Edge emission of CdS crystals stimulated by infrared

Excitation in the band edge region of CdS (wave numbers $\text{v}\text{≈ 20,000}\text{cm}{}^{\mathrm{?1}}$) sensitizes the crystals in the far infrared range ($\text{v}\text{≈ 200}\text{cm}{}^{\mathrm{?1}}$). The resulting characteristic sensitized i.r. absorptions (SIRA) are used at T ≈ 5 K as a measuring probe for the degree of population of donors. These donors can be emptied by irradiation with red or near infrared light ($\text{v}\text{? 2000}\text{cm}{}^{\mathrm{?1}}$). At this stimulation process, the low energy series of the green edge luminescence is emitted.