Page 3 - AAS & AES & FES 01082016_Neat

energy absorbed, ∆E, is the difference between the energy of the higher energy state and the

lower energy state.

As shown schematically in Fig, this atom has four electronic energy levels. E 0 is the

ground state, and the other levels are higher energy excited states. If the exact energies of each

level are known, the three wavelengths capable of being absorbed can be calculated as follows:

The calculated wavelengths λ 1, λ 2, and λ 3 all arise from transitions from the ground state

to excited states. Absorption lines due to transitions from the ground state are called resonance
lines. It is possible for an electron in an excited state to absorb radiant energy and move to an

even higher excited state; in that case, we use the ∆E values for the appropriate energy levels
involved. As we will see, in AAS most absorptions do arise from the ground state.

Quantum theory defines the electronic orbitals in an atom and predicts the lowest energy
configuration (from the order of filling the orbitals). For example, the 11 electrons in sodium

1
6
2
2
have the configuration 1s 2s 2p 3s in the ground state. The inner shells (principal quantum
number, n = 1 and 2) are filled and there is one electron in the n = 3 shell. It is this outer shell
electron that is involved in atomic absorption transitions for sodium.
UV and visible wavelengths are the range of radiant energies absorbed in AAS. UV/VIS
radiation does not have sufficient energy to excite the inner shell electrons, only the electrons

in the outermost (valence) shell are excited. This is true of all elements: only the outermost

electrons (valence electrons) are excited in AAS.
While atomic spectroscopy considers the energy state of the atom and considers

quantized leaps from one state to another, a simplified picture can be developed for the electronic
transitions that are of interest in atomic absorption.

The number of energy levels in an atom can be predicted from quantum theory. The

actual energy differences of these levels have been deduced from studies of atomic spectra.
These levels have been graphed in Grotrian diagrams, which are plots for a given atom showing

energy on the y-axis and the possible atomic energy levels as horizontal lines. A partial
Grotrian diagram for sodium is shown in next Fig.

2