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The state-resolved collision-energy dependences of Penning ionization cross sections σ(E) were measured in an energy range (60<E<400 meV) for N2 and CO2 with He*23S by using a very high-intensity He* beam (1.8×1015 s−1 sr−1) and detecting energy-analyzed electrons as functions of time-of-flight of He*. The partial ionization cross sections for Π states (N+2B:A2Πu, CO+2B:X2Πg, A2Πu) were observed to increase more rapidly with the increase of the collision energy than those for Σ states (N+2B:X2Σ+g, B2Σ+u, CO+2B:B2Σ+u,C2Σ+g). In the studied energy range, the repulsive walls for end-on collisions were indicated to be harder than those for side-on collisions. The directional peculiarity of the potential surfaces was related to the anisotropy in the hybridization of He* orbitals interacting with the target molecules.

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Low-energy boron clusters are characterized by two-dimensional geometry. Aromaticity of these planar boron clusters was established in terms of topological resonance energy (TRE). All planar boron clusters were found to be highly aromatic with large positive TREs even if they have 4n pi-electrons. Aromaticity must therefore be the origin of unusual planar or quasi-planar geometry. Thus, the aromaticity concept is as useful in boron chemistry as it is in general organic chemistry. It is evident that the Hückel 4n + 2 rule of aromaticity should not be applied to such polycyclic pi-systems. Some of the boron clusters are in the triplet electronic state to attain higher aromaticity. Multivalency and electron deficiency of boron atoms are responsible for lowering the energies of low-lying pi molecular orbitals and then for enhancing aromaticity. For polycyclic pi-systems, paratropicity does not always indicate antiaromaticity.