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I.1.1 History of the investigation of complexes . . . . 3

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Contents

I Introduction 1

I.1 van der Waals complexes 3

I.1.1 History of the investigation of complexes . . . . 3

I.1.2 Complexes in planetary atmospheres and interstellar medium . . 5

I.1.3 Infrared spectroscopy of complexes . . . . 8

II Theory 11 II.1 Vibration-rotation energy 13 II.1.1 Vibrational energy . . . 13

II.1.2 Rotational energy . . . 16

II.2 Polyatomic molecules 19 II.2.1 Vibrational energy . . . 19

II.2.2 Rotational energy . . . 19

II.2.2.1 Linear molecules . . . 20

II.2.2.2 Symmetric top molecules . . . 21

II.2.2.3 Asymetric top molecules . . . 21

II.3 Application to ammonia and water 23 II.3.1 Ammonia . . . 23

II.3.1.1 Symmetry group (without inversion) . . . 23

II.3.1.2 Symmetry group (with inversion) . . . 25

II.3.1.3 Statistical weights . . . 28

II.3.2 Ammonia ≠ noble gas (NG) . . . 32

II.3.2.1 Selection rules . . . 33

II.3.2.2 Vibrational symmetries . . . 34

II.3.3 Water . . . 35

II.3.3.1 Symmetry group . . . 35

II.3.3.2 Statistical weights . . . 37

II.3.4 Water ≠ noble gas . . . 39

II.3.4.1 Vibrational symmetries . . . 40

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II.4 Absorption spectroscopy 43

II.4.1 Light-matter interaction . . . 43

II.4.1.1 Einstein coefficients . . . 43

II.4.1.2 Absolute intensities . . . 46

II.4.1.3 Line strength . . . 48

II.4.1.4 Selection rules . . . 48

II.4.2 Line profiles . . . 51

II.4.2.1 Natural width . . . 53

II.4.2.2 Pressure broadening . . . 53

II.4.2.3 Doppler width . . . 54

II.4.2.4 Voigt profile . . . 55

II.4.3 Lineshape fitting . . . 56

III Experiments 57 III.1 Supersonic expansion 61 III.1.1 Introduction . . . 61

III.1.2 Fundamentals of gas kinetic theory . . . 62

III.1.3 Velocity distributions . . . 62

III.1.4 Mean free path . . . 64

III.1.5 Effusive regime . . . 65

III.1.6 Supersonic regime . . . 65

III.1.7 Translational cooling mechanism . . . 66

III.1.8 Cooling mechanism of the internal degrees of freedom . . . 68

III.1.9 Thermodynamical approach . . . 69

III.1.9.1 Energy conservation . . . 69

III.1.9.2 Reversibility . . . 72

III.1.9.3 Thermodynamical parameters . . . 73

III.1.9.4 Clusters formation . . . 74

III.2 Cavity Ringdown Spectroscopy (CRDS) 75 III.2.1 Introduction . . . 75

III.2.2 Principle . . . 76

III.2.3 Sensitivity . . . 78

III.2.4 Cavity stability and modes . . . 79

III.2.4.1 Stability . . . 79

III.2.4.2 Longitudinal modes . . . 79

III.2.4.3 Transverse modes . . . 80

III.2.5 Bandwidth considerations . . . 81

III.2.6 CW-CRDS . . . 82

III.2.7 Instrumental set-up . . . 83

III.2.7.1 The laser source . . . 85

III.2.7.2 Gas injection . . . 86

III.2.7.3 Pumping system . . . 87

III.2.7.4 The injection system . . . 87

III.2.7.5 The cavity . . . 87

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III.2.7.6 The detector . . . 88

III.2.7.7 The ringdown detection . . . 88

III.2.7.8 Calibration . . . 88

III.2.8 Experimental conditions . . . 91

III.3 Fourier Transform Spectroscopy 95 III.3.1 Introduction . . . 95

III.3.2 The Michelson interferometer . . . 96

III.3.2.1 Effects of the finite resolution . . . 98

III.4 The microwave spectrometers 101 III.4.1 Chirped-pulse spectroscopy . . . 101

III.4.1.1 Chirped microwave pulse generation . . . 101

III.4.1.2 Microwave excitation pulse and molecular beam sample interaction region . . . 102

III.4.1.3 Detection . . . 102

III.4.2 Balle-Flygare spectroscopy . . . 102

III.4.3 Gas injection conditions . . . 104

IV Results 105 IV.1 Ammonia complexes 109 IV.1.1 NH

3

≠ NG . . . 109

IV.1.1.1 Introduction . . . 109

IV.1.1.2 Experimental conditions . . . 110

IV.1.1.3 Sub-band assignment . . . 112

IV.1.1.4 Rotational analysis . . . 118

IV.1.1.5 Sub-bands #2 and #4 . . . 123

IV.1.1.6 Conclusions . . . 128

IV.1.2

15

NH

3

≠ Ar . . . 130

IV.1.2.1 Introduction . . . 130

IV.1.2.2 Assignment . . . 131

IV.1.2.3 Rotational analysis . . . 131

IV.1.2.4 Conclusion . . . 134

IV.1.3

15

NH

3

. . . 134

IV.2 Water complexes 137 IV.2.1 (H

2

O)

2

. . . 137

IV.2.1.1 Introduction . . . 137

IV.2.1.2 Experimental details and observations . . . 139

IV.2.1.3 General features of the analysis . . . 143

IV.2.1.4 Results . . . 147

IV.2.1.5 Discussion . . . 155

IV.2.1.6 Conclusion . . . 158

IV.2.2 H

2

O ≠ Ar . . . 159

IV.2.2.1 Introduction . . . 159

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IV.2.2.2 Ab initio intermolecular potential energy surface . . . . 159

IV.2.2.3 Energy levels . . . 160

IV.2.2.4 Experiments and sub-band assignment . . . 163

IV.2.2.5 Rotational analysis . . . 168

IV.2.2.6 Conclusion . . . 180

IV.2.3 H

2

O ≠ Kr . . . 180

IV.2.3.1 Introduction . . . 180

IV.2.3.2 Experiments . . . 181

IV.2.3.3 Sub-band assignment . . . 183

IV.2.3.4 Rotational analysis . . . 185

IV.2.3.5 Conclusion . . . 192

IV.2.4 HDO ≠ N

2

O . . . 193

IV.2.4.1 Introduction . . . 193

IV.2.4.2 Experimental details . . . 193

IV.2.4.3 Ab initio calculations . . . 194

IV.2.4.4 Experimental results . . . 197

IV.2.4.5 Discussion . . . 200

IV.2.4.6 Conclusion . . . 203

IV.3 Predissociation lifetimes 205 IV.4 Ammonia FT spectra 209 IV.4.1 Introduction . . . 209

IV.4.2 Experimental conditions . . . 209

IV.4.3 Measurement of line parameters . . . 210

IV.4.4 Comparison with Kitt Peak . . . 210

IV.4.5 Conclusion . . . 213

V Conclusion and perspectives 215

Bibliography 222

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