Thermionic emission laser spectroscopy of stored C 60 −

Radiative cooling of fullerene anions in a storage ring

“…10). This observation correlates with the details of the absorption spectrum discussed in Appendix B: among the magic molecules, the HOMO-LUMO gap for the neutral molecule is largest for C 60 (∼ 1.7 eV), somewhat smaller for C 70 (∼ 1.3 eV) and for C 50 (∼ 1 eV) [38], and probably smallest for C 36 [39]. However, the strength of the cooling does not follow this order since there is a strong transition in the gap for the additional electron in the anions of the first two molecules.…”

“…The low-energy radiation is nearly two orders of magnitude stronger than the calculated contribution from vibrations [26]. A similar result is obtained for C − 70 where the single-electron line at 0.9 eV dominates the absorption below about 1.3 eV [38,44]. The measured decay curve is reproduced with two lines at 0.9 eV and 0.25 eV with intensities of 650 eV/s and 210 eV/s at 1 500 K. The prediction from the measured absorption strength at 0.9 eV is estimated in Appendix C to be about 400 eV/s, and the calculated intensity from infrared-active vibrations is less than 10 eV/s [26].…”

“…For hot C − 60 molecules, the single-electron transition at 1.15 eV dominates the absorption below about 1.5 eV [38,[40][41][42][43], and this line should therefore also dominate the radiation intensity. In an alternative analysis, we have simulated the measured decay spectrum in Figure 9c with a heat radiation composed of two lines, one at 1.15 eV and the other at 0.25 eV, and with a temperature dependence given by equation (B.1).…”

“…In order to fit the measurement in Figure 11b with the same parameters it is seen to be necessary to include a temperature dependence of the photon-absorption crosssection, as illustrated in Figure 11c. Such a dependence is expected since the laser frequency is very close to the centre of the absorption line of the additional electron in C − 60 and this line is strongly broadened at high temperatures [38]. If the integral of the line is constant, the absorption at the centre is weaker at high temperatures.…”

“…The main reason is that there is strong electronelectron correlation and also coupling of electronic and vibrational transitions, especially at high temperatures. The electronic absorption of radiation is therefore not limited to narrow lines corresponding to well defined transitions between single-electron states, and the distribution of oscillator strength depends on the temperature [38,48]. In reference [26] the thermal radiation was instead estimated from a classical dielectric model of the electromagnetic response of a fullerene molecule, and we have mainly compared our measurements to predictions from this model.…”