  Context: Over the past thirty years a wealth of observations of CO and other
molecules in optical/uv absorption in diffuse clouds has accumulated for which
no comparable CO emission line data exist. Aims: To acquire mm-wave J=1-0 CO
emission line profiles toward a substantial sample of commonly-studied
optical/uv absorption line targets and to compare with the properties of the
absorbing gas, especially the predicted emission line strengths. Methods: Using
the ARO 12m telescope we observed mm-wavelength J=1-0 CO emission with spectral
resolution R ~ 3x10^6 and spatial resolution 1' toward a sample of 110 lines of
sight previously studied in optical/uv absorption lines of CO, \HH, CH, etc.
Results: Interstellar CO emission was detected along 65 of the 110 lines of
sight surveyed and there is a general superabundance of CO emission given the
distribution of galactic latitudes in the survey sample. Much of the emission
is optically thick or very intense and must emanate from dark clouds or warm
dense gas near HII regions. Conclusions: Judging from the statistical
superabundance of CO emission, seen also in the total line of sight reddening,
the OB star optical/uv absorption line targets must be physically associated
with the large quantities of neutral gas whose CO emission was detected, in
which case they are probably influencing the absorbing gas by heating and/or
photoionizing it. This explains why CO/H2 and 12CO/13CO ratios differ somewhat
between $uv$ and mm-wave absorption line studies. Because the lines of sight
have been preselected to have AV < 1 mag, relatively little of the associated
material actually occults the targets, making it difficult for CO emission line
observations to isolate the foreground gas contribution.
