Isotope shift in the sulfur electron affinity: Observation and theory

The sulfur electron affinities (e)A(S) are measured by photodetachment microscopy for the two isotopes S-32 and S-34 (16 752.975 3(41) and 16 752.977 6(85) cm(-1), respectively). The isotope shift in the electron affinity is found to be more probably positive, (e)A(S-34) - (e)A(S-32) = +0.0023(70) cm(-1), but the uncertainty allows for the possibility that it may be either normal [(e)A(S-34) > (e)A(S-32)] or anomalous [(e)A(S-34) < (e)A(S-32)]. 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 (e)A(S-34) - (e)A(S-32) = -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.