Isotope Shift in the Spectrum of Mercury Hydride*

The isotope shift in the HgH spectrum for Hg198H–Hg200H has been measured for 698 lines in the A2Π−X2∑+ 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.