FORBIDDEN TRANSITIONS AND METASTABLE ATOMS

We have seen in Chapter 5 that the transition probability for electric dipole radiation, equation (4.23), is only non-zero if certain selection rules are satisfied. In particular the initial and final states of the system must have opposite parity, where the parity of an electron configuration is 

Many other atoms possess metastable levels, e.g. mercury and the inert gases. In gas discharges these play an important role since the population in the metastable levels can be very high and these atoms are more easily ionized than atoms in the ground state. However, forbidden magnetic dipole and electric quadrupole lines are difficult to observe in the spectra of laboratory sources since they are extremely weak in comparison with the allowed electric dipole transitions. Also, the metastable atoms are often quenched by collisions with other atoms or with the walls of the discharge tube before radiation can occur. By contrast, in the upper atmosphere, the solar corona, and in gaseous nebulae the densities of atoms are so low that collision pro- 

Metastable levels also occur in the energy-level schemes of simple molecules, e.g. N2, O2, NO, and CO. These metastable molecules are responsible for prominent emission 

The transition probabilities for magnetic dipole and electric quadrupole radiation are important since they can be combined with measurements of the absolute and relative intensities of forbidden lines emitted by nebulae, the aurora, or the solar corona to yield estimates of the number density, composition, and temperature existing in these various sources. We therefore proceed to obtain explicit expressions for these transition probabilities, making use of the expressions for the power radiated from the corresponding classical current and charge distributions which we obtained in sections 2.10 and 2.11. 

Breakdown of the electric dipole approximation. The expression for the transition probability for spontaneous emission of electric dipole radiation in a one-electron atom. 

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