Optically detected low-field ESR in the metastable state of Tm2+:SrF2. I. ESR signals and cross relaxation.

Abstract

ESR signals of ${\mathrm{Tm}}^{2+}$:${\mathrm{SrF}}_{2}$ are observed in the ground ${(}^{2}$${\mathrm{F}}_{7/2}$,${E}_{5/2}$) and metastable ${(}^{2}$${\mathrm{F}}_{5/2}$,${E}_{5/2}$) states in low magnetic fields (0--300 Oe) at liquid-helium temperatures. We achieved a very high detection sensitivity (\ensuremath{\sim}${10}^{9}$ spins in the ground state) using optical means; the population differences are created by circular dichroism and spin memory, and the change of the Faraday rotation is detected by a polarimeter. Several interesting phenomena were observed. The signals in the ground and metastable states appeared with opposite polarities. The preferential pumping from the metastable state was examined by a double pumping experiment. We also measured the Faraday rotation between 4000 and 7000 A\r{}. The experimental results suggest that the magnetization in the metastable state created by the preferential pumping from the ground state followed by the spin memory is in the opposite direction to that in the ground state. Anomalous behavior of the metastable-state signals was observed both in frequency and time domains. These are explained as an effect of the metastable-state--ground-state cross relaxation enhanced by the ground-state--ground-state cross relaxation.