C. Strömholm

Dissociative recombination and excitation of N2+: Cross sections and product branching ratios

The absolute dissociative recombination and absolute dissociative excitation rate coefficients and cross sections have been determined for N2+ and electrons for collision energies between 10 meV and 30 eV. The ion storage ring CRYRING has been used in combination with an imaging technique with a position-and-time-sensitive detector. Information is retrieved on the ion beam vibrational state populations and on the product branching in the dissociative recombination process at 0 eV collisions. A hollow cathode ion source has been used to lower the vibrational excitation in the ion beam; a more traditional hot-cathode ion source was used as well. The most important findings are the following. The rate coefficient for an N2+ ion beam (46%, v=0, 27% v=1) versus electron temperature (K) is α(Te)=1.75(±0.09)×10−7(Te/300)−0.30 cm3 s−1. The dissociative recombination rate is found to be weakly dependent on the N2+ vibrational level. At 0 eV collision energy, the v=0 product branching is found to be 0.37(8):0.11(6)...

Direct Determination of the Ionization-potential of Co By Resonantly Enhanced Multiphoton Ionization Mass-spectroscopy

Direct determination of the ionization potential of CO by resonantly enhanced multiphoton ionization mass spectroscopy

Rotational effects in HD+ dissociative recombination: Theoretical study of resonant mechanisms and comparison with ion storage ring experiments

The cross section for dissociative recombination of HD+ ions is calculated in the energy range 1 meV-13 eV, using an extended MQDT approach which includes simultaneously electronic, vibrational and rotational interactions. We analyse the effect of rotation on the low-energy resonance structure due to interfering bound Rydberg states, whereas at higher energy, above the ion dissociation limit, competition with dissociative excitation is introduced. The absolute cross section measured in the ion storage ring CRYRING is well reproduced in shape and size below 0.3 eV if the theoretical results are averaged with a 300 K Boltzmann distribution of ion rotational levels, in equilibrium with the beam tube at room temperature. Beyond this energy and up to the ion dissociation threshold (2.7 eV), the experimentally measured cross section is considerably larger than the theoretical one, although similarities in shape are still present. Above the dissociation limit, we obtain an overall good agreement between theory and experiment.

Photoionization and photodissociation of nitric oxide in the range 9–35 eV

Photoionization and photodissociation of NO has been studied using 9–35 eV synchrotron light and detection of fluorescence as well as of mass selected NO+, N+, O+, and O− ions. Rydberg series converging to all known states in NO+ below 24 eV have been analyzed and classified and several of them are found to be predissociated by various NO+ states as well as by NO ion pair states. The complex structure in the 950–1200 A excitation region is suggested to originate from interaction between a ‘‘new’’ NO valence state at T0=77 470 cm−1 and high vibrational levels of Rydberg series converging to the NO+ ground state.

A Storage Ring Study of Dissociative Excitation and Recombination of D3

Dissociative recombination and excitation of D3+ have been studied in CRYRING, a heavy-ion storage ring at the Manne Siegbahn Laboratory at Stockholm University. The measured cross section for dissociative recombination was used to deduce a 300 K rate constant of 2.7 × 10-8cm3s-1. This is a factor of four smaller than the corresponding value for H3+ measured earlier in CRYRING. Dissociative excitation into both the D and 2D channels (D + D or D2) were studied. The 2D channel occurs at energies below threshold for the ions dissociative states, which indicates that resonant enhanced dissociative excitation via autoionizing resonances takes place.

Dissociative photoexcitation of CH4 and CD4

Abstract Fluorescence emission spectra and ion-yield spectra of CH 4 and CD 4 have been measured using monochromatic synchrotron radiation. We have studied the X and A ionic states in the 12–30 eV region (400–1000 A). The visible fluorescence measurements focus upon the Rydberg series converging to vibrational levels of the A state at 22.39 eV; this structure is also reflected in some of the charged fragments including the parent ion indicating that these Rydberg states may autoionize. The character of the Rydberg states is discussed, and a comparison with negative-ion fragment spectra is made.

Studies of fluorescence from photoionization and photodissociation of N2 induced by 16-40 eV synchrotron radiation

Molecular nitrogen has been exposed to synchrotron radiation in the range 16-40 eV and the resulting fluorescence has been studied in various regions from 1100-8500 AA. Line structures are observed which are interpreted in terms of autoionization or dissociation of well known Rydberg states in N2 converging to the B, C and D states in N2+. In addition a new member of the Rydberg series converging to the D state is found.

Visible and VUV fluorescence from synchrotron radiation induced photoionization and photodissociation of molecular oxygen

Photon induced fluorescence from O2 has been studied in various emission regions 1150-6500 A in the excitation range 14-28eV using the Swedish synchrotron facility MAX in Lund. A rich line structure is observed which is interpreted in terms of autoionization and dissociation of Rydberg levels converging to the a, b, c, and B states in O2+. New features have been observed, for instance, concerning the (c 4Σu−) 3sσg Fano resonance which has been viewed in detail in different detection channels including O2+ and O+ ions.

Imaging spectroscopy of recombination fragments of

Dissociative recombination of vibrationally cold has been studied at the ion storage ring CRYRING, with an accurate measurement of the kinetic energy of the recombination fragments. When the incident electron energy exceeds , fragments are observed with a kinetic energy of only . The energetics suggest that this low-energy channel derives from either recombination of the state or recombination of the metastable state. High-quality ab initio calculations have been used to obtain the state vibrational relaxation time (10 ms), excitation energy (2.25 eV) and radiative lifetime (0.030 s). The dissociative recombination cross section has been obtained in a model calculation. The results imply that the state has too small a recombination cross section and too short a radiative lifetime to explain the experimental results, which were obtained after 15 s of storage in CRYRING. We conclude that the 120 meV channel is due to recombination of into the asymptotic limit. Possible mechanisms are discussed.