Cross sections for photoionization of fullerene molecular ions $\mathrm{C}{{}_n}^{+}$ with $n$ = 40, 50, 70, 76, 78, and 84

Absolute cross-section measurements are reported for single photoionization of $\mathrm{C}{{}_n}^{+}$ fullerene molecular ions ($n=40$, 50, 70, 76, 78, and 84) in the photon-energy range 18–70 eV. The experiments were performed by merging a mass and charge selected beam of $\mathrm{C}{{}_n}^{+}$ molecular ions with a beam of monochromatized synchrotron radiation and measuring the yield of $\mathrm{C}{{}_n}^{2+}$ product ions as a function of the photon energy. Oscillator strengths determined by integrating the measured cross sections over this energy range exhibit a linear dependence on $n$. The cross sections are parametrized by fits to three Lorentzian functions to represent plasmon excitations and a linear function for direct ionization. The highest-energy resonance in the data near 46 eV is similar to that previously observed in single photoionization of ${\mathrm{C}}_{60}$ and may be attributable to a harmonic of the dominant surface-plasmon resonance near 23 eV.

Figure 1

Experimental results for single photoionization of $\mathrm{C}{{}_n}^{+}$. Small solid squares represent energy-scan measurements and large solid circles with error bars are absolute cross-section measurements to which the scans are normalized. The $\mathrm{C}{{}_{60}}^{+}$ data are taken from Ref. [13].

Figure 2

Dimensionless oscillator strengths (open circles) determined by integrating the cross sections in Fig. 1 and applying Eq. (1). The line represents a linear least-squares fit to the data.

Figure 3

Least-squares fit of a linear and three Lorentzian functions using Eq. (2) to the cross section for single photoionization of $\mathrm{C}{{}_{70}}^{+}$. The four lowest-energy data points were excluded from the fitting procedure due to possible systematic effects in the recording of the photon flux.

Figure 4

Comparison of photoionization cross sections for single photoionization of $\mathrm{C}{{}_n}^{+}$ ions on the same scale along with curves representing fits of Eq. (2) to the measurements.