Collisional Line Mixing in an $N_{2}$-Broadened $CO_{2}$ Q Branch

Author Institution: Department of Physics, University of Maryland Baltimore County; Code 617, NASA/Goddard Space Flight CenterRotational collisional narrowing, or line mixing, has been observed in the ($11^{1}0,03^{1}0)_{1}-000$ band of $CO_{2}$ at $2076 cm^{-1}$ using a tunable diode laser spectrometer. Spectra of 1\% concentrations of $CO_{2}$ in $N_{2}$ were recorded at total pressures ranging from 100 to 720 torr. The experimentally determined absorption coefficients are up to 65\% lower in the Q-branch wing and 20\% larger inside the Q branch than those calculated assuming isolated Lorentz lines. A simple energy gap scaling law is used to determine the off-diagonal relaxation matrix elements from the known pressure-broadening coefficients. The spectra calculated using these matrix elements reproduce the observed absorption coefficients to within several percent. The accuracy of Rosenkranz's first-order theory for the calculation of line mixing will be discussed. Our observation of line mixing in an $N2$-broadened $CO_{2}$ Q branch demonstrates that line mixing may be important for a number of problems in atmospheric spectroscopy. The implications of Q-branch mixing for several satellite instruments that employ $CO_{2}$ Q branches for remote sounding of atmospheric temperature profiles will be discussed