄单晶中ff跃迁的自然光学活性

ErAl ₃（BO ₃）₄单晶的吸收光谱和自然圆二色性（NCD）光谱是在90 K下在10000 – 28200 cm ^-1范围内测量的。光谱在洛伦兹形状分量上分解，并且发现地面的Stark分量与激发的多重态之间的ff跃迁的自然光学活性（NOA）。NCD光谱使我们能够发现Er ³⁺的两个非等价位置的存在离子处于其激发态之一，这是由于在此状态下晶体对称性局部降低所致。发现振动线的NOA很大。该现象是基于电子跃迁的NOA的新量子力学公式得出的。讨论了电偶极子允许和奇偶禁止跃迁的NOA特性的原理差异。