Left panel (a): relation between the 3.4 μm aliphatic C–H stretch and the 6.85 μm aliphatic C–H deformation. Right panel (b): I_3.4/I_3.3 versus I_6.85/I_6.2. The results for R3 are indicated by the dashed open circle due to its low-quality spectrum, and this region has been excluded from the correlation analysis. While I_3.4 is not correlated with I_6.85, I_3.4/I_3.3 is positively correlated with I_6.85/I_6.2, confirming the aliphatic nature of the 6.85 μm band. That the 3.4 μm emitter and the 6.85 μm emitter are different in size explains why I_3.4 is not correlated with I_6.85.