Dynamics of van der Waals clusters: Theoretical and computational studies.

摘要

The marriage of two very powerful techniques---cryogenic matrix isolation spectroscopy and seeded supersonic molecular beams---has led to the development of a novel type of cryogenic matrix isolation spectroscopy in ultracold, near 0 K, He droplets. The technique known as helium nanodroplet isolation (HENDI) has seen tremendeous experimental interest over the past 20 years; this in turn has resulted in the availability of spectroscopic data for many molecules and clusters embedded in He clusters. The experimental findings have motivated a large number of theoretical calculations.;This dissertation focuses on theoretical and computational studies of the rotational dynamics of weakly bound van der Waals clusters with its main theme being the dynamics of molecules and small molecular dimers embedded in superfluid 4He nanodroplets.;The single molecular dopant systems studied were clusters of HCN-(He) N, HX-(He)N, where X = F, Cl, Br as well as NH3-(He)N, with N = 1 ≈ 20. Ground and excited state calculations were performed using the rigid body diffusion Monte Carlo (RBDMC) algorithm. For the excited state calculations a new approach was developed: adiabatic-node DMC (ANDMC).;The ANDMC method was used to study the renormalization of molecular rotational constants in He droplets. It revealed that the dynamics depend on a delicate interplay between the gas phase rotational constant value and the anisotropies in the potential energy interaction between the He atom and the dopant.;Also presented are the results of the first DMC simulations of the ammonia dimer doped into a small droplet of 4He. Further, a new approach to finding nodal surfaces for DMC simulations was developed that involved using a genetic algorithm (GA). This method was implemented to systematically and automatically compute nodal surfaces of excited states of the HCN- 4He complex and of the interchange tunneling splitting in the hydrogen-bonded HCl-HCl complex. The classical rotational dynamics of HX-4He complexes with X = F, Cl, Br, CN were studied to gain insight into quantum simulations and revealed highly chaotic dynamics for states with J > 0. Fractal Weyl law behavior in an open, chaotic Hamiltonian system is the subject of the final chapter.