Dissociative recombination (DR) of the water cluster ions H(+)(H(2)O)(3) and D(+)(D(2)O)(3) with electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). For the first time, absolute DR cross sections have been measured for H(+)(H(2)O)(3) in the energy range of 0.001-0.8 eV, and relative cross sections have been measured for D(+)(D(2)O)(3) in the energy range of 0.001-1.0 eV. The DR cross sections for H(+)(H(2)O)(3) are larger than previously observed for H(+)(H(2)O)(n) (n=1,2), which is in agreement with the previously observed trend indicating that the DR rate coefficient increases with size of the water cluster ion. Branching ratios have been determined for the dominating product channels. Dissociative recombination of H(+)(H(2)O)(3) mainly results in the formation of 3H(2)O+H (probability of 0.95+/-0.05) and with a possible minor channel resulting in 2H(2)O+OH+H(2) (0.05+/-0.05). The dominating channels for DR of D(+)(D(2)O)(3) are 3D(2)O+D (0.88+/-0.03) and 2D(2)O+OD+D(2) (0.09+/-0.02). The branching ratios are comparable to earlier DR results for H(+)(H(2)O)(2) and D(+)(D(2)O)(2), which gave 2X(2)O+X (X=H,D) with a probability of over 0.9.

Dissociative recombination of ammonia cluster ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for dissociative recombination of H+(NH3)2, H+(NH3)3, D+(ND3)2, and D+(ND3)3 in the collision energy range of 0.001-27 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 1000 K are calculated from the experimental data and compared with earlier results. The fragmentation patterns for the two ions H+(NH3)2 and D+(ND3)2 show no clear isotope effect. Dissociative recombination of X+(NX3)2 (X=H or D) is dominated by the product channels 2NX3+X [0.95+/-0.02 for H+(NH3)2 and 1.00+/-0.02 for D+(ND3)2]. Dissociative recombination of D+(ND3)3 is dominated by the channels yielding three N-containing fragments (0.95+/-0.05).

We report the investigation into the three-body fragmentation of the triatomic molecules NH2 and CH2 via the process of dissociative recombination (DR). Recently reported analysis of experiments into the DR of the similar system H2O indicated that the DR process is violent, involving a large degree of geometrical change and energy distribution from the initial attachment of the free electron by the parent ion to the final dissociation step. Comparison of data from NH2 and CH2 with that of H2O gives a similar picture of the DR process, though there are significant differences in the results, not only in the branching fraction between the expected decay channels, but also in the distribution of the available reaction energy over the product hydrogen atoms as well as in the molecular geometry at the point of dissociation.

We present dissociative recombination measurements, using the CRYRING ion storage ring, of H3+ ions produced in a supersonic expansion discharge source. Before and after the CRYRING measurements, the ion source was characterized in Berkeley using infrared cavity ringdown spectroscopy, and was found to exhibit a typical rotational temperature of ∼30 K. Our measurement of the dissociative recombination cross section using this ion source revealed resonances that had not been observed clearly in previous experiments that used rotationally hot ion sources. Based on the present measurements, we infer a thermal dissociative recombination rate coefficient for ions at interstellar temperatures of ∼ 2.6 × 10−7 cm3s−1. Our results are in general agreement with theoretical calculations of the dissociative recombination cross section by Kokoouline and Greene. We will review the enigma of the abundance of H3+ in the diffuse interstellar medium, and discuss the impact of these experiments, especially in the context of the recent observation of H3+ towards ζ Persei.

Experimental and theoretical cross sections for the resonant ion-pair formation (RIP) in electron collisions with rovibrationally cold H-3(+) ions are presented. Absolute cross sections for the RIP process producing H- ions are measured for center-of-mass energies between 2-20 eV using the CRYRING, heavy-ion storage ring. Theoretical cross sections are obtained using wave-packet propagation on both one- and two-dimensional models of relevant diabatic-potential energy surfaces and couplings of H-3(+) and H-3.

This paper presents the first dissociative recombination (DR) measurement of electrons with rotationally and vibrationally cold H-3(+) ions. A dc discharge pinhole supersonic jet source was developed and characterized using infrared cavity ringdown spectroscopy before installation on the CRYRING ion storage ring for the DR measurements. Rotational state distributions (T(rot)similar to30 K) produced using the source were comparable to those in the diffuse interstellar medium. Our measurement of the electron energy dependence of the DR cross section showed resonances not clearly seen in experiments using rotationally hot ions, and allowed calculation of the thermal DR rate coefficient for ions at interstellar temperatures, alpha(DR)(23 K)=2.6x10(-7) cm(3) s(-1). This value is in general agreement with recent theoretical predictions by Kokoouline and Greene [Phys. Rev. A 68, 012703 (2003)]. The branching fractions of the two breakup channels, H+H+H and H+H-2, have also been measured for rotationally and vibrationally cold H-3(+).

The dissociative recombination (DR) process of NH4+ and ND4+ molecular ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for DR of NH4+ and ND4+ in the collision energy range 0.001-1 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 2000 K are calculated from the experimental data. The absolute cross section for NH4+ agrees well with earlier work and is about a factor of 2 larger than the cross section for ND4+. The dissociative recombination of NH4+ is dominated by the product channels NH3+H (0.85+/-0.04) and NH2+2H (0.13+/-0.01), while the DR of ND4+ mainly results in ND3+D (0.94+/-0.03). Ab initio direct dynamics simulations, based on the assumption that the dissociation dynamics is governed by the neutral ground-state potential energy surface, suggest that the primary product formed in the DR process is NH3+H. The ejection of the H atom is direct and leaves the NH3 molecule highly vibrationally excited. A fraction of the excited ammonia molecules may subsequently undergo secondary fragmentation forming NH2+H. It is concluded that the model results are consistent with gross features of the experimental results, including the sensitivity of the branching ratio for the three-body channel NH2+2H to isotopic exchange.

Experimental data are presented from three different heavy-ion storage rings (ASTRID in Aarhus, CRYRING in Stockholm, and TSR in Heidelberg) to assess the reliability of this experimental tool for the extraction of absolute rate coefficients and cross sections for dissociative recombination (DR). The DR reaction between HD. and electrons has been studied between 0 and 30 eV on a dense energy grid. HD+ displays two characteristic local maxima in the DR rate around 9 and 16 eV. These maxima influence the data analysis at smaller collision energies. We conclude that resonant structures in the DR cross sections are reproduced among the experiments within the collision energy resolution. The absolute cross sections agree within the systematic experimental errors of 20% related to the measurement of the ion currents. Absolute thermal rate coefficients for HD. ions are given for an electron temperature range of 50-300 K. Results for the DR cross section and the thermal rate coefficients are compared to recent theoretical calculations including rotational effects, finding satisfactory agreement.

This article describes some of the most recent experiments at the ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm University. A comprehensive review of recent work at ion storage rings has recently been published,1 and there are several other reviews of recent date,2,3 as well as the proceedings from the previous dissociative recombination (DR) meeting at Nässlingen in Sweden.4 Finally, a brief review of three-body breakup dynamics has recently been published.5

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules1,2. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature3,4. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models5,6,7. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations5,6,7 of H3+ can be resolved.

* Department of Chemistry, † Department of Astronomy, University of California at Berkeley, Berkeley, California 94720, USA ‡ Gemini Observatory, 670 North A’ohoku Place, Hilo, Hawaii 96720, USA § JILA, University of Colorado and National Institute of Standards and Technology, Boulder, Colorado 80309, USA k Institute of Physics, Świetokrzyska Academy, 25 406 Kielce, Poland {Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University Prague V Holesovickach 2, Prague 8, Czech Republic # Department of Physics, SCFAB, Stockholm University, S-106 91 Stockholm, Sweden q Manne Siegbahn Laboratory, Stockholm University, S-104 05 Stockholm, Sweden

We have studied the dissociation dynamics of NO(+) ions in their ground, X (1)Sigma(+), and first excited metastable, a (3)Sigma(+) states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO(+) (X (1)Sigma(+),v=0) ions dissociate dominantly to the N((2)D)+O((3)P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N((4)S)+O((1)D) dissociation limit contributes 0(+/-2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation during the dissociation and the initial electron capture. We also report the distribution of fragment pairs from the DR reaction involving the metastable a (3)Sigma(+) state. This state is found to dissociate to nearly all of the energetically allowed product pairs. The lifetime of the a (3)Sigma(+) state is found to be 730(+/-50) ms, in agreement with earlier, sometimes indirect, observations. The experimental observations have been complemented with ab initio calculations on the different radiative decay processes both for the X (1)Sigma(+) and the a (3)Sigma(+) states. It is found that vibrational relaxation via infrared radiation is faster for NO(+) (a (3)Sigma(+),v>0) ions than the electronic decay of these metastable-state ions to the ground state. (C) 2003 American Institute of Physics.

We have studied the dissociation dynamics of NO+ ions in their ground, X 1Σ+, and first excited metastable, a 3Σ+ states, induced by the capture of electrons of variable collision energy in the dissociative recombination (DR) process. The branching over the different dissociation channels has been measured in a merged-beam experiment on the heavy-ion storage ring, CRYRING. In accord with previous observations, NO+ (X 1Σ+,v=0) ions dissociate dominantly to the N(2D)+O(3P) product limit at 0 and 1.2 eV collision energies. In contrast to earlier reports, the spin-forbidden N(4S)+O(1D) dissociation limit contributes 0(±2)% at 0 eV. At 5.6 eV a new channel coupled to the production of ground-state atoms becomes more important, but no increase in the production of ground-state product atoms was observed. All observed branching fractions compare very favorably with predictions from a simple statistical model, which is based on the multiplicity of each dissociation limit in combination with spin conservation during the dissociation and the initial electron capture. We also report the distribution of fragment pairs from the DR reaction involving the metastable a 3Σ+ state. This state is found to dissociate to nearly all of the energetically allowed product pairs. The lifetime of the a 3Σ+ state is found to be 730(±50) ms, in agreement with earlier, sometimes indirect, observations. The experimental observations have been complemented with ab initio calculations on the different radiative decay processes both for the X 1Σ+ and the a 3Σ+ states. It is found that vibrational relaxation via infrared radiation is faster for NO+ (a 3Σ+,v>0) ions than the electronic decay of these metastable-state ions to the ground state.

A detailed knowledge of the carbon chain’s spectroscopy is of considerable importance in astrophysics. More than two decades ago, Douglas1 put forward the suggestion that the unknown absorbing species responsible for over 150 diffuse interstellar bands, might be long carbon chain molecules, Cn. More recently, Tulej et al.2 measured the gas phase electronic spectrum of the C 7 − chain and found that the 5 narrow absorption bands in the lowest energy transition correlate quite well with the observed wavelengths of narrow diffuse interstellar bands. CRYRING was used to study collision processes between an electron and a negative ion cluster C 4 − . We made use of a grid to separate four neutral channels over the five that contribute to the total neutral 4C production. Only the detachment cross sections, leading to the production of C4, are presented here. Although the doubly charged negative ion C 4 −2 has received little previous attention, a resonance observed in the present work in the threshold region of the detachment cross section is associated with a short lived C 4 −2 state.

Much recent work in the field of dissociative recombination (DR) has been related to the break-up of polyatomic ions since, in spite of the apparent simplicity of DR, developing a general theory to predict product branching ratios for even the simplest polyatomic ions, e.g. X H 2 + , has proven to be difficult. Early models suggested that such ions would predominantly fragment to H + XH. However, all recent storage ring studies for such ions show a propensity for three body breakup, i.e. X + H + H.1−4 In the last year, the dynamics of some of these systems have been investigated, with results reported for H2O+ 5 and H 3 + ,6 and a brief review was also published.7 Using trajectory calculations, we have also investigated the dynamics on some of the potential surfaces involved in the DR of H2O+.8 The group at TSR looked at the dynamics in, and competition between, the two and three body fragmentation dynamics of H 3 + . Using a statistical model,9 they reached good agreement with the experimental data previously obtainedat CRYRING1 as a function of reaction energy.