  Isotope selective optical excitation of atoms is important for experiments
with neutral atoms, metrology, and work with trapped ions, including quantum
information processing. Polarization-enhanced absorption spectroscopy is used
to frequency stabilise a tunable external cavity laser diode system at 398.9 nm
for isotope selective photoionization of neutral Yb atoms. This spectroscopy
technique is used to measure isotope resolved dispersive features from
transitions within a see through configuration Ytterbium hollow-cathode
discharge lamp. This Doppler-free dichroic polarization spectroscopy is
realised by retro-reflecting a laser beam through the discharge and analysing
the polarization dependent absorption with balanced detection. The spectroscopy
signal is recovered using lock-in detection of frequency modulation induced by
current modulation of the external cavity laser diode. Here, we show the use of
polarization-enhanced absorption spectroscopy for isotope selectively loading
of $^{171}$Yb$^+$, $^{172}$Yb$^+$, or $^{174}$Yb$^+$ ions into an RF Paul trap.
