Quantum dynamics of molecules coupled to helium nanodroplets

本项目的目标是从实验和理论上研究分子与超冷氦纳米液滴环境耦合的超快动力学。特别是，该项目侧重于激光激发分子与量子流体团簇之间相互作用的细节，诱导液滴依赖的耗散和分子动力学的退相干。因此，不可逆分子动力学可以作为表征和描述纳米尺度上的集体现象（超流体、基本激发）的诊断方法。利用飞秒激光结合先进的探测方案（光电子、离子成像），将探测电子、振动和旋转波包。我们将研究不同的分子体系，它们与氦环境的相互作用强度不同，复杂程度也不同，从碱二聚体、碱土分子、碱卤化物到小的非均相配合物。理论上，时间相关的分子波包传播将利用已知的参数和分子的势能面来实现。至关重要的是，氦液滴引起的耗散和退相干将包括在内，从现象学方法开始识别相关的不可逆通道。首先，阻尼和退相干率被视为与实验结果一致的拟合参数。然后，将针对分子-液滴相互作用的微观模型，以便将阻尼和退相干时间尺度和机制与液滴的性质（激发谱，态密度）联系起来。现象学和微观方法都将在随机薛定谔方程的帮助下实现。在微观方法的情况下，我们必须确定浴槽相关（“响应”）函数，这些函数是从随时间的密度泛函计算中获得的。最终，将通过波包动力学确定氦液滴内部和表面超流体的直接特征。

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.