Photoionization and photofragmentation of multiply charged ${\text{Lu}}_3{\text{N@C}}_{80}$ ions

Relative cross sections for photoionization of endohedral fullerene ions ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{q+}$ $\left(q=1,2,3\right)$ have been measured employing the photon-ion merged-beam technique. The investigations include various ionization channels ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{+q\to +p}$ $(q=1,2,3$ and $p=2,3,4,5,6)$, in some cases accompanied by fragmentation of the carbon cage. Prominent structures related to the carbon $K$-shell ionization threshold were observed in the energy range 280–330 eV. These resonance structures have been analyzed in seven product channels and are compared with previously known absorption spectra of several fullerene species. Within the statistical significance of the data, we do not observe any signatures of the N $1s$ and Lu $3d$ thresholds in the energy ranges 390–435 eV and 1500–1700 eV, respectively. By comparing the energies for double ionization of ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{+}$ and ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{2+}$ at the carbon $1s$ $K$-shell threshold, we derive a value of $5.0\pm 0.4\text{Å}$ for the radius of the carbon cage.

Figure 1

Two primary-ion mass spectra of the same sample at different times: (a) recorded after 50 h of outgassing followed by 12 h of slowly increasing the heating power of the oven to 5 W; (b) recorded 48 h after (a) at a heating power of 12.75 W. The mass-resolving power was 200.

Figure 2

Product-ion mass spectrum obtained by sweeping the demerging magnet field while the photon energy remained at 300 eV. The mass scan could not be extended to include the primary ${\mathrm{Lu}}_3\mathrm{N}@\mathrm{C}{{}_{80}}^{+}$ beam because its intensity would have saturated the single-particle detector.

Figure 3

Relative cross sections for single (a), double (b), and triple (c) photoionization of ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{+}$ primary ions. Black square symbols with error bars represent the experimental data. Thick red lines are fitting curves composed of seven Voigt peak profiles (fine solid black lines), a constant background (fine light gray line), and a second order polynomial convoluted with a Gaussian (dashed black line) to model the $K$-shell threshold behavior. In the case of triple ionization, a wide eighth peak was needed for the modeling (dotted gray line). Vertical dashed lines illustrate the peak positions. We observe the same peak positions in the different product channels. The insets show the relative contributions of each resonance area. The largest peak area is normalized to 100% (see Table 2).

Figure 4

Relative cross sections of photoreactions for singly, doubly, and triply charged ${\mathrm{Lu}}_3\mathrm{N}$@${\mathrm{C}}_{80}$ ions. (a) ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{2+}\to {\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{3+}$ and (b) ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{3+}\to {\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{4+}$ both accompanied by the scaled fitting result from Fig. 3; (c) ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{+}\to {\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{78}{}^{3+}$ and (d) ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{2+}\to {\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{4+}$ both accompanied by the individually scaled fitting results from Fig. 3.

Figure 5

Comparison of photoabsorption of a range of fullerene species around the carbon $K$-shell ionization threshold with cross-section data from the present study. Photoabsorption of (a) ${\mathrm{C}}_{60}$ in the gas phase (black squares) [44] and solid ${\mathrm{C}}_{60}$ (orange squares) [45], (b) solid ${\mathrm{C}}_{70}$ [45], (c) solid ${\mathrm{C}}_{80}$ [46], and (e) graphite [45]. ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{+}$ cross-section data in panel (d): Single ionization (light-gray circles) and double ionization (black squares).

Figure 6

Photoreaction cross-section spectra in the energy range 1500–1700 eV of the ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{1+}$ initial state. (a) Black symbols: sum of the four individual spectra (b)–(e), (b) gray symbols: ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{3+}$ product ions, (c) black symbols: ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{4+}$ product ions, (d) gray symbols: ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{72}{}^{5+}$ product ions, (e) black symbols: ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{74}{}^{6+}$ product ions, and (f) solid orange line: prediction for the Lu photoabsorption cross section near the $M$ edge by Henke et al. [47] based on semiempirical calculations.