Knowledge Bank

Several years ago we used our molecular beam spectrometer to record the hyperfine spectrum of Lil. Attempts to fit the data to a set of molecular constants were close, but not quite satisfactory. Recently we have developed more refined techniques for fitting the lineshapes, and have refitted all the lines from the $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}Lil$ spectrum. With the improved line frequencies the data now fits nicely to a set of molecular constants, if a term representing the electric hexadecapole interaction of the iodine nucleus is included. A nuclear magnetic octupole interaction was also considered, but does not contribute significantly. To our knowledge this is the first time that a hexadecapole interaction significantly different from zero has been observed in a molecule.