Improved glow lamp - water vapor

Given the successful outcome of the first operation of the improved glow lamp with a low-pressure air fill (see there), I decided to go for water vapor. So I introduced two drops of tap water in the lamp and pumped the vapor out. The result is shown above, also at a few millibars (left) and below the millibar level (right). Like in previous case with air, there is plenty of UV-A emitted from the discharge, especially at a few millibars of pressure (left image). However, the source of this short-wave radiation is no longer N2, but OH formed from the dissociation of H2O in the plasma. The UV-A emission comes primarily from the A-X transitions in the OH molecule, which corresponds to a spectral band around 310 nm. The characteristic orange emission from sodium is also found around the electrodes at the lowest fill pressure (right image), but the aspect of the discharge is different than that in air because of the different nature of the gas fill (obviously) and the impurities introduced with the tap water.