Fig. 17.7. EELS spectra of Cm multilayers and monolayers on different substrates using incident electrons of 10 eV. The dashed line under each spectrum marks the zero level for the electron energy loss intensity. The broad feature below 1 eV in the third spectrum from the top is identified with the tlu plasmon. The second trace from the top is related to the third trace by a scale factor of ten [17.55],
~0.9 Á-"1 [17.21], The energy separation between the triplet and singlet excitons (0.29 eV) thus gives the exchange energy associated with this ex-citon. Other band-to-band transitions in the EELS spectra are reported at 2.7, 3.6, 4.5, and 5.5 eV [17.11,21,55], and the energies of these peaks are consistent with optical spectra (see §13.3.1). The EELS spectra for C70 are rather similar to those for C60 with structure in the optical conductivity derived from the C70 EELS spectra reported at 2.5, 3.2, 4.4, and 5.4 eV [17.21], Since the EELS and optical spectra both result in an excited electron and a hole that is left behind, both types of excitations give rise to similar excitonic states, except for differences in the selection rules cited above [17.28],
Because of the stronger interaction of electron probes with matter, EELS can also be used to investigate the electronic structure of monolayers on surfaces, while the more weakly interacting photon probes are more useful for studying bulk properties. Referring to Fig. 17.7, we see the effect of charge transfer from the Au (110) surface to an adsorbed monolayer of C60 through the large plasma background below 1 eV and the broad features in the EELS spectra at higher energies.
Deposition of a monolayer of K on the Au (110) surface prior to the C60 deposition greatly enhances the magnitude of the plasma absorption and shows the presence of a strong peak at 1.3 eV, which is within 0.1 eV of the EELS peaks in K3C60 [17.40] and in Rb3C60 [17.21], suggesting comparable charge transfer to that for the M3C60 alkali metal-doped fullerenes (namely, three electrons transferred to each adsorbed C60 molecule). When multiple K multilayers are deposited on the Au (110) surface prior to the monolayer C60 deposition, the EELS pattern in Fig. 17.7 changes drastically relative to the K monolayer spectrum, showing a much lower background absorption level and sharp EELS lines at 1.35, 2.6, 3.0, 4.3, and 5.8 eV, with the 1.35 eV peak in good agreement with EELS spectra for I^C^, Rb6C60 and Cs6C60 [17.55], suggesting a complete filling of the tlu band of C60 by charge transfer of up to six electrons from the multiple K layers on the Au (110) substrate [17.55],
EELS spectra, similar to the C60 multilayer spectrum in Fig. 17.7, are observed for thick C60 films on various substrates, such as Si (100) [17.56] and Rh (111) [17.54]. Some differences in the energies of the lower-lying electronic peaks have been reported for the case of monolayer C60 coverage on Rh (111) as compared with Si (100) [17.54], insofar as the blue shift of high-resolution electron energy loss spectroscopy (HREELS) vibrational features for one ML C60 coverage on Rh (111) has been interpreted in terms of C60 cation formation [17.54], indicative of electron transfer from C60 to the Rh (111) surface.
EELS spectra taken with high-energy electrons can probe excitations from the Is carbon core level to available states near the Fermi level. Such core spectra (shown in Fig. 17.8 for C6q, C70, C76, and CS4) indicate four distinct spectral features in the figure below 289 eV. These features are associated with transitions from the carbon Is core level to tt* bands just above EF, and the structure above 290 eV is identified with the corresponding transitions to o* bands [17.23]. A broadening of the core level transitions is seen for both the C76 and Cg4 spectra in Fig. 17.8 and is attributed to differences in the structure and electronic states for the various isomers of C76 and C84 and to their lower symmetries. Although the spectra in Fig. 17.8 are strongly related to one another, shifts are seen in the frequency of the onset of the core level excitations from ~284.1 eV for C60 to ~283.7 eV for C84, as well as variations in the details of the spectra from one fullerene
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