DYNAMIC MEASUREMENT OF THE SURFACE-TENSION OF LIQUID-HELIUM WITH A TWO-DIMENSIONAL ELECTRON-PROBE

Electrons deposited above the free surface of liquid helium exhibit coupled electron-ripplon modes whose frequencies are simply related to the surface tension α of helium. The hydrodynamic expression for the ripplon dispersion relation for pure 4He is shown to be valid up to the wavevector range 105 cm-1 6 cm-1 of the coupled modes. For a small amount of 3He (X3 10-6-10-8) added to pure 4He, we measure the variation of the surface tension in the 60-600 mK temperature range. These experiments, which cover the frequency range ν 10-300 MHz, agree fairly well with previous static (capillary) or low-frequency (surface sound velocity) experiments.