  When an electron (or positronium atom) is injected into liquid helium with
nearly zero energy, a bubble quickly forms around it. This phenomenon (which
also occurs in liquid hydrogen, liquid neon and possibly in solid helium)
lowers the mobility of the electron to a value similar to that for a positive
ion. We estimate the radius of the bubble at zero pressure and temperature
based on the zero point energy of the electron. If the liquid is held in a
state of negative pressure, the bubble will expand beyond the radius at zero
pressure. We also estimate the negative pressure such that a bubble once formed
will grow without limit.
