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We report new observations of molecular anions with the Yebes 40m and IRAM30m 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 andC4H- and C6H- in L483. In addition, we report new lines of C6H- toward the sixtargeted sources, of C4H- toward TMC-1 CP, Lupus-1A, and L1527, and of C8H- andC3N- in TMC-1 CP. Excitation calculations indicate that the lines of anionsaccessible to radiotelescopes run from subthermally excited to thermalized asthe size of the anion increases, with the degree of departure fromthermalization depending on the H2 volume density and the line frequency. Wenoticed that the collision rate coefficients available for the radical C6Hcannot explain various observational facts, which advises for a revisitation ofthe collision data for this species. The observations presented here, togetherwith observational data from the literature, are used to model the excitationof interstellar anions and to constrain their abundances. In general, theanion-to-neutral ratios derived here agree within 50 % (a factor of two atmost) 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 canbe drawn. First, the C6H-/C6H ratio shows a tentative trend in which itincreases with increasing H2 density, as expected from theoretical grounds. Andsecond, it is incontestable that the higher the molecular size the higher theanion-to-neutral ratio, which supports a formation mechanism based on radiativeelectron attachment. Nonetheless, calculated rate coefficients for electronattachment to the medium size species C4H and C3N are probably too high and toolow, respectively, by more than one order of magnitude.

The abundance and excitation of molecular anions in interstellar clouds