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The present paper reports on a merged-beam experiment on mutual neutralization between ${\mathrm{Na}}^{+}$ and ${\mathrm{D}}^{\ensuremath{-}}$. For this experiment, we have used the DESIREE ion-beams storage-ring facility. The reaction products are detected using a position- and time-sensitive detector, which ideally allows for determination of the population of each individual quantum state in the final atomic systems. Here, the $4s, 3d$, and $4p$ final states in Na are observed and in all cases the D atom is in its ground state $1s ^{2}S$. The respective branching fractions of the states populated in Na are determined by fitting results from a Monte Carlo simulation of the experiment to the measured data. The center-of-mass collision energy is controlled using a set of biased drift tubes, and the branching fractions are measured for energies between 80 meV and 393 meV. The resulting branching fractions are found to agree qualitatively with the only available theoretical calculations for this particular system, which are based on a multichannel Landau-Zener approach using dynamic couplings determined with a linear combination of atomic orbitals model.

We apply near-threshold laser photodetachment to characterize the rotational quantum level distribution of OH- ions stored in the cryogenic ion-beam storage ring DESIREE at Stockholm University. We ...

The intrinsic radiative lifetimes of the $5{d}^{10}6{s}^{}^{2}S_{1/2}$ and $5{d}^{9}6{s}^{2} ^{2}D_{3/2}$ bound excited states in the platinum anion ${\text{Pt}}^{\ensuremath{-}}$ have been studied at cryogenic temperatures at the Double ElectroStatic Ion Ring Experiment (DESIREE) facility at Stockholm University. The intrinsic lifetime of the higher-lying $5{d}^{10}6{s}^{} ^{2}S_{1/2}$ state was measured to be $2.54\ifmmode\pm\else\textpm\fi{}0.10\phantom{\rule{0.16em}{0ex}}\mathrm{s}$, while only a lifetime in the range of 50\char21{}200 ms could be estimated for the $5{d}^{9}6{s}^{2} ^{2}D_{3/2}$ fine-structure level. The storage lifetime of the ${\text{Pt}}^{\ensuremath{-}}$ ion beam was measured to be a little over 15 min at a ring temperature of $13\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The present study reports the lifetime of an atomic negative ion in an excited bound state with an electron configuration different from that of the ground state.

The intrinsic lifetime of the upper level in the bound-bound $3{d}^{9}\phantom{\rule{0.16em}{0ex}}4{s}^{2}$ ${}^{2}{D}_{3/2} \ensuremath{\rightarrow} 3{d}^{9}\phantom{\rule{0.16em}{0ex}}4{s}^{2}$ ${}^{2}{D}_{5/2}$ radiative transition in ${\text{Ni}}^{\ensuremath{-}}$ was measured to be $15.1\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. The experiment was performed at cryogenic temperatures in one of the ion-beam storage rings of the Double ElectroStatic Ion Ring ExpEriment facility at Stockholm University. The storage lifetime of the ${\text{Ni}}^{\ensuremath{-}}$ ion beam was measured to be close to 5 min at a ring temperature of 13 K.

We use a novel electrostatic ion storage ring to measure the radiative lifetime of the upper level in the 3p 5 P 2 o 1/2 →3p 5 P 2 o 3/2 spontaneous radiative decay in S − 32 to be 503±54 ...

Resonant ion pair formation from collisions of electrons with ground state diatomic molecular ions has been observed and absolute cross sections measured. The cross section for HD+ is characterized by an abrupt threshold at 1.9 eV and 14 resolved peaks in the range of energies 0 less than or equal to E less than or equal to 14 eV. The dominant mechanism responsible fbr the structures appears to be resonant capture and stabilization, modified by two-channel quantum interference. Data on HF+ show structure correlated with photoionization of HF and with dissociative recombination of electrons with this ion.

Absolute cross sections and rate coefficients have been determined for dissociative recombination of electrons and ${\mathrm{CO}}^{+}$ ions for energies from 1 meV to 54 eV. We found values of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{2}$ at 1 meV and ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ at 1 eV, with an essentially $1/E$ energy dependence. Branching ratios over the final atomic product states have been determined using a position- and time-sensitive imaging system. At zero eV collision energy the predominant yield is to ground-state atomic fragments (76%). At higher collisional energies the branching ratio to the ground-state limit is reduced. A new limit, $\mathrm{O}{(}^{1}D)+\mathrm{C}{(}^{1}D),$ opens up and branching to the $\mathrm{O}{(}^{3}P)+\mathrm{C}{(}^{1}D)$ limit increases. Cross sections are also determined for dissociative excitation of ${\mathrm{CO}}^{+}.$ Thermal rate coefficients are deduced from the dissociative recombination (DR) data, and compared with measurements in the literature. Consideration of both the theoretical and spectroscopic data in the literature giving information about the potential curves along which DR may take place reveals both a paucity and disparity of the data.

The cross section for dissociative recombination of vibrationally cold ${\mathrm{D}}_{3}^{+}$ has been measured at the ion storage ring CRYRING. The rate constant at 300 K, $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}{\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}$, is a factor of 4.3 smaller than the corresponding value for ${\mathrm{H}}_{3}^{+}$ measured earlier in CRYRING. An electric field of 30 V/cm was introduced in the electron-ion interaction region. This had no measurable effect on the dissociative recombination cross section. This suggests that the cross sections measured in storage rings for ${\mathrm{H}}_{3}^{+}$ and its isotopic variants can be directly compared with theoretical results once such results become available.

We report experiments on dissociative recombination (DR) of HD{sup +} and H{sub 2}{sup +}. Product state information has been obtained over a wide range of electron energies with a position sensitive detector consisting of a graded absorber in combination with a surface-barrier detector. At low electron energies ({lt}3 eV) hydrogen atoms are formed preferentially in highly excited states ({ital n}{gt}2); at high electron energies ({gt}12 eV) both hydrogen fragments are excited. The dissociative recombination rate of H{sub 2}{sup +} has been measured also as a function of storage time in the energy range of 0 eV to 20 eV. We show that the H{sub 2}{sup +} beam is still vibrationally excited after 20 s at our experimental conditions. The H{sub 2}{sup +} ions relax vibrationally through interaction with electrons in the electron cooler. Vibrationally excited levels ({ital v}{ge}5) have DR rates that exceed the DR rate of the lower vibrational levels by two orders of magnitude. The latter observation has important consequences for the interpretation of previous DR experiments on H{sub 2}{sup +}. {copyright} {ital 1996 The American Physical Society.}

Absolute cross sections have been determined for the dissociative recombination and dissociative excitation of $^{4}\mathrm{HeH}^{+}$ for electron energies below 40 eV. The dissociative recombination cross section is in semiquantitative agreement with recent theoretical results by Sarpal, Tennyson, and Morgan [J. Phys. B 27, 5943 (1994)] and Guberman [Phys. Rev. A 49, R4277 (1994); in XIXth International Conference on the Physics of Electronic and Atomic Collisions, Whistler, Canada, AIP Conf. Proc. No. 360, edited by L. J. Dube, J. B. A. Mitchell, J. W. McConkey, and C. E. Brion (AIP, New York, 1995), p. 307]. The calculated resonant structure below a collision energy of 1 eV was not fully reproduced by the experiment. The quantum states of the dissociative recombination products at 0 eV collision energy have been determined; ground-state helium and excited hydrogen atoms (n=2) are dominantly formed, in agreement with recent predictions by Guberman. The dissociative excitation has an onset around 10 eV and follows the shape of the dissociative recombination cross section, illustrating that both processes start with the formation of doubly excited neutral states that lie in the ionization continuum as well as in the dissociation continuum. The dissociative excitation cross section is in quite good agreement with recent calculations by Orel and Kulander. \textcopyright{} 1996 The American Physical Society.

The dissociative recombination (DR) of CH{sub 5}{sup +} and CD{sub 5}{sup +} has been studied at the heavy-ion storage ring CRYRING. The fragmentation dynamics of the dominant reaction channel CH{sub 3}+H+H has been investigated using an imaging detector. The results indicate that a two-step process via the production of a CH{sub 4} intermediate, which has sufficient energy to fragment further to CH{sub 3}+H, may play an important role. Discrepancies between the present and earlier results obtained from storage ring measurements with those from flowing afterglow experiments are addressed. Newly measured branching fractions in the DR of CD{sub 5}{sup +} show an excellent agreement with branching fractions previously measured for CH{sub 5}{sup +}, and the absolute DR cross sections have been also measured over an interaction energy range between {approx}0 and 0.1 eV for both isotopologs.

We have developed a small purely electrostatic ion-beam trap which may be operated in thermal equilibrium at precisely controlled temperatures down to 10 K. Thus, we avoid magnetic field induced mixing of quantum states and may effectively eliminate any influence from absorption of photons from blackbody radiation. We report the first correction-free measurement of the lifetime of the 1s2s2p {4}P{5/2}{0} level of 4He(-) yielding the high-precision result 359.0 +/- 0.7 micros. This result is an essential proof-of-principle for cryogenic electrostatic storage rings and traps for atomic and molecular physics.

We have studied the dissociative recombination (DR) of molecular hydrogen ions with slow electrons over a range of collision energies from 0 to 400 meV. By employing a pulsed expansion source for rotational cooling and by exploiting super elastic collisions with near-0-eV electrons in a heavy ion storage ring for vibrational cooling, we observe a highly structured DR cross section, comparable to that reported for HD+. Using para-hydrogen-enriched ion beams, we identify for the first time features in the DR cross sections attributed to nu=0, J=even molecules (para-H2) and nu=0, J=odd (ortho-H2) molecules, separately. Indications are given that para levels have different DR rate coefficients from ortho levels for the first four vibrational levels at near-0-eV collisions.

We report an investigation into the three-body fragmentation of a triatomic molecule ${\mathrm{H}}_{2}\mathrm{O}$ via the process of dissociative recombination (DR) of ${\mathrm{H}}_{2}{\mathrm{O}}^{+}.$ At 0-eV center-of-mass reaction energy two competing three-body fragmentation channels are available, both leading to the production of ground state H atoms $\mathrm{H}{(}^{2}S)$ but differing in the excitation state of the O atom fragment: $\mathrm{O}{(}^{3}P)$ and $\mathrm{O}{(}^{1}D).$ Using time- and position-sensitive detectors in a triple-coincidence experiment, the fragments from the DR reaction are monitored and their momentum vectors and angular distribution recorded. From these data we determine that during the recombination process, oxygen atoms are produced in the ratio of 3.5(0.5):1 for $\mathrm{O}{(}^{3}P):\mathrm{O}{(}^{1}D),$ and find that the molecular geometry is not conserved and that the available reaction energy is randomly distributed between both hydrogen atoms. Quasiclassical trajectory calculations having the same initial starting conditions as those in the experiment have been carried out using calculated and measured ${\mathrm{H}}_{2}\mathrm{O}$ potential-energy surfaces, together with the knowledge of the various curve crossings, intersections, and estimations of the necessary coupling strengths. The results of these calculations, product state population distributions together with the geometries of the potential surfaces which contribute to the reaction, are compared to those observed in the experiment and allow questions to be answered on the dominance of three-body dissociation.

Absolute cross sections for the production of H - +D + in the collision of cold HD + with electrons have been measured in the storage ring CRYRING at Manne Siegbahn Laboratory, Stockholm University in Stockholm. The magnitude of this cross section (∼3 X 10 - 1 9 cm 2 above threshold), as well as its interference pattern (14 peaks in the interaction energy region 0-16 eV) were found to be identical to that observed for the production of D +H + , which was measured earlier in CRYRING.

Rate coefficients and absolute cross sections for center-of-mass energies between 0.0001 and 1 eV are reported for both resonant ion-pair formation and dissociative recombination in electron collisions with HF{sup +} ions. The heavy-ion storage ring CRYRING in Stockholm was used for these measurements. Notable is the fact that the dissociative recombination cross section is substantially smaller than that for most diatomic molecular ions. The recombination seems to have an underlying E{sup -1} energy dependence characteristic of the direct process in dissociative recombination, but both cross sections show structure, which may be attributed to contributions from different indirect processes. The cross sections have no observable energy thresholds. The ratio of the cross section for resonant ion-pair formation to that for dissociative recombination is about 0.25 at 10{sup -3} eV, with the ratio depending on the interaction energy, so the competition of the ion-pair process is much stronger than for other ions so far studied. The HF{sup +} ion is unique in the fact that the electron affinity of F, the binding energy of HF{sup +}, and energy of the atom pair [H(n=2)+F({sup 2}P{sub 3/2})] are the same within the rotational-energy spread of the HF{sup +} target. The resonant ion-pair formation process,more » e+HF{sup +}{yields}H{sup +}+F{sup -}, has some similarities to the photon process, h{nu}+HF{yields}H{sup +}+F{sup -}, and we discuss comparisons. We deduce thermal rate coefficients from our measurements and discuss them in the context of rate coefficients for other diatomic ions available in the literature.« less

Resonant ion-pair formation from the collisions of ${\mathrm{NO}}^{+}$ ions with electrons was studied using the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory of Stockholm University. The total cross section is measured for the formation of ${\mathrm{N}}^{+}{+\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ for electron energies 8--18 eV, and the results are compared with ion-pair formation in photoionization work. A peak in the cross section is observed at 12.5 eV, with a magnitude of $8.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}{\mathrm{cm}}^{2}.$ An attempt to extract the cross section for the reverse process of associative ionization is made.

Resonant ion-pair formation from collisions of electrons with electronic and vibronic ground-state diatomic molecular ions has been studied in the present work for ${\mathrm{HD}}^{+}$ and ${\mathrm{OH}}^{+}.$ The cross section for ${\mathrm{HD}}^{+}$ has a magnitude of the order of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19} {\mathrm{cm}}^{2}$ and is characterized by an energy threshold and 14 resolved peaks in the energy range up to 16 eV. A theoretical study confirms that the structures derive primarily from quantum interference of the multiple dissociation pathways. Measurements for ${\mathrm{OH}}^{+}$ reveal that the cross section for ${\mathrm{H}}^{+}$ and ${\mathrm{O}}^{\ensuremath{-}}$ formation is lower than ${10}^{\ensuremath{-}21} {\mathrm{cm}}^{2}$ at energies of 6 and 12 eV.