title: The Hydrated Excess Proton in the Zundel Cation H5 O2 (+) : The Role of Ultrafast Solvent Fluctuations.
abstract: The nature of the excess proton in liquid water has remained elusive after decades of extensive research. In view of ultrafast structural fluctuations of bulk water scrambling the structural motifs of excess protons in water, we selectively probe prototypical protonated water solvates in acetonitrile on the femtosecond time scale. Focusing on the Zundel cation H5 O2 (+) prepared in room-temperature acetonitrile, we unravel the distinct character of its vibrational absorption continuum and separate it from OH stretching and bending excitations in transient pump-probe spectra. The infrared absorption continuum originates from a strong ultrafast frequency modulation of the H(+) transfer vibration and its combination and overtones. Vibrational lifetimes of H5 O2 (+) are found to be in the sub-100 fs range, much shorter than those of unprotonated water. Theoretical results support a picture of proton hydration where fluctuating electrical interactions with the solvent and stochastic thermal excitations of low-frequency modes continuously modify the proton binding site while affecting its motions.
authors.affiliation: Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, 12489, Berlin, Germany.
authors.forename: Fabian
authors.initials: F
authors.lastname: Dahms
authors.affiliation: Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, 12489, Berlin, Germany.
authors.forename: Rene
authors.initials: R
authors.lastname: Costard
authors.affiliation: Department of Chemistry, Ben Gurion University of the Negev, P.O.B. 653, Beersheva, 84105, Israel.
authors.forename: Ehud
authors.initials: E
authors.lastname: Pines
authors.affiliation: Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, 12489, Berlin, Germany.
authors.forename: Benjamin P
authors.initials: BP
authors.lastname: Fingerhut
authors.affiliation: Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, 12489, Berlin, Germany. nibberin@mbi-berlin.de.
authors.forename: Erik T J
authors.initials: ET
authors.lastname: Nibbering
authors.affiliation: Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, 12489, Berlin, Germany. elsasser@mbi-berlin.de.
authors.forename: Thomas
authors.initials: T
authors.lastname: Elsaesser
keywords: Zundel cation; Zundel continuum; femtochemistry; hydrated protons; solvation dynamics