Herschel offers two types of spectroscopic capability. PACS and
SPIRE offer low to intermediate resolution spectroscopy covering
the full Herschel wavelength range. HIFI offers high-resolution
spectroscopy over the range from 157-625 μm (480-1910 GHz) using
heterodyne techniques, although there is a small gap in coverage
from 213-240 microns (1272-1430 GHz), between the HIFI 5b and 6a
sub-bands. Users can thus be able to select a wide range of
resolutions from Δλ/λ=20 to Δλ/λ=10 000 000 according to the
brightness of their source and the science that is required. The
main spectroscopic capabilities are summarised in Table 3.2. In
its highest resolution mode Herschel offers a velocity
resolution as high as 0.3km/s. The wavelength range covered by
Herschel has many thousands of lines of water, atomic
transitions and organic molecules. This allows Herschel to study
the chemistry of the interstellar medium, tracing water and
organic molecules in molecular clouds. Herschel is also be able
to study the chemistry of