Multiphoton absorption of TEA $CO_2$ (Transversely Excited at Atmospheric pressure $CO_2$) laser radiation has been studied using $CF_3Br$ molecules. $CF_3Br$ is known as a good candidate molecule for the laser isotope separation of $^{79}Br$/$^{81}Br$ and active molecule of far infrared laser ( ∼ 800㎛). Using the 9R branch lines of $CO_2$ laser, firstly, the dependency of multiphoton absorption on laser frequency was investigated. The observed multiphoton absorption spectrum showed the broadening and red-shift of absorption band in comparison with Fourier Transform-IR spectrum. The absorption cross section, σ was calculated using the data taken from simple absorption measurements and its dependency on pressure and laser fluence was studied. The average number of photons absorbed per molecule, ＜n＞, which indicates the degree of vibrational excitation of molecule, was also investigated. σ was found to be linearly dependent on the pressure of $CF_3Br$ molecule. As the laser fluence being increased, $\sigma$ and ＜n＞ exhibited three characteristic regions of absorption, linear saturation region, anharmonic bottlenecking region, quasi-continuum region. The buffer gas effect on absorption has been investigated using Ar, He, $CF_4$, and $SF_6$, and it showed that the addition of those buffer gas greatly enhanced the absorption.
Nonlinear refraction effect of TEA $CO_2$ laser in the polyatomic molecules is very important in studying the isotope selective multiphoton processes and optically pumped molecular lasers. It induces self-focusing and defocusing of TEA $CO_2$ laser pulse (1.3MW) in the medium of $CF_3Br$ molecules. Self-focusing of the 9R(30) line and self-defocusing of the 9R(26) and 9R(28) lines were observed. Nonlinear absorption cross sections ( σ ) were measured for various fluences of the laser beam in order to obtain accurate expression of nonlinear absorption coefficient, α. The beam size narrowing effect due to the nonlinear absorption ( α ) modified the far-field spatial profile of the beam, and from the distorted profile the nonlinear refractive index coefficient, $n_2$ was derived with an improved accuracy. The nonlinear refractive index coefficients for the three lines were $-7.9(\pm0.6)\times10^{-13}cm^2$ /Wtorr for 9R(26), $-3.7(\pm0.5)\times10^{-13}cm^2$/Wtorr for 9R(28) and $4.9(\pm0.4)\times10^{-12}cm^2$ /Wtorr for 9R(30).
