  Advances in ultra-intense laser technology are enabling, for the first time,
relativistic intensities at mid-infrared (mid-IR) wavelengths. Anticipating
further experimental research in this domain, we present high-resolution two
dimensional Particle-in-Cell (PIC) simulation results using the Large- Scale
Plasma (LSP) code that explore intense mid-IR laser interactions with dense
targets. We present the results of thirty PIC simulations over a wide range of
intensities (0.03 < $a_0$ < 39) and wavelengths ({\lambda} =780 nm, 3 {\mu}m,
and 10 {\mu}m). Earlier studies, limited to {\lambda} =780 nm and $a_0 \sim$ 1
[1, 2], identified super-ponderomotive electron acceleration in the laser
specular direction for normal- incidence laser interactions with dense targets.
We extend this research to mid-IR wavelengths and find a more general result
that normal-incidence super-ponderomotive electron acceleration occurs provided
that the laser intensity is not highly relativistic ($a_0 \lesssim 1$) and that
the pre-plasma scale length is similar to or longer than the laser wavelength.
Under these conditions, ejected electron angular and energy distributions are
similar to expectations from an analytic model used in [2]. We also find that,
for $a_0 \sim 1$, the mid-IR simulations exhibit a classic ponderomotive
steepening pattern with multiple peaks in the ion and electron density
distribution. Experimental validation of this basic laser-plasma interaction
process will be possible in the near future using mid-IR laser technology and
interferometry.
