Impurity Rydberg States as Probes of Local Dynamics in the Condensed Phase

Abstract

We report on structural dynamics in simple van der Waals solids (Ar, Ne, and H2), as driven by the excitation of an impurity (NO) Rydberg state. The resulting charge redistribution induces a local radial deformation of the medium (qbubbleq formation). This process can be characterized by steady-state spectroscopy and analyzed within the framework of the configuration coordinate model and harmonic approxn. Intermol. potentials describing the impurity-medium interaction are obtained, allowing us to establish observables for the dynamics of the process in a femtosecond pump-probe expt. In the very soft H2 (D2) environment, qbubbleq formation is a one-way process without recurrence of the cage motion and is complete in .apprx.1.5 ps (3 ps). The data are rationalized in terms of a continuum model, where the overdamped nature of the medium response is attributed to energy dissipation by emission of sound waves and through friction. In the case of solid Ne and solid Ar, the expansion mechanism is characterized by a short inertial response at early times (100-200 fs), followed by an oscillatory motion of the cage over 2-3 ps. These results are complemented by mol. dynamics simulations, which allow us to identify the inertial response. Our exptl. procedure establishes an approach for the study of electronic solvation dynamics in non-polar media, which is now being extended to liqs. [on SciFinder (R)]