Selected chemical physics studies of weakly bound systems

This dissertation includes three studies of weakly bound systems. The first is a study of the anharmonic vibrational spectra for hydrogen-bonded (NH3)(HF) using the ab initio Vibrational Self-Consistent Field (VSCF) method at the MP2/TZP level of theory. An important feature of the VSCF method is its inclusion of all combinations of pair-wise mode-mode coupling, which this study finds to be particularly important for accuracy. Frequencies and infrared intensities were calculated for the fundamental, first and second overtone, and all 1 + 1 combination transitions. The fundamental frequency, 3268 cm-1, agrees well with the gas-phase value, 3215 cm-1. The most important feature of the higher excitations is the relatively large intensity for the 1 + 1 combination of the HF stretch and intermolecular stretch, 3417 cm-1 at 80 km mol-1, which can guide experimentalists into currently unexplored, and increasingly anharmonic, regions of the (NH 3)(HF) potential.; The second study explains the lack of magic numbers for Ne+He n, even though they were predicted by diatomics-in-molecules (DIM) studies and they exist for the similar systems, He+n and Ar+Hen. The key to solving this mystery was a quantum Diffusion Monte Carlo (DMC) method using a DIM parameterization of the potential energy. Energy calculations show that rarely do the DMC walkers venture within 100 cm-1 of the minimum potential energy when n > 7. Zero point energy spreads the wave functions over multiple minima and this overwhelms the potential's influence in creating magic numbers.; The third study shows that electron impact ionization induces extreme fragmentation in size-selected krypton clusters containing as many as 4 atoms. The main fragmentation channel, accounting for at least 96% of all processes, produced Kr+2 . The remaining 4% resulted in Kr+2 production even though neutral Kr3 and Kr4 clusters were present. This is very different than the fragmentation of argon clusters, considered a representative system, which predominantly fragments to Ar+2 . Further experimental and theoretical studies are needed to confirm if this aggressive ionization technique accesses highly repulsive excited states available to Kr+2 , but not Ar+2 .