Page 90 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon
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4
Plasma Atomic Emission Spectrometry
4.1 Theory
4.1.1 Atomic Transitions
The probability of transitions from given energy levels of a fixed atomic population (e.g. between the
lower level i and upper level j) was expressed by Einstein in the form of three coefficients. These are
termed transition probabilities as follows:
Such transitions are illustrated in Fig. 4.1, and they can be considered as representing the ratio of the
number of atoms undergoing a transition to the number in the initial level. The intensity I of a
em
spontaneous emission line is related to A by the equation.
ji
When a system is in thermodynamic equilibrium the level population, i.e. the number of atoms N in the
j
excited state, is given by the Boltzmann distribution law:
where No is the number of atoms in the ground (unexcited) state with an energy E 0 = 0, g j and g are the
0
statistical weights of the jth (excited)
