Optical Emission from Argon Excited by Alpha Particles: Quenching Studies

Optical emission from argon gas excited with alpha particles has been studied to determine if electronic states of long lifetimes are excited by swiftly moving charged particles as has been observed in studies of ionization. The spectrum from argon showed a continuum which could be separated into three distinct bands with maxima at 1300, 1800, and 2000 Å. The light intensity of the 2200-Å band was readily quenched with the addition of about 2% of CH4, CO2, O2, and Kr. The 1800-Å band was quenched more slowly than the 2200-Å band by the impurity gases. The results support the notion that alpha-particle excitation of argon at high pressures leaves the argon in long-lived states of about 11.5 or 14.5 eV. The observed continua appear to arise mainly from Franck—Condon transitions between two excited states and the ground repulsive state of the argon dimer which forms as a consequence of the long lifetimes of the excited atoms.