EMISSION, ABSORPTION, AND FLUORESCENCE SPECTRA

Each line in the line spectra can be considered as monochromatic radiation. Because of the wave character of light, each line in the spectrum is characterized in terms of its wavelength (A)  

The recommended unit of wavelength is the nanometer (1 nm = 10 m). The traditional unit is the Angstrom (lnm=10A). Other widely used units are the micron (1 fim = 10 m) and the millimicron 

The distribution of the spectral lines of each individual element is not random. It was discovered first empirically and also later shown theoretically that the wavelengths of the lines of the simple atomic spectra can be fitted to simple series formulae with great accuracy. Furthermore, many of the lines in the simple spectra occur in small groups which are called multiplets, such as doublets of the alkali metals or triplets of the alkaline earths. There is also a constant difference between the wavenumbers of the two components of some doublets or two of the three components of some triplets. For example, the two lines of each doublet are separated by 17 cm in the atomic spectrum of sodium (Table 3). This has been shown by Ritz to be a direct consequence of a general rule named the combination principle. According to this principle, for each atom or molecule there is a set of spectral terms 

Nuclear Electron Ionization Valence Molecular Spin orientation 

An important advance in atomic spectroscopy was made in 1913 by Niels Bohr. According to the Bohr theory any atom is allowed only certain discrete and characteristic energy values. Absorption or emission of radiation is a result of a transition between two energy levels of the atom. These energy levels were found to be directly proportional to the empirical spectral terms of the atom. Planck showed the following relationship between the energy and frequency of a particular radiation  

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With gas-phase atoms or ions, there are no vibrational or rotational energy states. This means that only electronic transitions occur. Thus, atomic emission, absorption, and fluorescence spectra are made up of a limited number of narrow spectral lines. 

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