  We present the first complete 55-671 um spectral scan of a low-mass Class 0
protostar (Serpens SMM1) taken with the PACS and SPIRE spectrometers on board
Herschel. More than 145 lines have been detected, most of them rotationally
excited lines of 12CO (full ladder from J=4-3 to 42-41), H2O, OH, 13CO, HCN and
HCO+ . Bright [OI]63,145um and weaker [CII]158 and [CI]370,609um lines are also
detected. Mid-IR spectra retrieved from the Spitzer archive are also first
discussed here, they show clear detections of [NeII], [FeII], [SiII] and [SI]
fine structure lines as well as weaker H2 S(1) and S(2) pure rotational lines.
The observed line luminosity is dominated by CO (~54%), H2O (~22%), [OI] (~12%)
and OH (~9%) emission. A non-LTE radiative transfer model allowed us to
quantify the contribution of the 3 different temperature components suggested
by the 12CO rotational ladder (Tk(hot)~800 K, Tk(warm)~375 K and Tk(cool)~150
K). Gas densities n(H2)~5x10^6 cm^-3 are needed to reproduce the observed
far-IR lines arising from shocks in the inner protostellar envelope for which
we derive upper limit abundances of x(CO)~10^-4, x(H2O)~0.2x10^-5 and
x(OH)~10^-6. The lower energy submm 12CO and H2O lines show more extended
emission that we associate with the cool entrained outflow gas. Fast
dissociative J-shocks (v_s > 60 km s^-1) as well as lower velocity
non-dissociative shocks (v_s < 20 km s^-1) are needed to explain both the
atomic lines and the hot CO and H2O lines respectively. Observations also show
the signature of UV radiation and thus, most observed species likely arise in
UV-irradiated shocks. Dissociative J-shocks produced by an atomic jet are the
most probable origin of [OI] and OH emission and of a significant fraction of
the warm CO emission. In addition, H2O photodissociation in UV-irradiated
non-dissociative shocks can also contribute to the [OI] and OH emission.
