New Detections of Galactic Molecular Absorption Systems toward ALMA
  Calibrator Sources
  We report on Atacama Large Millimeter/submillimeter Array (ALMA) detections
of molecular absorption lines in Bands 3, 6 and 7 toward four radio-loud
quasars, which were observed as the bandpass and complex gain calibrators. The
absorption systems, three of which are newly detected, are found to be Galactic
origin. Moreover, HCO absorption lines toward two objects are detected, which
almost doubles the number of HCO absorption samples in the Galactic diffuse
medium. In addition, high HCO to H13CO+ column density ratios are found,
suggesting that the interstellar media (ISM) observed toward the two
calibrators are in photodissociation regions, which observationally illustrates
the chemistry of diffuse ISM driven by ultraviolet (UV) radiation. These
results demonstrate that calibrators in the ALMA Archive are potential sources
for the quest for new absorption systems and for detailed investigation of the
nature of the ISM.
