1. a Xerox Webster Research Center , 800 Phillips Rd., W114, Webster , NY , 14580 , U.S.A.
2. b Central Research and Development Department , E.I. DuPont de Nemours & Company , Wilmington , DE , 19898 , U.S.A.
3. c Department of Physics , University of Florida , Gainesville , FL , 32611 , U.S.A.