1. Wilson J. I. B, and Woods J., - J. Phys. D 5, 1700, 1972
2. Loutfy R., Sharpe J., Hsiao R., - J. App.Phys. 52, 5218, 1981
3. Loutfy R., Sharpe J., - J. Chem.Phys. 71, 1211, 1979
4. Griffiths J., - Colour and Constitution of Organic Molecules Chp. 6. Academic, London, 1976
5. Buckley R.W., - School Technology 12:2 28. 1978