Density of N2 (X 1Σ+g; υ=18) molecules in a dc glow discharge measured by cavity ringdown spectroscopy at 227 nm; validity domain of the technique

The high sensitivity cavity ringdown spectroscopy (CRDS) absorption technique is used to measure the absolute density of the ground state nitrogen molecules in the v " = 18 vibrational level in a dc glow discharge. The experimental conditions are 2.3 Torr nitrogen pressure, 100 mA current and a discharge tube of 3.6 cm diameter and 80 cm length. The excimer-pumped dye laser is tuned on the Lyman-Birge-Hopfield, N-2 (a (1)Pi (g); 8-X (1)Sigma (g); 18) band at 227 nm and absorption rates on spectrally-resolved rotational lines are obtained. The gas temperature deduced from the rotational distribution is 500 K. We measure a total density of (9 +/- 3.5) x 10(19) molecules m(-3) in the nu " = 18 vibrational level, equivalent to 0.2% of the total nitrogen density. So high density suggests that the nu " = 18 level should be located in the Treanor plateau of the vibrational distribution function. We also analyse the validity domain of the CRDS measurements, regarding the optical saturation due to the high intensity of the laser radiation inside the CRDS cavity and depletion of the absorbing species. If, for our measurements, the linear absorption conditions are fulfilled, we show that very often the optical saturation becomes dominant, inducing a non-single exponential decay of the ringdown signal and leading to an underestimated absorber density.