1. Åkerlind L 1962Ark. Fys. 22 65
2. Ammes L 1965 Ph.D. Thesis, Ohio State University Columbus
3. Bacis R and Collomb R 1973Phys. Rev. A8 2255
4. Carette P 1969 Ph.D. Thesis, University of Lille
5. Carette P and Blondeau J 1969CR Acad. Sci. B268 1743