  We use the photometric redshift method of Chakrabarti & McKee (2008) to infer
photometric redshifts of submillimeter galaxies with far-IR (FIR)
$\it{Herschel}$ data obtained as part of the PACS Evolutionary Probe (PEP)
program. For the sample with spectroscopic redshifts, we demonstrate the
validity of this method over a large range of redshifts ($ 4 \ga z \ga 0.3$)
and luminosities, finding an average accuracy in $(1+z_{\rm phot})/(1+z_{\rm
spec})$ of 10%. Thus, this method is more accurate than other FIR photometric
redshift methods. This method is different from typical FIR photometric methods
in deriving redshifts from the light-to-gas mass ($L/M$) ratio of
infrared-bright galaxies inferred from the FIR spectral energy distribution
(SED), rather than dust temperatures. Once the redshift is derived, we can
determine physical properties of infrared bright galaxies, including the
temperature variation within the dust envelope, luminosity, mass, and surface
density. We use data from the GOODS-S field to calculate the star formation
rate density (SFRD) of sub-mm bright sources detected by AzTEC and PACS. The
AzTEC-PACS sources, which have a threshold $850 \micron$ flux $\ga 5 \rm mJy$,
contribute 15% of the SFRD from all ULIRGs ($L_{\rm IR} \ga 10^{12}
L_{\odot}$), and 3% of the total SFRD at $z \sim 2$.
