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Theory of doubly resonant ionization by broad-band radiation applied to the determination of isotopic abundances.

We present a theoretical study of doubly resonant three-photon ionization with radiation of sufficiently broad bandwidth to overlap all hyperfine levels of the resonant intermediate states. We demonstrate that isotopic selectivity in ionization is apt to occur under a variety of circumstances, although the degree may vary from negligible to quite significant, depending on the relative values of atomic and radiation parameters. Our theory provides a satisfactory quantitative interpretation of recent experimental data in Sn.