Comparative Chemistry of Diffuse Clouds IV: CH
  We observed the 3335 MHz (9cm) F=1-1 line of CH toward a sample of diffuse clouds occulting compact extragalactic mm-wave continuum sources, using the old NRAO 43m telescope. Because radiofrequency observations of CH really must be calibrated with reference to a known CH abundance, we begin by deriving the relationships between CH, EB-V, H2 and other hydrides found by optical spectroscopy. No simple relationship exists between N(CH) and EB-V, since N(CH) is strongly bimodal with respect to reddening for EB-V < 0.3 mag and the typical range in the N(CH)/EB-V ratio is an order of magnitude or more at any given EB-V > 0.3 mag. However, N(CH)/N(H2) = 4.3 +/- 1.9 x 10^-8 in the mean and N(CH) varies as N(H2)^(1.00+/-0.06) for 10^{19} < N(H2) < 10^{21} cm^-2. If CH is a good predictor of H2, 40-45% of the hydrogen in the local diffuse/translucent ISM is in the molecular form at the accepted mean density, higher than previous estimates found in samples of lower-than-average mean density. Optical observations of the population ratios in the upper and lower halves of the CH lambda-doublet suggest that the brightness of the 3335 MHz CH line should be double-valued at a given CH column density in diffuse gas: double-valuedness is noticeable in our data when comparing CH with CO or HCO+. The CH brightness at 3335 MHz is mildly bimodal with respect to CO emission in our diffuse cloud data but much more strongly bimodal when comparing diffuse or translucent gas and dark gas. The CH Lambda-doublet is generally inverted in diffuse gas but we did not succeed in measuring the excitation temperature except toward 3C123 where we confirm one older value Tex ~ -10 K. 
H. Liszt (NRAO) and R. Lucas (IRAM)