High-resolution measurement of the dielectronic recombination of fluorinelike selenium ions

Hypersatellite Kα Production in Trapped Ar Ions at KK Trielectronic Recombination Energies

We report measurements of hypersatellite radiation of argon ions in the electron energy region of 5200 eV to 7500 eV. Here, we observed a strong enhancement of this hypersatellite Kαh production. Trielectronic recombination (TR) is discussed as a possible channel for Kαh production leading to this enhancement where main TR resonances are expected to occur. Data analysis was mainly based on the extracted intensity ratio of hypersatellite Kαh to Kα lines (Kαh/Kα). In addition, the collisional excitation and the collisional ionisation of the K-shell ions were modeled as main background processes of the Kα X-ray production. The Kαh/Kα intensity ratio shows a significant rise around 6500 eV electron energy by a factor of about two above the background level. This observation is compared with calculations of the expected electron energies for the resonant Kαh emission due to the KK TR process. The observed rise as a function of the electron collision energy, which occurs in the vicinity of the predicted TR resonances, is significantly stronger and energetically much wider than the results of theoretical calculations for the TR process. However, the experimental evidence of this process is not definitive.

Dissociative recombination of CH4(+)

CH4(+) is an important molecular ion in the astrochemistry of diffuse clouds, dense clouds, cometary comae, and planetary ionospheres. However, the rate of one of the common destruction mechanisms for molecular ions in these regions, dissociative recombination (DR), is somewhat uncertain. Here, we present absolute measurements for the DR of CH4(+) made using the heavy ion storage ring CRYRING in Stockholm, Sweden. From our collision-energy dependent cross-sections, we infer a thermal rate constant of k(Te) = 1.71(±0.02) × 10(–6)(Te/300)(−0.66(±0.02)) cm3 s(–1) over the region of electron temperatures 10 ≤ Te ≤ 1000 K. At low collision energies, we have measured the branching fractions of the DR products to be CH4 (0.00 ± 0.00); CH3 + H (0.18 ± 0.03); CH2 + 2H (0.51 ± 0.03); CH2 + H2 (0.06 ± 0.01); CH + H2 + H (0.23 ± 0.01); and CH + 2H2 (0.02 ± 0.01), indicating that two or more C–H bonds are broken in 80% of all collisions.

Dissociative recombination of OPCl+ and OPCl2+: pushing the upper mass limit at CRYRING

The dissociative recombination of OPCl+ and OPCl2+ has been studied at the storage ring CRYRING. The rate constants as a function of electron temperature have been derived to be 7.63 x 10(-7)(Te/300)(-0.89) and >1.2 x 10(-6)(Te/300)(-1.22) cm3s(-1), respectively. The lower limit quoted for the latter rate constant reflects the experimental inability to detect all of the reaction products. The branching fractions from the reaction have been measured for OPCl+ at approximately 0 eV interaction energy and are determined to be N(O+P+Cl)=(16+/-7)%, N(O+PCl)=(16+/-3)% and N(OP+Cl)=(68+/-5)%. These values have been obtained assuming that the rearrangement channel forming P+ClO is negligible, and ab initio calculations using GAUSSIAN03 are presented for the ion structures and energetics to support such an assumption. Finally, the limitations to using heavy ion storage rings such as CRYRING for studies into the dissociative recombination of large singly charged molecular ions are discussed.

Dissociative recombination study of N3+: Cross section and branching fraction measurements

We report an investigation into the dissociative recombination of the azide radical cation, N(3) (+). The reaction rate constant has been measured to be 6.47 x 10(-7) cm(3) s(-1) at room temperature. This value is smaller than those reported earlier for the ion-electron neutralization of N(3) (+) at nitrogen atmospheric pressure. A strong propensity to dissociate through the N(2)+N channel has been observed.

Dissociative recombination of the deuterated acetaldehyde ion, CD3CDO+: product branching fractions, absolute cross sections and thermal rate coefficient

Dissociative recombination of the deuterated acetaldehyde ion CD3CDO(+) has been studied at the heavy-ion storage ring CRYRING, located at the Manne Siegbahn Laboratory, Stockholm, Sweden. Product branching fractions together with absolute DR cross-sections were measured. The branching fractions were determined at a relative collision energy between the ions and the electrons of approximately 0 eV. With a probability of 34% the DR events resulted in no ruptures of bonds between heavy atoms (i.e. no breakage of the C-C bond or the C[double bond, length as m-dash]O bond). In the remaining 66% of the events one of the bonds between the heavy atoms was broken. The energy-dependent cross-section for the DR reaction was measured between approximately 0 and 1 eV relative kinetic energy. In the energy region between 1 meV and 0.2 eV the absolute cross section could be fitted by the expression sigma(E) = 6.8 x 10(-16)E(-1.28) cm(2), whereas in the energy interval between 0.2 and 1 eV the data were best fitted by sigma(E) = 4.1 x 10(-16)E(-1.60) cm(2). From these cross section data the thermal rate coefficient (as a function of the electron temperature), alpha(T) = 9.2 x 10(-7) (T/300)(-0.72) cm(3) s(-1) was obtained.

Dissociative recombination of ammonia clusters studied by storage ring experiments

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).

Dissociative recombination of protonated methanol

The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH3OH2+ and CD3OD2+ have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH3OH2+ and CD3OD2+, respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C-O bond, the major process being the three-body break-up forming CH3, OH and H (CD3, OD and D). The overall cross sections are best fitted by sigma = 1.2 +/- 0.1 x 10(-15) E(-1.15 +/- 0.02) cm2 and sigma = 9.6 +/- 0.9 x 10(-16) E(-1.20 +/- 0.02) cm2 for CH3OH2+ and CD3OD2+, respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 +/- 0.9 x 10(-7) (T/300)(-0.59 +/- 0.02) cm3 s(-1) (CH3OH2+) and k(T) = 9.1 +/- 0.9 x 10(-7) (T/300)(-0.63 +/- 0.02) cm3 s(-1) (CD3OD2+) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH3+ and H2O and subsequent dissociative recombination of the resulting CH3OH2+ ion to yield methanol and hydrogen atoms is therefore very unlikely.

Investigating the breakup dynamics of dihydrogen sulfide ions recombining with electrons

This paper presents results concerning measurements of the dissociative recombination (DR) of dihydrogen sulfide ions. In combination with the ion storage ring CRYRING an imaging technique was used to investigate the breakup dynamics of the three-body channel in the DR of 32SD2(+). The two energetically available product channels S(3P) + 2D(2S) and S(1D) + 2D(2S) were both populated, with a branching fraction of the ground-state channel of 0.6(0.1). Information about the angle between the two deuterium atoms upon dissociation was obtained together with information about how the available kinetic energy was distributed between the two light fragments. The recombination cross sections as functions of energy in the interval of 1 meV to 0.3 eV in the center-of-mass frame are presented for 34SH2(+). The thermal rate coefficient for the DR of 34SH2(+) was determined to be (4.8+/-1.0) x 10(-7)(T/300)(-0.72+/-0.1) cm3 s(-1) over this interval.

Dissociative recombination study of Na+(D2O) in a storage ring

The dissociative recombination of Na(+)(D(2)O) ion has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The cross section has been measured as a function of center-of-mass energy ranging from 1 meV to 0.1 eV and found to have an E(-1.37) dependence. The rate coefficient has been deduced to be (2.3+/-0.32)x10(-7)(T(e)/300)(-0.95+/-0.01) cm(3) s(-1) for T(e)=50-1000 K. The branching ratios have been measured at 0 eV. Of the four energetically accessible dissociation channels, three channels are found to occur although the channel that breaks the weak Na(+)-D(2)O bond is by far dominant.

Dissociative recombination of NH4+ and ND4+ ions: Storage ring experiments andab initiomolecular dynamics

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.

Extraordinary branching ratios in astrophysically important dissociative recombination reactions

Branching ratios of the dissociative recombination reactions of the astrophysically relevant ions DCO+, N2H+ and DOCO+ (as substitute for HOCO+) have been measured using the CRYRING storage ring at the Manne Siegbahn Laboratory at the University of Stockholm, Sweden. For DCO+, the channel leading to D and CO was by far the most important one (branching ratio 0.88), only small contributions of the CD + O and OD + C product pathways (branching ratios 0.06 each) were recorded. In the case of N2H+ the surprising result of a break-up of the N-N bond to N and NH (branching ratio 0.64) was found with the branching ratio of the N2 + H product channel therefore displaying a branching ratio of only 0.36. In the case of DOCO+, the three-body break-up into D + O + CO dominated (branching ratio 0.68), whereas the contribution of the CO2 + H channel was only minute (0.05). The remaining share (branching ratio 0.27) was taken by the pathway leading to OH + CO. For the dissociative recombination of N2H+ and DOCO+ also absolute reaction cross sections were obtained in the collisional energy range between 0 and 1 eV. From these cross sections it was possible to work out the thermal rate constants, which were found to be k(T) = 1.0+/-0.1 x 10(-7) (T/300 K)(-0.51+/-0.02) cm3 s(-1) and k(T) = 1.2+/-0.1 x 10(-6) (T/300 K)(-0.64+/-0.02) cm3 S(-1) for N2H+ and DOCO+, respectively.