On the scarcity of redshifted OH and millimetre-band molecular
  absorption
  Despite much searching, redshifted decimetre and millimetre-band absorption
by molecular gas remains very rare, limited to just six systems at z > 0.05.
Detection of these transitions can yield precise diagnostics of the conditions
of the star forming gas in the earlier Universe, the hydroxyl (OH) radical
being of particular interest as in the 18-cm ground state there are four
different transitions located close to HI 21-cm and thus detectable with the
Square Kilometre Array and its pathfinders. The four transitions of OH have
very different dependences on the fundamental constants, thus having much
potential in testing for any evolution in these over large look-back times. By
collating the photometry in a uniform manner, we confirm our previous
hypothesis that the normalised OH absorption strength is correlated with the
optical--near-infrared red colour of the sight-line. Applying this to the
published searches, we find that all, but one (J0414+054), have simply not been
searched sufficiently deeply. We suggest that this is due to the standard
selection of sources with reliable optical redshifts introducing a bias against
those with enough dust with which to shield the molecular gas. For the single
source searched to sufficient depth, we have reason to suspect that the high
degree of reddening arises from another system along the sight-line, thus not
being inconsistent with our hypothesis. We also show that the same optical
redshift bias can account for the scarcity of millimetre-band absorption.
