tailieunhanh - Carbon Materials for Advanced Technologies Part 3

Tham khảo tài liệu 'carbon materials for advanced technologies part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 50 of a strong electron-vibration interaction. Optical transitions between the HOMO and LUMO levels can thus occur through the excitation of a vibronic state involving the appropriate odd-parity vibrational mode 68 69 70 71 Because of the involvement of a vibrational energy in the vibronic state there is an energy difference between the lowest energy absorption band 67 and the lowest energy luminescence band 71 . Using these molecular states the weak absorption observed between 490 and 640 nm for Cfio in solution Fig. 6 67 is assigned to transitions between the singlet ground state So and the lowest excited singlet state Si associated with the tiu orbital and activated by vibronic coupling . For C70 molecular orbital calculations 60 reveal a large number of closely-spaced orbitals both above and below the HOMO-LƯMO gap 60 The large number of orbitals makes it difficult to assign particular groups of transitions to structure observed in the solution spectra of C70. UV-visible solution spectra for higher fullerenes Cn n 76 78 82 84 90 96 have also been reported 37 39 72 . Further insight into the electronic structure of fullerene molecules is provided by pulsed laser studies of C60 and C70. Such time-resolved studies of fullerenes in solution have been used to probe the photo-dynamics of the optical excitation luminescence spectra. The importance of these dynamic studies is to show that photo-excitation in the long-wavelength portion of the u V-visible spectrum leads to the promotion of Cgo from the singlet So ground state k4i into a singlet Si excited state which decays quickly with a nearly 100 efficiency 73 74 via an inter-system . sn Tn crossing to the lowest excited triplet state 71. This rapid singlet-triplet decay 33 ps 74 is fostered by the overlap in energy between the vibronic manifold of electronic states associated with the lowest singlet Si state and the corresponding vibronic manifold for the triplet 71 state and the very weak spin-orbit coupling .