COLLISION-INDUCED TUNNELING IN METHYL HALIDES

We use the technique of terahertz time-domain spectroscopy to investig ate the absorption and dispersion of spectrally dense methyl halide va pors, particularly in the low-and high-frequency spectral wings. For t he first time to our knowledge, it is possible Co observe essentially zero-frequency absorption resulting from molecular tunneling between t he two states of symmetry simultaneously, with absorption from the ent ire rotational manifold. We can obtain accurate fits to the measuremen ts on both the low-and high-frequency wings with our new molecular res ponse theory, This theory expands upon the basic van Vleck-Weisskopf a nd Lorentz theories by assuming a finite reorientation time of a molec ule to an external electric field during a collision. This line-shape theory is shown to eliminate the nonphysical Debye plateau of constant absorption at high frequencies inherent in the Debye and van Vleck-We isskopf theories. (C) 1997 Optical Society of America [S0740-3224(97)0 2011-0].