How do dipole transitions typically appear in X-ray spectra?

Dipole transitions are specifically linked to electric dipole moments and require a change in the dipole moment of a molecule or atom during the transition. In the context of X-ray spectra, these transitions are considered "forbidden" under certain conditions, particularly in heavier elements or in certain electronic configurations.

In X-ray spectroscopy, dipole transitions often occur in the higher energy range. However, the probability of these transitions happening for core electrons (which are typically involved in X-ray emissions) is significantly reduced owing to quantum mechanical selection rules. As a result, many dipole-allowed transitions that might be expected do not occur or are extremely weak, leading them to be classified as forbidden.

Because of this low probability, if dipole transitions are present in X-ray spectra, they may either be very weak or entirely absent, thus rendering them not easily observed. Hence, the characterization of these transitions as forbidden aligns well with how they manifest in the X-ray spectra, which makes the choice insightful regarding the nature of electronic transitions involved.