Precision measurements on Si-

High-precision measurements of the electron affinities (EA) of the three stable isotopes of silicon, ²⁸Si, ²⁹Si and ³⁰Si, have been performed at the cryogenic electrostatic ion-beam storage ring DESIREE. The quantum states of the ions were manipulated using laser depletion, and the ions were photodetached by laser photodetachment threshold spectroscopy. These EA values are the first reported for ²⁹Si⁻ and ³⁰Si⁻ and provide a reduced uncertainty for ²⁸Si⁻. The resulting EAs are EA(²⁸Si$) = 1.38952201(17)$ eV, EA(²⁹Si$) = 1.38952172(12)$ eV and EA(²⁹Si$) = 1.38952078(12)$ eV, with the corresponding isotope shifts IS(²⁹⁻²⁸Si$) = 0.29(16)$ micro eV and IS(³⁰⁻²⁸Si$) = 1.23(16) $ micro eV. In addition to these measurements, the resolution and signal-to-background level was sufficient to reveal the hyperfine structure splitting in the ²⁹Si⁻ isotope, which we report to be $1.8(4) micro eV.