Markus K. Scheller

Gas-Phase Multiply Charged Anions

Singly charged negative ions in the gas phase have attracted considerable experimental and theoretical attention over the past decades. However, the existence of free doubly or multiply charged negative ions, in particular those of small systems, has remained a curiosity and a matter of some controversy. Recent experimental and quantum mechanical studies show that multiply charged negative ions of small molecules and clusters can exist as isolated entities.

The structure of small doubly negative carbon clusters

Abstract Recently, experimental evidence has been obtained for the existence of dianionic carbon clusters as small as C2−7. So far, no theoretical evidence has been achieved for the electronic stability of C2−7, which is attributed to the underlying assumption of C2−7 having a linear structure. Linear, quasi-linear and trigonal planar atomic arrangements for C2−7 are discussed. At the latter structure C2−7 is predicted to be both stable with respect to vertical and adiabatic electron autodetachment and stable with respect to all framentation pathways. The reasons leading to the electronic stability of the trigonal C2−7 dianion are discussed and illuminated by drawing a connection with the structurally related MX2−3 alkali halides. Some remarks are made on larger dianionic carbon clusters.

Existence of doubly-negative charged ions and relation to solids

Some general aspects concerning dianions are discussed. The small LiF32- and related XY32- free dianions are predicted to be stable long-lived species. Quantitative results are given and the findings are illuminated by drawing a connection with crystals.

On the size‐dependence of the static self‐energy in propagator calculations

Various approximations are used currently to evaluate the static part Σ(∞) of the self‐energy or optical potential in molecular Green’s function calculations. Since its expansion is ruled out by the linked‐cluster theorem, one generally assumes a size‐intensive behavior of Σ(∞) in the thermodynamic limit of an infinite system. A detailed analysis of this size‐dependence property is conducted using the formulation of crystalline orbitals (CO) for stereoregular polymers. In spite of the linked‐cluster theorem, this study provides evidence for a logarithmic divergence with respect to the size of a chain for some forms of Σ(∞), computed with common approximation schemes. This is the direct outcome of the long‐range character of the Coulombic interaction and can be related to a violation in the number of particles within the system. A proper size‐intensive behavior implies an exact cancellation of the logarithmically divergent behavior of antigraphs. The conclusions drawn from CO analysis are confirmed by nume...

A construction principle for stable multiply charged molecular anions in the gas phase

Abstract A construction principle is discussed which allows us to design stable multiply charged molecular anions in the gas phase. A sequence of anionic potassium fluorides with two to seven excess electrons is systematically constructed on the basis of this principle. At the ab initio and ionic model level of theory the stability of these systems to electron detachment and fragmentation is investigated.

A new class of free stable doubly negative systems: Three Si2O2−5 isomers and their interconversion

The free dianionic Si2O2−5 system is studied by standard ab initio methods. We have located three distinct isomers in the Si2O2−5 system. All three Si2O2−5 isomers are found to be thermodynamically stable with respect to dissociation into monoanionic fragments. However, only two of them are predicted to be clearly stable with respect to vertical electron autodetachment. Minimal energy pathways which interconnect the three isomers have been studied together with the binding energy of the excess electron along them. The results give rise to an unusual nuclear dynamics problem. In particular, the associated barriers have been found to be high and wide enough to predict long lifetimes for two free dianionic species.

A new class of free stable doubly negative systems: First investigations on the SimO2−n series

Studies on several isomers of the C2O2−n and Si2O2−n series of dianionic systems are performed at the Hartree–Fock, configuration interaction, Mo/ller–Plesset perturbation theory and outer‐valence Green’s function levels of theory. All examined C2O2−n systems are predicted to be unstable with respect to electron autodetachment. However, the analogues silicon systems Si2O2−n turn out to be electronically more stable than the corresponding carbon species. For Si2O2−4 which marks the onset of electronic stability in the SimO2−n series the influence of the basis set and electron correlation on the geometrical parameters as well as on the electron detachment energy are studied in some detail. The possibility of decomposition via fragmentation or adiabatic electron loss is discussed. A simple criterion to determine whether a given structure may represent a promising candidate for a stable dianion of the SimO2−n type is suggested.

Correlation phenomena in the ionization of CN dimers

Abstract We present a theoretical analysis of the molecules cyanogen (NCCN), isocyanogen (CNCN) and diisocyanogen (CNNC) on the basis of Hartree-Fock (HF) calculations and of more sophisticated many-body computations represented by Greens function approaches such as ADC(3) and OVGF. Geometries, stabilities and binding as well as ionization properties are discussed and interrelated for the molecules mentioned above by use of orbital correlation diagrams and theoretical photoelectron spectra shown in this work.