Optical spectra are observed through a sampling orifice inserted directly into an inductively coupled plasma. The optical system does not use lenses, mirrors, or windows so that vacuum ultraviolet wavelengths as low as the ionization energy of Ar (78.7 nm) can be detected. These lines are readily observed with lateral spatial resolution. The intensity of Ar I 106.67 nm is studied for various plasma operating parameters and observation positions. The number density for the upper state for this line (<sup>3</sup>P<sup>1</sup>) is estimated to be of the order of 10<sup>11</sup> cm<sup>-3</sup> in the axial channel, which is similar in magnitude to the populations of the nearby metastable Ar levels measured previously. The Ar I line at 104.82 nm is self-reversed, indicating that Ar resonant radiation can definitely be trapped in the inductively coupled plasma. The results support the radiation trapping hypothesis but do not quantitatively determine the magnitude of energy transfer from the induction region to the axial channel by this mechanism.
R. S. Houk, Velmer A. Fassel, and Bryant R. LaFreniere, "Direct Detection of Vacuum Ultraviolet Radiation Through an Optical Sampling Orifice: Spatially Resolved Emission Studies of Argon Resonance Lines from an Inductively Coupled Plasma," Appl. Spectrosc. 40, 94-100 (1986)