A collision or transient molecular pair can possess a dipole even when the isolated molecules do not. The resulting opacity can be important in a dense molecular hydrogen atmosphere. Pair absorption per unit length scales approximately as the product of the two partner number densities, so its mass absorption coefficient is generally density-dependent. It supplies continuum absorption between ordinary spectral lines.
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Collision-induced absorption (CIA) and collision-induced emission (CIE) are phenomena that occur when molecules interact with one another during collisions, leading to changes in their energy states. These processes are particularly relevant in the context of molecular gases and contribute to their spectral properties. Here's a breakdown of each concept: ### Collision-Induced Absorption (CIA) - **Definition**: CIA refers to the absorption of light (electromagnetic radiation) resulting from the interactions between colliding molecules.