Discrete atomic energy levels lying between the first and second ionization potentials which are metastable against both auto-ionization and radiative decay have been observed in the alkali elements. These levels arise from the excitation of an electron from the outermost closed shell of the atom and have characteristic lifetimes in the microsecond region. A beam of metastable alkali atoms is produced by electron bombardment of neutral ground-state atoms and is detected by collecting the charged products of the decay. Stern-Gerlach magnetic-deflection experiments have been performed to show unambiguously that the signals arise from paramagnetic atoms and not from possible stray-photon effects in the apparatus. The excitation energies, electron-bombardment production cross sections, natural lifetimes, and the tentative spectroscopic assignments of these atoms are listed below: The energy and lifetime of the metastable lithium atom are in good agreement with the available theoretical estimates. The metastable states in the heavier alkali atoms are classified in relation to the known
Ib spectral terms. Magnetic and electric fields have been found to be effective in reducing the lifetime of the metastable atoms. The Zeeman quenching is discussed in a separate paper. The Stark quenching rate has been found to be proportional to the square of the electric field and to increase rapidly with atomic number
Z. It is suggested that the Stark quenching results from a second-order interaction which involves the product of the electric field and the spin-orbit or other magnetic operators.
Summary form only given. We have observed recombination into a large range of Rydberg levels in an ultracold plasma. The plasma is created by two-photon ionization of laser-trapped and -cooled metastable xenon atoms. By changing the intensity and detuning of the ionizing laser pulse, the initial temperature and density of the plasma can be varied. This allows us to access plasma coupling parameters (???) between 0.1 and 5. In this range of values, we previously reported unexplained heating of the expanding cloud, between 10 and 100 ???s after creation of the plasma. The recently observed recombination appears to provide much of the extra energy responsible for this behavior. The recombined atoms, up to 20% of the created plasma, populate energy levels down to n=40. They survive for /spl sim/1 ms indicating that high angular momentum states are present. However, standard scaling laws for three-body recombination do not agree with the observed dynamics. Furthermore, Rydberg atom formation is incomplete at the time when the plasma expansion velocity has reached its maximum value. Various effects are under consideration as possible remedies to these inconsistencies.
For understanding and optimization of the photo refractive gain it is important to know the basic parameters of photo-ionization and recombination processes. In the nanosecond regime the pulse energies required for maximum coupling gain in semiconductors easily reach the limit for saturation of the deep centers. especially if one of the recombination times is larger than the pulse duration. In that case, induced absorption (or bleaching) will appear due to a redistribution of the population of the different levels of the deep center and counterbalance photorefractive amplification. The present simultaneous study of the kinetics of the induced absorption and the photocurrent in an InP:Fe sample reveals a number of interesting parameters like electron and hole recombination times, ionization cross sections and Fe2+/Fe3+ concentrations.