1. R. F. Wallis, ed., Localized Excitation in Solids ( Plenum, New York, 1968 ).
2. W. Eisenmenger and A. H. Dayem, Phys. Rev. Lett. 18, 125 (1967).
3. N. S. Shiren, Phys. Rev. Lett. 11, 3 (1963).
4. P. A. Carruthers, Rev. Mod. Phys. 33, 92 (1961).
5. P. L. Taylor, A Quantum Approach to the Solid State (Prentice Hall, Englewood Cliffs, New Jersey, 1970 ).