Comparison of various two-photon excitation schemes for laser-induced fluorescence spectroscopy in atomic hydrogen

Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

“…This picture can be quantified by applying a rate-equation model for the laser-excited state population, as is, for instance, done in Ref. 19. This shows that for the H excitation scheme described here ''the average number of fluorescence photons emitted per ground-state atom'' saturates at Ϸ0.1 for a laser intensity around 10 9 W/cm 2 .…”

“…The excitation is monitored by detection of the nonresonant fluorescence on the Balmer-␣ transition at 656.3 nm from the 3d and 3s states to the 2p 2 P j states. Usually it is mentioned that the 3d←1s transition dominates over the 3s←1s transition due to a 7.56 times higher two-photon absorption cross section, 5,19,20 but actually it depends on the degree of saturation of the transition whether the relative population of 3d and 3s is determined by the relative transition strength or rather by the ratio of the statistical weights of the respective quantum states. Moreover, we found evidence for a pure statistical distribution of the population over all the nϭ3 sublevels from an accurate measurement of the lifetime of the excited state, 34 even though the applied laser intensity assures that the transition is far from saturation ͑see Fig.…”

“…Several schemes for twophoton excitation of H to nϭ3 and nϭ4 with detection of nonresonant fluorescence have been directly compared to each other. 19 These schemes differ in the number of laser beams and the wavelength combinations used for excitation, the intensity, polarization, and bandwidth of the laser beams, and the wavelength of the fluorescence light. This detailed comparison shows that the above described scheme for twophoton excitation of H with a pulsed, narrow-band 205 nm laser beam gives the largest average number of fluorescence photons per incident laser photon.…”

“…In the field of low-temperature plasma research, where the technique is also referred to as ''two-photon absorption laser induced fluorescence ͑TALIF͒, '' there has been a considerable effort on atomic hydrogen detection, with important contributions from the group of Miller 14 -18 and many others. [19][20][21][22][23] In these studies, the H LIF signal from the plasma is used to determine the hydrogen concentration, assuming that the LIF signal intensity is proportional to the ground-state density. The valuable density information gained in these experiments, however, is usually only qualitative in nature, due to one of the few disadvantages of the LIF technique.…”

Anomalous Atomic Hydrogen Shock Pattern in a Supersonic Plasma Jet

“…In the experiments reported here, ground-state hydrogen atoms are spatially probed by using a two-photon excitation laser-induced fluorescence (LIF) technique [12,13]. A 20 Hz Nd:YAG-pumped tunable dye laser is operated around 615 nm.…”