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

¹ Department of Physics, Iran University of Science & Technology, Narmak, Tehran, 16846-13114, Iran
² Laboratoire de Spectrométrie Physique, Université Joseph Fourier de Grenoble & CNRS, 38402 St Martin d'Hères, France

Corresponding author: Nader.Sadeghi@ujf-grenoble.fr

Received: 4 February 2008
Revised: 11 March 2008
Accepted: 28 March 2008
Published online: 30 May 2008

Abstract

A novel method, based on emission signal from the excitation transfer reaction: Ar*(³P₂) + N₂(X) N₂*(C³) + Ar, is proposed for the intensity calibration of the spectroscopic optical detection system in absolute scale. It is applied for the measurement of N(²P°) 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(²P°–⁴S°) line at 346.65 nm, whose transition probability is only 0.005 s⁻¹. The N(²P°) density variation in the SLA resembles those of N₂(A³ ) metastable molecules and electrons or the emission intensities of first positive (1⁺), second positive (2⁺) and first negative (1⁻) systems of N₂. 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 × 10¹⁷ m⁻³ at the maximum of the SLA. It is proposed that N(²P°) density results from a local equilibrium between its production: N₂(A³) + N(⁴S) N(²P) + N₂(X¹, v) and loss: N₂(X¹, v ≥ 10)+ N(²P) N₂ (A³) + N(⁴S) reactions, which strongly couple the atomic and molecular metastable states and hence recycle N₂(A³) metastable molecules produced in the SLA. The balance equation of N(²P°) density provides a N₂(X¹; v ≥ 10) density of 6.5 × 10²⁰ m⁻³ at the maximum of the SLA. This corresponds to 1% of the total N₂ molecules in vibrationally excited levels v ≥ 10.