Glow discharge

Glow Discharge Optical Emission Spectroscopy (GDOES), also known as Glow Discharge Spectroscopy, is a specific spectroscopic technique used for analyzing metallic and non-metallic solids. It allows the study of elemental composition, layer thickness, layer structure, concentration gradients, and mass occupancy. This method can detect very thin layers (< 50 nm) as effectively as thick layers of several hundred micrometers. Suitable materials for analysis include metals, semiconductors, glass, ceramics, and polymers. Introduced by Werner Grimm in 1968, this optical method has undergone continuous development and is now considered one of the most precise techniques for elemental analysis and layer thickness determination. The GDOES method involves three main steps: inserting the sample into a cylindrical hollow anode as a discharge source to generate plasma, exciting the sample atoms and ions through energy input, and finally, reading the emitted light using a spectrometer. By analyzing the specific wavelengths and intensities of the emitted light, valuable information on the elemental composition and concentration in the sample is obtained, enabling qualitative and quantitative analysis of chemical elements in various materials.


Briefly Explained

Simplified Operating Principle

The sample is inserted into the glow discharge source, where it comes into direct contact with the cathode, effectively acting as a cathode itself.

During the analysis, the glow discharge source contains argon gas at low pressure (0.5 hPa to 10 hPa). A high DC voltage is applied between the anode and the sample (acting as the cathode). This process releases electrons at the sample's surface and accelerates them towards the anode.

As the electrons gain kinetic energy, they release it through inelastic collisions with argon atoms, resulting in ionization of the argon atoms and the formation of argon cations and free electrons. This avalanche effect increases the density of charge carriers, transforming the insulating argon gas into a conductive state and forming a plasma—a mixture of neutral gas atoms and free charge carriers.

The argon cations are driven towards the sample surface due to the high negative potential present there. Upon impact with the sample surface, the argon cations transfer their kinetic energy to the atoms on the surface, causing them to be dislodged from the sample. This phenomenon is known as cathode sputtering or simply sputtering. The degradation of the sample surface occurs in a plane-parallel manner.

The atoms knocked out from the sample diffuse into the plasma, where collisions with high-energy electrons excite them to higher energy states. As these atoms return to their ground states, they emit light with characteristic wavelength spectra unique to each element.

The emitted light passes through the entrance slit and then onto a holographic grating, which directs it towards different detectors based on the diffraction angle of the wavelengths. The light is registered and measured by the detectors accordingly.

The intensity of each line is directly proportional to the concentration of the corresponding element in the plasma. For electrically non-conductive materials, high-frequency alternating voltage can be employed to generate plasma, allowing examination of non-metallic samples as well.

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