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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 rule is not made. [1]
The Laporte rule implies that s to s, p to p, d to d, etc. transitions should not be observed in atoms or centrosymmetric molecules. Practically speaking, only d-d transitions occur in the visible region of the spectrum. The Laporte rule is most commonly discussed in the context of the electronic spectroscopy of transition metal complexes ...
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).
Since both phosphorescence (transition from T 1 to S 0) and the generation of T 1 from an excited singlet state (e.g., S 1) via intersystem crossing (ISC) are spin-forbidden processes, most organic materials exhibit insignificant phosphorescence as they mostly fail to populate the excited triplet state, and, even if T 1 is formed ...
The weaker d–d transitions are potentially spin-allowed but always Laporte-forbidden. [2] Charge-transfer bands of transition metal complexes result from shift of charge density between molecular orbitals (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the transfer occurs from the MO ...
In this case, however, the ordinate is given by (α) 1/r, in which the exponent 1/r denotes the nature of the transition: [6], [7], [8] r = 1/2 for direct allowed transitions; r = 3/2 for direct forbidden transitions. r = 2 for indirect allowed transitions; r = 3 for indirect forbidden transitions
Principally, the electron can undergo a forbidden de-excitation from the excited triplet state, the electron can undergo a forbidden de-excitation to the ground state as radiative transition, called phosphorescence, or as non-radiative process. However, this occurs on a much slower time scale, being on the order of microseconds to seconds.
In molecular spectroscopy, a Jablonski diagram is a diagram that illustrates the electronic states and often the vibrational levels of a molecule, and also the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity . [ 1 ]