Nanosecond plasmas in liquids can initiate chemical processes that are
exploited in the elds of water treatment, electrolysis or biomedical applications. The
understanding of these chemical processes relies on unraveling the dynamics of the
variation of pressures, temperatures and species densities during the dierent stages
of plasma ignition and plasma propagation as well as the conversion of the liquid into
the plasma state and the gas phase. This is analyzed by monitoring the emission of
nanosecond pulsed plasmas that are generated by high voltages (HV) of 20 kV and
pulse lengths of 10 ns applied to a tungsten tip with 50 micrometer diameter immersed in water.
The spectra are acquired with a temporal resolution of 2 ns and the emission pattern
is modelled by a combination of black body radiation from the hot tungsten tip and
the pronounced emission lines of the hydrogen Balmer series. The data indicate two
contributions of the hydrogen line radiation that dier with respect to the degree of
self-absorption. It is postulated that one contribution originates from a recombination
region showing strong self absorption and one contribution from a ionization region
showing very little self-absorption. The emission lines from the ionization region are
evaluated assuming Stark broadening, that yielded electron densities up to 5 x 10^25
m^-3. The electron density evolution follows the same trend as the temporal evolution
of the voltage applied to the tungsten tip. The propagation mechanism of the plasma
is similar to that of a positive streamer in the gas phase, although in the liquid phase
field effects such as electron transport by tunneling should play an important role.