Preface

In the preface to Volume 1 [DAH 17], the importance of spectroscopy was emphasized, both from a theoretical and an instrumental point of view, for the analysis of observations of chemical species, molecules, radicals and ions. In the infrared (IR), using various types of spatial observation instruments, it is possible to detect molecules or chemical species (ions, radicals, macromolecules, nano-cages, etc.) present in the atmospheres of planets, Earth included, and their satellites, in interstellar media, comets or exoplanets, for example.

One of the most striking observations using ground-based instruments or embedded in space probes or telescopes was listed to show the diversity of discoveries that can lead to advances in the field of astrophysics or cosmology. Note, in particular, the observations mentioned in the preface to Volume 1 [DAH 17], that is:

And very recently, on September 14, 2015, the LIGO (Laser Interferometer Gravitational-Wave Observatory) detects for the first time, the distortions caused by gravitational waves in space-time, predicted by Einstein's theory of general relativity and generated by two black holes that collide nearly 1.3 billion light-years away.

This earned its authors, Barry C. Barish, Kip S. Thorne and Rainer Weiss, the Nobel Prize in Physics in 2017.

Advances in modern detection systems (Planck and Hubble telescopes) and large telescopes that are continually improved and programmed to be sent into space (NASA James Webb Space Telescope (2020), European Extremely Large Telescope (E-ELT) (2024)) can probe the universe to better understand its origin and what it comprises (less than 5% of visible matter, about 25% of dark matter and the rest of dark energy (70%) responsible for a force that repels gravity), to observe exoplanets or black holes, or to measure its expansion. All of these space observations lead astronomers and physicists to rework the cosmological model and revisit Einstein's equation as part of his theory of general relativity published in 1915. Similarly, planet exploration programs using robotic and communicating instruments, such as that of Mars Rover 2020, open the way to observations and analysis data that will have to be interpreted through theoretical models adapted to different areas of the electromagnetic spectrum such as that of IR spectroscopy, which is the focus of this volume.

Referring to the preface to Volume 1 [DAH 17], it should be recalled that spectroscopy not only makes it possible to determine the structure of chemical species (in the gas phase, liquid phase or solid phase) by applying the methods and tools of theoretical spectroscopy, but also helps to identify species (atoms, molecules, molecular fragments, radicals, etc.) in different environments (nano-cavities, media containing different species, ice surface, dust surface, etc.). The species themselves can be used as probes to characterize the environment (temperature, pressure, composition) and determine its nature by relying on the theoretical models developed to analyze the corresponding data.

This book describes the theoretical methods that are used in fundamental research to interpret the spectra of triatomic molecules observed in the infrared domain when these molecules are subjected to an environment where the temperature and the pressure modify their infrared spectra in the gas phase or in nano-cages. In this book, we describe the theoretical models that have been developed to study triatomic molecules in the gas phase as well as the modification of the infrared spectra of these molecules such as the displacement of the centers of bands or the modification of the rovibrational spectrum in nano-cages or on surfaces.

IR spectroscopic analysis is of fundamental interest to atmospheric physics. Ozone (O3) or water vapor (H2O) molecules, which are nonlinear triatomic molecules, given their role in energy exchanges with solar radiation and their implications for chemical equilibrium reactions with other minority constituents present in the atmosphere and in the clouds, are among the most studied gas phase molecules both experimentally and theoretically. Similarly, CO2 or N2O molecules, which are linear triatomic molecules and minor constituents of Earth's atmosphere, play a non-negligible role in the radiative budget. The CO2 molecule is a molecule that participates in global warming as a GHG (greenhouse gas).

This book is intended for Master's and PhD students, teachers and researchers, astronomers and astrophysicists who analyze the data corresponding to the interaction of electromagnetic radiation with matter in the infrared domain, in order to identify the chemical species and their environments.

The first part of the book, which consists of the first two chapters, describes the theoretical models developed for the study of triatomic vibrational-rotational spectra. It was partly inspired by the second year Master's courses (Master 2) in Molecular Physics by G. Amat at UPMC and those by J.M. Flaud and C. Camy-Peyret at DEA (Master 2), "Laser and Matter" at UPSUD, and laboratory research at CNRS, a research organization in France, involved in spectroscopic studies of triatomic molecules in the IR domain, either at UPMC (Group of G. Amat, L. Henry, J.M. Flaud and C. Camy-Peyret, A. Perrin, etc.) or UPSUD (Group of M. Barchewitz, G. Graner, C. Boulet, etc.). The second part, which includes the next three chapters, describes the theoretical models also developed in research to analyze the observations made on triatomic molecules when they are isolated in condensed phase media. This work was initiated in particular in the group Molecular Physics group of Besançon (L. Galatry, D. Robert, J. Bonamy, L. Bonamy, C. Girardet, A. Lahklifi, etc.) and continued, thereafter, in collaboration with researchers from laboratories in the Paris region (L. Abouaf, B. Gauthier, H. Dubost, P.R. Dahoo, etc.) to study molecules in different media and subjected to interactions, whose effects are particularly apparent at the nanometer scale, which modify the profile of the IR spectra of these molecules. The theoretical inclusion model or Lakhlifi-Dahoo extended model is explained with the programs which make it possible to calculate the IR spectra of the triatomics in nano-cages. Finally, in the third part, we present some applications of models, for the study of triatomic molecules, described in the second part.

In Chapter 1, we show how to use the symmetry properties of linear and nonlinear triatomic molecules to predict the structure of the vibrational-rotational IR spectrum, taking into account the symmetry of the states between which the possible transitions occur and that of the operator inducing these transitions (dipole moment or polarization tensor). The symmetry properties are also used to apply the contact transformation method to the vibrational-rotational Hamiltonian of nonlinear triatomic molecules in order to solve the eigenvalue equation to determine the energy levels of these molecules, in particular for the rotational degrees of freedom passing from the basis set constructed on the quantum numbers of the symmetric prolate and oblate rotors to the Wang basis set, eigenfunctions of the symmetry group D2 (or V).

In Chapter 2, as in Volume 1 [DAH 17], special emphasis is placed on the use of group theory to construct the vibrational-rotational Hamiltonian and to infer selection rules as a result of the interaction between light and molecules for electrical dipolar transitions in infrared spectroscopy. The theoretical models used in the context of the contact transformation are recalled to study the vibrational and rotational movements of linear and nonlinear triatomic molecules.

In Chapter 3, it is shown that a molecule trapped in a nano-cage of a clathrate crystal is subjected to an anisotropic force field due to the interaction with crystal water molecules. The theoretical inclusion model or Lakhlifi-Dahoo extended model described in this chapter makes it possible to determine the favorable trapping sites (cage structure) of the molecular species according to their structure and size. The Langmuir constants used for calculating the abundances of trapped species, in the Van der Waals-Platteeuw thermodynamic model, are determined and presented in the simple van't Hoff form. The results of the calculations concerning some triatomic molecules are presented. On the contrary, the vibrational and rotational energy levels of the trapped molecule are perturbed. The frequencies of the vibrational transitions are shifted, generally, by a few percent, while the rotational spectrum as well as the translation movement of the molecule's center of mass undergo important modifications. The inclusion model also makes it possible to calculate the modified spectrum, using an approach similar to that described in Chapter 3 of Volume 1 [DAH 17] concerning the calculation of the shifts and the widths of the spectral lines. To illustrate these calculations, we focused on CO2, a highly studied species because of its importance in planetary atmospheres.

In Chapter 4, the comparison of simulation results with high-resolution IR spectroscopy observations of samples diluted in solid media such as rare gas matrices revealed two trapping sites for C3 and O3, in rare gas matrices, a single site and a double site, in a face-centered cubic lattice. As for the molecule-matrix coupling, it is different at the two sites, the energy relaxation being affected by multiphonon direct transfer at one site and by another mechanism at the other. The presence of the molecule in the crystal lattice...