Excitation and ionization of atoms out of the 4 energy levels of the excited np(5)(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 np(5)(n + 1) p -> np(5)(n + 1) s emissions brought about by radiation trapping of atoms in np(5)(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.
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