title: Guest-host interactions investigated by time-resolved X‑ray spectroscopies and scattering at MHz rates: solvation dynamics and photoinduced spin transition in aqueous Fe(bipy)3(2+).
abstract: We have studied the photoinduced low spin (LS) to high spin (HS) conversion of [Fe(bipy)(3)](2+) in aqueous solution. In a laser pump/X-ray probe synchrotron setup permitting simultaneous, time-resolved X-ray diffuse scattering (XDS) and X-ray spectroscopic measurements at a 3.26 MHz repetition rate, we observed the interplay between intramolecular dynamics and the intermolecular caging solvent response with better than 100 ps time resolution. On this time scale, the initial ultrafast spin transition and the associated intramolecular geometric structure changes are long completed, as is the solvent heating due to the initial energy dissipation from the excited HS molecule. Combining information from X-ray emission spectroscopy and scattering, the excitation fraction as well as the temperature and density changes of the solvent can be closely followed on the subnanosecond time scale of the HS lifetime, allowing the detection of an ultrafast change in bulk solvent density. An analysis approach directly utilizing the spectroscopic data in the XDS analysis effectively reduces the number of free parameters, and both combined permit extraction of information about the ultrafast structural dynamics of the caging solvent, in particular, a decrease in the number of water molecules in the first solvation shell is inferred, as predicted by recent theoretical work.
authors.affiliation: Centre for Molecular Movies, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark. hald@fysik.dtu.dk
authors.forename: K
authors.initials: K
authors.lastname: Haldrup
authors.forename: G
authors.initials: G
authors.lastname: Vankó
authors.forename: W
authors.initials: W
authors.lastname: Gawelda
authors.forename: A
authors.initials: A
authors.lastname: Galler
authors.forename: G
authors.initials: G
authors.lastname: Doumy
authors.forename: A M
authors.initials: AM
authors.lastname: March
authors.forename: E P
authors.initials: EP
authors.lastname: Kanter
authors.forename: A
authors.initials: A
authors.lastname: Bordage
authors.forename: A
authors.initials: A
authors.lastname: Dohn
authors.forename: T B
authors.initials: TB
authors.lastname: van Driel
authors.forename: K S
authors.initials: KS
authors.lastname: Kjær
authors.forename: H T
authors.initials: HT
authors.lastname: Lemke
authors.forename: S E
authors.initials: SE
authors.lastname: Canton
authors.forename: J
authors.initials: J
authors.lastname: Uhlig
authors.forename: V
authors.initials: V
authors.lastname: Sundström
authors.forename: L
authors.initials: L
authors.lastname: Young
authors.forename: S H
authors.initials: SH
authors.lastname: Southworth
authors.forename: M M
authors.initials: MM
authors.lastname: Nielsen
authors.forename: C
authors.initials: C
authors.lastname: Bressler
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D007700:Kinetics; D055668:Photochemical Processes; D011789:Quantum Theory; D013052:Spectrometry, X-Ray Emission; D013816:Thermodynamics; D013997:Time Factors; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D005290:Ferric Compounds; D014867:Water