Abstract
Formation of chemical bonds is theoretically discerned by the presence of static nuclear configuration on a potential energy surface given within the Born-Oppenheimer framework. We here study dynamical chemical bonding for molecules residing in the electronic excited states that are in a densely quasi-degenerate electronic state manifold and thereby keep undergoing extremely frequent nonadiabatic transitions. For this type of the states, the notion of global potential energy surfaces based on the adiabatic representation loses the usual sense. Nonetheless, chemical bonding exists and associated chemical reactions certainly proceed, for which we call chemistry without potential surfaces. As such, we investigate the highly excited states of boron clusters, which have extraordinarily long lifetimes with neither ionization nor dissociation. The dynamical chemical bonds keep rearranging themselves without converging to a static structure, the vivid electron dynamics of which is tracked by means of the nonadiabatic electron wavepacket dynamics theory. To characterize the dynamical bonding theoretically, we propose the notion of hyper-resonance.
Highlights
Rigorous definition of chemical bond, irrespective of the kind such as covalent, ionic, and so on, is usually given such that nuclei in an atomic aggregate can all sit at a minimum on the quantum chemical potential energy surface, and thereby the classical forces working on the nuclei are everywhere zero
In contrast to the above quantum chemical definition, far more flexible and less rigorous definition of chemical bonding was proposed by Pauling in his The Nature of the Chemical Bond,[8] from which it is quoted “We shall say that there is a chemical bond between two atoms or groups of atoms in case that the forces acting between them are such as to lead to the formation of an aggregate with sufficient stability to make it convenient for the chemist to consider it as an independent molecular species.”
It has been theoretically found that alkali halides such as LiF placed in an appropriate continuous wave (CW) laser field can have a very long lifetime in the Franck–Condon region with energy higher than the dissociation limit.[11]
Summary
Rigorous definition of chemical bond, irrespective of the kind such as covalent, ionic, and so on, is usually given such that nuclei in an atomic aggregate can all sit at a minimum (local or global) on the quantum chemical potential energy surface, and thereby the classical forces working on the nuclei are everywhere zero. The CW laser field makes many copies of the so-called dressed-states from the original (undressed) one, concomitantly generating the corresponding potential curves of the ionic character, say, (Li+–F−, nhv), each of which can have an avoided crossing with that of the covalent nature (Li–F). Those dressed potential curves of the ionic nature can have high dissociation energy since they are lifted high above the original curve. This dynamical state confined in the Franck–Condon region can have a very long lifetime under an appropriate condition, which may be termed laser-assisted nonadiabatic chemical bonding
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