Knowledge Bank

The vibration-rotation overtone near $9050cm-1$ was studied using a Fourier transform spectrometer with resolution of $0.01cm-1$ and photometric precision of 0.02%. Absolute strengths of corresponding R-branch lines in isotopic ally pure samples were measured under identical conditions to minimize systematic errors in relative strengths. Line strengths for $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH4$ were observed to be smaller than those for $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH4$ by as much as 40%. (Similar results were found for $2\nu 3$ of methane.) The $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C:13C$ abundance ratios deduced from $3\nu 3$ spectra of planetary atmospheres will now require modifications.