  We report results from a series of PIC simulations carried out to investigate
the interaction of intense laser light with overdense plasma and in particular,
aspects of the harmonic emission spectrum generated in these interactions.
Recent observations of emission in this range by Watts et al. (2002) show
emission above the 20th harmonic extending to the 37th with conversion
efficiencies of order 10^-6 with this spectrum showing distinct, if variable,
modulation of between two and four harmonics. This spectral feature proved
highly sensitive to the incident intensity, with more pronounced modulation at
higher intensities. These authors also reported results from a set of model PIC
simulations which showed a modulated emission spectrum across the entire
harmonic range, but with a wider modulation bandwidth. In the results to be
presented we report on features of the emission spectrum across a wide
parameter range and its dependence on the detailed structure of the plasma
density profile. While we do see evidence of a modulated spectrum, this appears
to be sensitive not only to the incident intensity but to the plasma density
profile. In addition plasma emission present further complicates the harmonic
emission spectrum. Our results lead us to conclude that attempts to interpret
modulation in emission spectra in terms of one particular theoretical model and
from this to infer the dynamics of the critical surface may be misguided.

Plasma Physics