Low-energy reactions of CF<SUB>2</SUB><SUP>2+</SUP> dications with atoms and molecules

State-selective, energy- and angle-resolved collisional interactions of CF<SUB>2</SUB><SUP>2+</SUP> dications with atomic and molecular gases are studied at low collision energies. Single-electron transfer measurements help resolve the prevailing ambiguity vis-a-vis the lowest ionization energy of CF<SUB>2</SUB><SUP>2+</SUP>, yielding a value of 20.55 ± 0.5 eV. Ground state (<SUP>1</SUP>Σ <SUB>g</SUB><SUP>+</SUP>) CF<SUB>2</SUB><SUP>2+</SUP> captures an electron from Ar, D<SUB>2</SUB>, N<SUB>2</SUB> and CF<SUB>4</SUB> predominantly into the ground 6a<SUB>1</SUB> orbital of CF<SUB>2</SUB><SUP>+</SUP> but in CF<SUB>2</SUB><SUP>2+</SUP>-O<SUB>2</SUB> and H<SUB>2</SUB>O collisions, the dominant capture channel is the excited (4b<SUB>2</SUB>) orbital of CF<SUB>2</SUB><SUP>+</SUP> . Data are qualitatively interpreted using 'reaction windows' calculated using the Landau-Zener and extended over-the-barrier models

Electron capture processes in slow collisions of Ne

Energy-gain spectra and absolute total cross-sections for single-, double-, and triple-electron capture processes in collisions of Ne6+ ions with CO2 and H2O at laboratory impact energies between 450 and 2400 eV, have been studied experimentally by means of a translational energy-gain spectroscopy technique. The energy-gain spectra for single-electron capture show that the dominant reaction channels are due to capture into the n = 4 state of Ne5+, in agreement with classical over-the-barrier model calculations. In both cases, contributions due to transfer excitation into the 2s2p (1,3P) 3 l states are also detected. The energy-gain spectra are interpreted qualitatively in terms of the reaction windows, which are calculated using the single-crossing Landau-Zener (LZ) model and the extended version of the classical over-the-barrier (ECOB) model. The energy dependence of cross-sections for electron capture are also measured and found to be slowly increased with increasing collision energy. The data for single-electron capture are also compared with theoretical results based on the multi-channel Landau-Zener (MCLZ) model

Charge exchange and dissociative processes in collisions of slow He2+ ions with H2O molecules

Experimental and theoretical studies of one-electron capture in collisions of He2+ ions with H2O molecules have been carried out in the range 0.025-12 keV amu(-1) corresponding to typical solar wind velocities of 70-1523 km s(-1). Translational energy spectroscopy (TES), photon emission spectroscopy (PES), and fragment ion spectroscopy were employed to identify and quantify the collision mechanisms involved. Cross sections for selective single electron capture into n=1, 2, and 3 states of the He+ ion were obtained using TES while PES provided cross sections for capture into the He+(2p) and He+(3p) states. Our model calculations show that He+(n=2) and He+(n=3) formation proceeds via a single-electron process governed by the nucleus-electron interaction. In contrast, the He+(1s) formation mechanism involves an exothermic two-electron process driven by the electron-electron interaction, where the potential energy released by the electron capture is used to remove a second electron thereby resulting in fragmentation of the H2O molecule. This process is found to become increasingly important as the collision energy decreases. The experimental cross sections are found to be in reasonable agreement with cross sections calculated using the Demkov and Landau-Zener models