H. Steger

Single-photon ionization of C60- and C70-fullerene with synchrotron radiation: determination of the ionization potential of C60

Abstract The photoionization threshold region of C60 has been studied using single-photon ionization with synchrotron radiation. Mass selection is realized with a time-of-flight mass spectrometer. The ionization potential of C60 has been determined to 7.58+0.04−0.02 eV.Significant structure in the photoionization yield is seen in the energy range up to 1 eV above threshold.

Direct double ionization of C60 and C70 fullerenes using synchrotron radiation

Abstract The double photoionization threshold region of C 60 and C 70 has been studied using single-photon ionization with synchrotron radiation. Mass selection is realized with a reflectron time-of-flight mass spectrometer (RETOF). The ionization potentials for double ionization of C 60 and C 70 have been determined to be 19.0±0.03 and 18.84±0.03 eV, respectively. In the 6 eV range above the IP covered in this report, the doubly charged ion yield spectra exhibit no significant structure and rise continuously and nearly linearly as a function of photon energy. A change of slope occurs in the C 2+ 60 signal 1.3 eV above threshold which could be due to participation of an excited C 2+ 60 state.

Single-photon ionization of higher fullerenes C76, C78 and C84. Determination of ionization potentials

Abstract The ionization of higher fullerenes by single-photon excitation with synchrotron radiation has been investigated in a molecular beam experiment using the time of flight technique. Ionization potentials for the higher fullerenes C 76 , C 78 and C 84 have been determined from photoionization efficiency spectra for these molecules. The measured values are 7.10±0.10 eV for C 76 , 7.05 ± 0.10 eV for C 78 and 7.15±0.10 eV for C 84 .

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOFs capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH3 monomer from protonated clusters (NH3)n − 2NH4+. Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.

Ionisation and fragmentation dynamics of highly fluorinated fullerenes C60F46,48 and C70F54,56 after excitation with synchrotron radiation

Highly fluorinated fullerenes of the composition C60Fx with x = 46, 48 and C70Fx with x = 54, 56 have been ionised by single photon absorption. The resulting ions have been detected by means of time-of-flight mass spectrometry. For all species under investigation, the careful analysis of the photo-ion yield curves at threshold revealed identical ionisation energies of 12.0 eV with an uncertainty of ±0.1 eV for the C60-based molecules and ±0.2 eV for the C70-based ones.

Metastable decay of argon clusters after photoionisation at high excess energies

Recently Märk and collaborators [1, 2] reported the metastable emission of large fractions from argon and neon cluster ions after electron impact ionisation at high excess energies. The decay was interpreted as the result of an intra-cluster excitation of a metastable state by one of the electrons involved in the ionisation process. Here we report the first direct observation of such a correlated two electron process during photoionisation of argon clusters using synchroton radiation and the TPEPICO technique. We observe at least two distinct maxima of the metastable TPEPICO spectrum at around 27 eV and 28.5 eV, the former being consistent with the previously reported energetic threshold for electron impact ionisation [1, 2].

TPEPICO studies near ionization threshold of argon and krypton clusters

Single photon ionization of argon and krypton clusters has been studied in the region between threshold and the ionization potential of the corresponding atom. Synchrotron radiation from the electron storage ring BESSY is used to ionize the clusters; threshold-photo-electron-photoion-coincidence (TPEPICO)-time-of-flight technique is used to detect ions correlated with the emission of zero-kinetic-energy-electrons. The spectra of the clusters in the range ofn=2 to 15 are discussed in view of the extensive fragmentation taking place in these systems. In order to characterize the properties of the clusters a method using scaling laws is applied. The principles and the deduction of Hagenas scaling parameter Γ* are briefly reviewed. Using Γ* an experimentally derived mean cluster size for molecular beams can be assigned. This allows one to clearly demonstrate the systematic variations of the measured spectra due to cluster fragmentation. As a general feature it is observed that, in the range studied, the peak in the measured ionization rate for a cluster ion (fragment) of a given size shifts to higher photon energies as the mean cluster size is increased.

The role of excitons in argon cluster ion decay

Argon clusters show metastable decay with ejection of several monomers when they are photoionized at energies where, in addition to ionization, excitonic states can be populated. When photoions are detected in coincidence with threshold photoelectrons (TPEPICO) resonancelike peak shapes in the signal as a function of photon energy for both the metastable and the spontaneous fragmentation channel are observed. Contributions of different excitonic states to the signal can be resolved and their dependence on cluster size is studied. Several possible excitation processes are being discussed.

Fullerene ionization dynamics after single photon excitation with synchrotron radiation

Ionization dynamics of free C60 is investigated after single photon excitation with synchrotron radiation in the energy range between 8.9 eV and 27.6 eV. Upper limits for the time constants describing possible delayed ionization are obtained from peak shape asymmetries in time of flight spectra as a function of excitation energy. The results clearly indicate that delayed ionization is not observed after single photon excitation in contrast to the results obtained in ns-laser MPI experiments.

PHOTOIONIZATION THRESHOLD STRUCTURE OF FULLERENES: VIBRATIONAL EXCITATION

The threshold region of the photoionization spectra of fullerenes shows distinct changes of slope which are typical for the opening of additional channels due to vibrational excitation. Typically, after evaporation from a hot oven, C60 is excited in all its vibrational modes. Through coupling with the ionization transition only the two highly symmetric modes (breathing mode and tangential stretch mode) can be excited or de-excited. An analysis of the C60 data reveals very good agreement of the observed structure with the expected energetic positions of various combinations of excitation and de-excitation of these two modes.