Isotope-selective ionization utilizing field-free alignment of isotopologues using a switched nanosecond laser pulse

We propose and numerically simulate a method of laser isotope separation based on field-free alignment of isotopologues, utilizing an intense "switched" nanosecond (ns) laser field which is slowly turned on and rapidly turned off at the peak with the falling time of 200 fs. The femtosecond (fs) laser-induced alignment of isotopologues including heavy isotopes is severely disturbed by ionization because of their small ionization potential. Our simulations for I⁷⁹Br and I⁸¹Br isotopologues demonstrate that the switched ns laser field can make isotopologues well-aligned with the reduced ionization probability at the laser intensity [I₀ = 2 TW/cm² (= 2 × 10¹² W/cm²)] which is an order-of-magnitude lower than a typical intensity (I₀ = 20 TW/cm²) for field-free alignment induced by a fs laser field. In our simulations, IⁱBr isotopologues (i = 79 or 81) aligned with the switched ns laser field are selectively ionized by an intense fs laser pulse at an appropriate time delay from the alignment pulse.