Collision-induced tunneling in methyl halides

We use the technique of terahertz time-domain spectroscopy to investigate the absorption and dispersion of spectrally dense methyl halide vapors, particularly in the low-and high-frequency spectral wings. For the first time to our knowledge, it is possible Co observe essentially zero-frequency absorption resulting from molecular tunneling between the two states of symmetry simultaneously, with absorption from the entire rotational manifold. We can obtain accurate fits to the measurements on both the low-and high-frequency wings with our new molecular response theory, This theory expands upon the basic van Vleck-Weisskopf and Lorentz theories by assuming a finite reorientation time of a molecule 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-Weisskopf theories. (C) 1997 Optical Society of America [S0740-3224(97)02011-0].