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.

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The neutron scattering function for liquid 3 He at 120 mK and pressures of 0, 0.5, 1, and 2 MPa has been determined for wave vectors in the range 3 nm -1 < Q < 20 nm _1 . This represents the first experimental information on the density dependence of the zero-sound frequency and damping at finite wave vectors, and should serve as a useful test of theories on excitations in neutral Fermi liquids. The results are in qualitative agreement with extensions of the Landau Fermi-liquid theory to finite wave vectors.PACS numbers: 67.50.Dg Neutron inelastic scattering offers a unique method to examine directly the elementary excitations in liquid 3 He on a microscopic scale, i.e., for wave vectors of order 10 nm -1 . Through their spin-dependent interaction with the 3 He nuclei, neutrons probe the nuclear spin fluctuations, in addition to the density fluctuations which are probed by the spin-averaged interaction. In the neutron scattering function, one distinguishes three components: the excitation of single particle-hole (p-h) pairs, a collective density mode (zero sound), and a broad distribution corresponding to multiple p-h excitations. The current theoretical and experimental situation in this area has recently been reviewed by Glyde. l Previous neutron-scattering experiments 2 " 5 have shown that the single p-h spectrum resembles the one expected for a noninteracting Fermi gas but with an effective mass m* ~ 3mo which lowers the energy of the p-h band compared with that of the noninteracting system. For Q < 10 nm -1 , the single-pair spin fluctuations are enhanced and are well described by the paramagnon model. 6 The zero-sound mode is well resolved for wave vectors less than 10 nm -1 and shows anomalous dispersion, i.e., the energies are above those corresponding to linear dispersion co^coQ, where Co is the ultrasonic zero-sound velocity. It broadens considerably with increasing wave vector even before the mode overlaps with the p-h band. All the results published so far at saturated-vapor pressure (SVP) are in good general agreement with the predictions of the polarization-potential theory developed by Pines and co-workers. 7 Here the potential parameters, obtained by a fitting to the neutron-scattering results and to the Landau parameters, have been used to calculate transport properties for both normal and superfluid 3 He which are in excellent agreement with measured values. 8 The effective interactions in the polarization-potential theory are expected to be sensitive to the density, as discussed by Aldrich and Pines 7 and as shown by the Landau parameters, which are the low-(? limits of the effective interactions.While in an earlier experiment 9 the scattering function for pressurized liquid 3 He could only be measured for wave vectors larger than 12 nm -1 , the present experiment explores the wave-vector range 3 nm ~l < Q < 20 nm" 1 . This therefore represents the first measurement of the zero-sound mode at elevated pressures.The measurements were made at the time-of-flight spectrometer 10 IN6 at the high-flux...

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