Helium

Time-resolved absolute intensity measurements have been made
on the Balmer series lines of a residual hydrogen impurity in a hollowcathode
helium afterglow plasma. From these measurements, the excited
state populations were calculated and revealed the existence of population
inversions in the excited states of hydrogen. In the hydrogen
afterglow the maximum average state population shifted upwards from
n = 3 at the beginning of the afterglow to progressively higher states
until in the late afterglow the maximum population was at n = 6. The
effects of the helium metastables, both atomic and molecular, were
shown to offer no detectable contribution to the observed inversions.
Comparison with a theoretical model for a recombining pulsed hydrogen
plasma for which inversions had been predicted are favorable and
recomb ~nation processes are assumed responsible for the inversions in
the lower discrete levels in the hydrogen impurity.

The purpose of this experimental study of the helium afterglow
in a hollow cathode discharge is to identify the mechanisms which
cause an apparent unequal acoustic modulation of the red and violet
wings of observed optical spectrum lines. The unequal red-violet effect is traceable to the fact that curved
spectrum lines are formed at the exit slit of a monochromator which
is equipped with straight entrance and exit slits. Curved spectrum
lines emphasize the red and violet wings differently, depending on
the light contributions from different portions of the entrance slit, so
that spatial variations in the optical image due to acoustic standing
waves show unequal integrated light intensities for the modulation of
each wing. Only the central portion of the radial standing wave in the afterglow
(Bessel function J (kr) distribution) was explored, and the ratio
of the red to violet amplitude calculated as 1 : 1.58 agrees qualitatively
with experimental results.