High selectivity (>10) for separation of ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}$ from other Zr isotopes was observed during triple resonant ionization of Zr vapors. Linearly polarized lasers were used to prepare aligned states, from which further excitation to a presently discovered J=0 level could be suppressed for even-mass isotopes, by a suitable choice of relative laser polarization. Photoionization of the odd-mass isotope ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}$ could be retained by use of transitions with relatively strong hyperfine interactions, because of associated population redistributions in the magnetic sublevels. The dependence of the even-mass-isotope signals on relative laser polarization followed sinusoidal forms, which are in excellent agreement with predictions derived using only geometric components of the transition dipole moment. The isotopic selectivity for ${}_{}{}^{91}{}_{}{}^{}\mathrm{Zr}$ can be increased by use of saturating fluences, because of the effects of saturation, population trapping in the even-mass isotopes and strong hyperfine interactions in the odd-mass isotopes. For the same reasons, biases in even:odd isotope ratio measurements cannot always be eliminated by polarization scrambling or magic angles.

• Received 13 January 1993

DOI:https://doi.org/10.1103/PhysRevA.47.4946