Linewidth studies on the NI( 4S− 4P) resonance multiplet

Abstract

We have measured the linewidths of the NI multiplets [2p 2 3p 4D 0−2p 23s 4P, λ8691 A ̊ ; 2p 2 3p 4P 0 −2p 23s 4P, λ8212 A ̊ ; 2p 2 3s 4P−2p 3 4S 0, λ1200 A ̊ ] produced in the dissociative excitation of N 2 by energy electrons. The infrared transitions excite the N( 4P) resonance state by cascade and they account for > 50% of the total N( 4P) cross section at 100 eV. Both the i.r. and v.u.v. lines are found to be highly Doppler broadened ( ∼ 25 times the thermal Doppler line width). These results indicate that dissociative excitation of N 2 produces N ( 4P) atoms with sufficient kinetic energy so that the λ1200 A ̊ resonance radiation [2p 2 3s 4P −2p 3 4S 0] emitted by these excited atoms would be optically thin in the Earth's upper atmosphere. We also found that the line strength ratios for the resolved components of the λ1200 A ̊ triplet excited by dissociative excitation differ from those predicted by the multiplicities of the states involved and used in current entrapment models; the intensity ratios also vary with the energy of the incident electron. These developments introduce new complications into the analysis of the terrestrial ultraviolet dayglow.