The densities and electron temperatures in transient plasmas produced
by high-voltage pulsed discharges in liquid helium below 4.2K, at 1 at
m, and cryogenic helium gas near 4.2K, at 1 atm, have been measured by
time-resolved emission spectroscopy up to 3.8 mu s after the discharg
es. The densities are measured using Stark broadening of line profiles
. The electron temperatures are deduced from the ratios of total spect
ral line intensities to continuum intensities. Gaseous plasmas with re
latively high density and low electron temperature are obtained in cry
ogenic environments. Typical density and electron temperature at 0.2 m
u s after breakdown are 10(18) cm(-3) and 35,000 K, respectively. Mech
anisms of the dominant spectral line broadening at an early stage afte
r the discharges in liquid helium are discussed in detail.