Molecular Dications Research Articles

Activation energies for fragmentation channels of anthracene dications—experiment and theory

We have studied the fragmentation of the polycyclic aromatic hydrocarbon anthracene (C14H10) after double electron transfer to a 5 keV proton. The excitation energies leading to the most relevant dissociation and fission channels of the resulting molecular dication were directly determined experimentally. Density functional theory calculations were performed to explore the potential energy surfaces on which the fragmentation dynamics proceed. There is clear experimental evidence for a dominance of fission into C11H+7–C3H+3 over C2H+2 loss. The energetic ordering of the dissociation and fission channels and the kinetic energy releases are in good agreement with the theoretical results. It can be concluded that the unique combination of experiment and theory presented here is an excellent tool to study the fragmentation of complex molecular ions in unprecedented detail.

Bipolarons and polaron pairs in oligopyrrole dications

A series of oligopyrrole dications, n-Py2+, n=1–12, 15, 18, 21, 24, 27 are studied with hybrid density functional theory at the B3PW91/6-31+G(d,p) level. Geometry optimization and electronic structure of oligopyrrole dications are revisited using spin-restricted and spin-unrestricted methods. It is found that the ground states of short oligopyrrole dications n-Py2+ (n<7) are spin-restricted singlet ground states. Thus, the charge carrier in these molecular dications has characteristics of a bipolaron. In contrast, medium size oligopyrrole dications (7⩽n⩽15) have a spin-unrestricted singlet ground state and display charge carriers that have a polaron pair character. With increasing oligomer length, the spin-unrestricted singlet and the triplet become degenerate such that for even longer oligopyrrole dications (n-Py2+,n⩾18) the ground state is a triplet and the charge carrier is predominantly a polaron pair.

Effect of fluoro substitution on the fragmentation of the K‐shell excited/ionized pyridine studied by electron impact

Fragmentation of the pyridine ring followed by K-shell excitation/ionization has been studied with 2-fluoropyridine (2FPy) by electron impact. Ab initio molecular orbital (MO) calculations were also carried out to investigate the electronic states correlating with specific fragment ions. The fragment ions are produced characteristically at the N 1s edge, while the spectra observed at the F 1s and C 1s edges exhibit a small difference from that at the valence ionization. The production of the C(4)H(2)(+), C(4)H(3)(+) and C(4)H(2)F(+) ions indicates that the cleavage of the N-C6 and C2-C3 bonds or the N-C2 and C5-C6 bonds is likely to occur after the N 1s excitation/ionization. Ab initio MO calculations indicate that the former fission is likely to proceed through the n(N)(1)π(2)(1)π(3)(2) and n(N)(0)π(2)(2)π(3)(2) excited states of the parent molecular dication. On the other hand, the breakage of the N-C2 and C4-C5 bonds, which specifically proceeds at the N 1s edge for 2-methylpyridine, does not occur for 2FPy. The present calculation reveals that the products of this channel are unstable by the electronegativity of fluorine and that the relative energy of the Auger-final states of 2FPy is lowered by the reorganization and electron correlation effects.

Probing dissociative molecular dications by mapping vibrational wave functions

We present high-resolution photoelectron--Auger-electron coincidence spectra of methane (${\mathrm{CH}}_{4}$). Since the vibrational structure in the photoelectron spectrum is resolved, the Auger spectra corresponding to different vibrational levels can be separated. The seven final states of ${\mathrm{CH}}_{4}^{2+}$ are either dissociative or metastable, but in any case are populated in a repulsive part of their potential-energy curve via the Auger decay. The Auger line shapes can therefore be obtained by mapping the vibrational wave functions of the core-hole state into energy space. We have implemented this connection in the data analysis. By simultaneously fitting the different Auger spectra, detailed information on the energies of the dicationic states and their repulsive potential-energy curves is derived.

Lifetime and kinetic energy release of metastable dications dissociation

A new method for the determination of dynamical features of the molecular dication dissociation processes, following the single photon double ionization, investigated by time-of-flight mass spectrometry technique has been developed. The method is based on an extension of the generalized simulated annealing statistical methodology, previously applied in other fields. Here it is described and applied, as an example, to the case of the dissociation of the CO22+ dication giving CO++O+ ion fragments. The results are consistent with previous determination of the metastable lifetime of the dication, but the analysis also provides additional information about the dynamics of the reaction.

Dissociation of methanol dications produced by electron capture with and without transfer ionization

Double ionization and subsequent ion-pair dissociation induced by low-energy collisions of Ar8 + were studied for various methanol isotopomers to understand how the reactions of transient methanol dications correlate with the charge states of the scattered ions. Single-hit time-of-flight (TOF) spectra of recoil ions excluding multi-hit events show that long-lived molecular dications always correlate with Ar7 + scattered ions, indicating that they are formed only by double-capture collisions that accompany transfer ionization. The double-hit TOF measurements of fragment ions show that the branching ratios of CH/OH cleaving largely depend on the charge states of scattered ions. The H/D substitution effect on the hydrogen migration indicates that a concerted motion of atoms is essential for this reaction pathway.

Multicomponent Direct Detection of Polycyclic Aromatic Hydrocarbons by Surface-Enhanced Raman Spectroscopy Using Silver Nanoparticles Functionalized with the Viologen Host Lucigenin

Silver nanoparticles (NPs) functionalized with the molecular assembler bis-acridinium dication lucigenin (LG) have been used as a chemical sensor system to detect a group of polycyclic aromatic hydrocarbon (PAH) pollutants in a multicomponent mixture by means of surface-enhanced raman scattering (SERS). The effectiveness of this system was checked for a group of PAHs with different numbers of fused benzene rings, namely anthracene, pyrene, triphenylene, benzo[c]phenanthrene, chrysene, and coronene. In order to determine the host capacity of this sensor system, the self-assembly of the LG viologen on a metallic surface has been checked by analyzing SERS intensities of PAH bands at different LG concentrations. The NP-LG-analyte affinity is derived from the analysis of PAH band intensities at different concentrations of pollutants, the adsorption isotherm of each PAH on NP-LG cavities has been studied, and the corresponding adsorption constants have been evaluated. The limit of detection at trace-level concentration is confirmed by the presence of their characteristic fingerprint vibrational bands. The SERS spectra of PAH mixtures confirm that LG viologen dication shows a higher analytical selectivity to PAHs constituted by four fused benzene rings, mainly pyrene and benzo[c]phenanthrene, in agreement with their higher affinity which is also related to their better fit into the intermolecular LG cavities. As a conclusion, SERS spectra recorded on modified NP-LG surfaces are a powerful chemical tool to detect organic pollutants.

A simple ‘statistical’ approach for fragmentation studies of doubly ionized cytosine, thymine and uracil bases

A simple statistical model describing the dissociation of molecular dications into correlated fragment pairs has been developed. This model is based on a combinatory approach in which all possible fragments are enumerated and is refined by taking into account the initial structure of the parent molecule, considering the number of chemical bonds to be broken to give rise to the fragments. We show how this model can be used as a tool to help interpret experimental results of coincidence experiments. It shows that dissociation of doubly ionized molecules upon 100 keV proton irradiation is dominated by statistical processes but it also enables easy identification of the dissociation products originating from non-statistical processes requiring further investigation, possibly conveying information on the radiation–molecule interaction itself.

Communication: Delayed asymmetric Coulomb fission of molecular clusters: Application of a dielectric liquid-drop model

Delayed asymmetric Coulomb fission in size-selected molecular dication clusters has been recorded for the first time. Observations on (NH(3))(n)(2+) clusters show that fragmentation accompanied by charge separation can occur on a microsecond time scale, exhibits considerable asymmetry, and involves a kinetic energy release of ∼0.9 eV. The fission process has been modeled by representing the fragments as charged dielectric spheres and the calculated maximum in the electrostatic interaction energy between the fragments gives a good account of the measured kinetic energy release. A simple kinetic model shows that instrumental factors may contribute to the observation of asymmetric fragmentation.

The formation of NH+following the reaction of N22+with H2

The nitrogen molecular dication (N22+) has been proposed as a minor but significant component of the ionosphere of Saturn's moon Titan with an abundance comparable to that of several key monocations. It has also been suggested that the reactions of N22+ with H2 can provide a source of N2H2+ in Titan's atmosphere. This paper reports the results from experiments, using a position-sensitive coincidence technique, which reveal the chemical reactions forming pairs of monocations following collisions of the N22+ dication with H2(D2) at a centre-of-mass collision energy of 0.9(1.8) eV. These experiments show, in addition to single electron-transfer processes, a bond-forming pathway forming NH+ + H+ + N and allow an estimate to be made of the reaction cross section and the rate coefficient for this reaction. The correlations between the product velocities revealed by the coincidence experiments show that NH+ is formed via N atom loss from a primary encounter complex [N2H2]2+ to form NH22+, with this triatomic daughter dication then fragmenting to yield NH+ + H+. A computational investigation of stationary points on the lowest energy singlet and triplet [N2H2]2+ potential energy surfaces confirms the mechanistic deductions from the experiments and indicates that the formation of NH+ occurs solely, and efficiently, from the reaction of the c3Σ+u excited electronic state of N22+.

Double ionization of cycloheptatriene and the reactions of the resulting C7Hn2+ dications (n = 6, 8) with xenon

The formation and fragmentation of the molecular dication C(7)H(8)(2+) from cycloheptatriene (CHT) and the bimolecular reactivities of C(7)H(8)(2+) and C(7)H(6)(2+) are studied using multipole-based tandem mass spectrometers with either electron ionization or photoionization using synchrotron radiation. From the photoionization studies, an apparent double-ionization energy of CHT of (22.67 ± 0.05) eV is derived, and the appearance energy of the most abundant fragment ion C(7)H(6)(2+), formed via H(2) elimination, is determined as (23.62 ± 0.07) eV. Analysis of both the experimental data as well as results of theoretical calculations strongly indicate, however, that an adiabatic transition to the dication state is not possible upon photoionization of neutral CHT and the experimental value is just considered as an upper bound. Instead, an analysis via two different Born-Haber cycles suggests (2)IE(CHT) = (21.6 ± 0.2) eV. Further, the bimolecular reactivities of the C(7)H(n)(2+) dications (n = 6, 8), generated via double ionization of CHT as a precursor, with xenon as well as nitrogen lead, inter alia, to the formation of the organo-xenon dication C(7)H(6)Xe(2+) and the corresponding nitrogen adduct C(7)H(6)N(2)(2+).

Time-resolved pump-probe experiments at the LCLS

The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs.

Double Ionization of Nitrogen from Multiple Orbitals

In intense femtosecond laser fields, molecules could be tunnel ionized from multiple valence orbitals, which produces molecular ions in different electronic states. In this article, we have used a reaction microscope to study double ionization of nitrogen by intense femtosecond laser pulses. It is found that some doubly charged molecular ions N(2)(2+) are metastable while others dissociate through charge symmetric dissociation (N(2)(2+) --> N(+) + N(+)) or charge asymmetric dissociation (N(2)(2+) --> N(2+) + N). The kinetic energy releases and angular distributions of atomic ions are measured for the dissociation channels. With the aid of the CASSCF and MRCI calculations, the electronic states are identified and the contributions of the valence orbitals are discussed for these dissociated molecular dications.

Carbon−Carbon Coupling Reactions of Medium-Sized Nitrogen-Containing Dications

The reactivity of molecular dications C(m)H(n)N(2+), generated by (dissociative) double ionization of different heteroaromatic compounds, with methane, ethane, and nitrogen is investigated using a triple-quadrupole mass spectrometer. About a quarter of the set of dications studied undergo carbon-carbon coupling reactions with methane, whereas only nonspecific reactions leading to the formation of monocations (such as proton-, electron- and hydride transfer as well as Coulomb explosions) occur in the presence of either ethane or nitrogen. Here, we report the reactivity observed for 42 different C(m)H(n)N(2+) dications formed via electron ionization of eleven different heteroaromatic precursors. The potential relevance of these results is discussed in the context of the growth of hydrocarbon species and their nitrogen-containing homologues at extremely low temperatures and pressures, such as in interstellar environments or the ionospheres of planets and moons, Saturn's moon Titan in particular.

Ion–ion coincidence experiments with low extraction fields

The fragmentation of a polyatomic molecular dication has been investigated by means of a Monte Carlo simulation which can take into account realistic experimental conditions. The simulation applied to the three body process CH 4 2+ → CH 2 + + H + + H shows that the low angular acceptance, consequence of the low extraction field, does not prevent the distinction between the initial charge separation and the synchronous concerted decay mechanisms, which lead to the same final state. The comparison of the results with recent experiments [1] confirms that an initial charge separation with a following loss of a hydrogen atom describes better the experimental observations.

Trace Detection of Triphenylene by Surface Enhanced Raman Spectroscopy Using Functionalized Silver Nanoparticles with Bis-Acridinium Lucigenine

Surface enhanced raman scattering (SERS) of triphenylene (TP) has been recorded on Ag nanoparticles functionalized with the molecular assembler bis-acridinium lucigenine dication (LG) which approaches the adsorbate to the metal surface allowing for its detection. Structural information on the host and the analyte can be extracted from the SERS spectra of LG and LG/TP complex. The acridinium planes in LG are staggered, so cavities into which hydrophobic TP can be allocated are created. Moreover, the orientation of LG with respect to the metal surface changes from tilted to perpendicular when concentration of TP increases. However, perpendicular orientation of TP with respect to the metal surface is preferred according to the in-plane enhanced bands recorded in the SERS spectrum. The dependence of the Raman signal of TP on LG concentration has been checked, and trace concentrations of TP have been detected by this technique which therefore can be used as a chemical sensor of organic pollutants.

Specific fragmentation of the K-shell excited/ionized pyridine derivatives studied by electron impact: 2-, 3- and 4-methylpyridine

Fragmentation of the pyridine ring upon K-shell excitation/ionization has been studied with gaseous 2-, 3- and 4-methylpyridine by the electron-impact method. Ab initio molecular orbital (MO) calculations were also carried out to explore electronic states correlating with specific fragments. Some specific fragmentation channels were identified from the ionic fragments enhanced characteristically at the N 1s edge. Yields of the C(2)HN(+) and C(5)H(5)(+)/C(5)H(6)(+) ions show that the fission of the N-C2 and C4-C5/C5-C6 bonds of the ring is likely to occur after the N 1s excitation and ionization. Ab initio MO calculations for the 2-methylpyridine molecule indicate that the dissociation channels to produce these ions are only accessible through the excited states of the parent molecular dication, which can be formed by Auger decays after the N 1s ionization. Fragment ions via hydrogen rearrangement are produced as well, but the rearrangement is not a phenomenon specific to the K-shell excitation/ionization.

Cationic rare-earth-metal methyl complexes: a new preparative access exemplified for Y and Pr

A new procedure for the generation of cationic methyl complexes of rare-earth metals has been developed for yttrium and praseodymium as two examples from the rare-earth series. The reactions of the rare-earth-metal tetramethylaluminates [Ln(AlMe(4))(3)] of yttrium and praseodymium with three differently sized crown ethers in thf, namely [12]crown-4, [15]crown-5 and [18]crown-6, respectively, led to the compounds [YMe([12]crown-4)(2)](2+)[AlMe(4)](-)(2) (1), [PrMe([12]crown-4)(2)](2+)[AlMe(4)](-)(2) (2), [YMe([15]crown-5)(thf)(2)](2+)[AlMe(4)](-)(2) (3), [Pr([15]crown-5)(2)](3+)[AlMe(4)](-)(3) (4), [YMe([18]crown-6)(thf)](2+)[AlMe(4)](-)(2) (5) and [PrMe([18]crown-6)(thf)(2)](2+) [AlMe(4)](-)(2) (6). With the exception of 4, all compounds contain molecular di-cations with a Ln-CH(3) unit bonded to one (3, 4, 6) or two (1, 2) crown-ether molecules. The compounds were characterised by determination of their crystal structures, by elemental analyses and in the case of the yttrium compounds by variable temperature NMR spectroscopy. The latter gives insights into molecular dynamic processes and methyl group exchange between cations and anions in solution.

Growth Of Doubly Ionized C,H,N Compounds in the Presence of Methane

The molecular dications C(6)H(5)N(2+) generated via dissociative double ionization of 2- and 4-picoline, respectively, react with methane to form the C-C coupled products C(7)H(7)N(2+) concomitant with liberation of molecular hydrogen. Multipole-based mass spectrometric experiments and photoionization studies using synchrotron radiation demonstrate that this bond-forming reaction involves the corresponding dications in their electronic ground states. The reactions might hence be of relevance in the context of the growth of hydrocarbon species at extremely low temperatures and pressures, such as in the atmosphere of Titan. Whereas the parent [(CH(3))C(5)H(4)N](2+) dications of both picolines also show C-C coupling to a limited amount, the molecular dication of aniline, [C(6)H(5)NH(2)](2+), is almost unreactive toward methane.

Double Photoionization of CO2 Molecules in the 34−50 eV Energy Range

The double photoionization of CO(2) molecules has been studied in the 34-50 eV photon energy range, by the use of synchrotron radiation and detecting electron-ion and electron-ion-ion coincidences. Three processes have been observed: (i) the formation of the CO(2)(2+) molecular dication, (ii) the production of a metastable (CO(2)(2+))* that dissociates, with an apparent lifetime of 3.1 micros, giving rise to CO(+) and O(+) ions, and (iii) the dissociation leading to the same products, but occurring with a lifetime shorter than 0.05 micros. The relative dependence on the photon energy of the cross section for such processes has been measured. While for the production of the molecular dication a threshold is observed, in agreement with the vertical threshold for double ionization of CO(2), for the dissociative processes the threshold appears to be lower than that value, indicating the presence of an indirect dissociation, probably leading to the formation of CO(+) together with a neutral autoionizing oxygen atom.