The isotope shift in the HgH spectrum for Hg<sup>198</sup>H-Hg<sup>200</sup>H has been measured for 698 lines in the <i>A</i><sup>2</sup>II-<i>X</i><sup>2</sup>Σ<sup>+</sup> system. The variation of the isotope shift with rotational frequency of the line is different for the various branches of a band, and greatly exceeds the variation predicted by the usual rotational and vibrational mass effects alone. This phenomenon is a consequence of the different nuclear volumes of the mercury isotopes. The results are evidence that the total electron probability density at the mercury nucleus increases with rotational frequency at a faster rate for the ground state than for the excited state.
TERENCE L. PORTER and SUMNER P. DAVIS, "Isotope Shift in the Spectrum of Mercury Hydride," J. Opt. Soc. Am. 52, 1206-1206 (1962)