We obtain a series of incoherent-inelastic-neutron-scattering spectra covering the complete frequency range of vibrations of hydrogen atoms in water over a temperature range of -15 to 80°C and also in ice at 20 K for reference. Comparison of these frequency spectra with those generated by computer molecular dynamics with flexible water molecules indicates that the proton motion in a free water molecule is substantially affected by formation of hydrogen bonds in the condensed phases.PACS numbers: 61.12.Fy, 35.20.Gs, 61.25.Em As a result of the existence of spin incoherence in neutron scattering from protons, neutrons are uniquely suited for probing the single-particle motions of protons in hydrogen-containing substances. This feature is particularly valuable for the case of water because of the fact that the hydrogen motion is sensitively affected by the formation of hydrogen bonds which is so characteristic of this substance. Although traditionally Raman scattering has been used extensively to study the hydrogenbond dynamics in water, 1 " 4 the quantitative theoretical prediction of the resulting Raman spectra of water has been difficult. This is due to the fact that the intensity of the Raman bands is related to the fluctuation spectra of the polarizability tensor of the molecules which is difficult to calculate when every molecule is interconnected to its neighbors by the hydrogen-bond network. 5 On the other hand, the incoherent neutron scattering spectrum singles out the vibrational and diffusive motions of the hydrogen atoms which can be directly simulated on a computer with molecular dynamics. 6 The analysis of the neutron spectrum in terms of a computer molecular dynamics (CMD) simulation serves the valuable purpose of refining the intramolecular and intermolecular potential model for water. 7 In this paper we report for the first time measurements of incoherent neutron scattering spectra of water extending to an energy transfer of 600 meV at reasonably small momentum transfer and at a range of temperatures including that of a supercooled state. 8 Such measurements have become possible because of the high intensities of epithermal neutrons produced by a pulsed spallation source. In parallel with the neutron experiments we have developed a new series of CMD simulations of water taking into account the internal vibrational motion of the molecule. Previous CMD studies of water used mainly potential models applicable to rigid molecules; thus the results give little information on the intramolecular part of the spectra. 6,9 The combination of experiment and simulation allows us to unambiguously identify and understand the features of the vibrational spectrum in the 50-500-meV range. We must qualify that although the neutron spectra are easier to interpret in terms of the proton motion, the Raman scattering is still superior in its energy resolution compared with the present stage of development of the neutron scattering technique.The experiment was carried out at the intense pulsed neutron source (IPNS) at t...

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