1. Ip, W.-H., and Mendis, D. A., Nature, 245, 197 (1973).
2. Welch, A. R., and Judge, D. L., J. chem. Phys., 57, 286 (1972).
3. Herzberg, G., Proc. R. Soc., A 262, 291 (1961).
4. Herzberg, G., and Johns, J. W. C., Proc. R. Soc., A 295, 107 (1966).
5. Hinteregger, H. E., Ann. Geophys., 26, 547 (1970).