Direct Optical Detection of the Ground-State Population Changes of Neodymium in Ethylsulfate Crystals

A method is described for optically detecting population changes in paramagnetic salts and applied to the ground state of neodymium ethylsulfate and neodymium in lanthanum ethylsulfate. By observing the transmitted right or left circularly polarized light, the population of one of the ground Zeeman levels is selectively monitored. The ground-state population is changed by paramagnetic resonance and thermal relaxation processes. Knowing the ground state, the circular polarization of the light, and the selection rules, information about the excited states can be obtained. The spin-lattice relaxation time between Zeeman sublevels of the ground state is also obtained. After pulsing the microwave resonance transition, the decay of the transmitted light is observed; the decay curve is related to the spin-lattice relaxation curve. Relaxation-time measurements in the temperature range of 1.87 to 2.16°K were made in both the pure and dilute salt. The values ranged from 17 msec for the lower temperature to 11 msec for the higher temperature. The slope of the relaxation time versus the temperature $T$ indicates a temperature dependence for the relaxation time of $T^{-2.5\mathrm{}}$ in this temperature range for the 3% neodymium in lanthanum ethylsulfate and $T^{-1.7\mathrm{}}$ for the neodymium ethylsulfate crystal. The crystal $c$ axis was oriented parallel to the magnetic field of 4780 Oe; the microwave frequency was 23.6×${10}^9$ cps.