At room temperature, electrons in noble gasses move about freely, limited only by collisions with the weakly interacting atoms.
[2] At low temperatures, electrons injected into liquid helium do not move freely as one might expect, but rather form small vacuum bubbles around themselves.
The electron is forbidden from entering the liquid for the same reason hydrogen atoms are stable: quantum mechanics.
The pressure-volume term is the amount of energy needed to push the helium out of the bubble.
A theoretical prediction has been made based on the analysis of the equation above,[5] that the 2S electron bubble exhibits a startling morphological instability under a wide range of ambient pressures.