Knowledge Bank

A multiple reflection Raman tube was used with a quartz spectrograph, giving a dispersion of $25.7cm-1/mm$ at $\lambda 2537$, to study the fine structure of the $\nu 3$ band of methane at a pressure of four atmospheres. Seventy-five lines in the band were resolved, and evidence for most of the fifteen theoretically predicted sub-branches was found. Various combination relations were used to calculate the molecular constants from the frequencies of selected, unblended, Raman lines and the infrared frequencies of Nielsen and $Nielsen.1$ The analysis gave three different rotational constants $B1+,B10$ and $B1-$ for the upper vibrational state and, surprisingly, three different values for the ground state. The average rotational constants are $B1=5.207,D1=1.6\times 10-4$ and $B0=5.244$, and $D0=1.3\times 10-4$. The values of $\mu 0$ and $\zeta 3$ were found to be 3018.1 $cm-1$ and 0.055. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ A. H. Nielsen and H. H. Nielsen, Phys. Rev. 48, 864 (1935)