Measurement of metastable and resonance level densities in rare-gas plasmas by optical emission spectroscopy

Abstract

Excitation and ionization of atoms out of the 4 energy levels of the excited np5(n + 1)s configuration of rare gases play an important role in many low temperature rare-gas plasmas. We compare two optical methods for measuring the number densities of atoms in these excited levels in an inductively coupled plasma under a variety of operating conditions (600 W, 1–25 mTorr). The first method is a standard white light absorption technique, whereas the second method exploits changes in the effective branching fractions of np5(n + 1)p → np5(n + 1)s emissions brought about by radiation trapping of atoms in np5(n + 1)s levels. The branching fraction method was found to produce results that agree well with the direct white light absorption method for both argon and neon plasmas using little more than a low-resolution spectrum of the plasma glow.