Quantum dynamics of molecules coupled to helium nanodroplets

点击翻译按钮获取中文摘要

The goal of this project is to investigate both experimentally and theoretically the ultrafast dynamics of molecules coupled to an environment of ultracold helium nanodroplets. In particular, the project focuses on the details of the interaction between laser-excited molecules and quantum fluid clusters, inducing droplet-dependent dissipation and decoherence of molecular dynamics. Thus, the irreversible molecular dynamics serves as diagnostics for characterizing and describing collective phenomena (superfluidity, elementary excitations) on the nano-scale. Using femtosecond lasers in combination with advanced detection schemes (photoelectron-, ion-imaging), electronic, vibrational and rotational wave packets will be probed. Different molecular systems will be studied that feature different interaction strengths with the helium environment as well as various degrees of complexity, ranging from alkali dimers, alkaline earth molecules, alkali halides, to small heterogeneous complexes.Theoretically, time dependent molecular wave packet propagation will be implemented utilizing known parameters and potential energy surfaces for the molecules. Crucially, dissipation and decoherence induced by the helium droplet will be included, starting from a phenomenological approach to identify the relevant irreversible channels. First, damping and decoherence rates are seen as fit parameters chosen to agree with experimental findings. Then a microscopic modeling of molecule-droplet interaction will be aimed at in order to relate damping and decoherence time scales and mechanisms to properties of the droplets (spectrum of excitations, density of states). Both the phenomenological and microscopic approach will be implemented with the help of stochastic Schr ¨odinger equations. In the case of the microscopic approach we have to determine bath correlation (“response”) functions which are obtained from a time-dependent density functional calculation. Eventually, direct signatures of superfluidity in and on the surface of helium droplets will be identified through wave packet dynamics.