It can be explained by the fact that there is a local peak of the benzene absorbance in the vicinity of the unperturbed position (in the absence of the magnetic field) of the resonance line of the 200Hg isotope. Therefore, the positive response is observed for the 198Hg and196Hg isotopes with the unperturbed position of the resonance line at a frequency higher than that of the 200Hg isotope, and the negative response is observed for the 202Hg and204Hg isotopes with a lower frequency of the resonance line.
absence of the magnetic field