Decoupling NO production and UV emission intensity over the E-H mode transition in a low-pressure inductively coupled plasma device

TL;DR

This study investigates how UV emission and NO production are decoupled during the E-H mode transition in a low-pressure inductively coupled plasma, revealing increased excitation despite decreased ground state NO density.

Contribution

It introduces a combined experimental and modeling approach to analyze NO excitation and UV emission during plasma mode transitions, highlighting the role of nitrogen metastables.

Findings

UV emission intensity increases with power despite lower ground state NO density in H-mode.

Excitation of NO(A) by nitrogen metastables is enhanced in H-mode, compensating for reduced NO density.

Ground state nitric oxide density decreases significantly during the E-H transition.

Abstract

A low-pressure double-inductively coupled plasma device is used to study the fundamental plasma parameters, plasma chemistry, and UV photon emission from the first excited state of nitric oxide, NO(A), in gas mixtures of nitrogen and oxygen. In addition to the gas mixture, rf power and gas pressure are varied, and the E-H mode transition of the inductively coupled plasma is studied specifically. The gas temperature and UV photon emission are measured by optical emission spectroscopy (OES), the absolute density of the nitric oxide electronic ground state by laser-induced fluorescence (LIF), as well as electron density and electron temperature by a multipole resonance probe (MRP). A simple collisional-radiative model for UV emission from NO(A) is developed, which takes the measured densities of ground state nitric oxide and electrons, as well as the electron temperature and neutral gas temperature, as input parameters. The results reveal the links between the absolute densities of ground state nitric oxide, the excitation of this species driven by electron impact and collisions with nitrogen metastables, quenching of the nitrogen metastables, and the resulting UV photon emission rate. The density of ground state nitric oxide is shown to increase with power, while the discharge remains in E-mode, and to decrease significantly with the transition into H-mode, when sufficient rf power is deposited in the discharge. Despite the lower densities of ground state nitric oxide in H-mode, the UV photon emission intensity increases continuously with higher rf powers and over the E-H transition. This effect is shown to be caused by increased excitation of NO(A) by nitrogen metastables in H-mode, which is sufficient to overcompensate the decrease in ground state nitric oxide density.