Optical pumping by excitation of molecular vibrations

A new optical pumping mechanism is observed for dilute Yb³⁺ impurities in a single crystal of yttrium ethyl sulphate at low temperatures. The polarization of Yb³⁺ spins is reduced by illumination with near infrared radiation. This reduction in spin polarization is caused by excitation of molecular vibrations of the ligand molecules surrounding the paramagnetic impurity.