Isotope Effect of Pairs of Coupled Tunneling Systems

Using dielectric rotary echo experiments we have studied the coherent tunneling of strongly coupled Li ${}^{+}$ ions in KCl crystals. Since our experiments were performed at resonance frequencies of about 1 GHz, we have investigated tunneling systems with energy splittings ${E/k}_B\le 50$ mK. Measurements of the Rabi frequency for samples doped with different lithium isotopes give a conclusive proof that the low-energy states arise from pairs of coupled Li ${}^{+}$ ions. We discuss the tunnel splitting and the microscopic arrangement of such pairs.