COCHISE observations of argon Rydberg emission from 2 to 16 micrometers

Infrared (2 to 16 micrometers emission from atomic Ar Rydberg atoms, excited in a coventional low-pressure microwave discharge plasma, has been observed in the cryogenic COCHISE facility. The observed spectrum is very complex but is identifiable using the detailed energy level structure and optical selection rules for Arl. Detailed transition probabilities for dipole-allowed transitions between 2 and 16 micrometers were estimated using the Coulomb approximation. Comparisons of observed and simulated spectra show that substantial LWIR emission (approx. 12 micrometers) arises from Rydberg states near the ionization limit. These states must have kinetically enchanced populations in order to account for the observed emission intensities. The results are interpreted in terms of a radiative collisional cascade sequence initiated by dissociative recombination of Ar2(+) with energetic electrons in the active discharge. The implications of these observations for atmospheric infrared signatures are discussed.