Diagnostics of atoms by laser spectroscopic methods in plasmas and plasma-wall interaction studies (vacuum ultraviolet and two-photon techniques)

Abstract

We describe in this paper the measurement of light atom concentrations in plasmas and their detection in plasma-wall interaction experiments. VUV radiation or multiphoton techniques are required due to the large energy differences between the ground state and the excited states. We discuss briefly commonly applied schemes of nonlinear optics to generate narrow-bandwidth tunable VUV radiation with particular emphasis on high-order stimulated anti-Stokes Raman scattering in hydrogen gas. Applications of absorption and laser-induced fluorescence spectroscopy both to discharges and experiments with ion beams on targets are described. The discussion of multiphoton techniques focuses on the detection of sputtered atoms by resonantly enhanced multiphoton ionization (REMPI) and the detection of O and H in radiofrequency excited plasma reactors. Methods of absolute calibration and limitations by collisional effects are considered in this context. Examples of the detection of atomic hydrogen by light mixing are outlined in the conclusion.

Measurement and modeling of a diamond deposition reactor: Hydrogen atom and electron number densities in an Ar∕H2 arc jet discharge

“…Absorption measurements are made using the Balmer-␤ transition: the choice of a transition within the Balmer series is dictated in part by the experimental convenience of the visible wavelength region, and because H-atom-containing plasmas rapidly become optically thick to Lyman series radiation. 41 Measurements obtained using the H Balmer-␤ line can be directly compared with the outcomes of the model of the arc jet because the chemical kinetic scheme treats H͑n =1͒ and H͑n =2͒ independently. The n = 2 level is optically coupled to the ground state through emission of Lyman-␣ radiation ͑10͒, but, because of the very high Lyman-␣ oscillator strength, this radiation is effectively trapped by reabsorption in the plasma.…”

A novel diagnostic technique for H−(D−) densities in negative hydrogen ion sources

“…A very precise method is two-photon excitation laser-induced fluorescence (LIF) [42], with the drawback of being very complex and expensive. An alternative is the analysis of absolute line emission of H γ [41], where the effective excitation from atomic hydrogen is the dominant excitation path.…”

Determination of the degree of dissociation in an inductively coupled hydrogen plasma using optical emission spectroscopy and laser diagnostics

“…In practice it is more convenient to apply two-photon absorption with the aid of powerful commercial dye laser systems in combination with frequency doubling and mixing in nonlinear optical crystals. Ground state atoms are excited by simultaneous absorption of two UV laser photons to a higher electronic level according to the two-photon selection rules [11,12].…”