WIDE RANGE ANALYSIS OF OZONE GAS CONCENTRATION IN...

WIDE RANGE ANALYSIS OF OZONE GAS CONCENTRATION IN

ULTRAVIOLET REGION

MASLINA BINTI YAACOB

UNIVERSITI TEKNOLOGI MALAYSIA

WIDE RANGE ANALYSIS OF OZONE GAS CONCENTRATION IN

ULTRAVIOLET REGION

MASLINA BINTI YAACOB

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Doctor of Philosophy (Electrical Engineering)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

JUNE 2016

iii

Specially dedicated to my beloved parents, Yaacob and Hamidah; my husband,

Ahmad Muhaimin and my son, Abdullah Muhammad for their prayers, patience and

support

iv

ACKNOWLEDGEMENT

First and foremost, the deepest gratitude of all shall be bestowed to Allah The

Almighty and The Merciful for all the insight which He gave to us that lead to the

completion of this project.

I would like to express my sincere gratitude and appreciation to my main

supervisor, Assoc. Prof. Dr. Mohd Haniff Ibrahim for his knowledge, kindness and

patience throughout this project. I would also like to express my thanks to my co-

supervisor, Dr. Nor Hafizah Ngajikin for her advice, time and motivation throughout

this project.

I would like to acknowledge Professor Dr. Elfed Lewis and Dr. Gerard Dooly

for their assistance during my attachment and use of their facilities at Optical Fibre

Sensor Research Centre (OFSRC), University of Limerick, Ireland. I also indebted to

Universiti Tun Hussein Onn Malaysia (UTHM) for funding my Ph.D. study.

Also, special words of thanks to all Lightwave Communication Research

Group (LCRG) members especially Dr. Tay Ching En Marcus, Dr. Mohd Rashidi

Salim, Dr. Michael David and Nabihah Hussin for the help and support during of the

period this project. Last but not least, an expression and gratitude to all individual

who involved directly or indirectly in making this project.

v

ABSTRACT

The purpose of this research is to investigate the development of optical gas

sensor employing absorption spectroscopy technique for ozone concentration

measurement. Additionally, wide range analysis has been conducted to improve

range of ozone concentration measurement using ultraviolet light absorption.

Simulation of ozone absorption cross section in ultraviolet region was conducted via

Spectralcalc.com® simulator. Simulation result for ozone absorption cross section

was then verified by comparison with result from previous studies, showing small

percentage of difference less than 3.05 %. In addition, the simulator was also used to

investigate the effect of pressure and temperature on ozone absorption cross section.

Simulation result showed ozone absorption cross section to exhibit negligible effect

of pressure and temperature from 0.1 atm until 2.0 atm and from 293 K until 305 K,

respectively. Next, path length of gas cell that suits with detection range of ozone

monitor was determined through Spectralcalc.com® simulator. Finally, transmissive

type gas cell is fabricated at optimum length of 10 cm. Based on the experiment

results wide range analysis was conducted at 10 cm gas cell by consideration of less

relative error of concentration. It was observed that wavelengths at 232 nm, 233 nm,

234 nm, 235 nm, 236 nm, 284 nm, 285 nm, 286 nm and 287 nm exhibit capability to

measure ozone concentration using ultraviolet light absorption at high concentration

value with wide range of concentration measurement from 619 ppm until 932 ppm.

Of this, 285 nm was chosen due to its high resolution value at 17 ppm. The sensor

exhibits fast response time and recovery time, both at 20 s. Peak of ozone absorption

cross sections were observed in both experiment and simulation, located at 260.45

nm and 255.44 nm, respectively. Meanwhile, the values of peak of ozone absorption

cross section were observed in experiment and simulation at 164.37 × 10-23

m2

molecule-1

and 114.86 × 10-23

m2

molecule-1

, respectively. Significantly, this research

has successfully demonstrated possibility of conducting wide-range analysis

employing consideration of less relative error concentration. Particularly, vast

improvement range of ozone concentration measurement has been achieved by

wavelength selection which is far from the peak of ozone absorption cross section.

vi

ABSTRAK

Tujuan penyelidikan ini adalah untuk mengkaji pembangunan penderia gas

optik berdasarkan teknik spektroskopi penyerapan untuk mengukur kepekatan ozon.

Selanjutnya analisis perluasan julat telah dijalankan bagi memperbaiki julat

pengukuran kepekatan ozon menggunakan penyerapan cahaya ultraungu. Simulasi

keratan rentas penyerapan ozon di rantau ultraungu dijalankan melalui penyelaku

Spectralcalc.com®. Keputusan simulasi bagi keratan rentas penyerapan ozon

kemudiannya disahkan melaui perbandingan dengan kajian terdahulu. Keputusan

menunjukkan peratusan perbezaan yang kecil, kurang daripada 3.05%. Penyelaku ini

juga digunakan untuk mengkaji kesan tekanan dan suhu terhadap keratan rentas

penyerapan ozon. Keputusan simulasi menunjukkan kesan tekanan dan suhu

terhadap keratan rentas penyerapan ozon boleh diabaikan, masing-masing dari 0.1

atm sehingga 2.0 atm dan dari 293 K hingga 305 K. Seterusnya, panjang sel gas yang

dapat mengukur julat kepekatan ozon sepadan dengan alat pengukur ozon ditentukan

melalui penyelaku Spectralcalc.com®. Sel gas jenis transmissive difabrikasi pada

panjang optimum 10 cm. Berdasarkan keputusan eksperimen, analisis perluasan julat

dijalankan pada sel gas 10 cm dengan pertimbangan nilai ralat relatif kepekatan yang

kecil. Didapati bahawa panjang gelombang 232 nm, 233 nm, 234 nm, 235 nm, 236

nm, 284 nm, 285 nm, 286 nm dan 287 nm menunjukkan keupayaan mengukur

kepekatan ozon menggunakan penyerapan cahaya ultraungu pada nilai kepekatan

Panjang gelombang 285 nm dipilih kerana nilai resolusinya yang tinggi pada 17 ppm.

Penderia ini mempamerkan masa tindak balas dan masa pemulihan yang pantas,

masing-masing pada 20 s. Puncak keratan rentas penyerapan ozon diperhatikan

daripada keputusan eksperimen dan simulasi masing-masing terletak pada 260.45 nm

dan 255.44 nm. Manakala, nilai puncak keratan rentas penyerapan ozon daripada

keputusan eksperimen dan simulasi masing-masing adalah 164.37 × 10-23

m2molekul

-

1 dan 114.86 × 10

-23 m

2molekul

-1. Kajian ini telah berjaya menunjukkan

kemungkinan menjalankan analisis perluasan julat berdasarkan pertimbangan nilai

kepekatan ralat relatif yang kecil. Julat pengukuran kepekatan ozon berdasarkan

penyerapan cahaya ultraungu telah ditambahbaik dengan pemilihan panjang

gelombang yang terletak jauh dari puncak keratan rentas penyerapan ozon.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE OF RESEARCH

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xii

LIST OF ABBREVIATIONS xv

LIST OF SYMBOLS xvii

LIST OF APPENDICES xix

1 INTRODUCTION 1

1.1 Background of Research 1

1.2 A Review on Ozone Sensor 3

1.3 Problem Statement 12

1.4 Significant of the Research 13

1.5 Objective of the Research 14

1.6 Scope and Limitation of the Research 14

1.7 Overview of the Thesis 15

2 LITERATURE REVIEW 17

2.1 Introduction 17

2.2 The Nature of Light 17

2.3 Spectrum of Electromagnetic and Type of

Spectroscopy 20

viii

2.4 Ultraviolet Absorption Process: The Nature of

Electronic Excitation 22

2.5 Derivation of Beer's Law 25

2.5.1 Beer's Law (Gas Phase) 29

2.5.2 Limitation of Beer's Law 32

2.6 Consideration of Less Relative Error of

Concentration in Absorption Measurement 33

2.7 Instrumentation 35

2.7.1 Ultraviolet Source 40

2.7.2 Ultraviolet Detector 41

2.7.3 Ultraviolet Lens 42

2.7.4 Gas Cell Material 43

2.8 A Review on Optical Sensor Based on Absorption

Spectroscopy for Ozone Measurement 44

2.9 Parameter of Performence 50

2.10 Summary 51

3 RESEARCH METHODOLOGY 52

3.1 Introduction 52

3.2 Frame Work of Research 52

3.3 Summary 57

4 SIMULATION OF OZONE ABSORPTION

WAVELENGTH 58

4.1 Introduction 58

4.2 Simulation of Ozone Absorption Wavelength 59

4.3 Analysis of Pressure Effect 64

4.4 Analysis of Temperature Effect 67

4.5 Wide Range Analysis by Different Path Length and

Optimization of Gas Cell 69

4.6 Wide Range Analysis by Different Wavelength 74

4.7 Summary 76

ix

5 DEVELOPMENT OF ABSORPTION SPECTROSCOPIC

OZONE GAS SENSOR 78

5.1 Introduction 78

5.2 Fabrication of Gas Cell 78

5.3 Installation of Ozone Sensing System 79

5.4 Characterization of Absorption Spectroscopic

Ozone Gas Sensor 83

5.5 Resolution Analysis 93

5.6 Determination of Ozone Absorption Cross Section 95

5.7 Wide Range Analysis Based on Less Relative Error

of Concentration 98

5.8 Summary 104

6 CONCLUSIONS, CONTRIBUTIONS AND FUTURE

WORK 105

6.1 Conclusions 105

6.2 Contributions 108

6.3 Future work 109

REFERENCES 110

Appendices A- E 118 - 124

x

LIST OF TABLES

TABLE NO. TITLE PAGE

1.1 Summary on type of ozone gas sensor and its response time

and range of ozone concentration measurement. 7

2.1 Ultraviolet subdivision in electromagnetic spectrum

(Manap, 2011). 21

2.2 Type of spectroscopy in each region of electromagnetic

spectrum (Banwell and McCash, 1994). 22

2.3 Summary of commercially available broadband sources

from Ocean Optic (2016). 40

2.4 Summary on the performance comparison of optical sensor

based on absorption spectroscopy. 47

2.5 List of parameter used to characterize sensor performance

(Bochenkov and Sergeev, 2010). 50

4.1 Determination of ozone absorption cross section by

Daumont et al., (1992) at specific experiment condition. 59

4.2 List of inputs to be set in Observer tab and Gas Cell tab to

simulate ozone absorption cross section in ultraviolet

region. 61

4.3 Comparison of ozone absorption cross section, (σ) between

previous experiment and this simulation work (in unit of

10-23

m2moleculce

-1). 62

4.4 Percentage of difference of ozone absorption cross section

between previous experiment and this simulation work (in

unit of %). 63

5.1 List of equipment and its specifications used in light

transmission and detection, gas circulation system and data

acquisition. 81

xi

5.2 Characteristics of sensing system using 10 cm gas cell. 93

5.3 List of parameter and calculated ozone absorption cross

section based on experiment results of 10 cm gas cell. 96

xii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Propagation of an electromagnetic wave in free space. 18

2.2 The region of electromagnetic spectrum. 21

2.3 Electronic transitions with vibrational transitions

superimposed. (Rotational levels, which are very closely

spaced within the vabrational levels, are omitted for clarity)

(Lampman et al., 2010). 23

2.4 Absorption cross-section of ozone spectrum at 293K for

complete coverage of 231 nm until 794 nm range (Burrows

et al., 1999). 24

2.5 Experimental setup for Beer's Law derivation by Smith

(2002). 25

2.6 Twyman-Lothian curve based on absolute transmittance

error 0.001 (Marcus et al., 2014). 34

2.7 A single beam instrument. Radiation from filter or

monochromator passes through either the reference cell or

sample cell before striking the photodetector (Skoog et al.,

2007). 36

2.8 A double beam instrument. The beam is alternately sent

through reference and sample cell before striking a single

photodector. (Skoog et al., 2007). 37

2.9 Multichannel spectrometer with an array detector based on a

grating spectrograph (Skoog et al., 2007). (Note that λ1 > λ2

> λ3). 38

2.10 A multichannel miniature fiber optic spectrometer. The

fiber optic cable transports the light beam from the cell

xiii

holder on the left to spectrograph and detector on right

(Skoog et al., 2007). 39

2.11 The internal components of the HR-4000 spectrometer

(Ocean Optics, 2008). 42

3.1 Flow chart of research methodology. 56

4.1 Setting input parameter in Spectralcalc.com® gas cell

simulator at (a) Observer tab and (b) Gas Cell tab. 60

4.2 Transmittance output for 950.2907 ppm of ozone using

Spectralcalc.com® gas cell simulator. 62

4.3 Output transmittance from Spectralcalc.com® gas cell

simulator due to 950.29 ppm of ozone at various pressure. 64

4.4 Analysis of pressure effect toward ozone absorption cross

section at a) 253.65 nm b) 289.36 nm and c) 296.73 nm with

output transmittance from Spectralcalc.com® gas cell

simulator. 66

4.5 Analysis of temperature effect toward ozone absorption

cross section at a) 253.65 nm b) 289.36 nm and c) 296.73

nm with output transmittance from Spectralcalc.com® gas

cell simulator. 68

4.6 Calculated ozone concentration for 3.285 cm, 6.285 cm and

9.285 cm gas cell. 70

4.7 Transmittance output from Spectralcalc.com® using 3.285

cm gas cell at various ozone concentration. 71

4.8 Calculated and simulated ozone concentration for 3.285 cm,

6.285 cm and 9.285 cm gas cell. 72

4.9 Percentage of difference in transmittance between

theoretical calculation and simulation for 3.285 cm, 6.285

cm and 9.285 cm gas cell. 73

4.10 Calculated ozone concentration at vary wavelength using

3.285 cm gas cell. 76

5.1 Optical fiber ozone sensor based on absorption

spectroscopy. 79

5.2 Experimental set up for ozone sensing system based on

absorption spectroscopy technique. 81

xiv

5.3 Counts of intensity when 10 cm gas cell is filled with 932

ppm ozone and without ozone. 84

5.4 Measured transmittance as 932 ppm of ozone is flowed into

10 cm gas cell. 85

5.5 Measured transmittance when ozone generator is switched

off. 86

5.6 Transmittance measurements for four different ozone

concentrations range from 619 ppm until 932 ppm using 10

cm gas cell. (Refer to Appendix C for all transmittance

value of fifteen different ozone concentration) 87

5.7 Ozone concentration reading from ozone monitor and

measured transmittance from sensing system at 254 nm

using 10 cm gas cell. 88

5.8 Zoom in x-axis based on transmittance value in Figure 4.7

for determination of (a) response time and (b) recovery time

starting at the 90th

second and 1050th

second, respectively 89

5.9 Temperature and ozone flow rate measurement at various

ozone concentrations using 10 cm gas cell. 91

5.10 Transmittance measurement for fifteen steps of ozone

concentration at various wavelength using 10 cm gas cell. 94

5.11 Calculated ozone absorption cross section from

experimental and simulation result based on 10 cm gas cell. 97

5.12 Transmission value at specific ozone concentration for (a)

first and (b) second group of wavelength used in wide range

analysis based on 10 cm gas cell. 100

5.13 Determination of total range of ozone concentration by

consideration of transmittance value between 0.25 until 0.5

for experiment and simulation result at various wavelengths

using 10 cm gas cell. 102

xv

LIST OF ABBREVIATIONS

Ag2WO4 - Silver Tungstate

AOT40 - Accumulated exposure of ozone concentration over a

threshold of 40 ppbv

Au/TiO2-WO3 - Gold//Titanium dioxide-Tungsten Trioxide

CCD - Charge Coupled Devices

CuAlO2 - Copper Aluminium Oxide

e.s.r - Electron spin resonance

FUV - Far ultraviolet

HC - Hydrocarbon

HITRAN - High resolution transmission

In2O3 - Indium oxide

KI - Potassium iodide

LED - Light Emitting Diodes

MUV - Middle ultraviolet

MV - Medium voltage

n.m.r - Nuclear magnetic resonance

NASA - National Aeronautics and Space Administration

NO2 - Nitrogen Dioxide

NOx - Oxide of Nitrogen

NUV - Near ultraviolet

O2 - Oxygen

O3 - Ozone

PAN - Peroxyacetyl nitrates

PDA - Photodiode array

PEO/RbI/I2 - Polyethylene oxide/RubidiumIodide/Iodine

PMMA - Polymethylmethacrylate

ppb - Part per billion

xvi

ppm - Part per million

Pt/TiO2-WO3 - Platinum/Titanium dioxide-Tungsten Trioxide

PTFE - Polytetrafluoroethylene

SIG - Southern Industrial Gas

SnO2 - Tin dioxide

UV - Ultraviolet

VOC - Volatile organic compounds

VUV - Vacuum ultraviolet

WO3 - Tungsten oxide

ZnO - Zinc Oxide

xvii

LIST OF SYMBOLS

A - Absorbance

c (ppm) - Concentration of gas in ppm

c - Speed of light, 3 x 108 m s

-1

c(molm-3) - Concentration in mol m-3

dI - Amount of light absorbed in dl

dl - Thickness of an infinitesimally thin slab of sample

E - Photon energy in Joules (J)

f - Frequency (s-1

or Hertz)

h - Planck's constant, 6.63 x 10-34 J s

I - Intensity after the light passed through the sample

Io - Intensities before the light passed through the sample

V

n - Concentration

0I

I - Transmittance, (Tr)

l - Path length in m

n - Amount of substances in mol

NA - Avogadro number, 6.022 x 1023

molecule mol-1

P - Pressure of gas in atm

R - Gas constant, 8.205746 x 10-5

atm m3 mol

-1 K

-1

T - Temperature in K

Tr - Transmittance

V - Volume of gas in m3

w - Wavenumber (cm-1

)

Δc /c - Relative error of concentration (%)

ΔTr - Absolute error of transmittance

ε - Absorptivity in m2 mol

-1

xviii

λ - Wavelength

ρ - Ozone density, 2.144 kg m-3

σ - Absorption cross section of sample in m2 molecule

-1

σs - Absorption cross section obtained in simulation work

ω - Molar mass of ozone, 48 ×10-3

kg mol-1

xix

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Ozone Chemical Sampling Information and

Exposure Limit

118

B Schematic Used in Ocean View to Capture

Transmittance

119

C Transmittance Value for Fifteen Different Ozone

Concentration

120

D Ozone Output Test Report (for Model EXT50) 121

E List of Publication 122

CHAPTER 1

INTRODUCTION

1.1 Background of Research

In 1785, ozone was discovered by Dutch scientist Martinus van Marum from

observation on air exposed to electrical spark had characteristic smell and

demonstrated redox properties. Later, the Swiss researcher Christian Schonbein in

1840 explained this phenomenon as formation of a special gas. In 1865, J.L. Soret

named it ozone when this new gas was clearly identified as molecule containing

three atom of oxygen. Ozone was named based on the Greek word "ozo'" which

meant "smell".

Ozone is a gas of dark blue colour at normal temperature and pressure and

dark-blue coloured liquid at temperature below -111.5 °C (Roshchina and Roshchina,

2003). It has pungent smell that can be detected by nose as sign of toxic gas.

Chemical symbol for ozone is O3 as formation of three atom of oxygen. Ozone has

characteristic powerful oxidising property that is suitable for sterilization and

bleaching. However, ozone is unstable and quickly reverts to oxygen.

Ozone can be produced by human or natural activity; therefore, ozone can be

originated from many source. Naturally, thunder storm, ultraviolet irradiation of air

and forest excretions are the main contributors to ozone generation (Roshchina and

Roshchina, 2003). Meanwhile, increase in ozone level cause by human activity is due

to waste from manufacturing industry (Leman et al., 2010), indirect ozone discharge

2

from automobile exhaust (Park et al., 2009) and any technology that produces

ultraviolet irradiation such as photocopier, laser printer (Arshak et al., 2007), and

computer. Significant case study by Leman et al., (2010) show ozone is one of the

gas produced from welding process that is common in manufacturing industry. The

case study was done in two car component manufacturer located in Shah Alam,

Malaysia indicate ozone level varying from 0.001 until 0.13 ppm and 0.003 until

0.45 ppm for company A and company B, respectively.

Ozone is one of the gases that are naturally present in the atmosphere. It has

both beneficial and damaging effects. At stratospheric layer, ozone plays significant

role for life on Earth as it filters harmful ultraviolet radiation emitted by the Sun.

However, when ozone is photochemically produced in the troposphere (at low

altitudes and ground level), it can be toxic and results in significant physiological and

ecological damage. Ozone may cause different kinds of serious health consequences

such as irritation to the eyes, pulmonary oedema, and air passages, causing numerous

respiratory problems (Arshak et al,. 2007), lung diseases and lung damage. The

exposure limits for ozone gas can be found in Appendix A. In addition, study of the

effect of ozone pollution associated with AOT40 (accumulated exposure of ozone

concentration over a threshold of 40 ppbv) index is done in area of Ahmednagar,

India. Measurement of ozone from January 2006 until December 2007 clearly

revealed that winter wheat and summer crop yield reduction by 10% and 15%,

respectively (Debaje et al., 2010).

Chemical reaction between oxide of nitrogen (NOx), which is emitted from

various sources, including motor vehicles and other industrial sources, and

hydrocarbon (HC) in presence of sunlight and oxygen (O2) forms ground level ozone

(O3), and volatile organic compounds (VOC’s) such as aldehydes and peroxyacetyl

nitrates (PAN) (Dooly, 2008). These secondary pollutants form a noxious mixture of

air known as photochemical smog have been identified as one of the primary

pollutants that degrade air quality. Ground-level ozone forms easily in the

atmosphere, normally in the warm urban atmosphere. There is study investigates the

secondary pollutants resulting from household product use in presence of ozone. The

use of trepenoid-cointaning cleaning products or air freshner combined with indoor

3

ozone produces substantial levels of secondary air pollutants specifically

formaldehyde and fine particulate mass (Singer et al., 2006).

Nevertheless, ozone also has the benefit due to its powerful oxidizing ability.

Ozone is effective to eliminate colour, taste and odour. In addition, ozone destroys

bacteria and viruses faster than other disinfection chemical. Hence, ozone is widely

used for sanitization in drinking water treatment, wastewater treatment, odour control

and air treatment (Buntat, 2010). Recently, the application of ozone is increasing in

many fields including fruit juice preservation (Cullen et al., 2010), food packaging

(Naitou and Takahara, 2008), fabrication of semiconductor wafer, fish plant

(Nakagawa et al., 2001) and alternative insecticides for the control of insects and

micro-organisms in stored products (Isikber and Athanassiou, 2015).

1.2 A Review on Ozone Sensor

With increasing use of ozone, there is a large demand to develop gas sensor

for ozone monitoring system and detection. In the past couple of years, there are

several type of gas sensor have been developed to determine ozone concentration

including electrochemical, metal oxide semiconductor, optical sensor based on

absorption in ultraviolet/visible region, carbon nanotube and reaction between ozone

and dye or Potassium Iodide (KI). Table 1.1 summarized type of ozone gas sensor

and its performance in term of range of ozone concentration measurement and

response time.

Ozone gas sensor that is based on reaction between ozone and indigo carmine

or azo dye orange I have advantages in term of simplicity, light, portable and passive

device (Maruo, 2007 and Maruo et al., 2010). Results of this reaction is colour-

fading of indigo carmine or azo dye orange I due to presence of ozone. However,

long response time is needed by this type of sensor for colour-fading reaction. In

addition, carbon nanotube based sensor also exhibits the limitation of long response

time (Park et al., 2009).

4

Work by Stergiou et al., (2010) demonstrates a proof-of concept for pH-

metric determination of ozone in an unbuffered potassium iodide (KI) solution. The

pH increase due to the reaction of ozone with an unbuffered KI solution, during

which hydroxyl ions are produced. The limitation of this method is sensitive to

presence of traces of acidic or basic gases in air sample (HCI and NH3). Thus, limits

the range of possible analytical applications.

Electrochemical based gas sensor operate by reacting with ozone and

producing an electrical signal proportional to the ozone concentration. A typical

electrochemical sensor consist of a sensing electrode (or working electrode or

anode), and a counter electrode (or cathode) separated by thin layer of electrolyte.

Because a current is generated in the process, the electrochemical sensor is also

described as an amperometric gas sensor or a micro fuel cell. Work by Stergiou et

al., (2009) demonstrate redox polymer electrolytes (Polyethylene oxide (PEO)/Rb/I2

redox polymer) can be used for development of easy-constructed, cost-effective and

ready-to-use sensors. However, sensor response to ozone has significant flow

dependence typical of electrochemical ozone measurement (Ebeling et al., 2009).

Besides, the life expectancy of the electrochemical sensor is limited as it highly

dependent on environmental contaminants, temperature and humidity to which the

sensor is exposed.

Recently, metal oxide semiconductor based gas sensor have received much

attention in ozone concentration measurement because of their simple structure,

ready modification, ability to detect various gases at low concentration, easy

implementation, small size and light weight. The sensor operates based on the fact

that adsorption or desorption of gas molecule on the metal oxide surface leads to

change in electric resistance (Belaqziz et al., 2014). This change in resistance is

measured electrically and is proportional to the concentration of ozone being

measured. Since ozone is an oxidizing gases, n-type semiconductor where the

majority charge carriers are electrons and upon interaction with ozone resulting in an

increase of electrical resistance. Conversely, a p-type semiconductor with positive

holes being the majority charge carriers showing a decrease of electrical resistance in

the presence of ozone (Fine et al., 2010). Various type of metal oxide semiconductor,

5

such as SnO2, In2O3, ZnO, WO3, Fe2O3 (Debliquy et al., 2011), NiO (Demin et al.,

2008), SmFeO3 (Mori et al., 2012) and In-Ga-Zn-O (Chen et al., 2015) have been

widely used for ozone concentration measurement.

One of the main problems associated with metal oxide semiconductor sensors

is that they require operation at high temperature (Ollitrault et al., 2015; Rocha et al.,

2016), leading to high energy consumption. Work by Chen et al., (2014) demonstrate

Pt/TiO2-WO3 thick film sensor exhibits a relatively high sensitive to 2.5 ppm of

ozone gas at room temperature when irradiated using 460 nm light source. However,

a drawback of this sensor is long response time. The sensor based on SnO2 thin film

for efficient detection of ozone at room temperature without activation using UV

radiation or catalyst has been demonstrated by Belaqziz et al., (2014). Disadvantage

of this sensor require complex fabrication process through a heat treatment of film

sensor at 300 for 1 hour. The thermal treatment at high temperature helps to improve

the film density, the grain growth, the quality and the stability of the sensible

material (Acuautla et al., 2014). On the other hand, an easy and low temperature way

to prepare ozone gas sensor based on ZnO nanorods via hydrothermal process has

been successfully demonstrated by Catto et al., (2015). Results show ZnO nanorods

display long term stability over a 6 month period at optimal temperature of 250 °C

and exhibit a good sensitivity to ozone at room temperature when exposed to

ultraviolet illumination. However, the sensor suffer from long response time between

40 s and 44 s and recovery varying between 9 and 11 minutes, depending on ozone

concentration level.

An optical sensor based on absorption spectroscopy for ozone concentration

measurement has clear advantage compared to above mentioned sensor particularly

in response time. Work by Degner et al., (2010) demonstrate fast response time in

milisecond range for ozone concentration measurement from tenth of ppb until 100

ppm using 4 cm reflective gas cell indicates that optical measurement principle

provides fast response. Basically, optical sensor based on absorption spectroscopy

operates by measuring the ratio of incident and transmitted light intensity after

travelling through gas cell filled with ozone. Ozone absorb light due to electronic

6

excitation within its molecules. For quantity analysis, Beer law is used to relate

between light absorption and ozone concentration.

Ozone can absorb light at two main regions which are ultraviolet and visible

region. It exhibits a strong absorption band in ultraviolet region compared to the

visible region which centred at about 254 nm and 603 nm, respectively. Work by

O'Keeffee et al., (2007) successfully demonstrated the use of 5 cm transmissive gas

cell for ozone concentration measurement based on optical absorption in the

ultraviolet and visible region at 254 nm and 603 nm, respectively.

In addition, optical sensor have several advantages such as light, durable,

small size, immune to electrical and electromagnetic interferences and able to

remotely monitor ozone concentrations thus the control electronics can be placed

away from harsh environments. By taking into account these characteristics, optical

sensor are robust and highly suitable for in situ measurement as ozone is often

produced in electrochemically harsh environments.

Overall, all type of gas sensor have their own advantages and disadvantages.

These sensors are used to be applied in many application. Among of them, optical

sensor based on absorption spectroscopy exhibit excellent performance particularly

offer fast response time. Besides, optical sensor well suit to be used in harsh

environment for wide range application such as detection of natural hazard ozone

(Aoyagi et al., 2012), and detection of ozone produced by predischarge phenomena

on medium voltage (MV) electrical equipment (Maria and Bartalesi, 2012) as well as

monitoring ozone concentration in the atmosphere of printing process (Yu et al.,

2012), food industry to prolong the shelf-life of food (O'Keeffe et al., 2008) and

industrial process control application (Degner et al., 2010).

Besides the above mentioned sensor type, there are ozone sensors using other

principal of operation such as fluorescence (Felix et al., 2011), thermal

decomposition heat (Nakagawa et al., 2001) and optical sensor based on purely

organic phosphor (Lee et al., 2015).

7

Table 1.1: Summary on type of ozone gas sensor and its response time and range of ozone concentration measurement.

No. Author

(Year)

Type of sensor or principal of

operation

Ozone concentration

measurement

Response time

1 Maruo

(2007)

Reaction of ozone with indigo

carmine (dye)

Several ppb 1 hour

(Colour-fading reaction occurs in

the nano-pores between ozone and

indigo carmine)

2 Maruo

et al.,

(2010)

Reaction of ozone with azo dye

orange I

400 ppb 1 hour

(Ozone detection paper fades from

pink to white)

3 Stergiou

et al.,

(2010)

Reaction of ozone with

unbuffered solution of KI

55.5 until 166.5 μg (-)

4 Park

et al.,

(2009)

Carbon nanotube 50 ppb until 1 ppm 100 s

5 Ebeling

et al.,

(2009)

Electrochemical 100 ppb After 1 minute

(attainment of 90% of steady state

response)

6 Stergiou

et al.,

(2009)

Electrochemical 55.5 until 277.5 μg

(for PEO/Rb/I2 redox

polymer)

(-)

7 Arshak

et al.,

(2007)

Metal oxide semiconductor

(Material: Mixtures of In2O3, ZnO

and SnO2)

0 until 500 ppb Response time : 240 s

Recovery time : 340 s

(For sensor 3: 90In2O3:3ZnO:7SnO2

%Mol)

8

No. Author

(Year)


operation

Ozone concentration

measurement

Response time

8 Belaqziz et al.

(2014)


(Material: SnO2-TX film)

500 ppb Response time : 15 s

Recovery time : 12 minutes

9 Silva et al.,

(2014)


(Material: Ag2WO4)

80 until 930 ppb Response time : 7 s


(At low concentration of 80 ppb)

10 Catto et al.,

(2015)


(Material: ZnO)

0.06 until 1.19 ppm Response time :

Between 40 and 44 s

Recovery time :

Between 9 and 11 min

(Depending on ozone level)

11 Acuautla et al.,

(2014)


(Material: ZnO)

5 ppb until 500 Response time : < 4 minutes

Recovery time : < A few minutes

12 Rocha et al.,

(2016)


(Material: ZnO)

100 ppb Response time : 9.6 s

Recovery time : 45.6 s

(Sensor with heat treated at 120 °C)

13 Chen et al.

(2014)


(Material: Pt/TiO2-WO3)

0.5 until 2.5 ppm Response time : 890 s


(For 0.5 wt% Pt/TiO2-WO3(1:4))

14

Wu et al.,

(2015)


(Material: Au/TiO2-WO3)

1.0 until 7.5 ppm Response time : 450 s


(For 0.1 wt% Au/TiO2-WO3(3:1))

9

No. Author

(Year)


operation

Ozone concentration

measurement

Response time

15 Klaus et al.,

(2015)


(Material: In2O3)

50 until 220 ppb Recovery Time : Approx. 2.5 min

(For small particle)

Recovery Time : Approx. 5.3 min

(For large particle)

16 Baratto et al.

(2015)


(Material: CuAlO2)

70 until 350 ppb Response time : 3 minutes

Recovery time : 53 minutes

(For sample A: 0.13wt% CuAlO2 at

400°C)

17 Chein et al.,

(2010)


(Material: ZnO)

1 ppm until 2.5 ppm Response time : 45 s

Recovery time : 5 s

(Under 2.5 ppm ozone)

18 Starke and

Coles, (2002)

Metal oxide

semiconductor

SnO2 doped

with 2 wt.%

Pt

25 until 250 ppb

(at operating temperature of

120°C)

40 s until 60 s

In2O3 50 until 500 ppb


60°C)

25 s until 45 s

WO3 200 until 1000 ppb


180°C)

15 s

10

No. Author

(Year)


operation

Ozone concentration

measurement

Response time

19 O'Keeffee

et al.,

(2007)

Optical sensor

based on

absorption

spectroscopy

Ultraviolet

region

(254 nm)

0 until 0.97 mg/liter 1 s

Visible

region

(603 nm)

25 until 126 mg/liter

20 Hawe et al.,

(2007)

Optical sensor

based on

absorption

spectroscopy

Visible

region

(603 nm)

Ozone levels down to 500

ppm

(-)

21 Maria et al.,

(2008)

Optical sensor

based on

absorption

spectroscopy

Ultraviolet

region

(254 nm)

0.1 until 10 ppm Few seconds

22 Degner et al.,

(2010)

Optical sensor

based on

absorption

spectroscopy

Ultraviolet

region

(255 nm)

Some tenth of ppb until about

100 ppm

(For 4 cm reflection type gas

cell)

msecond range

Some ppb until about 10 ppm

(For 40 cm reflection type

gas cell)

msecond range

11

No. Author

(Year)


operation

Ozone concentration

measurement

Response time

23 Aoyagi et al.,

(2012)

Optical sensor

based on

absorption

spectroscopy

Ultraviolet

region

(280 nm)

0.01 until 1 ppm (-)

24 Teranishi

et al.,

(2013)

Optical sensor

based on

absorption

spectroscopy

Visible

region

(600 nm)

7.1 until 68.4 g/m3 (-)

25 Jodpimai

et al.,

(2016)

Optical sensor

based on

absorption

spectroscopy

Visible

region

(605 nm)

0 until 180 g/m3 (-)

Notes:

KI Potassium Iodide TX film Triton (X-100)

PEO/RbI/I2 Polyethylene oxide/RubidiumIodide/Iodine Ag2WO4 Silver Tungstate

Pt/TiO2-WO3 Platinum/Titanium dioxide-Tungsten Trioxide In2O3 Indium oxide

Au/TiO2-WO3 Gold//Titanium dioxide-Tungsten Trioxide ZnO Zinc Oxide

CuAlO2 Copper Aluminium Oxide SnO2 Tin dioxide

Pt Platinum WO3 Tungsten oxide

12

1.3 Problem Statement

Demand of ozone application in various fields illustrates the requirement to

develop an ozone gas sensor with a wide range of concentration measurement and

fast response time. Optical sensor based on absorption spectroscopy exhibit response

time of milisecond range makes the sensor well suited compared to other method to

be adopted in ozone concentration measurement.

From literature, number of efforts has been worked out by previous

researcher to improve range of ozone concentration measurement based on optical

sensor. Work by Degner et al., (2010) demonstrate range of ozone concentration

measurement using 4 cm reflective gas cell (up to 100 ppm) is ten time more than 40

cm reflective gas cell (up to 10 ppm) indicate shorter gas cell provide wide range of

ozone concentration measurement. However, as gas cell is shortened from 40 cm to 4

cm resulting in deterioration of resolution from 3 ppb to 30 ppb. Besides, previous

work by O’Keeffe, et al. (2007) demonstrated that optical sensor based on visible

absorption allow wide range of ozone concentration measurement from 25 until 126

mg/liter, while effect of ozone absorption in ultraviolet region lead to narrowed range

of ozone concentration from 0 until 1 mg/liter. However, work by Teranishi et al.,

(2013) based on ozone absorption in visible region demonstrate effect of the

presence of nitrogen oxide (NO2 and/or NO3) on the ozone concentration

measurement this region is unavoidable. Thus ultraviolet region is preferable

compared to visible region for ozone concentration measurement as cross sensitivity

between ozone and nitrogen oxide (NO2 and/or NO3) in visible region cannot be

ignored especially for ozone concentration generated from air. However, it is already

known the sensor suffers from narrow range of ozone concentration measurement in

ultraviolet region.

In previous work, ozone concentration measurement in ultraviolet region is

typically done near peak absorption such as 253.7 nm (Darby et al., 2012), 254 nm

(Yu et al., 2012; Maria and Bartalesi, 2012; O'Keeffe et al., 2007) and 255 nm

(Degner et al., 2009). Thus, this research is carried out to investigate effect of

13

particular wavelength instead of wavelength near peak absorption in order to

improve the range of ozone gas measurement in ultraviolet region.

1.4 Significant of the Research

In order to improve range of ozone concentration measurement in ultraviolet,

wide range analysis based on less relative error of concentration is presented.

Experiment results show wavelengths from 232 nm until 236 nm and from 284 nm

until 287 nm exhibit capability to provide total range of ozone concentration

measurement as wide as 313 ppm which is from 619 ppm until 932 ppm. Meanwhile,

wavelength near the peak absorption at 260 nm demonstrate the range of ozone

concentration measurement of zero based on consideration of less relative error of

concentration. This indicate wavelengths that located far from peak absorption

demonstrate capability to provide wide range of ozone concentration measurement

based on consideration of less relative error of concentration. Meanwhile,

improvement of 52.78 % from 36 ppm to 17 ppm has been observed in resolution

analysis when wavelength 285 nm is used instead of wavelength 265 nm. Therefore,

285 nm is highly recommended to be chosen due to its high resolution value. In

previous work, Aoyagi et al., (2012) had demonstrated measurement of ozone

concentration in ultraviolet region by employing wavelength of 280 nm using 20 cm

gas cell to achieve range of ozone concentration measurement from 0.01 until 1 ppm

at resolution less than 0.1 ppm. Wide range analysis based on consideration of less

relative error of concentration show the capability of wavelength 285 nm to measure

high concentration over a wide range of 619 ppm until 932 ppm for measurement of

ozone concentration in ultraviolet region using a compact gas cell of 10 cm with less

relative error of concentration.

14

1.5 Objective of the Research

The research objectives can be specified as follow:

To determine range of ozone absorption wavelength in ultraviolet region.

To improve range of ozone concentration measurement in ultraviolet region.

To characterize developed optical sensor based on absorption spectroscopy

performance in terms of its range of ozone concentration measurement,

response and recovery time, resolution and operating temperature.

1.6 Scope and Limitation of the Research

In order to achieve the objectives of the research, the scope of work have been

identified as follow:

Analysis of suitability of Spectracalc.com® simulator in obtaining ozone

absorption cross section in ultraviolet region.

Investigation of pressure (0.1 until 2.0 atm) and temperature (293 until 305

K) effect towards ozone absorption cross section for accurate ozone

concentration calculation.

Analysis of path length of gas cell and value of ozone absorption cross

section at different wavelength in order to improve range of ozone

concentration measurement in ultraviolet region using Spectracalc.com®

simulator.

Optimization of path length of gas cell in order to obtain range for ozone

concentration measurement up to 1000 ppm (due to limitation of ozone

monitor).

Determination of transmittance value for less relative error of concentration

based on Twyman-Lothian equation.

15

Selection of equipment to be used in arrangement of optical sensor based on

absorption spectroscopy.

Development of optical sensor based on absorption spectroscopy using ozone

transmissive type gas cell limited to optimum path length of 10 cm and

sensing system characterization.

Consideration of Twyman-Lothian Equation in wide rage analysis for 10 cm

gas cell in order to obtain suitable wavelengths that exhibit capability of

ozone concentration measurement over wide range in ultraviolet region with

less relative error of concentration.

1.7 Overview of the Thesis

This thesis is devoted towards development of absorption spectroscopic

ozone gas sensor for ozone concentration measurement in ultraviolet region. In order

to develop the sensor, overview on whole chapter is briefly described as follow.

In Chapter 2, the background of the theory of absorption spectroscopy will be

described in detail by discussion on the nature of light, the electromagnetic spectrum,

ultraviolet absorption process, derivation of Beer's Law and limitation of Beer's Law

as well as consideration of less relative error of concentration in absorption

measurement (Twyman-Lothian Equation). Then, review on components that

involved for spectroscopic instrumentation and definition of performance parameters

will be explained. Additionally, a review on optical sensor based on absorption

spectroscopy for ozone measurement from previous work will be discussed in detail

in term of sensor performance.

In Chapter 3, research methodology and frame work of research will be

discussed in detail in order to achieve research objective and to solve research

problem. These include explanation of sequence of step to obtain ozone absorption

wavelength in ultraviolet region for ozone concentration measurement, approach to

improve range of ozone concentration measurement in ultraviolet region and

characterization of developed optical sensor based on absorption spectroscopy.

16

In chapter 4, discussion will be focused on simulation of ozone absorption

wavelength to obtain ozone absorption cross section in ultraviolet region. Then,

analysis to investigate effect of pressure and temperature toward ozone absorption

cross section will be continued. The accurate value of ozone absorption cross section

in ultraviolet region is essential to determine correctly ozone concentration. Next,

wide range analysis will be done to improve range of ozone concentration

measurement in ultraviolet region by investigating effect of different path length of

gas cell and effect of value of ozone absorption cross section at different wavelength

in ultraviolet region. Besides, determination of optimum path length of gas cell will

be carried out in order to obtain gas cell that can provides range of ozone

concentration measurement up to 1000 ppm due to limitation of ozone monitor used

in this research work.

In Chapter 5, discussion will be focused on explanation of development of

ozone gas sensor based on absorption spectroscopic technique. Discussions cover

important topics include fabrication of gas cell, installation of ozone sensing system,

characterization of absorption spectroscopic ozone gas sensor, resolution analysis,

determination of ozone absorption cross section and wide range analysis based on

Twyman-Lothian Equation.

Finally, Chapter 6 remarks the overall conclusions, contributions of this

thesis and recommendation for future work.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

In this chapter, the background of the theory of absorption spectroscopy will

be explained in detail by discussion on the nature of light, the electromagnetic

spectrum, ultraviolet absorption process and derivation of Beer's Law. Also

discussion on limitation of Beer's Law and consideration on Twyman-Lothian

Equation. Finally, review on components that involved for spectroscopic

instrumentation and definition of parameter of performance. Additionally, a review

on optical sensor based on absorption spectroscopy for ozone measurement from

previous work will be discussed in detail in term of sensor performance.

2.2 The Nature of Light

Interaction between energy and sample to conduct an analysis is known as

spectroscopy (Hashim, 2009). Dispersion of an object light into its component

colours (i.e. energies) is regarded to spectroscopy. An object can be seen in colour is

due to selective absorption of light rays. In order to understand the way an object

absorbs light, it is important to know properties of light.

Light is an electromagnetic wave which is composed of electric and magnetic

fields that are always perpendicular to each other. Light propagates through space in

18

z direction as shown in Figure 2.1. In absorption spectroscopy, interaction between

electric wave of light and matter is usually measured to yield absorbance spectra.

Figure 2.1 Propagation of an electromagnetic wave in free space.

There are many types of light that exist around us. Therefore, characteristics

of light in terms of wavelength, frequency, wavenumber and energy are applied to

differentiate types of light. Here, definition of each light characteristic is briefly

explained by referring to Figure 2.1.

Wavelength (λ) is the distance between two consecutive peaks a wave. and

each types of light have their own wavelengths. Second characteristic of a light is its

frequency (f). Frequency is the number of complete cycle a wave undergoes per

second. For instance, Figure 2.1 shows the wave undergoes almost two cycles. Unit

for frequency is Hertz (Hz) or cycles s-1

. Wavelength, frequency, and speed of light

can be related to each other based on following relationship:

fc (2.1)

where

c is speed of light, 3 x 108 m s

-1

f is frequency (s-1

or Hertz)

λ is wavelength

x

y

z

Time

1st

cycle

2nd

cycle

19

From this equation indicate that different types of light travel at same speed

although they have different wavelengths and frequencies. Wavenumber is the third

characteristic that is used to describe light and can be defined reciprocal of the

wavelength as follows:

1w (2.2)

where

w is wavenumber (cm-1

)

λ is wavelength

Equation (2.2) shows that wavenumber is number of cycles per unit length.

By substituting Equation (2.2) into (2.1) yield

wfc (2.3)

and by rearranging Equation (2.3) gives

cwf (2.4)

Thus equations (2.1) until (2.4) demonstrate that light can be characterised by

their wavelength, wavenumber or frequency. Finally, the energy of light photon and

frequency is related based on following relationship:

hfE (2.5)

where

E is photon energy in Joules (J)

h is Planck's constant, 6.63 x 10-34

J s

f is frequency (s-1

or Hertz)

20

By substituting Equation (2.4) into (2.5), yield

E = hcw (2.6)

which shows that photon energy also depends on the wavenumber.

2.3 Spectrum of Electromagnetic and Type of Spectroscopy

Light is one part of electromagnetic spectrum. As discussion in previous

Section 2.2, different types of light have different wavelength, frequency,

wavenumber and energy. Based from these differences, light can be divided into six

main regions. Figure 2.2 shows these region across electromagnetic spectrum

consists of γ-ray, X-ray, visible and ultraviolet, infra-red, microwave and radio

frequency. As illustrated the wavenumber, frequency and the energy decrease as

wavelength increase from γ-ray to radio frequency region. Regarding to the region of

interest for ozone concentration measurement in ultraviolet region, knowledge of

ultraviolet subdivision in electromagnetic spectrum is important for determination of

light source to be used in future. The ultraviolet subdivision in electromagnetic

spectrum is shown in Table 2.1 (Manap, 2011). Ultraviolet radiation may be

subdivided into four regions which are near ultraviolet (NUV), middle ultraviolet

(MUV), far or vacuum UV (FUV or VUV; 200–10 nm), and extreme ultraviolet

(EUV). When considering the effect of ultraviolet radiation on human health and the

environment, the range of ultraviolet wavelengths is often subdivided into UVA (also

called Long Wave or "blacklight"), UVB (also called Medium Wave) and UVC (also

called Short Wave or "germicidal").

Spectroscopic is a technique that uses the interaction of electromagnetic

radiation with sample to perform an analysis. The list of wavelength and frequency

ranges for region of electromagnetic spectrum that are related to different types of

spectroscopy is shown in Table 2.2. The last column of the table lists the type of

energy change that serve as the basic for the various spectroscopic technique. From

21

the table, electronic spectroscopy takes place for ultraviolet and visible region is due

to energy change of electron distribution. Detailed explanation on energy change of

electron distribution particularly due to ultraviolet absorption is continued in next

section.

Figure 2.2 The region of electromagnetic spectrum.

Table 2.1: Ultraviolet subdivision in electromagnetic spectrum (Manap, 2011).

Name Abbreviation Wavelength

Ultraviolet A, long

wave or black light

UVA 400 nm - 320 nm

Near NUV 400 nm - 300 nm

Ultraviolet B or

medium wave

UVB 320 nm- 280 nm

Middle MUV 300 nm - 200 nm

Ultraviolet C, short

wave or germicidal

UVC 280 nm - 100 nm

Far FUV 200 nm -122 nm

Vacuum VUV 200 nm - 10 nm

Extreme EUV 121 nm - 10 nm

Vis

ible

Ult

ravio

let UVC

UVB

UVA

Orange

Blue

Green

Yellow

Red

Violet

γ- ray

X-ray

Ultraviolet

and Visible

Infra-red

Microwave

Radio

frequency

Hig

h w

Hig

h f

Hig

h e

ner

gy

Short

λ

Low

w

Lo

w f

Low

ener

gy

Long λ

22

Table 2.2: Type of spectroscopy in each region of electromagnetic spectrum

(Banwell and McCash, 1994).

Region of

spectrum

Approximate

range of f and λ

Type of

Spectroscopy

Type of Energy

Change

γ-ray 3 1018

until 3 1020

Hz

λ : 100 pm until 1pm

- Energy change

of nuclear

configuration

X-ray 3 1016

until 3 1018

Hz

λ : 10 nm until 100 pm

- Energy change

of electron

distribution

Ultraviolet

and visible

3 1014

until 3 1016

Hz

λ : 1 μm until 10 nm

Electronic

spectroscopy

Energy change

of electron

distribution

Infra-red 3 1012

until 3 1014

Hz

λ : 100 μm until 1μm

Vibrational

Spectroscopy

Energy change

of configuration

Microwave 3 1010

until 3 1012

Hz

λ : 1 cm until100 μm

Rotational

Spectroscopy

Energy change

of orientation

Radio

frequency

3 106

until 3 1010

Hz

λ : 10 m until 1cm

Nuclear magnetic

resonance (n.m.r)

and electron spin

resonance (e.s.r)

spectroscopy

Energy change

of spin

2.4 Ultraviolet Absorption Process: The Nature of Electronic Excitation

Atoms and molecules can absorb electromagnetic radiation only at particular

energies (i.e: wavelengths). In order for absorption to occur, there are a few types of

absorption mechanisms for the effective interaction between molecules and

electromagnetic radiation. In the case of ultraviolet spectroscopy, the transitions that

results in the absorption of electromagnetic radiation in this region are transitions

between electronic energy level (Lampman et al., 2010). As molecule absorbs

23

energy, an electron is promoted from a state of low energy (ground state) to a state of

higher energy (excited state).

Ozone is one of these gases that has significant absorption in ultraviolet and

visible regions. Ozone absorbs radiation in the these regions due to major electronic

excitation within ozone molecule. As ozone absorbs ultraviolet radiation, valence

electrons at lower energy level are excited to a higher energy level. In most

molecules, the energy differences between electronic levels vary from 125 to 650

kJ/mole (Lampman et al., 2010).

Usually, ultraviolet absorption occurs over a wide range of wavelength

because molecule also have many excited modes of rotation and vibration at room

temperature. The energy levels for these rotational-vibrational excitation are quite

closely spaced and can be considered as being packed on top of electronic levels.

Figure 2.3 shows a molecule simultaneously undergoes electronic (E0 to E1) and

rotational-vibrational (V1 to V1E) excitation. The excitation of rotational-vibrational

levels is "superimposed" on the electronic level. Observation from these types of

combined transition is absorption spectrum of a ozone consists of a broad band of

absorption in ultraviolet region centered near the wavelength of the major transition.

Figure 2.3 Electronic transitions with vibrational transitions superimposed.

(Rotational levels, which are very closely spaced within the vabrational levels, are

omitted for clarity) (Lampman et al., 2010).

En

ergy

E1

E0

V3E

V2E

V1E

V4

V3

V2

V1

Electronic excited

state

Electronic ground

state

Vibrational levels

Vibrational levels

24

Ozone spectrum in ultraviolet and visible region can be presented by plotting

graph of absorption cross section versus wavelength. The absorption cross section

determine how strong specific molecule absorb light at specific wavelength. The

absorption cross section reported by Burrows et al., (1999) shows that ozone gas

absorbs light at many wavelength in ultraviolet and visible region as shown in Figure

2.4. From the graph, ozone absorbs light in three regions known as the Hartley band

(spectral region: 231 until 300 nm), the Huggins band (spectral region: 300 until 370

nm) and the Chappuis band (370 until 794 nm) (Burrows et al., (1999)). Two

wavelength regions, which are Hartley band and Chappuis band have been reported

to have large value of absorption cross section in ultraviolet and visible regions,

respectively. Burrows et al., (1999) shows absorption cross section of ozone in the

Hartley band near peak absorption is found to be 1150 x 10-20

cm2

molecule-1

at

wavelength 253.65 nm. Meanwhile, absorption cross section of ozone in Chappuis

band near peak absorption is 47000 x 10-24

cm2 molecule

-1 at wavelength 611.97 nm.

Figure 2.4 Absorption cross-section of ozone spectrum at 293K for complete

coverage of 231 nm until 794 nm range (Burrows et al., 1999).

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