Influence of molecular motions on NQR echoes

The influence of thermal molecular motions on spin echo decay in pure nuclear quadrupole resonance (NQR) is considered. Our calculations show that the Hahn echo decay is caused by dipole-dipole interaction of the nuclear spins and is strongly affected by molecular mobility that can lead to the shortening of the echo decay with increased temperature. Slow molecular motion yields an exponential τ³ time dependence, while fast motion yields an exponential decay. The outlined theory allows us to explain an unusual shortening of the ³⁵Cl NQR echo decay on heating in thiourea-C ₂Cl₆ inclusion compound.