Partial photoionization cross sections of C60 and C70: A gas versus adsorbed phase comparison

Abstract

We have performed high resolution measurements of the photoelectrons emitted from the valence shell of C70, in the gas and adsorbed phase on a metal surface, in order to derive branching ratios and the partial photoionization cross sections of the two highest occupied molecular orbitals, HOMO and HOMO-1. The comparison between the two phases and the adsorbed phase shows an interesting and unexpected difference that can be attributed to a small orbital shift in solid C70. Density Functional Theory calculations within the Local Density Approximation (LDA) show good agreement for both data sets and give a plausible explanation for the observed difference. IntroduktionThe discovery of the family of carbon molecules dubbed the 'fullerenes' [1] and the breakthrough in their synthesis [2] have stimulated an extensive research activity in this field. The best known among the fullerenes is the C60 molecule, which consists of 60 carbon atoms arranged on a nearly spherical shell with the highest possible symmetry, point group Ih. Recently, we have studied some of its particularly interesting properties by photoelectron spectroscopy [3]. Our results revealed strong oscillations in the partial photoionization cross sections of the two highest occupied molecular orbitals, HOMO and HOMO-1, reflecting the molecular geometry and symmetry of the electronic charge distribution.These oscillations, which were for the first time discovered in solid C60 and interpreted as a solid state effect [4,5], were later also observed for free C60 molecules [6]. Their interpretation was basically interpreted within a standing wave model of the photoelectron wave in a box like potential [7] being extended to a jellium like potential in a subsequent experimental and theoretical study [8]. It is the spherical like structure of the C60 molecule which gives rise to shell like distribution of the delocalized carbon valence electrons. However, one may look to the problem also from the view point of symmetry. The valence hole states HOMO and HOMO-1 have different symmetry gerade (g) and ungerade (u), a situation which may be compared to homonuclear diatomic molecules. Here the g and u states show also pronounced oscillations in their partial photoionization cross sections and even more pronounced oscillations in their photoelectron diffraction intensity behavior in the molecular frame. These oscillations which were predicted more than forty years ago by Cohen and Fano [9] have been experimentally corroborated since the last ten years only [10,11]. Regarding photoionization this was even more recently [12]. The showcase example exhib-

Attosecond structures from the molecular cavity in fullerene photoemission time delay

“…This produced HOMO and HOMO-1 to be of 2h (l = 5) and 2g (l = 4) character, respectively, with each having a radial node-a result known from the quantum chemical calculation [20] supported by direct and inverse photoemission spectra [21], and from energy-resolved electron-momentum density measurements [22]. TDLDA predicted oscillatory photoemission cross sections of HOMO and HOMO-1 in C 60 which agreed well with the experiment [23] and with quantum chemical calculations [23,25]. Figure 1 shows very good agreement between measurements and TDLDA ratio of HOMO and HOMO-1 cross sections for the four low-energy oscillations.…”

VUV photoionization dynamics of the C60 buckminsterfullerene: 2D-matrix photoelectron spectroscopy in an astrophysical context

“…Note that comparing the branching ratios between different electronic states intensities between this work and the simulated PES at 14 eV of Gao et al is not straightforward because of the kinetic energy effect on the partial cross sections and the possible presence of autoionization resonances in the TPES. Lastly, a comparison with the measured PES from Korica et al 68 is presented which was collected at 50 eV and compares reasonably well with our TPES in terms of shape and energies although the lower resolution of this spectrum precludes a more precise comparison. For instance, the first band of Korica et al is noticeably shifted to the red by about 400 meV which could either be a resolution effect or due to other experimental issues.…”

Vibrational branching ratios in the photoelectron spectra of N2 and CO: interference and diffraction effects

“…While weak EX-AFS oscillations in absorption cross sections due to diffraction are nowadays a standard tool for local structure determination in solid state, surface science and biological studies, comparable studies in photoelectron emission in the gas phase are just in their infancy. In fact up to now only two cases have been thoroughly explored: the diatomic homonuclear case, especially N 2 4,11,61,64,69,74,75,77 , after the Cohen-Fano prediction 82 , and the HOMO/HOMO-1 oscillations in C 60 and related systems [86][87][88][89][90][91][92][93][94] . It is becoming clear, however, that such interference/diffraction patterns are an ubiquitous phenomenon, although more complex in nonsymmetrical molecules and further complicated in the valence shell by mixing of different atomic orbitals (i.e.…”