A microwave plasma source for VUV atmospheric photochemistry

TL;DR

This paper demonstrates that a microwave neon plasma source can efficiently generate VUV photons for planetary atmospheric photochemistry, enabling studies of nitrogen-rich atmospheres like Titan without other energy interactions.

Contribution

It introduces a windowless VUV photon source using microwave plasma discharges, specifically designed for planetary atmospheric photochemistry experiments, with experimental flux estimation and application to Titan's atmosphere.

Findings

VUV photon flux ranges from 2x10^13 to 4x10^14 photons/sec/cm^2.

The source effectively produces Titan-relevant compounds like HCN and C2N2.

Neon resonance lines at 73.6 and 74.3 nm behave differently under various conditions.

Abstract

Microwave plasma discharges working at low pressure are nowadays a well-developed technique mainly used to provide radiations at different wavelengths. The aim of this work is to show that those discharges are an efficient windowless VUV photon source for planetary atmospheric photochemistry experiments. To do this, we use a surfatron-type discharge with a neon gas flow in the mbar pressure range coupled to a photochemical reactor. Working in the VUV range allows to focus on nitrogen-dominated atmospheres ({\lambda}<100nm). The experimental setup makes sure that no other energy sources (electrons, metastable atoms) than the VUV photons interact with the reactive medium. Neon owns two resonance lines at 73.6 and 74.3 nm which behave differently regarding the pressure or power conditions. In parallel, the VUV photon flux emitted at 73.6 nm has been experimentally estimated in different conditions of pressure and power and varies in a large range between 2x1013 ph.s-1.cm-2 and 4x1014 ph.s-1.cm-2 which is comparable to a VUV synchrotron photon flux. Our first case study is the atmosphere of Titan and its N2-CH4 atmosphere. With this VUV source, the production of HCN and C2N2, two major Titan compounds, is detected, ensuring the suitability of the source for atmospheric photochemistry experiments.