Probing transient species in plasmas and photodissociation reactions using infrared and raman laser spectroscopy

PROBING TRANSIENT SPECIES IN PLASMAS AND PHOTODISSOCIATION REACTIONS USING INFRARED AND RAMAN LASER SPECTROSCOPY LI PENG NATIONAL UNIVERSITY OF SINGAPORE 2004 PROBING TRANSIENT SPECIES IN PLASMAS AND PHOTODISSOCIATION REACTIONS USING INFRARED AND RAMAN LASER SPECTROSCOPY LI PENG A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2004 i ACKNOWLEDGMENT First of all I wish to express my sincere gratitude to Dr Fan Wai Yip, my supervisor, who provided much guidance and help Thank you for your effort, patience as well as teaching me how to use good English I like to acknowledge Dr Loh Kian Ping for his help in my project and occasional guidance for my work also I am grateful to the members of my group; Tan Yen Ling, Li Shu Ping, Tan Hua, Chong Thiam Seong, Wong Ling Keong, Royston Cheng Kum, Lim Kok Peng, Lee Wei Te and Wong Ling Kai Thank you for your help and company these past few years I would like to thank Mr Conrado Wu of the Chemistry department Glassblowing workshop for fabricating all the glassware equipment for my experiments; Mr Tan Choon Wah from the Physics department workshop and Mr Rajoo and Mr Guan from the Chemistry department workshop for their technical support I also would acknowledge the support from Mr Teo Leong Kai, Mr Sim Hang Whatt and Mr Lee from the Chemistry department lab supply room and Mdms Adeline Chia and Patricia Tan from the Physical Chemistry laboratory Lastly I wish to acknowledge the National University of Singapore for offering me a research scholarship and providing me the opportunity to pursue my degree here ii Table of Contents Acknowledgment i Table of Contents ii Summary vi Chapter Introduction 1.1 Physical properties of plasmas 1.1.1 What is a plasma? 1.1.2 Plasma temperature 1.1.3 Debye length and plasma sheath 1.2 DC glow discharge - 1.2.1 General characteristics of a dc glow discharge 1.2.2 Physical structure of a dc glow discharge 1.3 Applications of plasma diagnostic techniques -10 1.3.1 Applications of TDLAS in plasmas 12 1.3.2 Applications of OES in plasmas 16 1.3.3 Applications of FTIR absorption spectroscopy in plasmas 18 1.3.4 TDLAS, OES and FTIR diagnostics of hydrocarbon plasmas 19 1.3.5 Hydrocarbon plasmas doped with N and S elements 24 1.3.6 Silicon nitride films and plasmas 25 1.4 Objectives of the project -26 References -28 Chapter Experiments and Theory 34 1.1 Discharge cell 34 2.2 Tunable infrared diode laser 37 2.2.1 Introduction to diode lasers 37 2.2.2 Characteristics of IR diode lasers 40 2.2.3 Modulation of TDLAS 43 2.3Experimental setup of TDLAS system -49 2.3.1 Infrared laser coldhead and Laser Control Module (LCM) 50 iii 2.3.2 Optics 50 2.3.3 Scanning and modulation 51 2.3.4 Calibration of the spectrum 53 2.3.5 Sensitivity of the TDLAS system 55 2.4 Optical Emission Spectroscopy (OES) 56 2.4.1 Basic principles 56 2.4.2 Experimental setup of OES 59 2.5 Fourier transform infrared (FTIR) absorption spectroscopy 61 2.6 Computational Chemistry -62 References -64 Chapter The Energy Distributions of CO Produced in an Acetone-Containing Discharge 67 3.1 Introduction 67 3.2 Experiments and frequency calculations -69 3.2.1 Experimental setup 69 3.2.2 CO rovibrational frequency calculations 70 3.3 Results -72 3.3.1 The FTIR spectrum of acetone/argon discharge 72 3.3.2 Translational temperature 76 3.3.3 Rotational temperature 79 3.3.4 TDLAS of C2D2 and CN 80 3.4 Discussion 83 References -87 Chapter Diatomic CN and CS Transient Species in CH3CN and CH3SCN dc Discharges 89 4.1 Introduction 89 4.2 Experimental section -91 4.3 Rovibrational line strengths and concentrations of CN and CS -93 4.3.1 Determination of the vibrational band intensity of CN and CS 93 4.3.2 Determination of the individual rovibrational line intensities of CN and CS 95 4.3.3 Determination of the absolute concentrations of CN and CS 98 iv 4.4 The CN radical in CH3CN discharge 101 4.5 The CN and CS transient species in CH3SCN discharge 115 4.6 Summary 135 References - 137 Chapter The SiN Radical and other transient species in SiCl4/N2 dc discharges 139 5.1 Introduction 139 5.2 Experimental section - 140 5.3 Results - 142 5.4 Discussion - 150 5.5 Summary 154 References 155 Chapter Vibrational Spectroscopy and 266 nm Photochemistry of NCNCS and CNCN 156 6.1 Introduction 156 6.1.1 Photochemistry of thiocyanate (X-NCS) compounds 156 6.1.2 Principles of CARS spectroscopy 159 6.1.3 Experimental setup of CARS spectroscopy 162 6.1.4 Objectives of the project 166 6.2 Experimental section - 167 6.2.1 Synthesis and photolysis of NCNCS 167 6.2.2 CARS experiment of NCNCS and CNCN 167 6.2.3 UV/VIS absorption spectrum of NCNCS 168 6.2.4 Calculations 169 6.3 Results and discussion 170 6.3.1 Infrared and CARS spectra of NCNCS 170 6.3.2 Infrared and CARS spectra of CNCN 175 6.3.3 266nm photodissociation of NCNCS 180 6.3.3 Potential energy of NCNCS 183 6.4 Summary 186 References - 187 Appendices 189 v A The SRS DDDA data acquisition program that controls the diode laser and collects the TDLAS spectrum - 189 B The QBASIC program that calculate the rovibrational frequencies of CO at different vibrational levels 190 C The QBASIC program that calculate the rovibrational line intensities of CN - 191 D The SRS DDDA data acquisition program that collects the CARS spectrum - 192 vi SUMMARY The work in this thesis is directed towards understanding the chemistry of free radical and transient species primarily in plasmas and flash photolytic reactors using infrared and Raman laser-based techniques The introduction of the general properties of glow discharge and the applications of tunable infrared diode laser absorption spectroscopy (TDLAS), optical emission spectroscopy (OES) and FTIR absorption spectroscopy in the plasma diagnostics were presented in the first chapter The principle of infrared diode laser, OES and the experimental set up of the dc discharge cell, TDLAS, OES and FTIR absorption spectroscopy as well as Gaussian 98 calculation methods were delivered in Chapter In Chapter 3, the translational, rotational and vibrational distributions of CO in an acetone/argon dc plasma have been characterized by using TDLAS and FTIR absorption spectroscopies A broad vibrational distribution of CO was observed with gradually decreasing intensities from the fundamental band to υ = 12 ← 11 When nitrogen was added to the plasma, the distribution became narrower The rotational distribution can generally be fitted to a Boltzmann distribution within each vibrational level although the rotational temperature is highest for the lowest vibrational quantum number In Chapter 4, the plasma chemistry of transient species in CH3CN and CH3SCN dc discharges was studied semi-quantitatively using TDLAS, OES and FTIR spectroscopies with focus on CN and/or CS transient species The vibrational spectra of CS and CN in these plasmas have been recorded using TDLAS and the concentrations of CN and CS were determined aided by vibrational intensity calculations performed at UB3LYP/6311+G** level of theory in Gaussian 98 It was also found that under high plasma current CHAPTER 1ntroduction – CHAPTER – Introduction 1.1 Physical properties of plasmas 1.1.1 What is a plasma? When a sufficiently high voltage is applied across two electrodes immersed in a gaseous medium, atoms and molecules of the medium will break down electrically, forming electron-ion pairs and permitting current to flow The phenomenon of current flowing through a gaseous medium is termed a “discharge” Irving Langmuir and his collaborators were the first to study the phenomena in the discharge in the early 1920’s and it was Langmuir who gave the ionized gas the name of ‘plasma’ [1] The plasma is considered the fourth state of matter beside solid, liquid and gas In fact, most of the observable matter in the universe is in the plasma state [2] Plasmas can be divided into high-temperature plasma and low-temperature plasma and a further subdivision of the low-temperature plasma relates to local thermodynamic equilibrium plasma (LTE plasma) and non-local thermodynamic equilibrium plasma (non-LTE plasma) (Table 1.1) [3] CHAPTER 1ntroduction Table 1.1 Classification of plasmas Taken from reference [3] Low-temperature Plasma LTE plasma Te ≈ Ti ≈ Tt ≤ 104 K (Tt gas temmperature) High-temperature Plasma Non-LTE plasma (cold plasma, glow discharge) Ti ≈ Tt ≈ 300 K Te ≈ Ti ≥ 107 K Ti