Abstract
The coupled lattice and charge dynamics induced by phonon excitation in polycrystalline acetylsalicylic acid (aspirin) are mapped by femtosecond x-ray powder diffraction. The hybrid-mode character of the 0.9 ± 0.1 THz methyl rotation in the aspirin molecules is evident from collective charge relocations over distances of some 100 pm, much larger than the sub-picometer nuclear displacements. Oscillatory charge relocations around the methyl group generate a torque on the latter, thus coupling electronic and nuclear motions.
Highlights
The interplay of electronic and nuclear motions in molecular systems is at the heart of numerous processes in physics and chemistry
We present a study of phonon driven charge relocations in polycrystalline aspirin by femtosecond x-ray powder diffraction
The linear absorption spectra of aspirin molecules diluted in liquid solvents and of aspirin crystals display a similar pattern of electronic absorption bands, pointing to a minor electronic coupling of aspirin molecules in the crystal and a localized character of the underlying electronic excitations.[15]
Summary
The interplay of electronic and nuclear motions in molecular systems is at the heart of numerous processes in physics and chemistry. Femtosecond x-ray diffraction has been applied to make such behavior directly visible.[1,2,3] In the prototype material potassium dihydrogen phosphate (KH2PO4, KDP), coherent vibrational motions along a transverse-optical (TO) phonon coordinate induce a relocation of electronic charge within the PO4 groups and, to lesser extent, between the Kþ ion and the PO4 groups.[2,3] The length scale of charge relocation is on the order of 100 pm, i.e., a chemical bond length, while the nuclear elongations along the TO phonon coordinate are in the sub-picometer range This hybrid character of the TO phonon is very similar to the behavior of low-frequency soft-modes in crystalline ferroelectrics which display a strong coupling to the electronic system and undergo a pronounced frequency down-shift upon the phase transition from a para- to a ferroelectric phase of the material.[4,5,6,7,8,9]. The behavior observed here is a direct manifestation of a dynamic hybrid-mode response
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