DepartmentofAppliedPhysics;
ChongqingUniversity;
Chongaing400044;
China;
KeyLaboftheEducationMinistryforPhotoelectricTechnologyandSystem;
ChongqingUniversity;
Chongqing400044;
China;
DepartmentofChemistry;
ChongqingUniversity;
Chongqing400044;
ChinaManuscriptreceived25November2002;