Experimental verification of a simple method to avoid isotopic biases resulting 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. PAYNE et al. [Spectrochim Acta 46B, 1439 (1992)] proposed that these anomalies 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-broadened width of a selected resonant transition exceeds the laser bandwidth. The laser is detuned from resonance by an amount greater than the laser bandwidth but less than the power-broadened width. The data obtained in our study confirm the prediction that a slightly detuned resonant ionization scheme effectively eliminates odd/even isotope ionization biases and that operating at adequate laser intensity produces a fairly wide detuning regime over which faithful odd/even isotope ratios 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 ³P₀ → ³P₁ resonance in tin (Sn) at a laser power density of 10⁸ W cm^-2.