We present terahertz spectroscopy and analysis of two commercially available
quinacridone pigments in the 0.5–4.5 THz range. Our results show a clear
distinction between quinacridone red and magenta pigments. We reveal four definite
absorptions in the terahertz regime common to both pigments, but offset between
the pigments by ∼ 0.2 THz. The lowest-energy line in each pigment is observed to
increase in frequency by ∼ 0.1 THz as the temperature is reduced from 300 to 12 K.