Absolute intensity calibration of emission spectra: application to the forbidden 346 nm nitrogen line for N(2P°) metastable atoms density measurement in flowing afterglow

Abstract

A novel method, based on emission signal from the excitation transfer reaction: Ar*( 3 P 2 ) + N 2 ( X ) N 2 *( C 3 ) + Ar, is proposed for the intensity calibration of the spectroscopic optical detection system in absolute scale. It is applied for the measurement of N( 2 P°) metastable atoms density in the Short Lived Afterglow (SLA) of a 440 Pa nitrogen discharge produced by a 433 MHz resonant cavity. This density is deduced from the absolute intensity of the forbidden N( 2 P°– 4 S°) line at 346.65 nm, whose transition probability is only 0.005 s −1 . The N( 2 P°) density variation in the SLA resembles those of N 2 ( A 3 ) metastable molecules and electrons or the emission intensities of first positive (1 + ), second positive (2 + ) and first negative (1 − ) systems of N 2 . It first decays after the discharge zone up to a minimum and hence increases by almost a factor of thirty to reach a maximum value of 6 × 10 17 m −3 at the maximum of the SLA. It is proposed that N( 2 P°) density results from a local equilibrium between its production: N 2 ( A 3 ) + N( 4 S) N( 2 P) + N 2 ( X 1 , v ) and loss: N 2 ( X 1 , v ≥ 10)+ N( 2 P) N 2 ( A 3 ) + N( 4 S) reactions, which strongly couple the atomic and molecular metastable states and hence recycle N 2 ( A 3 ) metastable molecules produced in the SLA. The balance equation of N( 2 P°) density provides a N 2 ( X 1 ; v ≥ 10) density of 6.5 × 10 20 m −3 at the maximum of the SLA. This corresponds to 1% of the total N 2 molecules in vibrationally excited levels v ≥ 10.