Physics: Negative ion formation in complex heavy systems

Dr Alfred Msezane from Clark Atlanta University’s Department of Physics lavishes us with his knowledge about an intriguing aspect of physics, which focusses on his research on negative ion formation in complex heavy systems

The investigation of negative ion formation in low-energy electron elastic collisions with complex heavy systems through the total cross sections (TCSs) calculation provides a novel and robust approach to producing unambiguous and definitive relevant atomic and molecular data for the first time. The novelty and generality of the Regge-pole approach is in the extraction of the anionic binding energies (BEs) from the calculated TCSs for the complex heavy systems; for ground state collisions these BEs yield the definitive theoretically challenging to calculate electron affinities (EAs). Much has been reported and discussed in the published literature about negative ions in general from various perspectives, including their nature and applications. The major objective here is the production of reliable data for complex heavy systems, such as the lanthanide and actinide atoms and fullerene molecules through low-energy electron elastic scattering TCSs calculations. This presentation toward quality data production is organised for convenience in the following four inter-connected subtopics: I. Overview and novel robust theoretical approach; II. Ground state negative ion formation in complex heavy systems: electron affinity extraction; III. Metastable and excited states negative ion formation in fullerene molecules: new insights; and IV. Negative ion formation in the lanthanide and actinide atoms: determination of reliable EAs.

I.1 Overview

One of the most challenging and lingering problems in atomic and molecular physics and still continues to plague both experiments and theory alike when exploring negative ion formation in complex heavy atoms and fullerene molecules is the determination of the unambiguous and reliable electron affinities (EAs) of the atoms and molecules involved. Indeed, the published literature abounds in ambiguous and unreliable EAs for the lanthanide and actinide atoms. Also calculating the EAs of complex heavy systems is a formidable task for conventional theoretical methods due to the presence of the large and diverse intricate electron configurations. The use of theoretical methods that account correctly for the important physics, viz. electron-electron correlation effects and core-polarization interaction is fundamental to the reliable investigation and understanding of negative ion formation in complex heavy systems. This is the adopted approach in the investigation here.

Essentially, many existing experimental measurements and sophisticated theoretical calculations have considered the anionic BEs of the electron attachment to metastable and/or excited anionic states leading to stable negative ion formation to correspond to the EAs of the considered complex heavy atoms, such as the lanthanide and actinide atoms. Indeed, this is contrary to the usual meaning of the EAs found in the standard measurement of the EAs of such complex systems as atomic Au and Pt as well as of the fullerene molecules from C20 through C92. In these systems, the EAs correspond to the anionic binding energies for electron attachment to the ground state of the formed negative ions. Therefore, there must be a consistent and definitive meaning of the EA to avoid the proliferation of ambiguous and confusing meaning of the EAs of these complex heavy systems.

Unfortunately, progress toward a theoretical understanding of the fundamental mechanism underlying low-energy electron scattering from complex heavy atoms, including fullerene molecules, leading to stable negative ion formation has been very slow. In the lanthanide and actinide atoms, the presence of many intricate and diverse electron configurations that characterise low-energy electron interactions in these systems leads to computational complexity. This renders very difficult to obtain unambiguous and reliable electron affinities (EAs) for complex heavy systems using conventional theoretical methods consisting of large notoriously slow converging expansions. In particular, electron affinities calculated using structure-based theoretical methods tend to be riddled with uncertainties.

I.2 Novel and robust theoretical method

In recent years, the Regge-pole methodology has proved to be essential to the determination of reliable negative ion formation in low-energy electron collisions with complex heavy systems through the TCSs calculation. Regge-poles, singularities of the S-matrix, are generalised bound states within the complex angular momentum (CAM) description of scattering; they are, therefore, appropriate for the present investigations. The great advantage of the Regge-pole calculated electron elastic total cross sections (TCSs) is the extraction from them of the energy positions of the characteristic Ramsauer-Townsend (R-T) minima, shape resonances and the dramatically sharp resonances manifesting ground, metastable and excited states negative ion formation. The novelty and generality of the Regge-pole approach used here is in the extraction of the binding energies (BEs) of the anionic ground states from the calculated elastic TCSs of the complex heavy systems; these BEs have been identified with the measured EAs.

Within the CAM theory, the calculation of the TCS embeds fully the essential electron-electron correlation effects. Its calculation uses the Avdonina-Belov-Felfli (ABF) potential which accounts for the vital core-polarization interaction. The ABF potential has the appropriate asymptotic behaviour and accounts properly at low electron impact energy for the polarization interaction (both ground and excited states). It has five turning points and four poles connected by four cuts in the complex plane. The presence of powers of the charge Z as coefficients of the r and r2 (r is the radial distance) in the ABF potential ensures that spherical and non-spherical atoms and fullerenes are correctly treated. Also appropriately treated are small and large systems.

I.3 Accomplishments

The EAs provide a stringent test of theoretical methods when the calculated EAs are compared with those from reliable measurements. Accurate and reliable atomic and molecular EAs are essential for understanding chemical reactions, whose importance and vast utility in terrestrial and stellar atmospheres as well as in device fabrication, catalysis, organic solar cells and drug delivery are well-documented.

Unfortunately, the published literature abounds in ambiguous and difficult to interpret EA values for complex heavy systems, particularly for the experimentally difficult to handle radioactive actinide atoms. Entirely new in the field of electron-cluster/fullerene collisions, the Regge-pole method has been benchmarked on the measured EAs of atomic Au and Pt as well as of C60 and C70 fullerene molecules yielding an outstanding agreement. The method requires no assistance whatsoever from either experiment or other theory to accomplish the remarkable feat. Indeed, very recently, it has been demonstrated for the first time that the ground state anionic BEs extracted from our Regge-pole calculated elastic scattering TCSs for C20 through C92 fullerenes matched excellently the measured EAs for these fullerene molecules. This is an unprecedented theoretical achievement; existing theoretical calculations are still struggling to go beyond the C60 fullerene because the EAs are at the heart of the fullerene shell model potentials.

In our research, the characteristic R-T minima, shape resonances and the ground, metastable and excited anionic BEs are extracted from the calculated TCSs for complex heavy systems, focusing mainly on the ground state anionic BEs. In the process, the following have been exposed and elucidated: 1) Novel mechanism for creating long-lived metastable atomic negative ions by exploiting the orbital collapse mechanism in the lanthanide and actinide atoms, impacting significantly the polarization interaction; 2) Manifestation of polarization-induced fullerene-like behaviour in the TCSs for the large actinide atoms Pu and Lr due to the size effects; 3) Multiple functionalisation of large fullerene molecules through the rich negative ion resonances in their TCSs; and 4) Effective use of Regge Trajectories to probe electron attachment at the fundamental level in multi-electron systems. They are also used to delineate and identify ground, metastable and excited states negative ion formation through the anionic BEs. And importantly, these Trajectories are essential in assessing the role of relativity in the TCSs calculation.

Recently, the conundrum in the measured EAs of the complex heavy atoms Eu, Tb, Tm, Nd and Nb has been clarified and resolved through the scrutiny of the calculated electron scattering TCSs using our robust Regge pole methodology. It has been concluded that the measured and previously calculated EAs for the investigated atoms, including the most recent measurements of the EAs of Eu and Nb correspond to the BEs of excited anions of these atoms. This demonstrates the importance of our research.

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Dr Alfred Msezane

Department of Physics,

Clark Atlanta University

Tel: +1 404 880 8663

amsezane@cau.edu

www.cau.edu/department-of-physics/