Energy states rules governing transition between

The selection rules governing transitions between electronic energy levels are the spin rule (AS = 0), according to which allowed transitions must involve the promotion of electrons without a change in their spin, and the Laporte rule (AL = 1 for one photon). This parity selection rule specifies whether or not a change in parity occurs during a given type of transition. It states that one-photon electric dipole transitions are only allowed between states of different parity [45],... 

The transition between the possible electronic energy states is governed by certain selection rules initially derived empirically. These are ... 

Moseley found that each K spectrum of Barkla contains two lines, Ka and K(3, and that the L spectra are more complex. Later important work, especially by Siegbahn,38 has shown that M, N, and O spectra exist and are more complex in their turn. Relatively numerous low-intensity lines are now known to exist in all series. Fortunately, the analytical chemist can afford to ignore most of these low-intensity lines in his practical applications of x-ray methods at present. It generally suffices for him to know that x-ray spectra at their most complex are enormously simpler than emission spectra involving valence electrons, and that most x-ratr lines are satisfactorily accounted for on the basis of the simple selection rules that govern electron transitions between energy states. 

If a charge exchange process, A + + B- A -f- B +, occurs when the distance between the two particles is large, we expect that no transfer of translational energy takes place in the reaction and that the same selection rules govern the ionization as in spectroscopic transitions. This means that if the molecule B is in a singlet state before the ionization, the ion B + will be formed in a doublet state after ionization of one electron without rearrangements of any other electrons, at least for small molecules. 

It would thus seem that promotion of a given electron in a molecule could result either in a singlet or a triplet excited state depending on the amount of energy added. However, this is often not the case because transitions between energy levels are governed by selection rules, which state that certain transitions are forbidden. There are several types of forbidden transitions, two of which are more important than the others. 

As already evident from the previous section, symmetry properties of a molecule are of utmost importance in understanding its chemical and physical behaviour in general, and spectroscopy and photochemistry in particular. The selection rules which govern the transition between the energy states of atoms and molecules can be established from considerations of the behaviour of atoms or molecules under certain symmetry operations. For each type of symmetry, there is a group of operations and, therefore, they can be treated by group theory, a branch of mathematics. 

THE RULES GOVERNING THE TRANSITION BETWEEN TWO ENERGY STATES... 

The strength or intensity of absorption is related to the dipole strength of transition D or square of the transition moment integral M m , and is pressed in terms of oscillator strength / or integrated molar extinction jfe Jv. A transition with /= 1, is known as totally allowed transition. But the transitions between all the electronic, vibrational or rotational states are not equally permitted. Some are forbidden which can become allowed under certain conditions and then appear as weak absorption bands. The rules which govern such transitions are known as selection rules. For atomic energy levels, these selection rules have been empirically obtained from a comparison between the number of lines theoretically... 

For molecules, the rules governing the transition between two given energy states are... 

In any allowed electronic energy transfer process, the overall spin angular momentum of the system should not change. This statement is known as Wigner s spin conservation rule. The rule is applicable whether the transfer occurs between an excited atom or a molecule and another molecule in its ground state or in the excited state. In an electronic transition between the energy states of the same molecule also, spin is necessarily conserved. But the phenomenon is governed by rules for dipole-dipole interaction. 

Predissociation is governed not only by the intersection of the potential energy curves (Franck-Condon principle) but by the selection rules which specify the types of state between which transitions may take place. These are treated fully by Herzberg. Accidental predissociation is said to occur when the dissociation takes place by two radiationless transitions via the intermediacy of a third state. 

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