A 22GHz search for molecular absorption at z~3 with the upgraded ATCA
  We report a lambda > 1cm search for rotational molecular absorption towards quasars, now possible with the upgraded Australia Telescope Compact Array (ATCA). The targets were PKS 0201+113, PKS 0336-017 and Q 0537-286, where known damped Lyman-alpha absorption systems (DLAs) could cause redshifted molecular absorption in the 12mm band of the telescope. We place 3-sigma upper limits on any HCO+ 0-1 absorption features of < 30mJy per 3.4km/s channel. The non-detections could be attributed to the inherent low metallicities in DLAs leading to generally low H_2, and thus HCO+, column densities. In general, the detection of molecular rotational transitions in DLAs could be further hindered by a lower than expected CO-to-H_2 conversion ratio, whether due either to photoionization of carbon or its relative under-abundance at high redshift. 
S. J. Curran, M. T. Murphy, J. K. Webb (UNSW), Y. M. Pihlstroem (NRAO)