1. R. Boyd et al., Phys. Rev. Lett. 27 (1971) 1590
2. R. Boyd et al., Phys. Rev. Lett. 29 (1972) 955
3. S. Davis, Ph.D. Thesis, Rutgers University (1974) unpubl.
4. H. Clement and G. Graw, Phys. Lett. 57B (1975) 323
5. G. Van den Berghe et al., Phys. Lett. 38B (1972) 467 and private communication