| This thesis presents extensive studies on the quantum dynamics of diatom-diatom systems using primarily the time-dependent(TD) wave packet approach. The theoretical investigations include the photofragmentation of tetraatomic molecules and bimolecular diatom-diatom chemical reaction.; Specifically, the quantum mechanical study on direct photodissociation of H{dollar}sb2{dollar}0{dollar}sb2{dollar} is carried out using a time-dependent golden rule wave packet method on an empirical potential energy surface. The calculation shows that the rotation distributions are Gaussian-like and two OH radicals are highly correlated in rotation. Both the time-dependent and time-independent golden rule methods are employed to compute the vibrational predissociation decay widths for weakly bound clusters including D{dollar}sb2{dollar}HF, H{dollar}sb2{dollar}HF, HD-HF, and HF-HF. These quantum mechanical calculations reveal that the vibrational predissociation processes are very stereospecific and state specific, that the lifetimes depend critically on the type of mode being excited and the nature of the vibrational coupling, and the theoretical results are very sensitive to details of the interaction potential energy surface including the repulse of the PES.; The full-dimensional (6D) dynamics study for diatom-diatom chemical reactions is presented with the first initial state-selected total reaction probabilities, cross sections and thermal rate constants reported for H{dollar}sb2{dollar} + OH {dollar}to{dollar} H{dollar}sb2{dollar}O + H reaction. The effects of reagents rotation and vibration on reaction are examined in detail in this study. Our calculations reveal surprisingly sharp resonance-like features at low collision energies on the Schatz-Elgersma potential surface, and also demonstrate that the potential-averaged 5D (PA5D) treatment can produce reaction probabilities essentially indistinguishable from the full-dimensional (6D) treatment. |
