Production of small doubly charged negative carbon cluster ions by sputtering

Stability and Cooling of the C_{7}^{2-} Dianion

We have studied the stability of the smallest long-lived all carbon molecular dianion (C_{7}^{2-}) in new time domains and with a single ion at a time using a cryogenic electrostatic ion-beam storage ring. We observe spontaneous electron emission from internally excited dianions on millisecond timescales and monitor the survival of single colder C_{7}^{2-} molecules on much longer timescales. We find that their intrinsic lifetime exceeds several minutes-6 orders of magnitude longer than established from earlier experiments on C_{7}^{2-}. This is consistent with our calculations of vertical electron detachment energies predicting one inherently stable isomer and one isomer which is stable or effectively stable behind a large Coulomb barrier for C_{7}^{2-}→C_{7}^{-}+e^{-} separation.

A versatile setup for studying size and charge-state selected polyanionic nanoparticles

Using the example of metal clusters, an experimental setup and procedure is presented, which allows for the generation of size and charge-state selected polyanions from monoanions in a molecular beam. As a characteristic feature of this modular setup, the further charging process via sequential electron attachment within a three-state digital trap takes place after mass-selection. In contrast to other approaches, the rf-based concept permits access to heavy particles. The procedure is highly flexible with respect to the preparation process and potentially suitable for a wide variety of anionic species. By adjusting the storage conditions, i.e., the radio frequency, to the change in the mass-to-charge ratio, we succeeded in producing clusters in highly negative charge states, i.e., Ag₈₀₀ ^7-. The capabilities of the setup are demonstrated by experiments extracting electronic and optical properties of polyanionic metal clusters by analyzing the corresponding photoelectron spectra.

Isolated [B2(CN)6]2-: Small Yet Exceptionally Stable Nonmetal Dianion

We report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B₂(CN)₆]^2- in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) ─a remarkably high value of electronic stability and a ∼2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN)₃]^•- is energetically favorable. Quantum chemical calculations demonstrate a D_3d staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances.

Cresting the Coulomb Barrier of Polyanionic Metal Clusters

Combining photoelectron spectroscopy with tunable laser pulse excitation allows us to characterize the Coulomb barrier potential of multiply negatively charged silver clusters. The spectra of mass- and charge-selected polyanionic systems, with z=2-5 excess electrons, show a characteristic dependence on the excitation energy, which emphasizes the role of electron tunneling through the barrier. By evaluating experimental data from an 800-atom system, the electron yield is parametrized with respect to tunneling near the photoemission threshold. This analysis results in the first experimentally based potential energy functions of polyanionic metal clusters.

Dianion diagnostics in DESIREE: High-sensitivity detection of Cn2- from a sputter ion source

A sputter ion source with a solid graphite target has been used to produce dianions with a focus on carbon cluster dianions, C_n^2-, with n = 7-24. Singly and doubly charged anions from the source were accelerated together to kinetic energies of 10 keV per atomic unit of charge and injected into one of the cryogenic (13 K) ion-beam storage rings of the Double ElectroStatic Ion Ring Experiment facility at Stockholm University. Spontaneous decay of internally hot C_n^2- dianions injected into the ring yielded C_n^- anions with kinetic energies of 20 keV, which were counted with a microchannel plate detector. Mass spectra produced by scanning the magnetic field of a 90° analyzing magnet on the ion injection line reflect the production of internally hot C₇^2- - C₂₄^2- dianions with lifetimes in the range of tens of microseconds to milliseconds. In spite of the high sensitivity of this method, no conclusive evidence of C₆^2- was found while there was a clear C₇^2- signal with the expected isotopic distribution. This is consistent with earlier experimental studies and with theoretical predictions. An upper limit is deduced for a C₆^2- signal that is two orders-of-magnitude smaller than that for C₇^2-. In addition, C_nO^2- and C_nCu^2- dianions were detected.

A detailed-balance model for thermionic emission from polyanions: The case of fullerene dianions

A detailed-balance model for thermionic emission from polyanions has been developed and applied to fullerene dianions. The specificity of this delayed decay process is electron tunneling through the repulsive Coulomb barrier (RCB). An analytical expression of the RCB is derived from electrostatic modeling of the fullerene cage. The reverse process, namely, electron attachment to the singly charged anion, is described by a hard sphere cross section weighted by the Wentzel-Kramers-Brillouin tunneling probability. This simple expression leads to a very good agreement with a measured time-resolved kinetic energy distribution of C₈₄^2-. Electron binding energy is reduced when the fullerene cage size decreases, leading to an almost zero one for C₇₀^2- and a negative one for C₆₀^2-. Extension of the model to these systems of interest is discussed, and model outputs are compared with the experimental data from the literature.

Laser cooling of molecular anions

We propose a scheme for laser cooling of negatively charged molecules. We briefly summarize the requirements for such laser cooling and we identify a number of potential candidates. A detailed computation study with C_{2}^{-}, the most studied molecular anion, is carried out. Simulations of 3D laser cooling in a gas phase show that this molecule could be cooled down to below 1 mK in only a few tens of milliseconds, using standard lasers. Sisyphus cooling, where no photodetachment process is present, as well as Doppler laser cooling of trapped C_{2}^{-}, are also simulated. This cooling scheme has an impact on the study of cold molecules, molecular anions, charged particle sources, and antimatter physics.

Formation of dianions in helium nanodroplets

The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60)n(2-) and (C70)n(2-) were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He(-) ion. As well as the prospect of studying other dianions at low temperature using helium nanodroplets, this work opens up the possibility of a wider investigation of the chemistry of He(-), a new electron-donating reagent.

Ground and excited states of the diatomic dianion Cl2(2-)

The QCISD(T)/aug-cc-pVQZ and CIS/aug-cc-pVQZ calculations have been carried out to obtain potential energy curves (PECs) of the Cl2(2-) diatomic dianion in order to address possibility of its formation in the merged beam fragmentation of Cl2(-) questioned based on the observation of the Cl(-)+Cl+e(-) channel. Results show that two of the excited states, namely A(1)Σg and a(3)Σg are metastable with PECs having wells deep enough to suite several bound states, with minima located at Re=2.8280 Å and Re=2.5972 Å, and Coulomb barriers of 1648.288 and 1403.835 cm(-1) heights located at 4.0320 and 3.6130 Å, respectively. Transition probabilities and tunneling predissociation lifetimes corresponding to these metastable states are also calculated and analyzed. Ground state X(1)Σg and excited states B(1)Σu, C(1)Πg and D(1)Πu calculated for this dianion are all repulsive. Calculated Franck-Condon factors suggest that Cl2(2-) can be produced in its excited states via an electron impact process initiating from the ground states of Cl2 and Cl2(-) .

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

Recent progress towards understanding the repulsive Coulomb barrier in multiply-charged anion using photoelectron spectroscopy is discussed.

Guiding electron emissions by excess negative charges in multiply charged molecular anions

Using photoelectron imaging, we show the effects of excess negative charges on the directions of outgoing electrons in multiply charged anions. Photoemissions are observed to occur either in a perpendicular or parallel direction, depending on the molecular configurations and origins of the detached electrons. Detachment of the π electrons from biphenyl-disulfonate dianions is shown to occur in a perpendicular direction due to the Coulomb repulsion from the two terminal charges, whereas detachment from the sulfonate groups in linear aliphatic disulfonates occurs in parallel directions.

First observation of a tetra-anionic metal cluster, Al(n)(4-)

The production of aluminum cluster tetra-anions, and thus the first observation of a tetra-anionic metal cluster in the gas-phase, is reported. The aluminum cluster polyanions were generated by use of the "electron-bath technique." The smallest tetra-anion observed was Al(215) (4-), containing 14% fewer atoms than expected from classical estimates of the tetra-anion appearance size.

Imaging intramolecular coulomb repulsions in multiply charged anions

The properties of multiply charged anions are dominated by intramolecular Coulomb repulsion (ICR). Using photoelectron imaging, we show the effect of ICR on photoelectron angular distributions for a series of dianions, -O2C(CH2)_{n}CO_{2};{-} (D_{n};{2-}). The observed photoemission band of D_{n};{2-} was due to a perpendicular transition from the charged end group. However, photoemission intensities were observed to peak along the laser polarization for smaller n due to the strong ICR that forces electrons to be emitted along the molecular axis. This emission pattern weakens with increasing n and at D112- the angular distribution reverses back to peak at the perpendicular direction due to the reduced ICR.

Dimensional scaling treatment of stability of atomic anions induced by superintense, high-frequency laser fields

We show that dimensional scaling, combined with the high-frequency Floquet theory, provides useful means to evaluate the stability of gas phase atomic anions in a superintense laser field. At the large-dimension limit (D-->infinity), in a suitably scaled space, electrons become localized along the polarization direction of the laser field. We find that calculations at large D are much simpler than D=3, yet yield similar results for the field strengths needed to bind an "extra" one or two electrons to H and He atoms. For both linearly and circularly polarized laser fields, the amplitude of quiver motion of the electrons correlates with the detachment energy. Despite large differences in scale, this correlation is qualitatively like that found between internuclear distances and dissociation energies of chemical bonds.

Stabilization of excess charge in isolated adenosine 5'-triphosphate and adenosine 5'-diphosphate multiply and singly charged anions

Multiply charged anions (MCAs) represent highly energetic species in the gas phase but can be stabilized through formation of molecular clusters with solvent molecules or counterions. We explore the intramolecular stabilization of excess negative charge in gas-phase MCAs by probing the intrinsic stability of the [adenosine 5'-triphosphate-2H](2-) ([ATP-2H](2-)), [adenosine 5'-diphosphate-2H](2-) ([ADP-2H](2-)), and H(3)P(3)O(10)(2-) dianions and their protonated monoanionic analogues. The relative activation barriers for decay of the dianions via electron detachment or ionic fragmentation are investigated using resonance excitation of ions isolated within a quadrupole trap. All of the dianions decayed via ionic fragmentation demonstrating that the repulsive Coulomb barriers (RCB) for ionic fragmentation lie below the RCBs for electron detachment. Both the electrospray ionization mass spectra (ESI-MS) and total fragmentation energies for [ATP-2H](2-), [ADP-2H](2-), and H(3)P(3)O(10)(2-) indicate that the multiply charged H(3)P(3)O(10)(2-) phosphate moiety is stabilized by the presence of the adenosine group and the stability of the dianions increases in the order H(3)P(3)O(10)(2-) < [ADP-2H](2-) < [ATP-2H](2-). Fully optimized, B3LYP/6-31+G* minimum energy structures illustrate that the excess charges in all of the phosphate anions are stabilized by intramolecular hydrogen bonding either within the phosphate chain or between the phosphate and the adenosine. We develop a model to illustrate that the relative magnitudes of the RCBs and hence the stability of these ions is dominated by the extent of intramolecular hydrogen bonding.

Small gas-phase dianions of Zn3O42−, Zn4O52−, CuZn2O42−, Si2GeO62−, Ti2O52−and Ti3O72−

We have searched for new species of small oxygen-containing gas-phase dianions produced in a secondary ion mass spectrometer by Cs+ ion bombardment of solid samples with simultaneous exposure of their surfaces to O2 gas. The targets were a pure zinc metal foil, a copper-contaminated zinc-based coin, two silicon-germanium samples (Si(1-x)Ge(x)(with x= 6.5% or 27%)) and a piece of titanium metal. The novel dianions Zn3O(4)(2-), Zn4O(5)(2-), CuZn2O(4)(2-), Si2GeO(6)(2-), Ti2O(5)(2-) and Ti3O(7)(2-) have been observed at half-integer m/z values in the negative ion mass spectra. The heptamer dianions Zn3O(4)(2-) and Ti2O(5)(2-) have been unambiguously identified by their isotopic abundances. Their flight times through the mass spectrometer are approximately 20 micros and approximately 17 micros, respectively. The geometrical structures of the two heptamer dianions Ti2O(5)(2-), and Zn3O(4)(2-) are investigated using ab initio methods, and the identified isomers are compared to those of the novel Ge2O(5)(2-) and the known Si2O(5)(2-) and Be3O(4)(2-) dianions.

New stable multiply charged negative atomic ions in linearly polarized superintense laser fields

Singly charged negative atomic ions exist in the gas phase and are of fundamental importance in atomic and molecular physics. However, theoretical calculations and experimental results clearly exclude the existence of any stable doubly-negatively-charged atomic ion in the gas phase, only one electron can be added to a free atom in the gas phase. In this report, using the high-frequency Floquet theory, we predict that in a linear superintense laser field one can stabilize multiply charged negative atomic ions in the gas phase. We present self-consistent field calculations for the linear superintense laser fields needed to bind extra one and two electrons to form He-, He2-, and Li2-, with detachment energies dependent on the laser intensity and maximal values of 1.2, 0.12, and 0.13 eV, respectively. The fields and frequencies needed for binding extra electrons are within experimental reach. This method of stabilization is general and can be used to predict stability of larger multiply charged negative atomic ions.

Small gas-phase dianions produced by sputtering and gas flooding

We have extended our previous experiment [Schauer et al., Phys. Rev. Lett. 65, 625 (1990)] where we had produced small gas-phase dianion clusters of C(n) (2-)(n > or =7) by means of sputtering a graphite surface by Cs(+) ion bombardment. Our detection sensitivity for small C(n) (2-) could now be increased by a factor of about 50 for odd n. Nevertheless, a search for the elusive pentamer dianion of C(5) (2-) was not successful. As an upper limit, the sputtered flux of C(5) (2-) must be at least a factor of 5000 lower than that of C(7) (2-), provided that the lifetime of C(5) (2-) is sufficiently long to allow its detection by mass spectrometry. When oxygen gas (flooding with either O(2) or with N(2)O) was supplied to the Cs(+)-bombarded graphite surface, small dianions of OC(n) (2-)(5< or =n < or =14) and O(2)C(7) (2-) were observed in addition to C(n) (2-)(n > or =7). Similarly, Cs(+) sputtering of graphite with simultaneous SF(6) gas flooding produced SC(n) (2-)(6< or =n< or =18). Mixed nitrogen-carbon or fluorine-carbon dianion clusters could not be observed by these means. Attempts to detect mixed metal-fluoride dianions for SF(6) gas flooding of various Cs(+)-bombarded metal surfaces were successful for the case of Zr, where metastable ZrF(6) (2-) was observed. Cs(+) bombardment of a silicon carbide (SiC) wafer produced SiC(n) (2-) (n=6,8,10). When oxygen gas was supplied to the Cs(+)-bombarded SiC surface, small dianions of SiOC(n) (2-) (n=4,6,8) and of SiO(2)C(n) (2-) (n=4,6) as well as a heavier unidentified dianion (at mz=98.5) were observed. For toluene (C(7)H(8)) vapor flooding of a Cs(+)-bombarded graphite surface, several hydrocarbon dianion clusters of C(n)H(m) (2-)(n> or =7) were produced in addition to C(n) (2-)(n> or =7), while smaller C(n)H(m) (2-) with n< or =6 could not be observed. BeC(n) (2-) (n=4,6,8,10), Be(2)C(6) (2-), as well as BeC(8)H(m) (2-) (with m=2 and/or m=1) were observed for toluene vapor flooding of a Cs(+)-bombarded beryllium metal foil. The metastable pentamer (9)Be(12)C(4) (2-) at mz=28.5 was the smallest and lightest dianion molecule that we could detect. The small dianion clusters of SC(n) (2-), OC(n) (2-), BeC(n) (2-), and SiO(m)C(n) (2-) (m=0,1,2) have different abundance patterns. A resemblance exists between the abundance patterns of BeC(n) (2-) and SiC(n) (2-), even though calculated molecular structures of BeC(6) (2-) and SiC(6) (2-) are different. The abundance pattern of SC(n) (2-) is fairly similar to that of C(n) (2-).

Multiply-charged negative clusters of adenosine-5'-monophosphate in the gas phase

Multiply-charged noncovalent cluster anions of adenosine-5'-monophosphate (AMP) were formed by electrospray ionization (ESI). Ions in higher charge states were observed when the ions were accumulated in an ion trap with helium buffer gas before detection. We determined the smallest size (n(a)) or appearance size as a function of charge state (q), i.e., n(a) = 4 for q = 2, n(a) = 8 for q = 3, and n(a) = 13 for q = 4. The relation between n(a) and q can be described by a charged droplet model. When the size is larger than n(a) for a given q, the fragmentation pathway of an anion cluster is dominated by loss of neutral fragments. In contrast, when the size approaches the appearance size, only charged fragments are formed.

Electron localization–delocalization transitions in dissociation of the C4− anion: A large-Danalysis

We present a study, employing high level ab initio methods, of electron localization-delocalization transitions along the dissociation path of the C4- anion to C2 and C2-. We find that at the equilibrium geometry, the symmetrical and nonsymmetrical configurations of the linear C4- anion are almost isoenergetic. However, along a collinear dissociation path, the dipole moment drops abruptly to zero when the separation between the two middle carbon nuclei reaches about R = 2.15 angstroms. The dipole moment remains zero until about R = 2.78 angstroms, and then continuously increases as dissociation proceeds. This behavior is analogous to critical phenomena: The abrupt drop to zero of the dipole moment resembles a first-order phase transition, the later steady rise resembles a continuous phase transition. We show that a simple sub-Hamiltonian model, corresponding to the large-dimension limit for an electron in the field of four collinear carbon atoms, exhibits both kinds of phase transitions along the dissociation path.

The Repulsive Coulomb Barrier along a Dissociation Path of the Be Dianion

We present ab initio calculations of the repulsive Coulomb barrier for several geometrically stable isomers of the BeC(2-)(4) dianion. We describe how the deformation of certain isomers can account for the experimental Coulomb explosion images of the dianion. For the most stable linear isomer, C(-)(2)BeC(-)(2), we examined the electron tunneling process along the dissociation path to obtain C(-)(2) plus BeC(-)(2). We found the crossing point for autodetachment to be R(c)(dis)= 3.25 A. R(dis) is the bond length between C(-)(2) and BeC(-)(2); at this point, the electron tunneling energy is equal to the maximum of the repulsive Coulomb barrier. In the framework of the Wenzel-Kramer-Brioullin theory, the electron-loss lifetime of the metastable C(-)(2)BeC(-)(2) dianion at the equilibrium geometry, R(dis) = 1.64 A, was estimated to be about 5 ms. This lower limit is in agreement with the experimental results in which the BeC(2-)(4) dianion has a lifetime much longer than 5 micros.

Gas-phase stability of derivatives of the closo-hexaborate dianion B(6)H(6)(2-)

B(6)H(6)(2-) does not represent a stable gas-phase dianion, but emits spontaneously one of its excess electrons in the gas phase. In this work we address the question whether small stable gas-phase dianions can be constructed from the parent B(6)H(6)(2-) dianion by substitution of the hydrogens with appropriate ligands. Various hexa-, tetra-, and disubstituted derivatives B(6)L(6)(2-), B(6)H(2)L(4)(2-), and B(6)H(4)L(2)(2-) (L = F, Cl, CN, NC, or BO) are investigated with ab initio methods in detail. Four stable hexasubstituted B(6)L(6)(2-) (L = Cl, CN, NC, or BO) and three stable B(6)H(2)L(4)(2-) (L = CN, NC, or BO) gas-phase dianions could be identified and predicted to be observable in the gas phase. The trends in the electron-detachment energies depending on various ligands are discussed and understood in the underlying electrostatic pattern and the electronegativities of the involved elements.

Cyclic carbon cluster dianions and their aromaticity

Cyclic carbon cluster dianions (CC(2))(2-)(n)(n = 3-6) are investigated by ab initio methods with regard to their geometric properties, electronic stability, and aromaticity. The unique wheel-like structures of these dianions consist of a n-membered carbon ring, where a C(2) unit is attached to each carbon atom. All investigated dianions represent stable gas-phase dianions. While the smallest member of this family (CC(2))(2-)(3) is clearly aromatic, the aromatic character decreases rapidly with increasing ring size. The geometries and the aromaticity of the cyclic clusters (CC(2))(2-)(n)(n = 3-6) can be nicely explained using resonance structure arguments.