Far-infrared foreign and self-broadened rotational linewidths of high-temperature water vapor

We report what is believed to be the first measurement of rotational line broadening of high-temperature water vapor for transition frequencies that range from 1.0 to 2.5 THz (33 to 83 cm-1). Water-vapor rotational transitions were measured in a near-stoichiometric propane–air flame at 1490 K with THz time-domain spectroscopy. Linewidths were measured for 29 pure rotational transitions in both ground vibrational and ν2=1 vibrational levels with lower state energies ranging from 80 to 2390 cm-1. Measured widths show a dependence on rotational state energy that roughly follows a thermal distribution with variation between various J, Ka, Kc rotational quantum numbers.