Knowledge Bank

Accurate values of the ground state molecular rotational constant $Ao(=h8\pi cIa$ where $Ia$ is the principal moment of inertia about the symmetry axis) for axially symmetric molecules can not be determined from pure rotation spectra in the far infrared and microwave regions or from normal parallel bands in the ordinary and near infrared regions. Moreover, from an individual perpendicular band, e.g., $\nu 4$ of any methyl halide molecule, a least squares analysis of the data yields only a linear combination of $A0$ with the coefficient $A\ast \ast \ast \zeta 42$ of the first-order Coriolis essential resonance term. However, for two suitably related bands such as $\nu 4$ and $2\nu 4$, in which the linear combinations of $A0$ and $A\ast \ast \ast \zeta 42$ for each band are different but known, a simultaneous analysis of lines of both bands permits solution for $A0$ directly. We are in the process of applying this technique to the spectra of the methyl halides. A simultaneous analysis of about 150 lines of $\nu 4$ (perpendicular), 20 lines of $2\nu 4$ (parallel component), and 200 lines of $2\nu 4$ (perpendicular component) of methyl bromide $(CH3Br)$ has given a preliminary value of $A0=5.1268\pm 0.0008cm-1$ with 95% confidence.