Double ENDOR measurements in CaF2:Tm2+

Abstract
The double ENDOR method has been used to measure the relative signs of the hyperfine interactions of the unpaired hole in Tm2+ (4f13) with (a) the 169Tm nucleus, and (b) the 19F ligand nuclei in CaF2. The sign of the interaction (a) is known from theory, hence the measurement determines the sign of the interaction (b); both components A2 and Ap are found to be positive. It has been shown that information about the relative signs of two hyperfine parameters may be obtained in a double ENDOR experiment only if the ENDOR signals are essentially transient, or have a significant transient component. The proportion of transient component is somewhat adjustable by suitable choice of experimental conditions. Rapid spin diffusion tends to decrease the proportion of transient component and hence tends to frustrate measurements of relative signs of hyperfine parameters.