The near-infrared and visible band systems of the diatomic molecules CaH, FeO, ScS, MnH, RhO, and RhN have been studied utilizing high resolution spectroscopy. Most of these metal-containing species are free radicals and as such provide edifying spectra from which information regarding electronic structure can be gleaned. Directly accessible phenomena modeled by effective Hamiltonian methods are molecular rotation, spin-orbit, spin-spin, spin-rotation, lambda-doubling, magnetic hyperfine interactions, and electric quadrupole interactions. In the case of CaH a deperturbation analysis was performed and in the case of RhN isotopic substitution was analyzed. For CaH, FeO, RhO, and RhN, the application of an external static electric field i.e. Stark effect) allowed determination of permanent electric dipole moments of the ground and excited states probed. For CaH and MnH, the application of an external static magnetic field (i.e. Zeeman effect) allowed determination of effective g-factors. Parameters obtained from the data indirectly allow a description of the molecular orbitals involved, and were also compared to ab initio calculations. Due to the transient nature of these gas phase molecules, production was accomplished via laser ablation of a metal sample and introduction of an appropriate reagent gas. Subsequent supersonic expansion and molecular beam formation then allowed probing of the molecules by Laser Induced Fluorescence (LIF). High-resolution spectra were facilitated by a Continuous-Wave (CW) ring laser. Complementary to this, low or medium resolution spectra were obtained with a pulsed-dye laser (controlled via a homemade Visual Basic 6.0 computer program). Several previously unobserved band systems have been detected as part of a project involving survey scans of Rh metal plus various gas reagents including CH4, SF6, NH3, and D2.

It is a great challenge in chemistry to clarify every detail of reaction processes. In older days chemists mixed starting materials in a flask and took the resul tants out of it after a while, leaving all the intermediate steps uncleared as a sort of black box. One had to be content with only changing temperature and pressure to accelerate or decelerate chemical reactions, and there was almost no hope of initiating new reactions. However, a number of new techniques and new methods have been introduced and have provided us with a clue to the examination of the black box of chemical reaction. Flash photolysis, which was invented in the 1950s, is such an example; this method has been combined with high-resolution electronic spectroscopy with photographic recording of the spectra to provide a large amount of precise and detailed data on transient molecules which occur as intermediates during the course of chemical reac tions. In 1960 a fundamentally new light source was devised, i. e. , the laser. When the present author and coworkers started high-resolution spectroscopic stud ies of transient molecules at a new research institute, the Institute for Molecu lar Science in Okazaki in 1975, the time was right to exploit this new light source and its microwave precursor in order to shed light on the black box.

Simple organometallic molecules, especially those with a single ligand, are the desired model systems to investigate the metal-ligand interactions. For such a molecule, a quantitative relationship between the geometry and the electronic configuration would be instructive to test the existing theories and to access more complicated systems as well. As a matter of fact, microwave spectroscopy could be the best approach to address this issue by measuring the pure rotational spectrum of a metal-containing molecule. By doing so, microwave spectroscopy can provide the most reliable bond lengths and bond angles for the molecule based on the rotational constants of a set of isotopologues. On the other hand, from the fine-structure and hyperfine-structure of the spectrum, microwave spectroscopy can also describe the electronic manifold, charge distribution and bonding nature of the molecule in a quantitative way. Fourier transform microwave spectrometers have been the most popular equipment to measure the pure rotational spectrum for three decades owing to the high resolution and super sensitivity. With the advances in digital electronics and the molecular production techniques, hyperfine structures of metal-containing molecules can be easily resolved even for the rare isotopologues in their nature abundance by this type of spectrometers. In this dissertation, molecules bearing metals in a wide range covering both the main group and transition metals were studied. By taking advantage of both the traditional and newly developed molecular production techniques in the gas phase (for example, metal pin-electrodes and discharged assisted laser ablation spectroscopy), we obtained spectra of molecules containing magnesium, aluminum, arsenic, copper and zinc. Our subjects include metal acetylides (MgCCH, AlCCH and CuCCH), metal dicarbides (CCAs), metal cyanides (CuCN, ZnCN) as well as other metal mono-ligand molecules. For the zinc metal, complexes with two simple ligands were also investigated, such as HZnCl and HZnCN. We strongly believe that researchers in different disciplines would benefit from our laboratory studies: theoretical chemists can use our experimental results for calibration; astrophysicists would interpret their telescope observations by matching our precisely measured frequencies; material scientists could find new functional materials by linking the bulky properties of certain materials with our spectroscopic results of the monomers.

The book reviews the results of vibration-rotational spectroscopy of molecules obtained recently by combining modern computational methods of quantum chemistry with the new techniques of high-resolution rotational and vibration-rotational spectroscopy. It shows for example that the tunneling vibration-rotational spectroscopy of the van der Waals complexes provides a new look at intermolecular forces while the high precision and sensitivity of the submillimeter-wave and Fourier transform microwave spectroscopy make it possible to study complex rotational spectra of molecules in excited vibrational states. New results of high level ab initio quantum chemical computations of vibrational and rotational energy levels and dipole moment functions of unusual molecules will be discussed together with the recent discovery of clustering of energy levels in asymmetric tops. Group theoretical analysis of floppy molecules, especially the tunneling effects in nonrigid molecules, will also be discussed.