The two-step laser-enhanced ionization (LEI) technique is believed to be more sensitive and selective in trace analysis than one-step LEI. As reported previously, for optimization of the two-step LEI configuration, factors involving laser intensity, transition probability of second-step excitation, collisional ionization rates, and transition linewidth should be taken into account. We have derived a simple theoretical model in relation to these parameters that is rough but suitable in practice for characterizing the ion yields enhanced by two-step LEI over one-step LEI. According to the model, each parameter affecting the ion enhancement has been systematically examined and compared to the theoretical evaluation. The findings of the relative ion enhancement as a function of each related parameter are consistent with the model prediction, except for the effect of transition linewidth, which is underestimated theoretically due to the difficulty of establishing the experimental conditions.
King-Do Su and King-Chuen Lin, "Ion Enhancement Effect of Laser-Enhanced Ionization Induced by One-Step Excitation and Two-Step Excitation," Appl. Spectrosc. 48, 241-247 (1994)