Capturing the Ultrafast Vibrational Decoherence of Hydrogen Bonding in Interfacial Water.

Vibrational sum-frequency generation (vSFG) measurements in the frequency and time domains reveal that the interfacial hydrogen bonded OH stretch at the water/calcium fluoride interface is composed of two populations oriented oppositely. The time-resolved vSFG free-induction decay suggested that, whereas the strongly hydrogen bonded OH vibrational stretches, centered near 3140 ± 11 cm-1, are oriented toward bulk water and lose their collective coherence within ∼70 ± 7 fs, the weakly hydrogen bonded OH species, centered near 3410 ± 12 cm-1, are pointed toward the interface and dephase within ∼50 ± 6 fs.