Vibrational Temperature of Excited Nitrogen Molecules Detected in a 13.56 MHz Electrical Discharge by Sheath-Side Optical Emission Spectroscopy

Optical emission spectroscopy parallel to the axis of a low-pressure radio-frequency discharge in nitrogen was realized, i. e. end-on instead of customary side-on measurements. A specially designed optical feedthrough was fabricated and installed co-planar with the anode elec- trode, allowing non-invasive observations perpendicular to the plasma sheath. The principal ex- cited species flowing towards the grounded electrode were identified and, by assuming Boltzmann distributions, the vibrational temperature of the molecules on the X ¹ Σ⁺ _g and C³ Π _u levels was estimated under various operating conditions.