NMR pulse studies of molecular solids:15N2and H2. II. Stimulated echoes and slow rotational motions

Abstract

For pt.I see ibid., vol.15, no.23, p.4881 (1982). Following a description of the formation of stimulated echoes in orientationally ordered crystals such as solid N₂ and solid H₂ the authors discuss the exploitation of these echoes for the detection of slow rotational motions in these systems. For solid N₂ at low temperatures where the molecular orientations remain fixed the 'natural' decay of the echoes is very slow as a result of the quenching of the nuclear spin flip-flop transitions between different molecules. The 'natural' decay is also very slow in the orientationally ordered phases of solid H₂ and the authors have used this property to search for residual rotational motion in the quadrupolar glass phase of solid H₂. Such motion would lead to a strong additional damping of the stimulated echoes. The experimental results show that the molecular orientations in the glass phase of H₂ are frozen for time scales up to 10^-2 s. The temperature dependence of the echo amplitudes in the glass transition region clearly indicates a collective freezing of the molecular orientations. A new model of an inhomogeneous dynamical freezing is proposed to account for the experimental results.