Excitation temperature (color-coded map) as a function of quantum number of upper level (x-axis) and H₂ volume density (y-axis) for six negative molecular anions as obtained from LVG calculations adopting a gas kinetic temperature of 10 K, a column density of 10¹¹ cm⁻², and a linewidth of 0.5 km s⁻¹. The references for the dipole moments are Brünken et al. (2007b) for C₂H⁻, Botschwina (2000) for C₄H⁻, Blanksby et al. (2001) for C₆H⁻, Botschwina et al. (1995) for CN⁻, Thaddeus et al. (2008) and Kołos et al. (2008) for C₃N⁻, and Botschwina & Oswald (2008) for C₅N⁻. For reference, the white dotted vertical line indicates the J level at which the energy is 10 K. The microwave and mm spectral regions observable with radiotelescopes are indicated. The small dark blue regions in the bottom-left corner of the C₄H⁻, C₆H⁻, and C₃N⁻ panels correspond to negative excitation temperatures.