Measurements of N₂ ⁺ ions in an inductively coupled plasma using saturated cavity ringdown spectroscopy

Abstract This paper presents a unique study of the bulk plasma characteristics in a low pressure inductively coupled nitrogen plasma. Saturated cavity ringdown spectroscopy (sat-CRDS) has been used to determine the absolute number densities and translational temperatures of N 2 + (X, ν = 0). The effect of saturation is readily accounted for by using an effective saturation parameter, S eff , and determined by a simple method employing measurements at two different gain settings of the detection system. The appropriateness of this method is confirmed by comparison with fitting individual ringdown data using a time-dependent saturation parameter, S ( t ), within the local approximation model for sat-CRDS; the two methods are in excellent agreement in returning absolute number densities and translational temperatures. N 2 + (X, ν = 0) number densities are determined across a matrix of pressure (10−100 mTorr) and radio-frequency power (200−400 W) conditions with maximum number densities of ca . 1.3 × 10 10 cm –3 while translational temperatures range from 600−1500 K.