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Photon Energy (10 cm )

Fig. 13.6. (a) Photoluminescence (PL) and optical absorption (OA) spectra of Cm in 6.6 x 10"6 molar methylcyclohexane (MCH) solution (features labeled M, for the PL spectra and M,< for the OA spectra). The PL spectra were taken with 330 nm (top) and 460 nm (bottom) excitation, respectively, (b) PL (features labeled F,) and OA (features labeled Fj,) spectra at 80 K for oxygen-free pristine ~4500 A thick Cg, films on a Suprasil (fused silica) substrate. The PL excitation wavelength was 488 nm at ~ 0.06 W/cm2 [13.15], The upper curve is the 10 K PL spectrum of Cm single crystal (features labeled C,) taken from Guss et al. [13.28], frequencies of the C60 molecule [13.15]. These choices of the zero-phonon transition energy E0 (y and the assignments of the PL and OA features agree quite well with those of Negri et al. [13.35]. The difference between and may be due to impurities and defects which localize the excitons [13.35,36], The vibrational frequencies obtained for features labeled M1 to Mg in PL, and Mr to M9, in OA, are identified with H-T active overtone (2(Oj) and combination (w, + Wj) vibrational modes.

In general, vibrational modes involved in strong first-order vibronic transitions also participate in strong combination and overtone vibronic transitions. Since the y4(Flu) mode is found to be the strongest H-T active vibrational mode [13.35], we identify features labeled Af7 to Mg and M7, to Mg, with combination modes and overtones involving the v4(Fu) mode. Although the transition between the zero vibration levels (at £0,o') is forbidden by symmetry, a weak peak at this energy is seen in the PL spectra [Fig. 13.6(a)] [13.15] and also in the OA spectra shown in the same figure [13.17], These E00, transitions are denoted by M0 in Fig. 13.6(a).

13.2.3. Dynamics of Excited States in Isolated C60 Molecules

We now review efforts to determine the dynamics of the low-lying excited states (Sj and Tx) in C60 and C70. Our discussion will center around the schematic energy level diagram shown in Fig. 13.7, which summarizes pertinent parameters for these low-lying excited states, as detected in transient

Fig. 13.7. Energy diagram for electronic states of (a) CM and (b) C70 based on experimental photophysical and transient absorption spectra [13.30,37]. Indicated in the figure are room temperature values for lifetimes ts of singlet states and r7 of triplet states, quantum efficiencies 4>r, and branching ratios for phosphorescence <t>p. In the figure, 5, and 7", denote the manifold of excitonic and vibronic states associated with the lowest excited configuration h9ut]u, and the higher-lying states S2, T2 refer to higher energy electronic configurations. Less is known about the photodynamics of the higher excited states. (See Table 13.3.)

Fig. 13.7. Energy diagram for electronic states of (a) CM and (b) C70 based on experimental photophysical and transient absorption spectra [13.30,37]. Indicated in the figure are room temperature values for lifetimes ts of singlet states and r7 of triplet states, quantum efficiencies 4>r, and branching ratios for phosphorescence <t>p. In the figure, 5, and 7", denote the manifold of excitonic and vibronic states associated with the lowest excited configuration h9ut]u, and the higher-lying states S2, T2 refer to higher energy electronic configurations. Less is known about the photodynamics of the higher excited states. (See Table 13.3.)

absorption and luminescence studies. Experimental values for the level positions and the lifetimes for radiative and nonradiative transitions are listed in Table 13.3. In pump-probe experiments [13.46], the CM molecule is first excited by a short-duration optical pulse from the electronic ground state (50) to some high-lying level from which nonradiative transitions to the lowest excited singlet state Sj occur rapidly. Upon populating 5! by this "pump" pulse, a very rapid (~ 1.2 ns) radiationless intersystem crossing to T, occurs, with a quantum efficiency 4>r close to unity, thereby depopulating S, [13.30,31,43] (see Table 13.3). By virtue of this pump pulse and the subsequent intersystem crossing, transient, time-dependent populations in S, and T, states are thereby created which allow the molecule to be further excited to higher-lying S„) and (7\-> Tn) states by a second, time-delayed, short duration optical pulse called the "probe" pulse. The higher cross sections for absorption from the excited singlet 5! state (as = 1.57 x 10~17 cm2) and the excited triplet Tx state (crr = 9.22 x 10~18 cm2) relative to the ground state S0 absorption cross section (oq = 2.87 x 10~18 cm2) have stimulated studies of absorption processes originating from optically excited states [13.37,41], Whereas transient absorption experiments provide powerful techniques for studying the energies of the excited states, emission studies are most useful for providing information on the lifetimes of the

Table 13.3

Photophysical and dynamical parameters for absorption and luminescence of Cjo and C,0 in solution.

Table 13.3

Photophysical and dynamical parameters for absorption and luminescence of Cjo and C,0 in solution.

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