Arecibo emission absorption studies of the interstellar medium

Arecibo emission-absorption studies of Hl, high velocity Hl, OH, and H2CO are presented. Errors in galactic Hl spectra caused by stray radiation into the far sidelobes of the antenna power pattern are estimated from calibration measurements and observations toward the moon. From observations toward 19 low galactic latitude continuum sources, the variation of Hl properties with latitude are explored. The observations of column density and integrated optical depth are consistent with high latitude studies when allowance is made for the well known deficiency of Hl in the solar neighborhood. The spin temperatures and column densities of low latitude absorption features are overestimated by a factor of two because of blending with low optical depth emission. A survey for emission and absorption from high velocity Hl shows that there is little cold, high velocity Hl. OH observations towards the nineteen low latitude continuum sources resulted in detection of 11 absorption features. In three cases, the ratio of the two OH main lines is not the thermal equilibrium ratio. H2CO absorption toward 3C 154 and 3C 123 was studied. Toward 3C 123, the H2CO line has a dispersion of only 0.13 km s-1. The abundance of H2CO relative to OH and CH varies by at least a factor of five between two otherwise similar regions