  High-precision measurements of the electron affinities (EA) of the three
stable isotopes of silicon, $^{28}$Si, $^{29}$Si and $^{30}$Si, 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 $^{29}$Si$^-$ and $^{30}$Si$^-$ and provide a
reduced uncertainty for $^{28}$Si$^-$. The resulting EAs are $EA(^{28}$Si$) =
1.38952201(17)$ eV, $EA(^{29}$Si$) = 1.38952172(12)$ eV and $EA(^{29}$Si$) =
1.38952078(12)$ eV, with the corresponding isotope shifts $IS(^{29-28}$Si$) =
0.29(16)$ micro eV and $IS(^{30-28}$Si$) = 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 $^{29}$Si$^-$
isotope, which we report to be $1.8(4) micro eV.
