Reduction of charge transfer loss in atomic laser isotope separation by production of excited ions through autoionization levels

A new technique to reduce charge transfer loss when extracting ions from photoionized plasma has been developed, thereby increasing the efficiency of atomic vapor laser isotope separation (AVLIS). By selecting autoionization levels, specific isotopes are ionized to the excitation levels of the ions. Cross sections for charge transfer between specific excited ions and ground state atoms are smaller than those between ground state ions and ground state atoms. The effectiveness of this technique was confirmed by measuring the difference in charge transfer cross sections between Gd++Gd and Gd+*+Gd. When the gadolinium atoms were ionized at the 53,616 cm(-1) autoionization level, the kinetic energy of the released electrons was measured with a time of flight electron spectrometer (energy resolution > 30 meV) and the produced ions were determined to be at levels of 4f(7)5d6s(10)D(13/2) and D-8(7/2). The charge transfer cross section for Gd+*(D-10(13/2) 1,935 cm(-1), D-8(7/2) 3,427 cm(-1))+Gd(D-9(J)) was 1.5x10(-18) m(2) at an impact energy of 250 eV, which was a smaller value than that for Gd+(D-10(5/2) 0 cm(-1))+Gd(D-9(J))(2.5x10(-18) m(2)). When specific isotopes of gadolinium were ionized by autoionization, the charge transfer loss could be reduced from 14% to 9% at an atomic density of 1x10(19) m(-3) in the typical isotope separation region. Because this effect is also expected in charge transfer between other heavy metallic ions, it should be possible to improve the ion collection efficiency for isotopes like uranium by selecting autoionization levels to produce specific excited ions.