We have made movies of electrons that are immersed in liquid helium.

1. Movies and Photographs of Electrons in Superfluid Helium Humphrey J. Maris, Brown University, DMR 0605355 We have made movies of electrons that are immersed in liquid helium. Electrons introduced into liquid helium normally form bubbles of diameter one sixth of a millionth of an inch (= 4 nm). By using a high amplitude sound wave we are able to expand the bubbles to a size large enough to see. In the last months we have worked on understanding where the electrons that we see come from. We believe that they result from the passage of cosmic rays through the cell. A cosmic ray excites the helium atoms that it passes near to. These helium atoms emit ultraviolet photons of energy around 16 eV. These photons can travel through the liquid without attenuation. When they strike the cell walls, electrons are photo-ejected and these are the electrons that we see. Photograph of a cloud of electrons in liquid helium. Each bright dot is an electron.

2. Movies and Photographs of Electrons in Superfluid Helium Humphrey J. Maris, Brown University, DMR 0605355 BROADER IMPACT1) Observation of the motion of individual electrons gives a new way to study how a liquid is moving. 2) We have worked with H.C. Materials, Inc., Illinois, to characterize a new piezoelectric material PMN-xPT that may have important practical applications. EDUCATION and OUTREACHThe movie has been distributed widely on the web. We have answered many e-mails from high school and undergraduate students asking for explanations of how the experiment works. Four graduate students have worked on this project during the past year. The most recent graduate, Wei Guo is now a posdoc at Yale.