Spatial distribution of emission in low pressure DC discharges in water and alcohol vapours*

Here we show the spatial distributions of emission in the DC breakdown in water vapour and alcohol vapours, for electrode gaps 1.1 and 3.1 cm. We recorded the 2D side-on distributions of emission using an ICCD camera. For the same \textit{pd}, at different electrode gaps $d$, the anatomy of the discharge changes. The radial discharge width is smaller at larger electrode gaps, as shown for the case of a gap of 3.1 cm, as compared to the gap of 1.1 cm. This difference is most noticeable at low pressures (large $E/N)$.At given electrode diameter to gap ratio the increased diffusional losses at lower pressures will violate \textit{pd} scaling, however, it is interesting that processes induced by heavy particles are more sensitive to changes in geometry of the discharge than electron induced processes. Under the investigated conditions, most of the emission comes from the cathode region due to the heavy-particle excitation. As the electrode gap increases the radial distribution of emission near the cathode becomes significantly narrower than that near the anode. We propose that the smaller radial discharge widths at larger gaps may be due to the change of electric field distribution in the vicinity of the cathode edges, in addition to the diffusion losses of charged particles near the discharge chamber walls. Consequently, processes of excitation and ionization in this region would be less prominent.