Models of the extended and more translucent environment of GMCs as a function of the ionization fraction. For N(HCN) = 10¹³ cm⁻², and n(H₂) = 5 × 10³ cm⁻³ we show: mean T_ex(J = 1–0, F_u -F_l) in the upper panel; line intensity ratios R₀₂ and R₁₂ in the middle panel; and total integrated line intensities HCN J = 1–0, 2–1, 3–2 in the lower panel. Continuous and dotted curves show model results for T_k = 30 and 60 K, respectively.