Population transfer for signal enhancement in pulsed EPR experiments on half integer high spin systems

High resolution pulse EPR techniques applied to half integer high spin systems, such as Mn²⁺ (S = 5/2), usually focus only on the central |-1/2〉→ |1/2〉 transition. The reason is that at high fields, where the zero field splitting is considerably smaller than the Zeeman interaction, the spectrum of this transition is intense and narrow. However, because the experiments are carried out at low temperatures, the low lying levels are heavily populated and the signal of the central transition is nevertheless diminished. This, in turn affects the sensitivity of the pulse EPR technique applied. A transfer of populations from the lower lying levels, which for Mn²⁺ are the |-3/2〉 and |-5/2〉 levels, to the |-1/2〉 level will therefore increase the sensitivity. Here we describe such an experiment, where a rapid magnetic field sweep over the |-3/2〉→ |-1/2〉 sub-spectrum is carried out, concomitantly with a low power microwave (mw) irradiation, which results in population inversion. After this sweep any pulsed EPR sequence can be applied to the central transition that now has a population difference that deviates from the equilibrium value. The feasibility of the experiment is demonstrated at W-band (95 GHz) on Mn ²⁺ doped in MgO for echo-detected EPR measurements and the dependence of the signal enhancement on the rate and range of the magnetic field sweep and on the mw power is described. The results are then accounted for theoretically by considering a simple fictitious spin 1/2 system. In addition, preliminary enhanced ⁵⁵Mn pulse ENDOR electron nuclear double resonance (ENDOR) spectra are presented.