Rotational excitation of HCO⁺ by H₂ and electrons in diffuse gas at moderate density in the LVG approximation. The horizontal axis of all panels is the column density per unit velocity of HCO⁺. The top panels show the brightness temperature above the cosmic microwave background of the J = 1 − 0 line, at left for fixed electron fraction x_e = n_e/n(H) = 1.4 × 10^-4 and varying hydrogen number density n(H) = 2n(H₂), at right for varying electron fraction at n(H) = 256   cm^-2. The lower panels show line brightness ratios in the same manner. At upper left the calculation for n(H)  = 256   cm^-3 is shown twice, for T_K = 15 (which lies below) and 35 K. Note that the lower panels in each column have a linear ordinate scale.