Metastable states in sputtered sample atoms: a tutorial review in perspective of elementary plasma processes

Abstract

In glow discharge (GD) plasma, the excitation/ionization of sputtered, analytically interesting atoms can occur by collisional interaction of the ground state of sputtered atoms M_(g) with the plasma particles (such as electrons and excited atoms/ions of plasma gas). Similarly, the excitation/ionization of sputtered atoms can also occur by collisional interaction of the metastable states of sputtered atoms M_(m) and plasma particles. Thus, the M_(m) can provide an alternative imperative route of excitation/ionization for sputtered atoms of elements in the plasma. The excitation/ionization via M_(m) can be particularly important for selective plasma processes, such as asymmetric charge transfer and Penning excitation. Extensive work has been done in analytical GDs regarding metastable states of plasma gases I_(m). However, so far, very modest effort has been extended in the direction of the M_(m). Knowledge of the cross-sections and rate constants of various collisional processes en-route of the M_(m) are valuable to provide broad physical data as input for theoretical calculations of ionization rates and accurate prediction of relative sensitive factors in mass spectrometry. All of these factors make it extremely important that detailed information about the metastable states of the analyte atoms is available. In this tutorial review paper, the metastable states of analyte atoms, types of metastability, populating/depopulating collisional and radiative processes, and their stringent role in the gas-phase collisional processes are reported and critically discussed in detail.