Single Photon Thermal Ionization of ${\mathrm{C}}_{60}$

We report on experiments which show that ${\mathrm{C}}_{60}$ can ionize in an indirect, quasithermal boiloff process after absorption of a single photon. The process involves a large number of incoherently excited valence electrons and yields electron spectra with a Boltzmann distribution with temperatures exceeding ${10}^4\mathrm{K}$. It is expected to be present for other molecules and clusters with a comparatively large number of valence electrons. The astrophysical consequences are briefly discussed.

Figure 1

Raw VMI transverse velocity distributions measured in coincidence with ${\mathrm{C}}_{60}^{+}$ at the photon energies given. The labels on the axes of panel (d) give the electron speeds parallel to the detector surface in units of $10^6\mathrm{m}/\mathrm{s}$. The images are normalized to identical total intensities.

Figure 2

(a) VMI image recorded at the photon energy $h\nu =17\mathrm{eV}$. (b)–(d) show electron counts versus different polar angles for low kinetic energy electrons (b) and for electrons emitted upon direct ionization of HOMO-1 (c) and HOMO (d).

Figure 3

Electron spectra deconvoluted from the VMI images measured in coincidence with ${\mathrm{C}}_{60}^{+}$ (a)–(h), ${\mathrm{C}}_{58}^{+}$ (i)–(k), and ${\mathrm{C}}_{56}^{+}$ (l) at several different photon energies. The dashed blue curves depict shifts of the HOMO and HOMO-1 with the photon energy. The electron spectra show that the relative intensities of HOMO and HOMO-1 vary as a function of photon energy, consistent with the observations made in [18, 19, 20]. The yellow areas highlight the low kinetic energy range ($E_k\le 3\mathrm{eV}$). The red curve in (j) is ${10}^{-3}+0.04\exp (-E_k/kT)$ with $kT=1.58\mathrm{eV}$.

Figure 4

The ionization process in a schematic, multilevel Jablonski diagram. Photoexcitation promotes the molecule to a highly excited electronic state, with the vibrational degrees of freedom (indicated as lines and grey areas) as spectators. During a time on the order of several hundred femtoseconds, the molecule moves across a number of states with increasingly lower electronic ground state energies and correspondingly higher vibrational excitation energy (red arrow). The motion should be understood as an average drift. During this drift, an electron may be emitted in a stochastic process (green arrow).

Figure 5

VMI spectra measured in coincidence with ${\mathrm{C}}_{60}^{+}$, ${\mathrm{C}}_{58}^{+}$, and ${\mathrm{C}}_{56}^{+}$, at the photon energies indicated. The photon energies at which the frames are recorded are given to the left, and the ions measured in coincidence are given at the top.