Photoionization and photofragmentation of the ${\mathrm{C}}_{60}^{+}$ molecular ion

Cross-section measurements are reported for single and double photoionization of ${\mathrm{C}}_{60}^{+}$ ions in the photon energy range 18–150 eV accompanied by the loss of zero to seven pairs of carbon atoms, as well as for fragmentation without ionization resulting in loss of two to eight pairs of C atoms in the photon energy range 18–65 eV. Absolute measurements were performed by merging a beam of ${\mathrm{C}}_{60}^{+}$ molecular ions with a beam of monochromatized synchrotron radiation. Product channels involving dissociation yielding smaller fullerene fragment ions account for nearly half of the total measured oscillator strength in this energy range. The sum of cross sections for the measured product channels is compared to a published calculation of the total photoabsorption cross section of neutral ${\mathrm{C}}_{60}$ based on time-dependent density-functional theory. This comparison and an accounting of oscillator strengths indicate that with the exception of ${\mathrm{C}}_{58}^{+}$, the most important product channels resulting from photoabsorption were accounted for in the experiment. Threshold energies for the successive removal of carbon atom pairs accompanying photoionization are also determined from the measurements.

Figure 1

Measured two-dimensional doubly charged product ion scans for a primary ion beam of ${\mathrm{C}}_{60}^{+}$ irradiated at a fixed photon energy of 65 eV [24]. The demerging magnet and spherical electrostatic plates deflect the product ions across the detector in orthogonal directions [25], permitting scans of the product space following photoabsorption.

Figure 2

Comparison of present results for single photoionization of ${\mathrm{C}}_{60}^{+}$ (solid triangles and open circles) with published results of Scully et al. (red open squares) [10].

Figure 3

Absolute measurements of cross sections for single photoionization of ${\mathrm{C}}_{60}^{+}$ and for single photoionization accompanied by the loss of up to seven pairs of C atoms. Open circles denote absolute cross sections determined from signal-ratio measurements.

Figure 4

Measured photon energy onsets for removal of two to seven pairs of C atoms accompanying single ionization of ${\mathrm{C}}_{60}^{+}$. The line represents a linear fit to the data.

Figure 5

Measured cross section for double photoionization of ${\mathrm{C}}_{60}^{+}$. The triangles represent a fine energy scan and the open circles with error bars designate absolute cross-section measurements to which the scan has been normalized. The inset shows the threshold energy region on an expanded scale and a linear regression analysis to determine the double-ionization threshold energy.

Figure 6

Measured absolute cross sections for double photoionization of ${\mathrm{C}}_{60}^{+}$ and for double photoionization accompanied by the loss of up to seven pairs of C atoms. Open circles denote absolute cross sections determined from signal-ratio measurements.

Figure 7

Cross-section measurements for fragmentation without ionization of ${\mathrm{C}}_{60}^{+}$ resulting in the loss of two to eight pairs of C atoms. Open circles denote absolute cross sections determined from signal-ratio measurements.

Figure 8

Sum of the measured cross sections for all 23 product ion channels resulting from photoabsorption by ${\mathrm{C}}_{60}^{+}$ ions (open circles) compared with the calculated photoabsorption cross section of neutral ${\mathrm{C}}_{60}$ from time-dependent density-functional theory (red curve) [33]. Note that the ionization potentials of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{60}^{+}$ are 7.65 and 11.35 eV, respectively.