Study On The Infrared Molecular Spectroscopy Of XH_n Type Molecules

Molecular spectroscopy has been systemic and mature with the development for one century, not only old but also powerful subject which is. It has been a window to understand the molecular structure, properties and interactions, furthermore have played an important role in photochemistry, astrophysical and environmental applications. The studies in astronomy and atmosphere require much information of molecular absorption spectra, which have stimulated spectroscopists to do more experimental and theoretical research. However there is still some trouble for experiments, especially in recording the absorption spectrum at super high temperature(above 2000 K). Nevertheless, the stellar atmospheric spectra is high temperature absorption. The advances in computer and quantum chemistry are greatly helpful to predict molecular structure, energies and properties. Precise potential energy surface(PES) and dipole moment surface(DMS) can now be derived for polyatomic molecules from high level electron correlation theory using large one-particle basis sets. Based on the ro-vibrational Hamiltonian and dipole moment model, the ro-vibrational energy levels, wavefunctions and intensities can be calculated from the PES and DMS with variable method or high level Van Vleck perturbation theory. The calculations can be greatly helpful to the assignment of experimental spectra, have insight into some special experimental phenomena, and simulate the isotopic spectrum or high temperature spectrum. As mentioned above, the dissertation mainly concentrates on experimental and theoretical research for molecular PES and DMS, especial for XH_n molecules.In Chapter one, we introduce background knowledge related to the theories, models and methods used in this thesis. The basis of molecular spectroscopic theory is concerned with Hamiltonian model, which is discussed briefly in normal and internal coordinate respectively. The vibrational Hamiltonian models of the XH stretching in XH_n(n ≥ 2), and the XH stretching and bending in XH_nY_4-n(n = 1, 2) are described. The (3n-6) dimensional Hamiltonian model of small molecules named with MORBID is also introduced. In the perturbation approach, an effective Hamiltonian in normal coordinate is obtained via successive vibrational and rotational contact transformations. The relations between spectroscopic and molecular force constants in second-order perturbation theory are also given in detail. The methods to construct the symmetry-adapted basis functions...