Low-pressure Discharge Plasma Research Articles

Doppler-free techniques in analytical spectroscopy

The application of Doppler-free spectroscopy techniques offers the possibility of spectrochemical analysis of isotopes. The basic requirements of Doppler-free spectroscopy are (i) the availability of tunable lasers with small line widths and (ii) the atomization of samples into vacuum or a low pressure atmosphere. With the exception of a few very light and heavy elements, the isotope line shifts are smaller than the Doppler widths. Therefore, the homogeneous line widths AVhom of atomic transitions have to be much smaller than the Doppler widths AvD of these lines and vacuum or low pressure atomization is required. The condition Avhom ~ AVD restricts the selection of atomizers which can be used in Doppler-free spectroscopy. Electrothermal atomization into vacuum or a low pressure noble gas, low-pressure discharge plasmas or analytical flames operating in the sub-atmosphere region (see e. g. [1]) are suitable methods. Basically, there are four Doppler-free techniques which, in principle, can be used in spectrochemistry: (1) excitation of species in an atomic beam at a right angle, (2) saturation spectroscopy, (3) off-resonance multiphoton spectroscopy and (4) resonant multiphoton spectroscopy (see e.g. [2]). Usually the techniques 1 3 use one laser, while technique 4 needs at least two laser systems. The restriction of method 1 is connected with the atomic beam atomization. However, the excitation is very efficient because the laser radiation is interacting with almost all atoms of the maxwellian velocity distribution in the beam. In Doppler-free off-resonant multiphoton spectroscopy we have again the interaction of all atoms with the laser radiation. However, because the atomic transition rates are very weak and dependent on I n (I: laser intensity, n: number of photons involved in the transition) the laser beam has to be focussed to get detectable signals. Therefore the interaction volume of laser radiation with the atomized sample is limited. In saturation and resonant multiphoton spectroscopy the lasers are interacting only with one atomic velocity group of the maxwellian distribution, which is a fraction of all analytes within the laser fields.

On Applicability of Laser-Induced Fluorescence from the Metastable State of Argon Ions to Ion Density Measurements

Laser-induced fluorescence (LIF) from an Ar+ metastable state was detected in low-pressure DC discharge plasmas. The dependence of the 3d4F and 4p2D population densities on the discharge current was examined with a collisional-radiative model (C-R model). The results show that information on the ion density can be obtained from LIF by using the C-R model when the electron temperature is known.

Infrared spectra (2–16 μm) of ArI Rydberg emission from a microwave discharge plasma

Infrared (2–16 μm) emission from Ar Rydberg atoms, excited in a conventional low-pressure microwave discharge plasma, has been observed in the cryogenic COCHISE reactor/spectrometer facility. The observed spectrum is very complex but is identifiable using the detailed energy level structure and optical selection rules for ArI. Detailed transition probabilities for dipole-allowed transitions between 2 and 16 μm were computed using the Coulomb approximation. Comparisons of observed and predicted spectra show that substantial long wavelength infrared (LWIR) emission (∼12 μm) arises from Rydberg states near the ionization limit; these states must have kinetically enhanced populations in order to account for the observed emission intensities. The results are interpreted in terms of a radiative/collisional cascade sequence initiated by dissociative recombination of Ar+2 with energetic electrons in the active discharge.

Power dissipated in low-pressure radio-frequency discharge plasmas

The experimental study of capacitive radio-frequency (rf) discharges was performed in mercury vapor at low pressures (10−4–10−2 Torr) and at rf applied voltage below 200 V. It was found that at pressures less than 10−2 Torr, plasma electrons gain rf energy basically through a noncollisional mechanism at the oscillating potential barrier formed in electrode sheaths. The rf power dissipated in this process turned out to be much larger than that received by electrons through their collisions with neutral atoms in the plasma bulk. Experimental results are in good agreement with the theoretical model of low-pressure rf discharges developed earlier by one of the authors.

Electron-impact-induced anisotropic etching of silicon by hydrogen

The etch rate of silicon in a hydrogen low-pressure discharge plasma can be strongly enhanced by electron bombardment, reaching presently up to ∼1000 A/min. The etch rate increases linearly with increasing electron current density and hydrogen pressure (range ∼0.05–0.7 mbar) and decreases with increasing temperature, yielding an activation energy of −4.2 kcal/mole in a temperature range of ∼80 to 300°C. The etching remains anisotropic within the whole pressure range studied.

Note on the dissociation of oxygen in a low-pressure discharge plasma

The oxygen atom concentration in the positive column of a low-pressure discharge has been investigated at different pressures and different discharge currents. The calorimetric method has been used for our measurements. It has been found that the atom concentrations in the H-form and in the T-form are very different.

A collisional-radiative model for helium and its application to a discharge plasma

For the collisional-radiative model applied to helium, two alternative formulations are presented in which non-linear process such as ionization due to metastable-metastable collisions may be included. The atomic data, including dielectronic,recombination, employed in the calculations are summarized. The collisional-radiative calculation is applied to a positive-column plasma for which both the nonlinear processes and radiation trapping are taken into account. The calculated population densities of excited levels for a low-pressure and low-current discharge plasma show good agreement with a measurement made using the hook method. The population density distribution among excited levels is interpreted in terms of excitation-ionization mechanisms in the plasma. This leads to a grouping of the excited levels into two phases for the discharge plasma considered: the corona phase applies to the lower-lying levels and the quasi-saturation phase to the higher-lying levels. The importance of the ladder-like excitation-ionization mechanism is noted for this class of plasmas.

An improved low-pressure dc discharge

We describe a scheme in which the primary beam electrons are electrostatically confined in a low-pressure dc discharge plasma which can substantially improve the density (300 or 400% in our case). The electrons are heated to 2 eV with a fraction thermalized to ∼10 eV by the extended presence of the primary beam electrons. Ion noise observed was not changed substantially from that without confinement. (It was slightly better than unconfined.) The plasma noise is strongly dependent on filament geometry. We also describe a second electrostatic confinement scheme in which both the primary electrons and the plasma are confined, resulting in a further density increase.

Orifice probe for plasma diagnostics: III. Experimental considerations

Experimental methods are presented for the detection and elimination of instrumental effects arising from field penetration and space charge for an orifice probe in low-pressure (<2×10−2 Torr) RF discharge plasmas in argon. A channel orifice, compared with a sharp-edged orifice, provides number density values which are too low. The creation of a dual-peak ion distribution by the insertion of a grid into the plasma is illustrated.

An ion flux probe for a low pressure discharge plasma

A new type of ion flux probe for measurements of ion flux as well as total mass flux at the boundary of a stationary low pressure discharge plasma has been constructed and tested. Fine apertures drilled in a thin metal plate inserted in the confining glass wall connect the discharge column with an external volume containing a pressure gauge. Atom and ion fluxes are derived from pressure measurements made at different electric potentials of the metal plate. The results are compared with measurements by Langmuir probe characteristics.