MICROSCOPIC DYNAMICS IN LIQUID DEUTERIUM - A TRANSITION FROM COLLECTIVE TO SINGLE-PARTICLE REGIMES

Well-defined (propagating) collective density fluctuations are found f or liquid deuterium up to large wave vectors at SVP conditions. Their phase velocities as well as the wave vector dependence of the excitati on linewidths and strengths indicate that what is experimentally obser ved is dominated by a continuation to large wave vectors of hydrodynam ic (sound-mode) excitations. For wave vectors 2.3 less than or equal t o Q less than or equal to 4 Angstrom(-1), the spectrum approaches that of a weakly interacting fluid. A transition to a regime dominated by single-particle (recoil and quantized rotation) motions is found to oc cur within 4.2 less than or equal to Q less than or equal to 4.5 Angst rom(-1), depending upon thermodynamic conditions.