  We demonstrate an efficient high-precision optical spectroscopy technique for
single trapped ions with non-closed transitions. In a double-shelving
technique, the absorption of a single photon is first amplified to several
phonons of a normal motional mode shared with a co-trapped cooling ion of a
different species, before being further amplified to thousands of fluorescence
photons emitted by the cooling ion using the standard electron shelving
technique. We employ this extension of the photon recoil spectroscopy technique
to perform the first high precision absolute frequency measurement of the
$^{2}$D$_{3/2}$ $\rightarrow$ $^{2}$P$_{1/2}$ transition in $^{40}$Ca$^{+}$,
resulting in a transition frequency of $f=346\, 000\, 234\, 867(96)$ kHz.
Furthermore, we determine the isotope shift of this transition and the
$^{2}$S$_{1/2}$ $\rightarrow$ $^{2}$P$_{1/2}$ transition for $^{42}$Ca$^{+}$,
$^{44}$Ca$^{+}$ and $^{48}$Ca$^{+}$ ions relative to $^{40}$Ca$^{+}$ with an
accuracy below 100 kHz. Improved field and mass shift constants of these
transitions as well as changes in mean square nuclear charge radii are
extracted from this high resolution data.
