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Encyclopedic overview
When an electron leaves a helium atom, it leaves an electron hole in its place. This causes the helium atom to become positively charged.
In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle denoting the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or crystal lattice the negative charge of the electrons is balanced by the positive charge of the atomic nuclei, the absence of an electron leaves a net positive charge at the hole's location.
Excerpted from Wikipedia’s “electron hole” article, available under the CC BY-SA 4.0 licence.