Restgasen

Restgasen, or residual gases, are the gases that remain in a vacuum system after evacuation. They arise from outgassing of chamber walls and components, desorption of adsorbed molecules, and small leaks or vapors from seals, pumps, and lubricants. In high or ultra-high vacuum, the restgas is governed by a balance between pumping speed and the rate at which surfaces desorb or release gases.

The composition of restgasen varies with material, temperature, and cleanliness. Common components include hydrogen (H2), water vapor (H2O), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2), oxygen (O2), and various hydrocarbons. At room temperature or after initial pumping, water and hydrocarbons often dominate; with bakeout or cryogenic pumping, lighter species such as H2 and CO become more prominent. The total pressure in vacuum systems is typically in the range from about 10^-3 to 10^-9 mbar, with higher-quality systems reaching 10^-9 mbar or lower.

Restgasen are routinely monitored using a residual gas analyzer (RGA), a mass spectrometer that identifies gases by their mass-to-charge ratio. Mitigation strategies include bakeout to desorb contaminants, selecting low-outgassing materials, avoiding oil vapors and leaks, and using dry or ion pumps and, in some cases, cryopumps to remove rest gases.

In experiments, restgasen can influence surface chemistry, cause film deposition, affect contamination of samples, and interfere with sensitive measurements. Effective control of restgases is essential for achieving and maintaining desired vacuum conditions and measurement quality.