Astrophysical masers may be pumped either radiatively or collisionally.

These rely on different types of spectral lines-recombination lines and collisionally excited lines.

Collisionally excited lines are generally much stronger than recombination lines, and so have historically been used to determine abundances.

Even the hardest laboratory vacuum on Earth is still too dense for forbidden line emission to occur before atoms are collisionally de-excited.

The spectral lines formed by these photons are called collisionally excited lines (often abbreviated to CELs).

The neutral particles in this case are mostly hydrogen molecules in a cloud that would undergo gravitational collapse if it were not collisionally coupled to the plasma.

CID (also called collisionally activated dissociation - CAD) is a method used to fragment molecular ions in the gas phase.

However, recent studies have found that abundances derived from recombination lines seen in the spectrum of NGC 6543 are some three times higher than those derived from collisionally excited lines.

If gas molecules near the surface of a moon collide with each other many times before escaping into space, the gas is said to be "collisionally thick," and thus is thought of as a true atmosphere.

Collision-induced dissociation (CID), also called collisionally activated dissociation (CAD), involves the collision of an ion with a neutral atom or molecule in the gas phase and subsequent dissociation of the ion.