  We analyse mid-infrared (MIR) spectroscopic properties for 19 ultra-luminous
infrared quasars (IR QSOs) in the local universe based on the spectra from the
Infrared Spectrograph on board the Spitzer Space Telescope. The MIR properties
of IR QSOs are compared with those of optically-selected Palomar-Green QSOs (PG
QSOs) and ultra-luminous infrared galaxies (ULIRGs). The average MIR spectral
features from ~ 5 to 30um, including the spectral slopes, 6.2um PAH emission
strengths and [NeII] 12.81um luminosities of IR QSOs, differ from those of PG
QSOs. In contrast, IR QSOs and ULIRGs have comparable PAH and [NeII]
luminosities. These results are consistent with IR QSOs being at a transitional
stage from ULIRGs to classical QSOs. We also find that the colour index
alpha(30, 15) is a good indicator of the relative contribution of starbursts to
AGNs for all QSOs. Correlations between the [NeII] 12.81um and PAH 6.2um
luminosities and those between the [NeII], PAH with 60um luminosities for
ULIRGs and IR QSOs indicate that both [NeII] and PAH luminosities are
approximate star formation rate indicators for IR QSOs and starburst-dominated
galaxies; the scatters are, however, quite large (~ 0.7 to 0.8 dex). Finally
the correlation between the EW(PAH 6.2um) and outflow velocities suggests that
star formation activities are suppressed by feedback from AGNs and/or
supernovae.
