title: Coherent low-frequency motions of hydrogen bonded acetic acid dimers in the liquid phase.
abstract: Ultrafast vibrational dynamics of cyclic hydrogen bonded dimers and the underlying microscopic interactions are studied in temporally and spectrally resolved pump-probe experiments with 100 fs time resolution. Femtosecond excitation of the O-H and/or O-D stretching mode gives rise to pronounced changes of the O-H/O-D stretching absorption displaying both rate-like kinetic and oscillatory components. A lifetime of 200 fs is measured for the v=1 state of the O-H stretching oscillator. The strong oscillatory absorption changes are due to impulsively driven coherent wave packet motions along several low-frequency modes of the dimer between 50 and 170 cm(-1). Such wave packets generated via coherent excitation of the high-frequency O-H/O-D stretching oscillators represent a clear manifestation of the anharmonic coupling of low- and high-frequency modes. The underdamped low-frequency motions dephase on a time scale of 1-2 ps. Calculations of the vibrational potential energy surface based on density functional theory give the frequencies, anharmonic couplings, and microscopic elongations of the low-frequency modes, among them intermolecular hydrogen bond vibrations. Oscillations due to the excitonic coupling between the two O-H or O-D stretching oscillators are absent as is independently confirmed by experiments on mixed dimers with uncoupled O-H and O-D stretching oscillators.
authors.affiliation: Max-Born-Institut fur Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2A, D-12489 Berlin, Germany. heyne@mbi-berlin.de
authors.forename: Karsten
authors.initials: K
authors.lastname: Heyne
authors.forename: Nils
authors.initials: N
authors.lastname: Huse
authors.forename: Jens
authors.initials: J
authors.lastname: Dreyer
authors.forename: Erik T J
authors.initials: ET
authors.lastname: Nibbering
authors.forename: Thomas
authors.initials: T
authors.lastname: Elsaesser
authors.forename: Shaul
authors.initials: S
authors.lastname: Mukamel
mesh_terms: D019342:Acetic Acid / Q000737:chemistry*; D003198:Computer Simulation; D019281:Dimerization; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D009038:Motion; D044367:Phase Transition; D012996:Solutions; D014732:Vibration
chemical_list: D012996:Solutions; D006859:Hydrogen; D019342:Acetic Acid