  To provide constraints on the chemical processes responsible for the observed
columns of organic species, we used NOEMA to observe the sight line toward
NRAO150 in the 2mm spectral window. We targeted the low excitation lines of
o-H2CO 2(1,1)-1(1,0) and p-H2CO 2(0,2)-1(0,1) as well as the nearby transitions
of CS(3-2) and c-C3H2. We combined these data with previous observations to
determine the excitation conditions, column densities, and abundances relative
to H2 in the different velocity components. We performed non-LTE radiative
transfer calculations including collision cross sections with ortho and para H2
and with electrons. New collision cross sections with electrons were computed
for ortho and para formaldehyde. The c-C3H2 line profiles are very similar to
those of HCO+ and CCH, while the CS absorption features are narrower and mostly
concentrated in two main velocity components at V = -17 and -10 km/s. H2CO
absorption lines present an intermediate pattern with absorption in all
velocity components but larger opacities in the two main velocity components.
The ortho-to-para ratios of H2CO and c-C3H2 are consistent with the statistical
value of 3. While the excitation temperature of all c-C3H2 velocity components
is consistent with the CMB, the two strong components detected in CS show a
clear excess over the CMB indicating that CS resides at higher densities than
other species along this particular sightline, n(H2) ~ 2500 cm-3 while n(H2) <
500 cm-3 for the other velocity components. We detected faint absorption from
o-H213CO and C34S allowing us to derive isotopic ratios: o-H2CO/o-H213CO = 61
and C32S/C34S = 24. The excitation of the 4.8GHz line of formaldehyde is
sensitive to the electron fraction and its excitation temperature is predicted
to be lower than the CMB at low and moderate electron fractions, x(e)< 6E-5,
and to rise above the CMB at high electron fractions, > 1e-4.
