Knowledge Bank

Recent measurements $\mathrm{<mrow\; data-mjx-texclass="ORD">1,2</mrow>}$ of absolute strengths of the $2\nu 3$ singlets Q1, RO, and R1 for $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH4$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH4$, under identical experimental conditions designed to minimize errors in derived relative strengths, demonstrated that isotopic substitution for the central atom in a spherical top can affect these strengths dramatically. In an effort to explain these apparently anomalous experimental results, vibration-rotation interactions in the multiplets $\{m\nu 1+n\nu 3\}$ with m + n = 1,2,3,\ldots have been considered. Preliminary theoretical results indicate that the observed isotopic effects, including possible variation with rotational quantum number, can be accounted for satisfactorily.