NMR pulse techniques have been used to study the orientational ordering of ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}_2^{}{}_{}{}^{}$ molecules physisorbed on graphite. A small hysteresis (~2.5%) and small discontinuity in the evolution of the NMR echo amplitudes which depend directly on the local order parameters indicates that the order-disorder transition is first order.

• Received 15 March 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.658