W. Kamke

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.

Photoionisation studies of homogeneous argon and krypton clusters using TPEPICO

The photoionisation threshold region of homogeneous Argon and Krypton clusters Arn and Krn forn up to 24 formed in a free jet expansion has been studied in detail, using the threshold photoelectron photoion coincidence (TPEPICO) time of flight technique. Measurements performed at a variety of different expansion conditions (nozzle temperature and stagnation pressure) demonstrate that fragmentation of larger clusters contributes substantially to the shape of the TPEPICO spectra even for the smallest clusters and at all photon energies higher than about 200 meV to 400 meV above the ionisation threshold. The determination of ionisation potentials for these cluster ions is discussed and careful estimates are given and compared with recent theoretical values.

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 .

On the photoionization and fragmentation of ammonia clusters using TPEPICO

Measurements of the appearance potentials of ammonia cluster ions (NH3)n+ and (NH3)n−2NH4+ using the threshold photoelectron photoion coincidence (TPEPICO) time-of-flight method are performed. The results agree well with other available data, however, solvation energies derived from these and from thermochemical data show significant discrepancies. The energetics for ammonia clusters are elucidated from various points of view.

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.

Exciton absorption and intramolecular Penning ionisation in small molecular clusters I: (N2O)n and C6H5CN·(N2O)n

Photoionisation efficiency (PIE)-spectra are measured for small (N2O)n-clusters,n=2...8, using synchrotron radiation from the electron storage ring BESSY at Berlin. Ionisation thresholds are reported. Resonances in the wavelength-range 97...100 nm forn>2 are attributed to exciton absorption with N2O-Rydberg levels, series I and II parentage. This interpretation is supported by measurements of the PIE of mixed benzonitrile · (N2O)n clusters where benzonitrile serves as a monitor for photon absorption in N2O at energies above the ionisation limit of benzonitrile.