lieve this is due to disparities in the excitation energy thresholds
for the vapor species.
The emissions from atomic sodium above the breakdown
threshold are more temporally spiked and usually, but not
always, appear as the active electrode transitions to a negatively
biased state and emits electrons. Occasionally the emission
occurs at the transition when the active electrode becomes
positive. The pronounced temporal spike behavior could be
due to excited sodium atoms that are produced rapidly at
the vapor–electrode or vapor–liquid interfaces during voltage
reversals and which are field ionized and desorbed into the gas
phase within the vapor layer.
The emissions from both excited hydroxyl radicals and
hydrogen atoms also occasionally exhibit spiked temporal
behavior but not to the same extent as the sodium lines. Moreover, the emissions from OH and H tend to peak later during
the time when the active electrode is negative. We believe that
this behavior is connected with the simultaneous vaporization
and dissociative excitation of water molecules requiring both
significant integrated power (from the discharge voltage and