An Etymological Dictionary of Astronomy and Astrophysics
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A → selection rule obeyed by → discrete transitions. Among them are: rigorous selection rules for → electric dipole transitions (→ permitted) requiring: 1) ΔJ must be 0 or ± 1 with J = 0 ↔ 0 forbidden. 2) ΔM_J = 0, ± 1. 3) → Parity change, i.e. even ↔ odd.

→ rigorous; → selection; → rule.

Any of a set of rules specifying the relationships between the → quantum numbers that characterize the initial and final states of a quantum-mechanical system in a → discrete transition. Transitions that do not agree with the selection rules are called → forbidden and have considerably lower probability. There are several types of selection rules (→ rigorous selection rule, → LS coupling, etc.) for → electric dipole transition (→ permitted), → magnetic dipole (forbidden), electric → quadrupole (forbidden), etc.

Selection, verbal noun of → select; → rule.

The process of choosing a site for an astronomical observatory based on meteorology, seeing conditions, and access to the site.

→ site; → selection.