A gas of Rydberg atoms

With the development of far infrared and millimetre wave amplifiers and detectors, in which Rydberg atoms are used as the active medium, researchers have been led to study the possibility of making a dense gas of highly excited atoms. Unfortunately, there are fundamental constraints 

An excimer is defined as a molecule which exists only in a highly excited state and possesses no stable ground state (see section 2.27). 

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