Collective excitations in liquid4He: I. Experiment and presentation of data

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Abstract
We report neutron scattering measurements of the dynamic structure factor S(Q, omega ) of liquid 4He in the collective excitation regime. The use of the IN6 time-of-flight spectrometer at the Institut Laue-Langevin has enabled us to cover the wavevector region from 0.3 to 2.1 AA-1 in a continuous manner with an accurately calibrated energy scale and an energy resolution of approximately 0.1 meV; temperatures between 1.24 and 4.95 K were examined at saturated vapour pressure. In the wavevector region investigated, sharp excitations are seen that are unique to superfluid 4He; it is the purpose of the present paper to examine their temperature dependence and to discuss measurements of the multiphonon continuum scattering at higher energies. At small wavevectors in the 'phonon' region, the excitations are relatively unaffected by the lambda transition, whereas at larger Q a sharp component disappears at or close to Tlambda . The temperature variation of S(Q, omega ) is much more rapid in the superfluid phase than in the normal phase. In paper II of this series, the results for S(Q, omega ) will be compared with several models for the temperature variation of the 4He excitation spectrum.