Complex organic molecules in diffuse clouds along the line of sight to
  Sgr B2
  Up to now, mostly relatively simple molecules have been detected in
interstellar diffuse molecular clouds in our galaxy, but more complex species
have been reported in the diffuse/translucent medium of a z = 0.89 spiral
galaxy. We aim at searching for complex organic molecules (COMs) in diffuse
molecular clouds along the line of sight to Sgr B2(N), taking advantage of the
high sensitivity and angular resolution of the Atacama Large
Millimeter/submillimeter Array (ALMA). We use data acquired as part of the
EMoCA survey performed with ALMA. To analyse the absorption features of the
molecules detected towards the ultracompact HII region K4 in Sgr B2(N), we
calculate synthetic spectra for these molecules and fit their column densities,
line widths, centroid velocities, and excitation temperatures. We report the
detection of CH3OH, CH3CN, CH3CHO, HC3N, and NH2CHO in Galactic center (GC)
diffuse clouds and CH3OH and CH3CN in a diffuse cloud in the Scutum arm. The
chemical composition of one of the diffuse GC clouds is found to be similar to
the one of the diffuse/translucent medium of the z=0.89 spiral galaxy. The
chemical processes leading to chemical complexity in the diffuse molecular ISM
appear to have remained similar since z=0.89. As proposed in previous studies,
the presence of COMs in diffuse molecular clouds may result from a cyclical
interstellar process of cloud contraction and expansion between diffuse and
dense states.
