  We report new observations of molecular anions with the Yebes 40m and IRAM
30m telescopes toward the cold dense clouds TMC-1 CP, Lupus-1A, L1527, L483,
L1495B, and L1544. We detected for the first time C3N- and C5N- in Lupus-1A and
C4H- and C6H- in L483. In addition, we report new lines of C6H- toward the six
targeted sources, of C4H- toward TMC-1 CP, Lupus-1A, and L1527, and of C8H- and
C3N- in TMC-1 CP. Excitation calculations indicate that the lines of anions
accessible to radiotelescopes run from subthermally excited to thermalized as
the size of the anion increases, with the degree of departure from
thermalization depending on the H2 volume density and the line frequency. We
noticed that the collision rate coefficients available for the radical C6H
cannot explain various observational facts, which advises for a revisitation of
the collision data for this species. The observations presented here, together
with observational data from the literature, are used to model the excitation
of interstellar anions and to constrain their abundances. In general, the
anion-to-neutral ratios derived here agree within 50 % (a factor of two at
most) with literature values, when available, except for the C4H-/C4H ratio,
which shows higher differences due to a revision of the dipole moment of C4H.
From the set of anion-to-neutral abundance ratios derived two conclusions can
be drawn. First, the C6H-/C6H ratio shows a tentative trend in which it
increases with increasing H2 density, as expected from theoretical grounds. And
second, it is incontestable that the higher the molecular size the higher the
anion-to-neutral ratio, which supports a formation mechanism based on radiative
electron attachment. Nonetheless, calculated rate coefficients for electron
attachment to the medium size species C4H and C3N are probably too high and too
low, respectively, by more than one order of magnitude.
