Progress towards a realistic theoretical description ofC60photoelectron-momentum imaging experiments using time-dependent density-functional theory

Abstract

We have studied theoretical photoelectron-momentum distributions of C60 using time-dependent density functional theory (TDDFT) in real time and including a self-interaction correction. Our calculations furthermore account for a proper orientation averaging allowing a direct comparison with experimental results. To illustrate the capabilities of this direct (microscopic and time-dependent) approach, two very different photo-excitation conditions are considered: excitation with a high-frequency XUV light at 20 eV and with a low-frequency 1R femtosecond pulse at 1.55 eV. The interaction with the XUV light leads to one-photon transitions and a linear ionization regime. In that situation, the spectrum of occupied single-electron states in Cso is directly mapped to the photoelectron spectrum. On the contrary, the IR pulse leads to multiphoton ionization in which only the two least-bound states contribute to the process. In both dynamical regimes (mono-and multiphoton), calculated and experimental angle-resolved photoelectron spectra compare reasonably well. The observed discrepancies can be understood by the theoretical underestimation of higher-order many-body interaction processes such as electron-electron scattering and by the fact that experiments are performed at Unite temperature. These results pave the way to a multiscale description of the C^o ionization mechanisms that is required to render justice to the variety of processes observed experimentally for fullerene molecules.

On the dynamics of photo-electrons in C60

“…However, neither the laser intensity nor the pulse duration are precisely known experimentally, while these two parameters strongly influence the final ionization stage and thus the Coulomb shift. This is particularly visible when comparing the dotted [72] and the dashed-dotted [10] PES for which the laser intensities differ by 25 %, and the pulse duration maybe by a factor of 3. Therefore, even if the laser parameters are very close in both experiments, the PES are almost out of phase.…”

“…This is in particular an important effect at low kin . Moreover, we should emphasize that if the calculations are done at 0 K, this is far from being the case in the experiments, in which the temperature of the C 60 jet ranges from 700 K to 850 K [10,18,72]. And we know that a finite ionic temperature tends to blur high electron energy peaks and thus reduces signal to background contrast [72].…”

“…It is worth mentioning that in [9], a theoretical PES calculated within TDLDA but in the jellium approximation is compared to an experimental PES using the same setup of [72] but at a slightly higher intensity (I = 2 × 10 13 W/cm 2 ) and measured at 90…”

“…Figure 8 shows the PAD for different photon frequencies, total PAD and selective for the group of HOMO and HOMO−1 levels. Note that the latter states are those which are experimentally analyzed the best [72]. In a given panel, each curve corresponds to one frequency and the number in parentheses is the β 2 resulting from a fit according to Eq.…”

Strong-field effects in the photoemission spectrum of theC60fullerene

“…There exist numerous investigations for C 60 [28,29] and other carbon nanostructures such as graphene [30] or nanotubes [31]. An elaborate description of C 60 with an involved orientation averaging is needed for detailed observables such as photo-electron spectra (PES) and photo-angular distributions (PAD) [32][33][34]. A key issue in the present investigations is the ponderomotive motion of the electron in the laser field associated with huge excursions of the electron [35].…”