Herschel observations of extra-ordinary sources: Detecting spiral arm
  clouds by CH absorption lines
  We have observed CH absorption lines ($J=3/2, N=1 \leftarrow J=1/2, N=1$)
against the continuum source Sgr~B2(M) using the \textit{Herschel}/HIFI
instrument. With the high spectral resolution and wide velocity coverage
provided by HIFI, 31 CH absorption features with different radial velocities
and line widths are detected and identified. The narrower line width and lower
column density clouds show `spiral arm' cloud characteristics, while the
absorption component with the broadest line width and highest column density
corresponds to the gas from the Sgr~B2 envelope. The observations show that
each `spiral arm' harbors multiple velocity components, indicating that the
clouds are not uniform and that they have internal structure. This
line-of-sight through almost the entire Galaxy offers unique possibilities to
study the basic chemistry of simple molecules in diffuse clouds, as a variety
of different cloud classes are sampled simultaneously. We find that the linear
relationship between CH and H$_2$ column densities found at lower $A_V$ by UV
observations does not continue into the range of higher visual extinction.
There, the curve flattens, which probably means that CH is depleted in the
denser cores of these clouds.
