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Infrared Spectroscopy of Atmospherical and Astrophysical Relevant Molecules

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Infrared Spectroscopy of Atmospherical and Astrophysical Relevant Molecules Università Ca’ Foscari Venezia Dottorato di ricerca in SCIENZE CHIMICHE, 22° ciclo (A. A. 2006/2007 – 2008/2009) Infrared Spectroscopy of Atmospherical and Astrophyysical relevant Molecules: Spectral analysis, Line parameter retrievals and Study of collisional decay processes Settore Scientifico-Disciplinare di afferenza: CHIM02, Chimica Fisica Tesi di dottorato di NICOLA TASINATO, 955380 Coordinatore del dottorato Tutore Prof. Paolo Ugo Prof. Paolo Stoppa TToo mmyy MMuumm NNaaddiiaa aanndd mmyy DDaadd OOlliivviioo…… INTRODUCTION AND THESIS OUTLINE 2 Introduction 2 Objectives and Thesis Outline 4 Part I: Theory 1. PRINCIPLES OF QUANTUM MECHANICS 10 1.1 Wave Equation and Approximations 10 1.2 Approximation Techniques 12 1.3 Time-Dependent Perturbation Theory 15 1.4 Angular Momentum Operators 16 1.5 The Harmonic Oscillator 20 2. MOLECULAR ROTATIONS AND VIBRATIONS 23 2.1 Molecular Vibrations 23 2.2 Vibrational Selection Rules 27 2.3 Moments of Inertia 30 2.4 The Rigid Symmetric and Asymmetric Rotors 32 2.5 The Effective Vibrational – Rotational Hamiltonian 36 2.6 Interactions 40 2.7 Nuclear Spin Statistical Weight 43 2.8 Linear Molecules 44 2.9 Symmetric Top Molecules 46 2.10 Asymmetric Top Molecules 47 I 3. MOLECULAR COLLISIONS AND SPECTRAL LINE SHAPES 50 3.1 The Absorption Coefficient 50 3.2 Line Intensities 51 3.3 Dipole Autocorrelation Function 53 3.4 Doppler Broadening 55 3.5 Collisional Broadening: the Lorentz Profile 56 3.6 The Voigt Profile 58 3.7 Collisional Narrowing: Galatry and Nelkin-Ghatak Profiles 59 3.8 Speed Dependence and Line Mixing 61 3.9 Semiclassical Theory of Pressure Broadening 63 3.10 Coherent Transients and Rapid Passage Signals 67 4. AB INITIO CALCULATIONS IN INFRARED SPECTROSCOPY 70 4.1 Potential Energy Surface 70 4.2 Electronic Structure Calculations 71 4.3 The Hartree – Fock Self Consistent Field 72 4.4 Basis Sets 75 4.5 Coupled Cluster Theory 77 4.6 Harmonic and Anharmonic Force Fields 79 Part II: Experimental and Computational Work 5. INSTRUMENTATION 84 5.1 Tunable Diode Laser Spectrometer 84 5.2 Fourier Transform Spectrometer 87 5.3 Supersonic Free Jet Expansion 91 5.4 Quantum Cascade Laser Spectrometer 94 6. VISUAL LINE-SHAPE FITTING PROGRAM 98 6.1. Visual-Line Shape Fitting Program Implementation: an Object Oriented Approach 99 6.2. Description of the Line-Shape Functions 100 II 6.3. The Levenberg – Marquardt Algorithm 102 6.4. Implementation of the Voigt Profile and the Complex Probability Function 104 6.5. Profiles Including Dicke Narrowing 108 6.6. Description of the User Interface 110 7. SULPHUR DIOXIDE LINE PARAMETERS IN THE 9.2 μM ATMOSPHERIC SPECTRAL 113 WINDOW 7.1. Experimental Procedure for Line Parameter Determinations: Characterization of 114 Instrumental Distortions and Data Inversion 7.2. Experimental Procedure for Sulphur Dioxide Vibrational Cross Section Measurements 120 7.3. Semiclassical Calculation of Self-Broadening Coefficients 121 7.4. Experimental Results and Discussion 124 8. VINYL FLUORIDE SPECTROSCOPY: VIBRATIONAL ANALYSIS, AB INITIO CALCULATIONS 139 AND LINE PARAMETER DETERMINATION 8.1. Experimental Details and Data Inversion 140 8.2. Computational Details 143 8.3 Description of the Spectrum and Assignment 144 8.4 Rotational Analysis 147 8.5 Integrated Band Intensities 149 8.6 Equilibrium Geometry and Harmonic Force Constants 152 8.7 Ab initio Anharmonic Force Field 153 8.8 Anharmonicity Constants and Vibrational Resonances 154 8.9 Line Shape Parameters 158 9. JET-COOLED DIODE LASER SPECTRUM AND FTIR INTEGRATED BAND INTENSITIES OF 169 CF3Br 9.1. Experimental Details 171 9.2. High Resolution Spectra: Description of the 2ν5 and ν2 + ν3 Band Regions 172 9.3 Rovibrational Analysis of 2ν5 and ν2 + ν3 176 9.4.Cross Section Measurements by Medium Resolution FTIR Spectra 179 III 9.5 Evaluation of Radiative Forcing and Global Warming Potential 182 10. FREQUENCY DOWN-CHIRPED QCL SPECTROSCOPY: TIME DEPENDENT MEASUREMENTS OF COLLISIONAL PROCESSES IN A DICKE NARROWED SPECTRAL LINE OF 187 WATER VAPOUR 10.1. Experimental Method and Data Inversion 189 10.2 Rapid Passage Signals and Chirp Rate Dependence of Collisional Processes: Water 192 Vapour 11. FREQUENCY DOWN-CHIRPED QCL SPECTROSCOPY: TIME DEPENDENT 201 MEASUREMENTS OF NITROUS OXIDE AND CARBON DIOXIDE COLLISIONAL RELAXATIONS 11.1. Experimental Procedure and Data Inversion 202 11.2. Theory: Modelling the Rapid Passage Signals 204 11.3 Rapid Passage Signals and Chirp Rate Dependence of Collisional Processes: Nitrous 206 Oxide and Carbon Dioxide CONCLUSIONS AND OUTLOOK 220 Summary 220 Possible Developments 226 Bibliographic Information & Appendices REFERENCES 230 APPENDIX A. CONTACT TRANSFORMATIONS 254 IV IInnttrroodduuccttiioonn aanndd TThheessiiss OOuuttlliinnee Introduction Spectroscopy is a field of chemical physics which studies the interaction of the electromagnetic radiation with matter. Infrared gas phase molecular spectroscopy uses the infrared light, which spans the electromagnetic spectrum from 14300 to 10 cm-1 [1], to probe a molecular gas sample. The interactions observed in the IR spectrum involve principally the energies associated with molecular structure change, and hence IR spectroscopy is widely used to infer information about the molecular structural parameters. Hence in the last decades the study of IR spectra has led to a fundamental understanding of molecular structures [2]. Although gas phase molecular spectroscopy dates back to the beginning of the 1800s, when it was used to explore the composition of the sun, it remains an active field, which during last years has become of essential importance in many disciplines, such as atmospheric chemistry and astrophysics [3]. Indeed, since the earlier experiments in the second half of the 1900, the study of Earth’s and planetary atmospheres and of the interstellar medium by means of spectroscopic techniques has rapidly grown up. Nowadays, remote sensing techniques are widely used to probe the atmosphere and retrieve the concentration profiles of a number of species. For instance, the satellites used for sounding the terrestrial atmosphere (e.g. AIRS, SCISAT-1) and to explore the solar system (Voyager1, Cassini-Huygens, ISO) mount spectrometers which provide large amount of spectral information at ever increasing quality in terms of spectral coverage, resolution and signal-to-noise ratio [4]. Among the various spectroscopic techniques which cover the whole spectral range from microwave to UV-visible, infrared spectroscopy plays a very significant role, in particular for remote sensing of the terrestrial atmosphere. Indeed, IR remote sensing permits to monitor and accurately retrieve the concentrations of the atmospheric constituents and trace pollutants, since almost all these molecules have strong vibration-rotation bands in the infrared spectral domain [5]. Further, IR molecular spectroscopy is playing a primary role in the study of the atmospheres of the so-called exo-planets, which are planets in solar systems other than the our. By using spectral retrieval methods, it is possible to determine aspects such as dayside temperature 2 profiles and atmospheric composition, from which information about their habitability can be obtained [6, 7]. Up to now, several exo-planets have already been discovered and their number is growing steadily [8]. In the coming decades many space- and ground- based facilities are planned, in order to search for new exo-planets of all dimensions, from massive young “hot Jupiters”, through large rocky super-Earths, down to the discover of exo-Earths [9]. Very recently, the presence of methane and water vapour in the atmosphere of the hot Jupiter HD189733b has been detected by infrared spectroscopy [10, 11]. On these bases, spectroscopic parameters turn out to be of fundamental importance for the remote sensing applications used in atmospheric and climate research, environmental monitoring, gas-phase analytics and astronomy. Indeed, only their accurate knowledge allows an accurate retrieval of concentrations and distributions of the gas phase molecular species. The spectroscopic parameters include either line by line parameters, i.e line positions, line strengths, pressure broadening coefficients and pressure induced shifts, or absorption cross sections when a line by line approach is not possible [12, 13]. On the other hand, the growing concern of scientific communities and international politicians about climate changes and environmental degradation related to the human activities has even more highlighted the requirements for a deeper knowledge of atmospheric chemistry and physics, in order to understand and predict the evolution of the Earth atmosphere. These studies are carried out by using radiative transfer models which, in addition to an atmospheric profile, require as input a set of spectroscopic data for an increasing number of molecules [14]. The existing spectroscopic data are collected into a number of different databases, among which there are HITRAN [15], GEISA [16], JPL [17]. Given the absorber amount and the atmospheric pressure and temperature profiles, the spectroscopic parameters allow the molecular absorption or emission to be computed at any frequency, assuming a reliable line shape function [14]. Within this framework, the aim of laboratory spectroscopy is to provide spectroscopic parameters for a wide variety of species of atmospheric, astrophysical and industrial importance. Furthermore, given the advances in infrared spectroscopy instrumentation, the spectral parameters require to be determined with the highest
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