  We present results of a systematic analysis of the XMM-Newton spectra of 40
quasars (QSOs) (z<1.72) from the Palomar-Green (PG) Bright Quasar Survey sample
(M_B<-23). The sample includes 35 radio-quiet quasars (RQQs) and 5 radio-loud
quasars (RLQs). The analysis of the spectra above 2 keV reveals that the hard
X-ray continuum emission can be modeled with a power law component with
<Gamma_(2-12)> = 1.89+/-0.11 and <Gamma_(2-12)> = 1.63^(+0.02)_(-0.01) for the
RQQs and RLQs, respectively. Below 2 keV, a strong, broad excess is present in
most QSO spectra. This feature has been fitted with four different models
assuming several physical scenarios. All tested models (blackbody, multicolor
blackbody, bremsstrahlung and power law) satisfactorily fitted the majority of
the spectra. However, none of them is able to provide an adequate
parameterization for the soft excess emission in all QSOs, indicating the
absence of an universal shape for this spectral feature. An additional cold
absorption component was required only in three sources. On the other hand, as
recently pointed out by Porquet et al. (2004) for a smaller sample of PG QSOs,
warm absorber features are present in ~50% of the QSO spectra in contrast with
their rare occurrence (~5-10%) found in previous studies. The XMM-Newton view
of optically-selected bright QSOs therefore suggests that there are no
significant difference in the X-ray spectral properties once compared with the
low-luminosity Seyfert 1 galaxies. Properties of the Fe Kalpha emission lines
are presented in a companion paper.
