  Far-UV photons (E<13.6 eV) from hot massive stars regulate, or at least
influence, the heating, ionization, and chemistry of most of the neutral
interstellar medium. Investigating the interaction between FUV radiation and
interstellar matter thus plays an important role in astrochemistry. We have
used ALMA to mosaic a small field of the Orion Bar where the critical
transition from atomic to molecular gas takes place. These observations provide
an unprecedented sharp view of this transition layer (~1" resolution or ~414
AU). The resulting images (so far in the rotational emission of CO, HCO+,
H13CO+, SO+, SO, and reactive ions SH+ and HOC+) show the small-scale structure
in gas density and temperature, and the steep abundance gradients. The images
reveal a pattern of high-density substructures, photo-ablative gas flows and
instabilities at the edge of the molecular cloud. We have also used the IRAM
30m telescope to carry out a line-survey of the illuminated edge of the Bar in
the mm domain. Our observations reveal the presence of complex organic
molecules that were not expected in such a harsh environment. In particular, we
have reported the first detection of the unstable cis conformer of formic acid
(HCOOH) in the ISM. The energy barrier to internal rotation (the conversion
from trans to cis) is approximately 4827 cm-1. Hence, this detection is
surprising. The low inferred trans-to-cis abundance ratio of 2.8+/-1.0 supports
a photoswitching mechanism: a given conformer absorbs a FUV stellar photon that
radiatively excites the molecule to electronic states above the interconversion
barrier. Subsequent fluorescent decay leaves the molecule in a different
conformer form. This mechanism, which we have specifically studied with ab
initio quantum calculations, was not considered so far in astrochemistry
although it can affect the structure of a variety of molecules in PDRs.
(abridged)
