title: Nanoscale probing of dynamics in local molecular environments.
abstract: Vibrational spectroscopy can provide information about structure, coupling, and dynamics underlying the properties of complex molecular systems. While measurements of spectral line broadening can probe local chemical environments, the spatial averaging in conventional spectroscopies limits insight into underlying heterogeneity, in particular in disordered molecular solids. Here, using femtosecond infrared scattering scanning near-field optical microscopy (IR s-SNOM), we resolve in vibrational free-induction decay (FID) measurements a high degree of spatial heterogeneity in polytetrafluoroethylene (PTFE) as a dense molecular model system. In nanoscopic probe volumes as small as 10(3) vibrational oscillators, we approach the homogeneous response limit, with extended vibrational dephasing times of several picoseconds, that is, up to 10 times the inhomogeneous lifetime, and spatial average converging to the bulk ensemble response. We simulate the dynamics of relaxation with a finite set of local vibrational transitions subject to random modulations in frequency. The combined results suggest that the observed heterogeneity arises due to static and dynamic variations in the local molecular environment. This approach thus provides real-space and real-time visualization of the subensemble dynamics that define the properties of many functional materials.
authors.affiliation: Department of Physics, Department of Chemistry, and JILA, University of Colorado , Boulder, Colorado 80309, United States.|Department of Chemistry, University of North Carolina , Chapel Hill, North Carolina 27599, United States.
authors.forename: Joanna M
authors.initials: JM
authors.lastname: Atkin
authors.affiliation: Department of Physics, Department of Chemistry, and JILA, University of Colorado , Boulder, Colorado 80309, United States.|Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Paul M
authors.initials: PM
authors.lastname: Sass
authors.affiliation: Department of Chemistry, University of Colorado , Boulder, Colorado 80309, United States.
authors.forename: Paul E
authors.initials: PE
authors.lastname: Teichen
authors.affiliation: Department of Chemistry, University of Colorado , Boulder, Colorado 80309, United States.
authors.forename: Joel D
authors.initials: JD
authors.lastname: Eaves
authors.affiliation: Department of Physics, Department of Chemistry, and JILA, University of Colorado , Boulder, Colorado 80309, United States.
authors.forename: Markus B
authors.initials: MB
authors.lastname: Raschke