Structural dynamics in quantum solids

Abstract

Structural dynamics in semi-quantum and quantum solids (Ne, H₂ and D₂) is reported, and compared to results in classical solids such as Ar. The structural dynamics is driven by excitation of the lowest Rydberg state of the NO impurity. The resulting charge redistribution induces a local radial deformation of the medium (‘bubble’ formation) around the impurity. The steady-state spectroscopic signatures of this process are presented and analysed in the configuration coordinate model and harmonic approximation. Intermolecular potentials describing the impurity-medium interaction are obtained. The dynamics of ‘bubble’ formation and the ensuing medium response are probed in real-time by femtosecond pump-probe spectroscopy. In the very soft H₂ and D₂ environments, ‘bubble’ formation is a one-way process without recurrence of the cage motion and is complete in ∼1–2 ps. In the case of solid Ne, the dynamics are characterized by an initial expansion of the matrix cage around the impurity, followed by a low frequency recurrence. The results in solid Ne are complemented by preliminary molecular dynamics simulations. Overall the trend observed is that the expansion becomes slower in the sequence Ar–Ne–hydrogens, which is counterintuitive. This is discussed in terms of the quantum (delocalised) character of the lighter media, which introduces an additional microscopic friction. These results establish our experimental procedure based on the use of low-n Rydberg states as a novel method for probing structural and electronic solvation dynamics in non-polar media.