The infrared absorption coefficient of H2SO4 vapor has been measured for the first time. The vapor was obtained from liquid acid held at the azeotrope. The temperature dependence of the absorption confirmed that the azeotrope concentration was accurate to better than 1%. The P, Q, and R branches of the measured band cover a spectral range of 1190 to 1260 cm−1. The peak absorption coefficient of 4.16 ±0.15 atm−1 cm−1 occurred at 1222 cm−1 within the Q branch, with an integrated absorption coefficient of 120 atm−1cm−2 over the entire band. Within the resolution of the experiment, 0.5 cm−1, no fine structure was observed on this band.
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