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The Laporte rule is a selection rule formally stated as follows: In a centrosymmetric environment, transitions between like atomic orbitals such as s-s, p-p, d-d, or f-f, transitions are forbidden. The Laporte rule (law) applies to electric dipole transitions , so the operator has u symmetry (meaning ungerade , odd).
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 ...
The Laporte rule is a rule that explains the intensities of absorption spectra for chemical species. It is a selection rule that rigorously applies to atoms, and to molecules that are centrosymmetric, i.e. with an inversion centre. It states that electronic transitions that conserve parity are forbidden. Thus transitions between two states that ...
As an example, consider the ground state of silicon.The electron configuration of Si is 1s 2 2s 2 2p 6 3s 2 3p 2 (see spectroscopic notation).We need to consider only the outer 3p 2 electrons, for which it can be shown (see term symbols) that the possible terms allowed by the Pauli exclusion principle are 1 D , 3 P , and 1 S.
Schematic of energy levels involved in two photons absorption. In atomic physics, two-photon absorption (TPA or 2PA), also called two-photon excitation or non-linear absorption, is the simultaneous absorption of two photons of identical or different frequencies in order to excite an atom or a molecule from one state (usually the ground state), via a virtual energy level, to a higher energy ...
For case b) the selection rules for quantum numbers , , and and for parity are the same as for case a). However for the rotational levels, the rule for quantum number J {\displaystyle J} does not apply and is replaced by the rule Δ N = 0 , ± 1 {\displaystyle \Delta N=0,\pm 1} .
A photon has spin 1, and when there is a transition with emission or absorption of a photon the atom will need to change state to conserve angular momentum. The term symbol selection rules are: ΔS = 0; ΔL = 0, ±1; Δl = ± 1; ΔJ = 0, ±1 .
These selection rules can be used for any crystal with the given crystal structure. KCl has a face-centered cubic Bravais lattice . However, the K + and the Cl − ion have the same number of electrons and are quite close in size, so that the diffraction pattern becomes essentially the same as for a simple cubic structure with half the lattice ...