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Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB- as well as several fragment anions. DNB-, (DNB-H)-, (DNB-NO)-, (DNB-2NO)-, and (DNB-NO2)- are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)- features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)- offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C5H4O- with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels. [ABSTRACT FROM AUTHOR]

Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of (H2O)nH3O+. Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with C60. Charged C60-water complexes are predominantly unprotonated; C60(H2O)4+ and (C60)2(H2O)4+ appear with enhanced abundance. Another intense ion series is due to C60(H2O)nOH+; dehydrogenation is proposed to be initiated by charge transfer between the primary He+ ion and C60. The resulting electronically excited C60+* leads to the formation of a doubly charged C60-water complex either via emission of an Auger electron from C60+*, or internal Penning ionization of the attached water complex, followed by charge separation within {C60(H2O)n}2+. This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of C60(H2O)n+. In addition to the loss of single water molecules, a prominent reaction channel yields bare C60+ for sizes n=3, 4, or 6. Ab initio Hartree–Fock calculations for C60-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C60+. For n=3, 4, or 6, fissionlike desorption of the entire water complex from C60(H2O)n+ energetically competes with the evaporation of a single water molecule. [ABSTRACT FROM AUTHOR]

We present a detailed study of the electronic structure and the stability of C60 dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least ∼1.5 eV. For C602- ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C602- ions is singlet. The observed spectrum corresponds to an electronic transition from a t1u lowest unoccupied molecular orbital (LUMO) of C60 to the t1g LUMO+1 level. The electronic levels of the dianion are split due to Jahn–Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm-1 corresponds to a transition between the Jahn–Teller ground states. Also transitions from pseudorotational states with 200 cm-1 and (probably) 420 cm-1 excitation are observed. We argue that a very broad absorption band from about 11 500 cm-1 to 13 500 cm-1 consists of transitions to so-called cone states, which are Jahn–Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C60- may also be a transition to a cone state. [ABSTRACT FROM AUTHOR]

Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered Hn+. The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered Hn+. The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered Hn+ up to n≅120 by their mass excess that distinguishes them from Hen+, mixed HemHn+, and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n=6, pronounced drops in the abundance of even-numbered cluster ions are seen at n=30 and 114, which suggest icosahedral shell closures at H6+(H2)12 and H6+(H2)54. Possible isomers of H6+ are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory [ABSTRACT FROM AUTHOR]

C602- and C702- dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C702- ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C602- ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200±30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s. [ABSTRACT FROM AUTHOR]

We have measured appearance energies of Arn+, n≤30, by electron impact of gas phase clusters. Quantum-chemical calculations have been performed to determine the adiabatic and vertical ionization energies of argon clusters up to n=4 and 6, respectively. The experimental appearance energy of the dimer ion approaches, under suitable cluster source conditions, the adiabatic ionization energy. The agreement with values obtained by photoionization and threshold photoelectron-photoion coincidence (TPEPICO) spectra demonstrates that autoionizing Rydberg states are accessible by electron impact. Appearance energies of larger clusters, though, exceed the TPEPICO values by about 0.5 eV. [ABSTRACT FROM AUTHOR]

A detailed experimental as well as theoretical investigation of the properties of the metastable dissociation Ar2+→Ar++Ar is presented. The mass-analyzed ion kinetic energy (MIKE) scan technique has been performed using a three sector field mass spectrometer. The possible mechanisms of the metastability of Ar2+ have been examined and the observed decay process is assigned to the II(1/2)u→I(1/2)g bound to continuum radiative transition, in agreement with earlier work. The calculation of the theoretical shape of the kinetic energy release distribution of fragment ions allowed us to construct the theoretical MIKE peak and compare it with the raw experimental data. The accuracy of various sets of potential energy curves for Ar2+ is discussed, as well as the way of production of the metastable Ar2+[II(1/2)u] electronic state by electron impact. Excellent agreement between the experimental data and theoretical model has been observed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have measured the kinetic energy released in the unimolecular dissociation of fullerene ions, Cn+→Cn-2++C2, for sizes 42≤n≤90. A three-sector-field mass spectrometer equipped with two electric sectors has been used in order to ensure that contributions from isotopomers of different masses do not distort the experimental kinetic energy release distributions. We apply the concept of microcanonical temperature to derive from these data the dissociation energies of fullerene cations. They are converted to dissociation energies of neutral fullerenes with help of published adiabatic ionization energies. The results are compared with literature values. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit arose from the fact that several isotopomers contribute to a decaying parent ion beam when the cluster size exceeds a certain value. Furthermore we developed a new data analysis. It allows us to model also fragment ion peaks that are a superposition of different decay reactions and thus we can determine the average kinetic energy release for all decay reactions of a given cluster ion. In a further step we used these results to determine the binding energies of cluster ions Rg[sub n] (n>=10) by applying finite heat bath theory. The smaller sizes have not been included in this analysis, because the validity of finite heat bath theory becomes questionable below n≈10. The present average kinetic energy releases and binding energies are compared with other experiments and various calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]