  We present Herschel-PACS observations of rest-frame mid-infrared and
far-infrared spectral line emissions from two lensed, ultra-luminous infrared
galaxies at high redshift: MIPS J142824.0+352619 (MIPS J1428), a
starburst-dominated system at z = 1.3, and IRAS F10214+4724 (F10214), a source
at z = 2.3 hosting both star-formation and a luminous AGN. We have detected
[OI]63 micron and [OIII]52 micron in MIPS J1428, and tentatively [OIII]52
micron in F10214. Together with the recent ZEUS-CSO [CII]158 micron detection
in MIPS J1428 we can for the first time combine [OI], [CII] and far-IR (FIR)
continuum measurements for photo-dissociation (PDR) modeling of an
ultra-luminous (L_IR > 10^12 L_sun) star forming galaxy at the peak epoch of
cosmic star formation. We find that MIPS J1428, contrary to average local
ULIRGs, does not show a deficit in [OI] relative to FIR. The combination of
far-UV flux G_0 and gas density n (derived from the PDR models), as well as the
star formation efficiency (derived from CO and FIR) is similar to normal or
starburst galaxies, despite the high infrared luminosity of this system. In
contrast, F10214 has stringent upper limits on [OIV] and [SIII], and an
[OIII]/FIR ratio at least an order of magnitude lower than local starbursts or
AGN, similar to local ULIRGs.
