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We present the mid-infrared (MIR) spectra obtained with the Spitzer InfraRedSpectrograph (IRS) for a sample of 52 sources, selected as infrared luminous,z>1 candidates in the Extragalactic First Look Survey (XFLS). The sampleselection criteria are f(24um) > 0.9mJy, nu fnu(24um)/nu fnu(8um) > 3.16 and nufnu(24um)/nu fnu(0.7um) > 10. Of the 52 spectra, 47 (90%) produced measurableredshifts based solely on the mid-IR spectral features, with the majority(35/47=74%) at 1.5<z<3.2. Keck spectroscopy of a sub-sample (17/47) agrees withthe mid-IR redshift measurements. The observed spectra fall crudely into threecategories -- (1) 33% (17/52) have strong PAH emission, and are probablypowered by star formation with total IR luminosity roughly a factor of 5 higherthan the local starburst ULIRGs. (2) 33% (17/52) have only deep silicateabsorption at 9.8um, indicative of deeply embedded dusty systems. The energeticnature of the heating sources in these systems can not be determined by thesedata alone. (3) The remainder 34% are mid-IR continuum dominated systems witheither weak PAH emission and/or silicate absorption. This third of the sampleare probably AGNs. From the silicate absorption feature, we estimate thatroughly two-thirds of the sample have optical depth tau(9.8um) > 1. Their$L_{1600\AA}$ and $L_{\rm IR}$ suggest that our sample is among the mostluminous and most dust enshrouded systems of its epoch . Our study has revealeda significant population of dust enshrouded galaxies at z~2, whose enormousenergy output, comparable to that of quasars, is generated by AGN as well asstarburst. This IR luminous population has very little overlap with sub-mm andUV-selected populations (Abridged).Comment: Accepted by the ApJ. In vol. 658, March, 200

SpitzerMid‐Infrared Spectroscopy of Infrared Luminous Galaxies atz∼ 2. I. The Spectra