Angular-resolved Photoelectron Spectroscopy of Superatom Orbitals of Fullerenes

Photoelectron angular distributions from both ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ were recorded for low laser intensity femtosecond and picosecond pulses. Rich structure is seen for electron kinetic energies that lie below the photon energy. Strong, broad peaks are observed for photoelectron energies corresponding to single-photon ionization of so-called superatom molecular orbitals (SAMOs). The very simple angular distributions measured for these peaks, the close similarity of the spectra observed from ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, and the comparison with time-dependent density functional theory provide strong support for the SAMO hypothesis.

Figure 1

Inverted VMI images for ${\mathrm{C}}_{60}$ after (a) 120 fs, 400 nm laser excitation ($3\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$) and (b) 120 fs, 800 nm ($3.7\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$). The laser polarization direction was parallel to the $z$-axis.

Figure 2

Angle-resolved PES for polar angle interval 0–10° obtained by exciting (a) ${\mathrm{C}}_{60}$ with 400 nm, 120 fs laser pulses of intensity $3\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$. (b) Calculated spectrum of excited states of ${\mathrm{C}}_{60}$ from TD-DFT calculations. The solid line was obtained by convoluting each state with a Gaussian function. The energy scale has been converted to photoelectron kinetic energy to compare to experimental PES obtained for 400 nm light. (c) Same as (a) but using 401 nm, 4 ps ($5{\mathrm{cm}}^{-1}$ bandwidth) laser light of intensity $7\times {10}^{10}\mathrm{W}/{\mathrm{cm}}^2$; (d) ${\mathrm{C}}_{70}$ 400 nm, 120 fs of intensity $3\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$ and (e) ${\mathrm{C}}_{60}$ 800 nm, 120 fs and intensity $3.7\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. The inset in (a) shows a time-of-flight mass spectrum obtained for the same laser conditions as in (a).

Figure 3

PADs corresponding to the marked peaks in Fig. 2 obtained by exciting with (a)–(d) 400 nm, 120 fs laser pulses of intensity $3\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$ and (e)–(h) 800 nm, 120 fs and intensity $3.7\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. The angular dependence was fitted using Eq. (1) to determine $\beta$.

Figure 4

Anisotropy parameter, $\beta$, as a function of photoelectron kinetic energy. The dashed lines show the minimum and maximum possible value of $\beta$ for a single-photon ionization process. Peaks I and IV are identified as arising from $s$-SAMO states based on their binding energies and $\beta \approx 2$. The solid line shows the calculated $\beta$ values obtained for the $p$-state using Eq. (2).