Pressure broadening effects on the lambda doublets of nitric oxide

In recent years several high resolution pressure broadening measurements have been performed on the fundamental band of NO. These works have assumed that the collision halfwidth of each component of the lambda doublet of NO was equal and neglected any spectral line coupling effects. This work examines the validity of these assumptions by examining, analytically, the effects of collision broadening on the lambda doublets of NO.

The Anderson-Tsao-Cumutte (ATC) pressure broadening theory is extended to a coupled spectral line theory which includes non-J] state diatomics. From this theory expressions for the coupled line absorption coefficient are obtained which include the dipole-dipole, dipole-quadrupole, quadrupole-dipole, and quadrupole-quadrupole intermolecular potentials. The resolved line approximation was added to the ATC theory in order to obtain expressions for the collision halfwidths of each lambda doublet component in the absence of line coupling effects. The expressions for the halfwidths given by the ATC theory were obtained when it was assumed the rotational eigenstates of NO are equivalent to those of a symmetric top molecule. The effects of collision broadening of NO by CO were calculated utilizing the three different cases above; coupled line formulation, resolved line approximation, and the ATC theory assuming symmetric top eigenstates.

The results confirmed that the equal halfwidth and no coupling assumptions are indeed valid approximations. The percent difference between the halfwidths of the doublet components was found to be no more than 2.8 percent. The difference between the halfwidths calculated by the ATC theory and that of the modified ATC, which incorporated non-∑ state eigenstates, was no more than 3.2 percent. The line coupling effects were found to be more pronounced in the lines of the 3/2 subband than those of the 1/2 subband due to the small doublet separation in the 3/2 subband. The coupled line effects were never found to exceed 1.42 percent of the absorption coefficient over a frequency range where the absorption was non-negligible at pressures as high as 600 Torr.