Absolute O3 and OH densities - measurement by two-beam UV-LED absorption spectroscopy in atmospheric pressure plasmas - thesis

Sep 4, 2026

Low temperature atmospheric pressure plasmas (APPs) provide a reactive particle environment and complex chemical-physical interactions near room conditions, enabling both fundamental studies and biomedical applications. This thesis focuses on ozone (O3) and hydroxyl (OH) diagnostics, where accurate density measurements are challenging due to sensitivities to non-plasma parameters and small plasma volumes. A two-beam Mach-Zehnder UV-LED absorption spectroscopy method was developed for O3 density in He-O2 capacitively coupled APPs and applied to biological samples, correlating spatial O3 distributions with bacterial inactivation. Duty-cycle and modulation effects were quantified, anticorrelations with gas temperature were analyzed through plasma chemistry models, OH densities were measured under H2O admixture, and O3 density contributions to CO2-CO conversion processes were examined.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages162

Reading Time6 h 48 min

AccessFree to download

Abstract

Introduction

Motivation

Thesis outline

Background

Kinetic theory and basic equation of nonthermal LTP

Elementary processes in gas phase

Modeling of non equilibrium low-temperature plasma

Atmospheric pressure plasmas

Diagnostic techniques

Two-beam UV-LED Absorption Spectroscopy

O3 density by UV-LED absorption spectroscopy

UV-LED Absorption setup and optical alignment

Data acquisition

Two-beam technique

Spatially resolved O3 density measurement

O3 Density Measurements

The APP source

O3 density and gas temperature

Model of gas temperature and O3 density

O3 density in pulse-modulated RF atmospheric pressure plasmas

Experimental setup

O3 density with kHz-frequency modulation of 13.56 MHz RF APP

O3 density with duty cycle modulation of 13.56 MHz RF APP

Spatial O3 density with duty cycle

Spatial O3 density in the effluent region of a co-axial DBD kHz-driven APPJ

Absorption principle and Abel inversion

Co-axial DBD kHz-driven APPJ and operational control system

Experimental setup

Surface O3 density distribution with correlation to the bacteria killing zone

Absolute O3 and OH densities measurement by two-beam UV-LED absorption spectroscopy in atmospheric pressure plasmas Apiwat Wijaikhum PhD University of York Physics September 2016 I would like to dedicate this thesis to my beloved family. Abstract Low temperature atmospheric pressure plasmas (APPs) create rich environment of reactive particle species and chemical-physical interactions at close-to-room temperature and ambient pressure which calls for a wide range of fundamental and application studies. APPs for biomedical applications is one of the emerging interdisciplinary researches. Its fundamental mechanisms have been studied using different numerical models and various diagnostic techniques. With hundreds of particle species and complex reactions, each species requires unique measurement techniques. In a typical APP, ozone (O 3), one of the key species in living-cells inactivation, is produced from the complex reaction chain of short-lived oxygen atoms and excited molecules. Measurement and theoretical predictions of O 3 densities can have high uncertainties. The measurements of O3 densities inside the small plasma volume are challenging due to the sensitivity to non-plasma parameters. In this work, two-beam UV-LED absorption spectroscopy has been developed by using a Mach-Zehnder configuration for O3 density measurements on the core of a homogeneous, He-O2 capacitively coupled, 13.56 MHz RF-driven APP. The improved technique allows for high-sensitivity measurement in the order of 10−3 absorption signal with 10−4 of uncertainty. The anticorrelation between O3 density and gas temperature was observed and described based on the plasma chemistry models. For controlling-parameter effect, the duty-cycle in frequency modulations showed a significant influence on the spatial profile of O 3 density in the plasma channel. From an application perspective, the developed technique was able to provide 2DO3 density distribution in the effluent region of a co-axial DBD kHz-driven APPJ when applied to biological samples. The correlation between radial O3 density profiles and bacterial inactivation areas was investigated. In the relatively realistic condition with higher H 2O vapour admixture, hydroxyl (OH) density, which is one of important radical species, can be measured using the UV absorption technique. Thus, the setup has been adjusted in order to measure both species. Furthermore, O 3 density in the CO 2-CO conversion 40.68 MHz RF-driven APP, an important process in chemical research, was observed. The O3 density as a function of plasma power and CO 2 concentration provided a significant contribution to the main production and destruction channels of the conversion processes. Table of Contents Abstract v Table of Contents vii List of Tables xi List of Figures xiii Acknowledgements xix Declaration xxii 1 Introduction 1 1.1 Motivation ................................... 1 1.2 Thesis outline ................................. 4 2 Background 7 2.1 Kinetic theory and basic equation of nonthermal LTP ............ 8 2.1.1 Boltzmann equation and distribution function ............ 9 2.1.2 Governing equation .......................... 10 2.2 Elementary processes in gas phase ...................... 12 2.2.1 Electron-atom collisions ....................... 12 2.2.2 Electron-molecule collisions ..................... 13 2.2.3 Heavy-particle collisions ....................... 16 2.3 Modeling of non equilibrium low-temperature plasma ............ 17 2.3.1 Continuum models .......................... 18 2.4 Atmospheric pressure plasmas ........................ 19 2.4.1 Fundamentals ............................. 19 2.4.2 Atmospheric pressure plasma sources for biomedical applications . 21 viii TABLE OF CONTENTS 2.4.3 Radio-frequency driven capacitively-coupled APPJ ......... 25 2.4.4 Dielectric barrier discharged (DBD) kHz-frequency driven APPJ .. 28 2.5 Diagnostic techniques ...

FAQs

What measurement technique is developed in the thesis for ozone density?

A two-beam UV-LED absorption spectroscopy method using a Mach-Zehnder configuration is developed to measure O3 density in atmospheric pressure plasmas.

Why are O3 density measurements considered challenging in APPs?

Uncertainties can be high because measurements are sensitive to non-plasma parameters and the small plasma volume makes accurate diagnostics difficult.

How are ozone and hydroxyl densities used in biological and chemical applications?

The method enables spatial O3 mapping in effluent regions and links radial O3 profiles to bacterial inactivation. With higher H2O vapour admixture, hydroxyl (OH) densities are measured, and O3 density variations are also used to analyze CO2-CO conversion channels.

Sep 4, 2026

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