Electronic structure and radiative transitions of excimer systems

Abstract

The interaction of the ground states of two closed-shell systems (atom or molecule) is usually repulsive except for a weak long-range van der Waals or electrostatic interaction. But the interaction of an excited state with the ground state of the same fragments can form a strong chemical bond. The bound excited state has been called an excimer or exciplex state and the bound to continuum transition defines an excimer emission system [2.1]. In the case of degenerate open-shell fragments (e.g., P-states, etc.) more than one molecular state will correlate with the degenerate asymptote. Some or all of the curves may be essentially repulsive, but there may also be a bound ground state. The excited state of the open-shell fragment interacting with a ground state fragment can also yield strongly bound molecular states which can be defined as excimer states. The analysis of excimer emission systems in this chapter assumes that the lower repulsive state always arises from ground state fragments which may, however, be degenerate. Rare gas atoms interacting in their ground state provides an example of a single repulsive curve arising from a non-degenerate asymptote. The interaction of two hydrogen atoms in their ground 2 S states will, on the other hand, yield two states a \(^1\Sigma _g^ + \) and a \(^3\Sigma _u^ + \) which are, respectively, strongly bound and repulsive. By generalizing the definition of an excimer emission system to include degenerate asymptotic ground states, we would include almost all possible chemical systems. In order to keep the review within limits, we have restricted consideration to the interactions of either rare gas or Group II atoms with other atomic species for which excimer emission systems have been found and were originally suggested as laser candidates [2.2]. Only two triatomic classes of excimers have been included which are again based on rare gas or Group II molecule fragments.