Separation of even-even from even-odd isotopes using ultrafast lasers

We propose a laser isotope separation mechanism in which selectivity arises from nuclear spin rather than isotope shifts, enabling the use of broadband ultrafast lasers. A Ramsey pulse sequence is applied to paramagnetic molecular isotopologues possessing two electronic states coupled by a dipole transition. For even-even isotopologues (nuclear spin $I = 0$), each electronic state is a single level and the time-reversed sequence returns all population to the ground state exactly. For even-odd isotopologues ($I > 0$), the hyperfine interaction splits each state into multiple levels with coupling amplitudes set by Wigner $6j$ symbols; incommensurate phase evolution during the dark interval prevents the echo from closing, trapping a fraction $P_m$ of the population in the excited manifold. In the impulsive limit ($\Omega \gg A_{\rm HF}$), $P_m$ depends only on the angular momentum quantum numbers $(J_g, J_m, I)$ and is independent of laser intensity or bandwidth. Density matrix simulations confirm $P_m = 0$ for $I = 0$ and $P_m \approx 0.23$-$0.47$ for $I > 0$ across representative systems including ${}^{235}$U, ${}^{87}$Sr, and ${}^{57}$Fe. Under realistic collisional conditions, single-pass enrichment exceeding 90% from natural feed is achievable without cascading.