Metastable atoms forbidden transitions

The four 4s levels play a key role in analytical glow discharges (e.g. for Penning ionization of the sputtered atoms) and they cannot easily be depopulated by radiative decay (due to forbidden transitions for the metastable levels, and to radiation trapping for the resonant levels). Therefore some additional loss processes are incorporated for these levels, in order to describe them with more accuracy ... 

By choosing a suitable atom density it is possible to place one atom on each potential minimum of the lattice. Such atoms isolated from their surrounding are excellent candidates for precise atomic clocks if one chooses a narrowband forbidden electronic transition to a metastable atomic state as clock transition. Since the atoms are (besides their zero-point motion) at rest no Doppler-effect contributes to line broadening or shift. There are intense investigations to realize an atomic clock... 

Figure 9 Three-level atoms with dipole transitions 1,2>- 3) in the Lambda configuration. The dipole forbidden transition between two metastable states 1> and 2) are coupled to each other via microwave transition (a), or Raman transition (b), or two-photon transition (c) with effective half Rabi frequency Qo Another optical field is resonantly coupled to the dipole transition 2)- 3> with half Rabi frequency Qi. The two cavity fields 012 are amplified from Rabi sidebands on the dipole transition 1)- 3>.

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-... 

In beam-foil experiments the velocities would be so great that no decay would be observed in any apparatus of convenient laboratory size. Similarly in the single-photon delayed-coincidence technique, the time required to obtain sufficient data would become quite prohibitive. The few reliable lifetime measurements that do exist have been made by the static afterglow technique. This was originally developed for experiments on the collisional destruction and diffusion of metastable atoms, which are discussed in detail in section 7.6. The difficulties encountered in the application of the afterglow and other methods to the experimental determination of the transition probabilities of forbidden lines have been reviewed by Corney (1973) and Corney and Williams (1972). 

In this chapter so far we have been concerned with forbidden transitions between low-lying metastable levels belonging to a single electronic configuration. Recently interest has focussed on the energetic metastable levels of hydrogen-like and. helium-like ions which emit radiation in the X-ray and vacuum ultraviolet region. Since these atoms are important in many atomic physics experiments, we... 

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