EXPERIMENTAL-VERIFICATION OF A SIMPLE METHOD TO AVOID ISOTOPIC BIASESRESULTING FROM HYPERFINE-STRUCTURE IN RESONANT IONIZATION MASS-SPECTROSCOPY

Hyperfine structure plays a major role in producing ''anomalous'' odd- to-even isotope ratios in the stepwise excitation, ionization and mass analysis of an element with broad bandwidth, long-pulsed lasers. PAYN E et al. [Spectrochim Acta 46B, 1439 (1992)] proposed that these anoma lies may be avoided by cautious application of a simple prescription. We test the prediction by conducting a one-color, two-step ionization experiment on Sn at sufficient laser intensity such that the power-bro adened width of a selected resonant transition exceeds the laser bandw idth. The laser is detuned from resonance by an amount greater than th e laser bandwidth but less than the power-broadened width. The data ob tained in our study confirm the prediction that a slightly detuned res onant ionization scheme effectively eliminates odd/even isotope ioniza tion biases and that operating at adequate laser intensity produces a fairly wide detuning regime over which faithful odd/even isotope ratio s are expected. Specifically, detunings of 5-10 cm-1 are sufficient to eliminate these biases in the two-photon stepwise (1+1) ionization by way of P-3O --> P-3(1) resonance in tin (Sn) at a laser power density of 10(8) W cm-2.