The role of intense line wings in the process of infrared multiple--photon excitation of polyatomic molecules

Abstract

The results of experimental testing the existence of intense Lorentzian--like wings with FWHM $\sim 4.5 cm^{-1}$ in the absorption spectra of polyatomic molecules in a gas phase are presented. Two independent experimental methods were used for evaluating the integral intensity of the line wings for a number of substances. In the first case, the cross--section of the far wings of absorption bands in a gas phase spectrum were measured. Then, these band wings were extrapolated inside the contour of absorption band. In the second case, the saturation degree of the linear spectrum of molecules was determined. Radiation of a pulsed $CO_2$--laser was used at low gas pressure ($\sim 16$ mtorr) and averaged excitation level of molecules ${<n>}\sim 0.1$ quanta/molecule. The values obtained by these two independent methods coincide for a variety of molecules. The average relative integral intensity of the line wings varied from $\sim 0.6%$ for $SF_6$ and $SiF_4$ to $\sim 90%$ for $(CF_3)_2O$ and $(CF_3)_2CO$.