Plasmas: laser-induced fluorescence

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

Two-dimensional distributions of ground-state NO were detected by planar laser-induced fluorescence during the process of NO removal in a corona radical shower system in NO/dry air mixtures [57,58], The authors observed that the density of NO molecules decreased not only in the plasma region formed by the corona streamers and the downstream region of the reactor, but also in the upstream region of the reactor. They explained this behaviour by oxidation with ozone, which is transported upstream by electrohydrodynamic flow. 

The industrial application of Plasma Induced Chemical Vapour Deposition (PICVD) of amorphous and microcrystalline silicon films has led to extensive studies of gas phase and surface processes connected with the deposition process. We are investigating the time response of the concentration of species involved in the deposition process, namely SiH4, Si2H6, and H2 by relaxation mass spectroscopy and SiH2 by laser induced fluorescence. 

Some of the most powerful tools for in situ discharge diagnostics are optical (62). Plasma-induced emission spectroscopy, laser-induced fluorescence, laser absorption, and laser optogalvanic spectroscopy have all been... 

Chang and Lin [50] determined vigabatrin in human plasma by capillary electrophoresis and laser-induced fluorescence after precolumn derivatization with 5-carboxytetramethylrhodamine succinimidyl ester. Optimal separation and detection were obtained with an electrophone buffer of 50 mM sodium borate (pH 9.5) containing 10 mM sodium dodecyl sulfate and a green He-Ne laser with fluorescence detection at 589 nm and excitation at 543 nm. The assay was rectilinear over the concentration range of 1.5-200 /iM and the lower limit of detection was 0.13 /iM. Both the... 

Musenga et al. [55] described a capillary electrophoresis method for determination of vigabatrin in human plasma after precolumn derivatization with 6-carbox yfluorescein-N-s ucc i n i m i d i d yl ester. Optimal separation and detection were obtained with 50 mM borate buffer (pH 9.0) containing 100 mM N-methylglucamine with laser-induced fluorescence detector (Aexc = 488 nm). The assay was rectilinear over the concentration... 

Optical spectroscopies. These techniques are the least intrusive in situ plasma diagnostic methods. The most commonly used techniques are emission spectroscopy, absorption spectroscopy, laser-induced fluorescence. 

Absorption spectroscopy and laser induced fluorescence (LIF), give access to the concentration of molecules, atoms, and ions in the ground state. LIF is enable to achieve highly spatial and time resolved analyses. This technique is thus particularly suitable to investigate composition changes in the plasma, and obtain spatial or time concentration profiles. Published results in fluorine plasmas using absorption [25-27] and LIF [28-32] mainly concern temperature measurements [25] or the quantification of CFV radicals [26-31] in fluorocarbon-based plasmas and SOx in SF6—02 discharges [32], Recently LIF has been used to measure plasma-surface interaction products [33]. 

New fusion applications include the concept of production of intense negative ion beams ( ) (for neutral beam injection for heating and diagnostics in tokamaks or other magnetically confined plasmas (26, 28)) by using photodissociation to ion pairs (e.g. NaLi + hVyy Na + Li") in supersonic molecular beams. Another promising concept is the use of laser induced fluorescence to monitor very low tritium concentrations (as little as 10 Tj/cm ) under fusion reactor conditions (29). 

Laser-Induced Fluorescence. Laser-induced fluorescence (Lif) provides, much as does ir spectroscopy, fingerprints of different organic molecules, which can be quantified by measuring fluorescence intensities. Selectivity is excellent, as both pump and fluorescence frequencies can be individually chosen for optimum performance, and it can be improved with measurements of fluorescence lifetimes and polarization behavior. The enhanced null-background sensitivity can achieve single-atom or single-molecule detection (256—258). Lif has important applications in gas analysis (259) and combustion and plasma diagnostics (260). 

Direct measurement of riboflavin, FMN, and FAD in plasma or erythrocytes may be made by HPLC, usually with fluorescence detection after protein precipitation or by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF). In a study of riboflavin status and FMN and FAD concentrations in plasma and erythrocytes from elderly subjects at baseline and after low-dose riboflavin supplementation, using both activation coefficient measurements and CE-LIF, it was concluded that concentrations of aU Ba vitamers except plasma FAD are potential... 

C Dumouseaux, S Muramatsu, W Takasaki et al. Highly sensitive and specific determination of pravastatin sodium in plasma by high-performance liquid chromatography and laser-induced fluorescence detection after immobilized antibody extraction. J Pharm Sci 83 1630, 1994. 

Hargis, P. J., Jr., and M. J. Kushner. Detection of CF2 radicals in a plasma etching reactor by a laser induced fluorescence spectroscopy. 

Figure 7. Detected light intensity at 126 nm from a sulphur plasma as a function of the time after the ablation pidse. The signal at about 10 ps is laser-induced fluorescence from the excited state. The signal before 6 ps is due to recombination light from atoms and ions. (From Ref. [37]).

Figure 9. The time dependence of the fluorescence light intensity and decay time for a laser-produced sulphur plasma a) Laser-induced fluorescence intensity from the 7s S°, state as a function of the delay time for the VUV laser pulse with respect to the ablation pulse, b) Measured radiative lifetime of the 5s state as a function of the delay. (From Ref. [37]).

Important plasma diagnostics include Langmuir probes, optical emission spectroscopy, laser induced fluorescence, absorption spectroscopy, mass spectrometry, ion flux and energy analysis, and plasma impedance analysis. A plasma reactor equipped with several of these diagnostics is shown in Fig. 51 [35, 160]. A capacitively coupled plasma is sustained between the parallel plates of the upper (etching) chamber. The lower (analysis) chamber is differentially pumped and communicates with the etching chamber through a pinhole on the lower electrode. 

---