A capacitive discharge in water vapour in the pressure range 133–150 Pa was studied. The discharge is an effective ecologically pure source of OH* hydroxyl emission in the VUV and UV spectral range (A  2Σ+ → X  2Π transition). The time profiles of densities of plasma species at different values of reduced electric field (E/N) were calculated using a kinetic model of the discharge. It is shown that E/N ∼ 190–200 Td is a breaking field for water vapour plasma, and with an increase in E/N a transition from electronegative to electropositive plasma takes place. The primary positive ions in the water vapour plasma are H3O+ ions, and the primary negative ions are OH− ions.

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We present the results of a study of emission from a low-density water vapor plasma in the vacuum ultraviolet (VUV) region of the spectrum. The plasma was formed in a longitudinal glow discharge. We have studied the spectral characteristics of the plasma and also the dependences of the relative energy characteristics of the hydroxyl (OH) emission band on the discharge current and the helium partial pressure in a He-H 2 O mixture in the spectral range 130-190 nm.

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