6533b857fe1ef96bd12b435d

RESEARCH PRODUCT

Sound conversion phenomena at the free surface of liquid helium. II. Experimental determination of acoustic coefficients and surface absorption coefficients

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The acoustic coefficients of reflection, transmission, and transformation of first-, second-, and gas sound waves incident normally on the free surface of liquid helium have been determined. The experimental results are compared with the theory developed in Part I of this paper. It is shown that equilibrium theories of boundary conditions disagree with the experiments. Nonequilibrium theories, however, which take into account that mass and energy transport processes occur at the liquid-vapor interface of helium II, lead to a very good description of the experimental data below 2 K. Within experimental accuracy the measurements fully confirm several kinetic models of evaporation and energy transport processes. Near the λ point, deviations from the theory have been found which might be attributed to influences of critical phenomena. The surface absorption coefficients of first-, second-, and gas sound waves could be deduced from the experimental data on the acoustic coefficients, and it is shown that, in agreement with the theory, appreciable energy dissipation occurs at the free surface of helium II when a second-sound or a gas sound wave is incident.