Mass / charge (Dalton)

Fig. 6.9. Comparison of normalized measured isotopic distributions for different charge states of CM and calculated distributions (filled bars) based on the natural abundance of 13C and the intensity of the 12C5913C peak in the mass spectra. The differences between the measured and calculated first peaks for pure 12C for the Cjf and Cj^ fullerene cations are due to contributions from singly charged fragments of CM„ i.e., C~{5 and CJ"2 (open bars on top of the filled bars). The mass spectra were taken at an electron energy of 200 eV and 300 fiA electron current [6.32], been used to estimate the energy necessary to add one electron to Negative surface ionization is proposed as the mechanism for forming the observed high concentration of C2m whereby the ratio of Cg0 to CM has been fit to the relation where (g2-/g_) = 4, due to the possibility of singlet and triplet states for the doubly ionized species, and AE = 2.2 eV has been attributed to the high thermal energies achieved in the carbon arc excitation method (~3000 K) [6.33],

6.4. Fullerene Contraction and Fragmentation

Closely related to studies of the growth and formation of fullerenes are studies of their contraction and fragmentation. Three main techniques are used for fragmentation studies: (1) photofragmentation, whereby incident photons excite fullerenes and thus promote subsequent fullerene fragmentation; (2) collision of charged C60 ions, which are used as projectiles impinging on surfaces; (3) energetic electron irradiation, which, similarly to

Fig. 6.10. Laser desorption negative ion mass spectrum of C60 and C70. The insets show fine structure in the mass spectra for the C60, C^", and C,n peaks associated with 13C isotopes, in good agreement with their natural abundances [6.33].

(1), excites the fullerene target and promotes fragmentation. Thermal fragmentation can also occur at elevated temperatures. The thermal dissociation of C60 into C58 and C2 has been observed above 900 K on both Si02 and highly oriented pyrolytic graphite (HOPG) surfaces using molecular beam reaction spectroscopy techniques [6.34], showing that the activation energy for this thermal dissociation is reduced from 5-12 eV in the gasphase [6.35-38] to ~2.5 eV on these two surfaces.

6.4.1. Photofragmentation

An experiment of historical significance is the study of photofragmentation of C60 and of endohedrally doped [email protected] and [email protected] [6.4]. The photofragmentation process tends to be a relatively gentle process and initially results in fullerene contraction, thereby shrinking the size of the fullerenes, without destroying them. In the photofragmentation process C2 units are expelled (as discussed in §6.3) and the mass of the fullerene drops until the fullerene bursts because of its high strain energy. The disintegration of C60 occurs at about C32, while with an endohedral dopant some reports claim

Fig. 6.11. C6„ photoion yield as a function of photon energy, displaying excitation of a giant plasmon resonance. Inset: The threshold region is shown on a magnified scale [6.41].

that the fullerenes burst at larger mass values, such as [email protected] and [email protected] [6.39], although other reports indicate that smaller metallofullerenes such as [email protected] are stable [6.40]. Early photofragmentation experiments provided strong evidence for a cage-like structure for C60 and focused strongly on study of the formation and disintegration mechanisms for fullerenes

Single-photon (UV) excitation experiments have been carried out using synchrotron radiation to provide an intense photon source above the ionization energy of 7.58 eV for C60 and above 7.3 eV for C70 [6.26], The photoion yield vs. ha> for the single-photon excitation is shown in Fig. 6.11, where a strong and broad plasmon resonance is seen. This strong feature is attributed to collective motion of the valence electrons of C^ [6.41], and the spectral line shape is in agreement with theoretical predictions [6.42].

Laser desorption of C2 species provides an important technique for fullerene contraction studies. Laser desorption of C2 can occur either at low photon energies, whereby several photons are absorbed by a single fullerene, or at high photon energies (above the ionization energy of 7.58 eV) where a single photon can produce electron ionization directly. For detection and measurement purposes, it is desirable to charge the C60 beam, which is normally emitted as a neutral beam from a C60 surface by laser ablation. The neutral beam can become charged by applying a second UV laser (~4 eV photon energy) to the incident neutral beam or by a variety of other means, while the beam is within a Fourier transform mass spectrometer. Neutral C60 molecules are often required to absorb several visible photons before they acquire enough energy to become ionized. The absorption of a photon normally results in strong vibrational excitations which can be quite energetic (~0.05-0.2 eV) and eventually may lead to thermionic emission or direct ionization [6.26], as discussed below.

For low photon intensities, the main effect of the laser beam is to des-orb fullerenes from the surface without causing much fragmentation [6.43], as shown in Fig. 6.12(a), where time-of-flight mass spectra (see §6.2) are shown for 20 mJ/cm2 photon irradiation at 248 nm. In this spectrum the peaks for the positive cations Cj, and C70 are well separated, and the inset showing the 12C5913Ci and ,2C58I3C2 mass peaks confirms the identification of the spectral line with Qj~0 (see §4.5). When the photon intensity increases to 30 mJ/cm2, as shown in Fig. 6.12(b), the intensity of the C84 line increases dramatically. The intensities for all the fullerenes between Cy,, and C84 and also the intensities of the mass peaks for and increase with increasing photon intensity. However, increasing the photon intensity to 90 mJ/cm2 [Fig. 6.12(c)] shows evidence for the formation of many higher-mass fullerenes and for their contraction by C2 emission. The relative intensities of the peaks between Cg0 and C70 suggest that these species are formed by the emission of C2 clusters from the Cy0 ion, and likewise the peaks below Cg0 are attributed to a similar contraction process by the ion. In contrast, the presence of relatively large amounts of high-mass fullerenes indicates the photoinduced absorption of C2 units by the stable Cj, and Cy0 fullerenes [6.43].

Delayed photoionization effects have also been reported for fullerenes. In this phenomenon, thermionic emission of electrons occurs from neutral fullerenes on a time scale of microseconds [6.44], This effect has been explained by a statistical model by Klots [6.42], based on the following physical mechanism. When a photon of energy less than the ionization energy (7.58 eV) is absorbed by C60, the photon energy is rapidly distributed among the vibrational degrees of freedom of the molecule. If several photons are consecutively absorbed, the temperature of the molecule rises, so that the molecule eventually has sufficient thermal energy for thermionic emission to occur. In such experiments a high-intensity laser of photon energy less than the ionization energy is used to energize the molecules, while a second laser with a different frequency is used for desorption to monitor the molecular ionization by the thermionic emission process [6.26],

6.4.2. Collision of Fullerene Ion Projectiles

In carrying out collision-induced fragmentation experiments, fullerenes are often used as the projectiles, and they are ionized so that the fullerene

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