The potential of laser-induced fluorescence spectroscopy of atoms is reviewed with emphasis on the determination of absolute densities. Examples of experiments with single-photon and two-photon excitation are presented. Calibration methods applicable with the different schemes are discussed. A new method is presented that has the potential to allow absolute measurement in plasmas of elevated pressure where collisional depletion of the excited state is present.
An attempt is made to unravel the influence of vibrational population distributions of molecular hydrogen in the electronic ground state on the formation of negative hydrogen ions in the volume of a magnetically confined low temperature plasma. The densities of rovibronically excited molecules are determined by laser-induced fluorescence spectroscopy and optical emission spectroscopy. Energy distributions of electrons are measured with a Langmuir probe and densities of negative ions are found by laser-induced photodetachment. A global model is presented that describes the stationary state of the molecular plasma. An overall view of the discharge is obtained on the basis of this model. This allows us to cross-check the obtained results for coherence. It also allows us to assess the relevance of specific processes and their dependences on the rovibronic molecular states and the discharge current. The influence of associative recombination of a hydrogen atom of the plasma bulk with a hydrogen atom adsorbed at the wall is discussed with respect to the influence of this process on the densities of other plasma particles, in particular the density of the negative ions.
Detection of excited electronic-ground-state hydrogen molecules with v(") up to 13 in a magnetic multipole plasma source was performed for the first time by laser-induced fluorescence with vacuum-ultraviolet radiation. The measurements are taken after fast shutoff of the discharge current. The rovibrationally excited molecules live longer than the plasma background light so that the fluorescence light can be detected with good signal-to-noise ratio. Absolute level populations are measured as well as decay times. The theoretically predicted suprathermal population of the vibrational distribution is clearly identified. The H- density is calculated on the basis of the measured populations and the measured electron energy distribution function. It is in excellent agreement with the H- density measured by photodetachment.
The negative ion density as a function of the hydrogen pressure (1–8mTorr) in the electron cyclotron resonance-driven version of the magnetic multipole volume source “Camembert III” is measured by means of the photodetachment technique. An optimum value is observed between 4 and 5mTorr, yielding a H− ion density of about 1.5×109cm−3 in the center of the source. The electron density monotonously increases in the range ∼(0.5–2.5)×1010cm−3 and the electron temperature decreases (∼1.25–0.5eV). The optimum pressure for H− production is equally reported for a conventional filamented multipole source, in which the influence of rovibrationally excited hydrogen molecules in the electronic ground state on the formation of H− is analyzed. The physical mechanism which determines the existence of this ion density maximum is discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.