Isotope biases in RIMS utilizing broad-band long-pulsed lasers

It has been shown (Fairbank et al 1990 and Lambropoulas et al 1990) that hyperfine structure plays a major role in producing `anomalous' odd to even isotope ratios in the stepwise excitation and ionization of an element with broad bandwidth lasers. This investigation is based on the suggestion (Payne et al 1991) that isotope biases may be avoided by detuning the laser from resonance by an amount greater than either the laser bandwidth or the hyperfine splitting, but less than the power broadened width. Our initial experiments have confirmed that detunings of 6 cm^-1 are sufficient to eliminate these biases in the two-photon stepwise ionization by way of the ³P₀→³P₁ resonance in Sn with a laser power density of 10⁸ W/cm². Also, we have seen that a 1+2 resonantly enhanced ionization scheme faithfully reproduces odd to even ratios, which can be attributed to beam geometry.