What occurs when free electrons and positive ions collide with each other?

When free electrons and positive ions collide with each other, recombination occurs. This process involves a free electron being captured by a positively charged ion, effectively neutralizing the charge of both particles. As a result of this collision, the electron and ion form a neutral atom or molecule, releasing energy in the form of a photon during this transition. Recombination is a crucial process in plasma physics, astrophysics, and many applications involving gases and charged particles, including the development of electricity in ionic and plasma systems.

In the context of the other choices: dissociation refers to the process where a compound breaks into two or more fragments, ionization involves the formation of ions from neutral atoms or molecules (typically through energy input), and fusion usually refers to the merging of atomic nuclei, a process related to nuclear reactions rather than the interaction of free electrons and positive ions. Understanding these definitions helps clarify why recombination is the correct answer in this scenario.