Measurements of Line Intensities for Some Lines of Ammonia in the v2, 2v2 and v4 Bands

Using Fourier Transform spectra (Bruker IFS 120 HR, resolution approximate to 0.004 cm(-1)) of NH3 in several branches of the v(2), 2v(2) and v(4) bands, absolute line intensities have been determined at room temperature (T = 295 K) for more than 400 rovibrational lines located in the spectral range 1000-1800 cm(-1). A non-linear least-squares multispectrum fitting procedure, including line mixing effects, has been used to retrieve the line intensities from eleven experimental spectra recorded at different pressures of pure ammonia. The present results are compared with previous measurements(1,2,3) in which the collisional absorption coefficient is a Voigt profile. The line intensities determined in the present work are smaller than those determined by these authors. For example, the difference in percent between our values and the values of C. Cottaz(1) is about 6%. For almost all lines and pressures considered in this work, it was shown that the first-order Rosenkranz absorption coefficient taking into account line-mixing effects is adequate to extract with sufficient accuracy the line intensities of NH3. An examination of these line intensities shows a pronounced J and K quantum numbers dependencies. Our results have been analyzed to obtain effective transition moments and correction parameters of symmetric and asymmetric partial bands of the fundamental v(4) band, as well as those of asymmetric partial bands of v(2) and 2v(2) bands. Since these bands are affected by the rotation-vibration interactions, the zero-order theory where the Hermann-Wallis factors are not included is not able to describe the intensity anomalies in these bands. As shown by Aliev et al.(4), it is necessary to include these interactions into theoretical line strengths expressions. These authors have developed a third-order theory of the line strengths of the vibrational transition from the ground vibrational state to the rovibrational levels of molecules with C-3v symmetry using the method of contact transformation applied to the electric dipole moment operator. A suitable expression for the line strength was derived to fit the experimental intensities.