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Study Notes: The Origin of AAS Spectra

Consider an uncombined or free atom of an element in its lowest or ground state of energy. The atom can absorb a photon of radiation of a specific or quantised energy content that exactly corresponds to the difference in energy between the ground state and a higher energy state. When this happens the atom is said to be excited (A*). The transfer of energy is illustrated by the following equation:

A + hv arrow right A*

Almost instantaneously the excited atom gives up its absorbed energy and in so doing returns, or relaxes, to its ground state (A).

The process of absorption and subsequent relaxation occurs continuously while the atom is radiated with energy.

The absorption of energy by a population of free atoms is characterised by its atomic absorption spectrum which is represented by a plot of absorbance versus wavelength.

An atomic absorption spectrum manifests itself as a set of discrete, very narrow peaks, often referred at as ‘lines’. The number, wavelength-position and intensity of atomic absorption lines are characteristic of the particular element.

The important concept to understand about AAS is that it involves the energy transitions of electrons to produce the excited state. The transitions come about as the result of absorption of radiation in the ultraviolet and visible regions.

Consider the atomic absorption spectra for sodium:

The spectrum of gaseous sodium atoms showing three distinct absorption lines at about 285, 330 and 590 nm.

The three lines represent the transition of a single outer electron from its 3s (ground state) orbital to outer (vacant) 3p, 4p and 5p orbitals (each with a different energy level) so creating different states of excitation. The energy of the absorbed photons associated with these transitions is exactly equal to the difference in energy between the ground and excited states for sodium.

Note that atomic spectra can also show lines produced from excited states (eg 3p to 5s transition for sodium) however these lines are generally not useful in AAS as they are weak. Atomic absorption spectra arise predominantly from ground state transitions to produce what are called resonance lines.

The atomic spectrum characteristics of each element contain a number of discrete lines - the resonance lines and other lines that arise from excited states.

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