Merged-beam Setup Research Articles

Single and multiple photoionization of Wq+ tungsten ions in charge states q=1,2,..,5: experiment and theory

Absolute cross sections for single and multiple photoionization of Wq+ ions have been measured in a photon-ion merged beam setup employing synchrotron radiation from the Advanced Light Source. The experimental data are compared to large-scale close-coupling calculations within the Dirac-Coulomb R-matrix (DARC) approximation.

Photoionization cross section calculations for the halogen-like ions Kr+ and Xe+

Photoionization cross section calculations on the halogen-like ions; Kr+ and Xe+ have been performed for a photon energy range from each ion threshold to 15 eV, using large-scale close-coupling calculations within the Dirac–Coulomb R-matrix approximation. The results from our theoretical work are compared with recent measurements made at the ASTRID merged-beam set-up at the University of Aarhus in Denmark and from the Fourier transform ion cyclotron resonance trap method at the SOLEIL synchrotron radiation facility in Saint-Aubin, France Bizau et al (2011 J. Phys. B: At. Mol. Opt. Phys. 44 055205) and the advanced light source Müller (2012 private communication), Aguliar et al (2012 J. Phys.: Conf. Ser. at press). For each of these complex ions our theoretical cross section results over the photon energy range investigated are seen to be in excellent agreement with experiment. Resonance energy positions and quantum defects of the prominent Rydberg resonances series identified in the spectra are compared with experiment for these complex halogen-like ions.

Photoionization of tungsten ions with synchrotron radiation

Photoionization experiments were performed with tungsten ions using a photon–ion merged-beams setup at the Advanced Light Source. Cross-sections σq,q+1 for photon-induced single ionization of Wq+ ions (q=1,2,3,5) and σq,q+2 for photon-induced double ionization of W+ ions have been explored in the energy range 20–70 eV. While σq,q+1 for the lower charge states is dominated by broad resonance features in the range of 4f and 5p excitations, the cross-section σ5, 6 shows much narrower peak features. Significant contributions from initial metastable states are evident. Double ionization is substantially enhanced at energies above the threshold for single ionization of a 4f electron in W+.

Photoionization study of Kr+ and Xe+ ions with the combined use of a merged-beam set-up and an ion trap

Photoionization cross sections of halogen-like Kr+ and Xe+ ions have been measured in the photon energy range extending up to 15 eV above the threshold. Two different devices were used, a merged-beam set-up and an ion trap. Combination of the two techniques allows for the extraction of the pure ground state ionization cross section on an absolute scale. Multiconfiguration Dirac–Fock calculations reproduce the magnitude of the direct photoionization cross sections well.

Experimental studies of electron collisions with atomic ions for astrophysical plasmas

We have measured dielectronic recombination (DR) and electron impact ionization (EII) for a range of astrophysically relevant atomic ions (Fe9+, Fe10+, O3+, and Mg6+). The experiments were performed in a merged beams setup at the heavy-ion storage ring TSR in Heidelberg, Germany. Here we discuss the importance of our data for astrophysical modelling and compare our experimental results with the previously recommended data and with modern theoretical calculations. We have parameterized our data using simple fiting formulae for convenient inclusion into astrophysical modelling codes.

Competitive processes in the associative ionization ofC−withC+,N+, andO+

Absolute integral cross sections have been measured for associative ionization reactions involving the ${\mathrm{C}}^{\ensuremath{-}}$ and ${\mathrm{C}}^{+}$, ${\mathrm{N}}^{+}$, and ${\mathrm{O}}^{+}$ reactants. These measurements, obtained using a merged-beam setup in the keV range, provide us with useful experimental information on the efficiency and mechanisms of molecular ion formation from ionic reactants. The relative magnitudes of the different cross sections are rationalized by considering the spin multiplicities of initial and final states, and the exothermicities of the detachment and transfer ionization channels. The very different production efficiencies of $\mathrm{C}{\mathrm{O}}^{+}$ ions via the ${\mathrm{O}}^{\ensuremath{-}}+{\mathrm{C}}^{+}$ and ${\mathrm{C}}^{\ensuremath{-}}+{\mathrm{O}}^{+}$ channels are explained by statistical and energetic considerations. The potential energy curves of CO and $\mathrm{C}{\mathrm{O}}^{+}$ have been calculated by quantum ab initio methods in order to characterize the reactive pathways leading to autoionization. Thermal rate coefficients are derived to serve the plasma physics community.

Reactive collisions between CH+ and O−

Integral cross sections were measured for two reactions occurring in CH+ + O- collisions: the formation of the carbon monoxide cation CO+ via a reactive ionization process and the formation of the (iso)formyl cation HCO+ (HOC+) via the associative ionization process. Both carbon monoxide and formyl cations are present in the interstellar medium, the latter one being quite abundant in dense clouds. Provided the oxygen anion would also be present in the interstellar environment, the large efficiency of the two reactive processes reported here would justify their inclusion in astrochemical models. The whole set of data was obtained by means of a merged-beam setup operating with keV beams.

Absolute measurements of photoionization cross-sections for ions

A merged-beam set-up for absolute measurements of photoionization cross-sections of ions is described. The facility is capable of recording cross-sections as low as 10 −19 cm 2 and has been used to study a large number of singly- and multiply-charged, atomic and molecular, positive and negative ions. It is based on a synchrotron radiation beam line fitted with an undulator at the storage ring ASTRID and a low-energy (∼2 keV) ion beam line. Photons in the energy range 15–200 eV are merged co-linearly with the target ions over a distance of 50 cm, and the absolute photoionization cross-section is determined from the resulting photoion yield with a typical accuracy of 10%. Different types of ion sources are available, thus permitting a large number of positive and negative, atomic and molecular, singly- and multiply-charged ions to be investigated. Emphasis is put on accurate determination of the absolute cross-sections, requiring calibration of photodiode and particle detectors together with measurements of the photon–ion overlap.

Abstraction and insertion mechanisms in reactive collisions of H $_\mathsf{2}^\mathsf{ + }$ and D $_\mathsf{2}^\mathsf{ + }$ with O $^\mathsf{-}$

Integral cross-sections were measured for the associative ionisation and reactive ionisation in collisions of H-2(+) and D-2(+) with O- by means of a merged-beam set-up operating with keV beams. The magnitude of the reactive cross-sections is quite large (10(-14) cm(2) at 10 meV), and surpasses the associative ionisation by an order of magnitude. The observed ratio is discussed in terms of insertion and abstraction mechanisms that prevail in the case of atom-diatom inelastic collisions. These measurements may be relevant to the understanding of some astrophysical objects such as the comets, where the presence of the water cation was highlighted.

Merged beam study of the associative ionization O− + D+/O+ + D− and C+ + D−

Absolute total cross sections have been measured for the associative ionization(AI) reactions O− + D+/O+ + D− and C+ + D− using a merged beam set-up. These originalmeasurements might be relevant to the understanding of several astrophysicalobjects. The internal excitation of the target ions is discussed, and structuresappearing in the different cross section curves are related to the interplay betweenAI and other non-associative reaction channels, such as detachment and multipleionization.

Merged beam study of the associative ionisation ( C + , N + and O + )+ O -

Total cross-sections have been measured for the associative ionisation of C+ + O-, N+ + O- and O+ + O- by means of a merged-beam set-up operating with keV beams. These original measurements might be relevant to the understanding of some astrophysical objects or laboratory-made plasmas (flames and etching plasmas). The magnitude of these cross-sections is particularly large whatever the associating system, as these are in the range of 1 X 10(-14) cm(2) at thermal energies. Their behaviour as a function of energy significantly differs from one system to another, and is characterised by the Wigner law at low energy, and a rapid fall-off at higher energy due to competition with non-associative ionisation processes.