A survey for redshifted molecular and atomic absorption lines I
  We are currently undertaking a large survey for redshifted atomic and molecular absorption ... only one clear and one tentative detection were obtained: HI absorption at z = 0.097 in PKS 1555-140 and OH absorption at z =0.126 in PKS 2300-189, respectively... In order to determine why no clear molecular absorption was detected in any of the 13 sources searched, we investigate the properties of the five redshifted systems currently known to exhibit OH absorption. In four of these, molecules were first detected via millimetre-wave transitions and the flat radio spectra indicate compact background continuum sources, which may suggest a high degree of coverage of the background source by the molecular clouds in the absorber. Furthermore, for these systems we find a relationship between the molecular line strength and red optical--near infrared (V-K) colours, thus supporting the notion that the reddening of these sources is due to dust, which provides an environment conducive to the formation of molecules. Upon comparison with the V-K colours of our sample, this relationship suggests that, presuming the reddening occurs at the host galaxy redshift at least in some of the targets, many of our observations still fall short of the sensitivityrequired to detect OH absorption, although a confirmation of the ``detection'' of OH in 2300-189 could contravene this. 
S. J. Curran, M. T. Whiting, M. T. Murphy, J. K. Webb, S. N. Longmore,   Y. M. Pihlstroem, R. Athreya, C. Blake