1. M. Ross, J. Chem. Phys. 60, 3634 (1974).
2. A. Driessen, J. A. de Waal, and I. F. Silvera, in Proc. International Conf. on Quantum Cystals, Fort Collins, Colorado, August 8?12 (1977), unpublished;
3. I. F. Silvera, A. Driessen, and J. A. de Waal, in press.
4. I. F. Silvera, in Proceedings LT14 (North-Holland, Amsterdam, 1975), Vol. 5, p. 123.
5. F. Simon, M. Ruheman, and W. A. M. Edwards, Z. Phys. Chem. B 6, 331 (1929).