Isotope shift in the sulfur electron affinity: Observation and theory

The sulfur electron affinities eA(S) are measured by photodetachment microscopy for the two isotopes S32 and S34 (16752.9753(41) and 16752.9776(85) cm^-1, respectively). The isotope shift in the electron affinity is found to be more probably positive, eA(S34)-eA(S32) =+0.0023(70) cm ^-1, but the uncertainty allows for the possibility that it may be either "normal" [eA(S34) eA(S32)] or "anomalous" [eA(S34) <eA(S32)]. The isotope shift is estimated theoretically using elaborate correlation models, monitoring the electron affinity and the mass polarization term expectation value. The theoretical analysis predicts a very large specific mass shift (SMS) that counterbalances the normal mass shift (NMS) and produces an anomalous isotope shift eA(S34)-eA(S32) =-0.0053(24) cm^-1, field shift corrections included. The total isotope shift can always be written as the sum of the NMS (here +0.0169 cm^-1) and a residual isotope shift (RIS). Since the NMS has nearly no uncertainty, the comparison between experimental and theoretical RIS is more fair. With respective values of -0.0146(70) cm^-1 and -0.0222(24) cm^-1, these residual isotope shifts are found to agree within the estimated uncertainties.