The effect of temperature upon an element

The principle of atomic absorption and flame emission is best understood through an experiment performed by Kirchhoff more than a century ago in which he demonstrated that incandescent gases absorb at the same wavelengths as they are able to emit. 

Chemical Analysis Second Edition Francis and Annick Rouessac 

CHAPTER 13 - ATOMIC ABSORPTION AND FLAME EMISSION SPECTROSCOPY 

The Maxwell-Boltzmann distribution function permits a calculation of the effect of temperature on each electronic transition. By designating Nq as the number of atoms in the fundamental state and Af as those in the excited state, one obtains  

Each atomic transition leads to an emission or an absorption of energy, which corresponds on the spectrum to a very narrow interval of wavelength. The uncertainty around the calculated value constitutes the natural bandwidth of the spectral line. The width is temperature dependent and can pass from 10 3 nm under ideal conditions, to about 0.002 nm at 3000 K. In reality, the line widths as observed through the monochromator of an instrument are much greater owing to the current technical limits of spectrometers. 

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