Thomas Sommerfeld

Temporary anions - calculation of energy and lifetime by absorbing potentials: the resonance

The calculation of energies and lifetimes of metastable molecules requires the treatment of both the continuum and correlation effects. We describe the complex absorbing potential approach incorporated within a configuration-interaction framework. The absorbing potential method allows a very efficient solution of the continuum problem, making possible a detailed study of the correlation effects that turn out to be surprisingly strong. The famous resonance is studied as a test case and much attention is paid to an internally balanced treatment of the metastable state. Our findings are rationalized within a simple model that is then used to understand the results of various previous studies.

Complex absorbing potentials in the framework of electron propagator theory. II. Application to temporary anions

In continuation of Paper I of this work we describe a practical application of the combination of complex absorbing potentials (CAPs) with Green’s functions. We use a new approach for calculation of energies and lifetimes of temporary anions, which emerge, e.g., from elastic scattering of electrons from closed-shell targets. This new method is able to treat the continuum and correlation effects simultaneously and reduces the problem to the diagonalization of a number of relatively small, complex symmetric matrices. The efficiency of the proposed method is demonstrated and its dependence on basis set and parameters characterizing the CAP is investigated using the 2Πg resonance state of N2− as an example. We also present the first correlated ab initio calculation of energies and lifetimes of resonances in elastic electron scattering from the organic molecule chlorobenzene. Our results for both examples are in good agreement with existing experimental values and other theoretical calculations. Possible futur...

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.

On the interatomic Coulombic decay in the Ne dimer

The interatomic Coulombic decay (ICD) in the Ne dimer is discussed in view of the recent experimental results. The ICD electron spectrum and the kinetic energy release of the Ne+ fragments resulting after Coulomb explosion of Ne2 (2+) are computed and compared to the measured ones. A very good agreement is found, confirming the dynamics predicted for this decay mechanism. The effect of the temperature on the electron spectrum is briefly investigated.

Coupling between dipole-bound and valence states: the nitromethane anion

Nitromethane is a prototypical example for a molecule that can bind an extra electron in two fundamentally different ways forming dipole-bound as well as valence anions. The classification of the electronic states as dipole-bound or valence does in fact suggest a diabatic viewpoint, and we investigate the coupling between these two electronic states of the nitromethane anion. The coupling element W is extracted from a cut through the two lowest adiabatic potential energy surfaces by fitting of a simple avoided crossing model potential, that is, W is effectively approximated as half the smallest splitting. High level ab initio calculations are performed to compute the two states along the cut. We discuss in particular how a balance between the two very different electronic states can be achieved, and how the temporary nature of the valence anion in a large region of the relevant nuclear coordinate space can be taken into account. The autodetachment lifetime following vertical electron attachment to the neutral is computed, but the calculation of the temporary anion state turns out to be too expensive for a study of the two adiabatic surfaces, and consequently, the second adiabatic state is only included at geometries where it lies below the neutral potential energy surfaces. We find a coupling matrix element of 30 meV. On the one hand, this value is much smaller than the vertical excitation energies underlining the need for a diabatic picture. On the other hand, this value suggests rapid transitions on a mass spectrometric timescale substantiating the notion that the dipole bound state provides an efficient doorway for attachment to the valence state.

Benchmark Calculations of the Energies for Binding Excess Electrons to Water Clusters.

State-of-the-art ADC(2), EOM-EA-CCSD, and EOM-EA-CCSD(2) many-body methods are used to calculate the energies for binding an excess electron to selected water clusters up to (H2O)24 in size. The systems chosen for study include several clusters for which the Hartree-Fock method either fails to bind the excess electron or binds it only very weakly. The three theoretical methods are found to give similar values of the electron binding energies. The reported electron binding energies are the most accurate to date for such systems, and these results should prove especially valuable as benchmarks for testing model potential approaches for describing the interactions of excess electrons with water clusters and bulk water.

A self-consistent polarization potential model for describing excess electrons interacting with water clusters.

A new polarization model potential for describing the interaction of an excess electron with water clusters is presented. This model, which allows for self-consistent electron–water and water–water polarization, including dispersion interactions between the excess electron and the water monomers, gives electron binding energies in excellent agreement with high-level ab initio calculations for both surface-bound and cavity-bound states of (H(2)O)(n)(-) clusters. By contrast, model potentials that do not allow for a self-consistent treatment of electron–water and water–water polarization are less successful at predicting the relative stability of surface-bound and cavity-bound excess electron states.

Doorway mechanism for dissociative electron attachment to fructose

Recently, the three sugars ribose, deoxyribose, and fructose have been shown to undergo dissociative electron attachment at threshold, that is, to fragment upon capture of a zero-energy electron. Here the electron acceptor properties of three fructose isomers are investigated in view of a doorway mechanism. Two key ingredients for a doorway mechanism, a weakly bound state able to support a vibrational Feshbach resonance, and a valence anion more stable than neutral fructose are characterized. Moreover, possible structures for the observed fragment anion (fructose-H2O)- are suggested.

Dipole-bound states as doorways in (dissociative) electron attachment

This communication starts with a comparison of dissociative recombination and dissociative attachment placing emphasis on the role of resonances as reactive intermediates. The main focus is then the mechanism of electron attachment to polar molecules at very low energies (100 meV). The scheme considered consists of two steps: First, an electron is captured in a diffuse dipole-bound state depositing its energy in the vibrational degrees of freedom, in other words, a vibrational Feshbach resonance is formed. Then, owing to the coupling with a valence state, the electron is transferred into a compact valence orbital, and depending on the electron affinities of the valence state and possible dissociation products, as well as on the details of the intramolecular redistribution of vibrational energy, long-lived anions can be generated or dissociation reactions can be initiated. The key property in this context is the electronic coupling strength between the diffuse dipole-bound and the compact valence states. We describe how the coupling strength can be extracted from ab initio data, and present results for Nitromethane, Uracil and Cyanoacetylene.

Ab initio calculation of energies and lifetimes of metastable dianions: The C22− resonance

Most small dianions known in the solid state and solutions cannot exist as isolated entities and decay in the gas phase by electron autodetachment. These dianions show rare-gas-like closed-shell electronic ground states and represent a new type of metastable system. Here we study the prototype closed-shell resonance C22− in the framework of the complex absorbing potential method. We investigate in detail a number of unsettled methodological issues. In particular, there is no “natural” choice of orbital set for closed-shell metastable states and we study several orbital sets as well as other basis set and correlation effects on resonance energy and width. Closed-shell resonances typically show several open decay channels and we compute partial widths for the three open channels of C22−. Finally, we study the complex potential energy curve and compare our bond lengths and vibrational frequencies with geometrical parameters which have been obtained ignoring the metastable character of C22−.