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Selection rule. In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in atomic nuclei, and so on. The selection rules may differ according to the ...
Rotational spectroscopy has primarily been used to investigate fundamental aspects of molecular physics. It is a uniquely precise tool for the determination of molecular structure in gas-phase molecules. It can be used to establish barriers to internal rotation such as that associated with the rotation of the CH.
The remaining two integrals contributing to the probability amplitude determine the electronic spatial and spin selection rules. The Franck–Condon principle is a statement on allowed vibrational transitions between two different electronic states; other quantum mechanical selection rules may lower the probability of a transition or prohibit ...
This polarisation effect is a powerful tool for astronomers to detect and measure stellar magnetic fields. The Zeeman effect ( / ˈzeɪmən /; Dutch pronunciation: [ˈzeːmɑn]) is the effect of splitting of a spectral line into several components in the presence of a static magnetic field. It is named after the Dutch physicist Pieter Zeeman ...
Forbidden mechanism. In spectroscopy, a forbidden mechanism ( forbidden transition or forbidden line) is a spectral line associated with absorption or emission of photons by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particular selection rule but is allowed if the approximation associated with that ...
Bragg's law. In many areas of science, Bragg's law, Wulff –Bragg's condition, or Laue–Bragg interference are a special case of Laue diffraction, giving the angles for coherent scattering of waves from a large crystal lattice. It describes how the superposition of wave fronts scattered by lattice planes leads to a strict relation between the ...
X-ray absorption near edge structure (XANES), also known as near edge X-ray absorption fine structure (NEXAFS), is a type of absorption spectroscopy that indicates the features in the X-ray absorption spectra of condensed matter due to the photoabsorption cross section for electronic transitions from an atomic core level to final states in the energy region of 50–100 eV above the selected ...
Surface-enhanced Raman spectroscopy. Raman spectrum of liquid 2-mercaptoethanol (below) and SERS spectrum of 2-mercaptoethanol monolayer formed on roughened silver (above). Spectra are scaled and shifted for clarity. A difference in selection rules is visible: Some bands appear only in the bulk-phase Raman spectrum or only in the SERS spectrum.