Dipole transitions and transition probabilities

The probability that a particular spectroscopic transition will occur is called the transition probability, or 

we discuss only some practical terms used to describe the probability of a transition, which is commonly called the transition strength. To a first approximation, transition strengths are governed by selection rules, which determine whether a transition is allowed or not (see Boxes 2.1 and 2.2). Practical measurements of transition strengths are usually described in terms of the well-known Einstein A and B coefficients, or the oscillator strength/. 

When interacting resonantly with a photon, an atom or molecule changes from one energy level to another while in an excited energy state it can also decay spontaneously to a lower state. The probability of an atom or molecule changing states depends 

on the nature of the initial and final state wave functions  

Electronic-state configurations are described by the standard quantum numbers or principal quantum number n, angular momentum quantum number L, spin quantum number S and total angular momentum quantum number J. In the so-called Russel-Saunders approximation of electron coupling, electronic states are commonly described symbolically by n For 

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