Bärbel Siegmann

Multi-fragmentation of C60 after collisions with Arz+ ions

We studied the fragmentation of C60 molecules in collisions with slow Ar+ ions (v ≈ 0.04–0.55 au) and with Arz+ ions (z = 2, 3) at somewhat higher velocities (v ≈ 0.3–0.95 au). The measured mass spectra show C60q+ ions (q ≤ 5), smaller C60−2mq+ fullerenes and C60−2mq+ + C n+ fission products. Special attention is devoted to the C60 multi-fragmentation process, i.e. the breakup of the C60 cage into several small Cn+ fragments (n = 1–29). Up to seven correlated small Cn+ fragments (n = 1–5) have been observed. The multi-fragmentation pattern depends on the projectile charge. Furthermore, it varies strongly with the projectile velocity for slow ions, whereas it becomes nearly constant for faster ions. This is in accordance with recent non-adiabatic quantum molecular dynamic calculations.

Multiple ionization and fragmentation of H2O in collisions with fast highly charged Xe ions

The multiple ionization and fragmentation of H2O in collision with 5.9 MeV u-1 Xe17+, Xe18+ and Xe43+ ions was studied using a position- and time-sensitive multiparticle detector which allows the coincident measurement of the momenta of correlated fragment ions. If all fragments from a particular fragmentation are detected, a kinematically complete study of the molecular break-up process is possible. For these Coulomb explosion processes the kinetic energy as well as angular correlations are determined.

Multiple ionisation and fragmentation of molecules

The multiple ionisation and dissociation of molecules, e.g. H2, N2, H2O, CH4 and C60, by fast ions was studied using a position- and time-sensitive multi-particle detector. The data obtained allow a clear separation of various reaction channels. Of special interest are the ‘Coulomb explosion’ processes like N2 → Nq+ +Nn+ or H2O → H+ + H+ +On+. For these reactions the coincident measurement of the momenta of correlated fragment ion yields a kinematically complete image of the molecular break-up process, and the fragmentation energy as well as angular correlations can be derived for each individual event. In the case of H2 as target molecule the CE model describes the fragmentation energy rather well, whereas in the case of more complex molecules, e.g. N2, CO and CH4, the simple CE model is insufficient to explain the measured energy and angular spectra. Better agreement was achieved with ab initio MCSCF calculations which take into account several molecular states of the fragmenting highly charged molecular ion.

Ion-impact induced excitation and fragmentation of C60

Abstract We studied the multiple ionization and fragmentation of C60 in collisions with 50– 300 keV H + , He+, Ne+, and 6.5– 300 keV ·z Ar z+ (z=1–3) ions using a time- and position-sensitive multi-particle detector. Several aspects of C2 evaporations, fullerene fission and multi-fragmentation processes are discussed. For Ar++C60 the multi-fragmentation patterns depend on the projectile velocity and especially for slow ions strong variations occur. This behaviour is in accordance with recent non-adiabatic quantum molecular dynamic (NA-QMD) studies (Phys. Rev. Lett. 86 (2001) 5258).

Ionization and fragmentation of small molecules in collisions with slow ions

Abstract We studied the fragmentation dynamics of small molecules, as e.g. N 2 or CH 4 , in collisions with slow He 2+ and Ar 3+ ions. A combination of position- and time-sensitive detectors allows the coincident measurement of the generated electrons and fragment ions as well as of those projectiles involved in a charge exchange process. The kinetic energy released by the molecular fragments clearly depends on the number of electrons stabilized at the projectile. For example, the measured N + +N + energy distributions in direct ionization processes are considerably broader than those corresponding to capture processes.

Orientation dependence of N2 and O2 multiple ionization in slow and fast collisions with highly charged Xe-ions

Abstract The influence of the molecular orientation on the multiple ionization of N 2 and O 2 was studied in collisions with highly charged Xe-ions at velocities of 0.2–0.3 and ≈15 a.u. The applied technique is based on a position- and time-sensitive multi-particle detector which allows to determine the ionization cross-section as a function of the orientation of the molecular axis. At lower collision energies, e.g. in collisions with 360 keV Xe 18+ , no anisotropy was found, whereas in collisions with 770 MeV Xe 18+ significant orientation effects appear at higher degrees of ionization.

The collision-induced electric dipole moment of H(n=2) in H-He, Ne and Ar collisions

The collision-induced electric dipole moment of H(n=2) in H(1s)-Ne and H(1s)-Ar collisions was measured in an energy range of 1 to 25 keV. For these systems we observe a positive electric dipole moment which corresponds to an electron lagging behind the proton. This behaviour is in contrast to recent measurements for the H-He systems, where a negative dipole moment corresponding to an electron moving in front of the proton was observed. A simple explanation for this difference is given.

Angular distribution of hydrogen fragment ions in H + -H 2 collisions

The angular distribution of H+ fragment ions produced in 5–25 keV H+–H2 collisions was investigated in coincidence with Lyman-α photons. The observed photons arise from electron capture to the projectile H(2p) state and/or from the fragmentation of the H2 molecule via 2sσg, 2pσu or 2pΠ u states of the excited H+2* ion. An analysis of the measured angular distributions has been performed to distinguish the separate degenerate channels available to an emitted Lyman-α photon. The results show similarities to the data of Lindsay et al. (1987) who measured the non-coincident angular distribution of H+ fragment ions within the same energy range.

Coherent excitation of H( n = 2) by electron capture in and collisions

The linear polarization and the collision-induced dipole moment of H(n = 2) in and collisions were measured in an energy range of 1 - 25 keV. We observe a positive electric dipole moment corresponding to an electron lagging behind the proton; however, in contrast to, for example, - He, Ne and Ar collisions the linear polarization nearly vanishes.

Coherent Excitation of H(n=2) in Few-Electron Collision Systems

Excitation processes in simply-structured one- and two-electron systems are both of fundamental importance and of practical relevance for a number of fields. Valuable information about the dynamics of such collisions can be extracted from a measurement of so-called coherence parameters. These parameters relate to the population of energetically degenerate states and are sensitive to the relative phases between the corresponding excitation amplitudes. In collisions involving hydrogenic atoms, not only the coherence between the energetically degenerate magnetic substates (magnetic quantum number m) but also between the near-degenerate angular momentum states (angular momentum quantum number l) with the same principle quantum number n becomes accessible1.