  We investigate the nature of the diffuse intra-cluster ultraviolet light seen
in twelve local starburst galaxies, using long-slit ultraviolet spectroscopy
obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble
Space Telescope (HST). We take this faint intra-cluster light to be the field
in each galaxy, and compare its spectroscopic signature with STARBURST99
evolutionary synthesis models and with neighboring star clusters. Our main
result is that the diffuse ultraviolet light in eleven of the twelve starbursts
lacks the strong O-star wind features that are clearly visible in spectra of
luminous clusters in the same galaxies. The difference in stellar features
dominating cluster and field spectra indicate that the field light originates
primarily from a different stellar population, and not from scattering of UV
photons leaking out of the massive clusters. We consider several scenarios to
understand the lack of observed O stars in the field, and their implications
for the origin of the field stellar population. If the field stellar
populations formed in situ, the field must either have an IMF which is steeper
than Salpeter (3.0-3.5) or a Salpeter slope with an upper mass cutoff of 30-50
solar masses. If star formation occurs primarily in star clusters, the field
could be composed of older, faded clusters, and/or a population which is coeval
with the luminous clusters but lower in mass. If the field is composed of
older, dissolving clusters, we estimate that star clusters (regardless of mass)
need to dissolve on timescales 7-10 Myr to create the field (abridged).
