  Far-infrared spectroscopy reveals gas cooling and its underlying heating due
to physical processes taking place in the surroundings of protostars. These
processes are reflected in both the chemistry and excitation of abundant
molecular species. Here, we present the Herschel-PACS far-IR spectroscopy of 90
embedded low-mass protostars from the WISH (van Dishoeck et al. 2011), DIGIT
(Green et al. 2013), and WILL surveys (Mottram et al. 2017). The $5\times5$
spectra covering the $\sim50''\times50''$ field-of-view include rotational
transitions of CO, H$_2$O, and OH lines, as well as fine-structure [O I] and [C
II] in the $\sim$50-200 $\mu$m range. The CO rotational temperatures (for
$J_\mathrm{u}\geq14)$ are typically $\sim$300 K, with some sources showing
additional components with temperatures as high as $\sim$1000 K. The H$_2$O /
CO and H$_2$O / OH flux ratios are low compared to stationary shock models,
suggesting that UV photons may dissociate some H$_2$O and decrease its
abundance. Comparison to C shock models illuminated by UV photons shows good
agreement between the line emission and the models for pre-shock densities of
$10^5$ cm$^{-3}$ and UV fields 0.1-10 times the interstellar value. The
far-infrared molecular and atomic lines are the unique diagnostic of shocks and
UV fields in deeply-embedded sources.
