Low pressure water vapour discharge as a light source: II. Electrical characteristics

Abstract

The electric field strength, electrode fall voltage, light emission characteristics and efficiency of a (Ar + H2O) dc discharge as functions of water vapour content, argon pressure and electric current are presented. The data show that the main processes of 306.4 nm OH band generation are (1) a collision between an excited argon atom and a water molecule with simultaneous excitation of OH into the A 2Σ+ state and (2) electron excitation of a ground state hydroxyl molecule produced by a quenching process from a water molecule. Electric field strength measurements make it possible to conclude that the light production efficiency of the plasma under study can reach 35 lm W−1. It is possible, with these data, to propose a model of the plasma in question having reasonable accordance with the experiment and show the way to further increase the efficiency.

Possible ways to increase the efficiency of a low-pressure water vapour discharge as a light source

“…Another discharge method is a mixture of a rare gas (Ne, Ar, Kr or Xe) and water vapour operating in a tube coated with phosphorescent material on the inside. It has been investigated 8,9 and has shown significantly better efficiency compared to a discharge without phosphor coating -the discharge with argon showing the best efficiency. The phosphor coating played two roles: First, it was an indicator of UV emission and second, it was the primary source of water molecules in the discharge, because the phosphor coating appeared to be a controllable temperature dependent absorber and emitter of water vapour.…”

Glow Discharge in the Mixture of a Rare Gas and Water Vapour: Properties and Application to Light Sources

“…One of such an opportunity could be a discharge on the base of a rare gas and water vapor mixture. It was shown [2][3][4] that such a discharge was a good emitter of hydroxyl molecule UV radiation 306.4 nm produced by excited hydroxyl molecules OH(A 2 Σ ). The intensity of the emission overwhelms the intensity of rare gas atomic lines and other possible dissociative products of water molecules (e.g., hydrogen and oxygen atoms).…”

“…The other decisive factors that make possible to model the plasma in question are as follows 4) . First of all the main value to make estimations is the water molecule concentration.…”

“…1.4·10 6 c -1 10) , 1.1·10 5 c -1 (this value takes into account super-fine structure of the mercury resonance line and radiation trapping 17) ), 2·10 14 cm 3 (this value is typical for a mercury fluorescent lamp at 40 C), ν 4.89 eV, ν 4.1 eV, consumed electric power per unit length ( ) 0.3 W/cm (typical value for a mercury fluorescent lamp 5, 6) ), ( ) 2.4 W/cm (at the current 0.3 A and electric field strength 8V/cm 4) ). Substituting these quantities one can obtain 10 14 cm 3 .…”

“…The hydroxyl concentration is proportional to the water molecule concentration 4) . However, the direct increase of water molecule content in the plasma by increasing the tube wall temperature does not give rise in the efficiency of the OH UV radiation of the (Ar water vapor)-discharge.…”