  We investigate the instantaneous star formation rates (SFR) and extinction
properties for a large (N=274), near-infrared (2.2 micron) + mid-infrared (24
micron) selected sample of normal to ultra-luminous infrared galaxies (ULIRGs)
[10^9<L_{IR}/L_{sun}<10^12.5] with <z>~0.8 in the Spitzer Extragalactic First
Look Survey. We combine Spitzer MIPS 24-micron observations with
high-resolution, optical Keck Deimos spectroscopy to derive optical
emission-line and infrared star formation rates (SFR_{opt} & SFR_{IR},
respectively). Direct comparison of these SFR diagnostics reveals that our
sample exhibits a wide range of extinction (1.0<Av<4.0 mag). This is after
removing spectroscopic and IRAC color-selected AGN candidates that account for
12% of the sample. Objects with SFRs of a few solar masses per year have Av
values consistent with those of normal spirals (Av~1.0 mag). By contrast, LIRGs
at z>1, which make up a large fraction of our sample, have SFR~100 M_{sun}/yr
and <Av>~2.5 mag. This translates to a 97% mean attenuation of the [OII]
forbidden line doublet, with the most extreme sources having as much as 99.7%
of their [OII] line flux extinguished by dust. Based on a SFR_{IR}/SFR_{opt}
diagnostic, we derive an IR-luminosity-dependent Av^{IR} function
[Av^{IR}=0.75*log(L_{IR}/L_{sun})-6.35 mag] that we use to extinction correct
our emission line luminosities. Application of this correction results in a
correlation between SFR_{IR} and SFR_{opt} that has a dispersion of 0.2 dex.
Investigation of the Av dependence on redshift reveals that for a fixed L_{IR},
there is no significant Av evolution. The mean attenuation of our sample is
intermediate between that of local optical/UV- and radio-selected samples and
has a marginally stronger L_{IR} dependence.
