Photoionization Study of Diatomic-Ion Formation in Argon, Krypton, and Xenon

Abstract

Ionization current at wavelengths of discrete absorption lines of five resonance series of argon, krypton, and xenon has been observed in the vacuum ultraviolet. This ionization is due to a collision process between electronically excited and ground-state atoms resulting in formation of a diatomic ion and an electron. Using the helium and argon continuum light sources, it was possible to identify over 20 reacting states for each gas, and to observe that the ionization is found at every absorption line of shorter wavelength than a definite ionization threshold. Ionization thresholds observed were: Ar, 14.710±0.009; Kr, 13.004±0.007; and Xe, 11.162±0.005 eV. The energies of the ionization thresholds are in good agreement with some electron-impact appearance potentials, but the number of excited electronic states found in this investigation is much larger. From the thresholds, the following lower limits for D(X2+) are obtained: Ar, 1.049; Kr, 0.995; and Xe, 0.967 eV. Ionization yields at argon absorption lines result in values of k4/k2 (ratio of de-exciting collisions to diatomic-ion-formation collisions) from 2.5 to 0.13 and values of τk2 (product of effective radiative lifetime and diatomic-ion-formation rate constant) of 3.6×10−18 to 1.28×10−16 cm3 atom−1.