Collisioninduced

Collision-induced describes a family of phenomena that arise when molecules interact during a collision in which the encounter perturbs energy levels or creates transient dipole moments, enabling effects that would be unlikely or forbidden for isolated molecules. The key idea is that the interaction during contact temporarily alters the system's properties, allowing energy transfer, absorption, emission, or dissociation that does not occur in the absence of a collision.

One well-known example is collision-induced absorption (CIA). Nonpolar molecules such as H2, N2, or O2 can acquire a temporary dipole moment during a collision, enabling infrared absorption and producing a broad, continuum-like spectrum. CIA plays an important role in the infrared opacity of dense planetary atmospheres and in various astrophysical contexts.

Collision-induced dissociation (CID) is common in mass spectrometry, where ions gain energy through collisions with neutral gas molecules, leading to fragmentation. CID is a central tool for structural analysis and sequencing of biomolecules, as the resulting fragment patterns reveal information about molecular architecture.

Collision-induced emission (CIE) and related collisional excitations occur when collisions elevate molecules to excited states that subsequently decay by photon emission. This mechanism is relevant in dense interstellar environments and other astrophysical settings where collisional processes compete with radiative ones.

Collisional effects also contribute to pressure broadening of spectral lines, where frequent collisions perturb energy levels and alter line shapes. Together, collision-induced phenomena are central to spectroscopy, atmospheric science, and astrochemistry, and are described using collisional cross sections and potential-energy surfaces.