Plot of log (3k ∫ T_RdV/8π³νSμ²) versus energy of the upper state in the transition (E_upper) for the transitions of vibrationally excited SiC₂ in Fig. 1. Three of the transitions (4₁₄–3₁₃, 6₁₅–5₁₄, and 7₁₇–6₁₆) fall on a line that indicates an excitation temperature of 13 K. From the ordinate intercept of the line, we calculate an upper limit to the column density of vibrationally excited SiC₂ N(ν₃ = 1) ≲ 2.6 × 10¹³ cm⁻² (see text). Two of the transitions sought (5₁₅–4₁₄ and 7₃₅–6₃₄) fall below their expected intensities.