Comparative chemistry of diffuse clouds. V. Ammonia and formaldehyde

Using the VLA and NRAO 140' telescopes we observed the λ1.3 cm (1, 1) and (2, 2) lines of NH₃ in absorption and emission toward the compact extragalactic continuum sources NRAO150 (B0355+508) and 3C 111 (B0415+379). These sources are occulted by some seven local diffuse and translucent clouds showing molecular absorption in OH, CO, HCO⁺ and C2H: for the four features having NH₃ absorption, we find rotational excitation temperatures 18-24 K, suggesting kinetic temperatures of at least 25-30 K. The abundance ratio N(NH_3)/N(HCO^+) is comparable to values quoted for the cyanopolyyne peak in TMC-1 (i.e., 2.5) in three of four cases where NH₃ was seen. For clouds with higher column density N(HCO^+) ⪆ 10¹²~~cm^-2, the NH₃ column density N(NH_3) is well correlated only with N(CS) (< N(NH_3)/N(CS)> ≈ 1.0) and N(H2CO) (< N(NH_3)/N(H2CO)> ≈ 0.4). N(H2CO) is well correlated with N(NH_3) and N(CS) (<N(H2CO)/N(CS)> ≈ 2.3) and the H2CO abundance relative to other species is like TMC-1, with (<N(H2CO)/N(HCO^+)> ≈ 2.3).