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The dependence of the excitation strength on the longitudinal mode structure of a pulsed, resonant excitation field is examined. As an experimental test case, resonance ionization signal fluctuations are studied with a two-step excitation of strontium. Successive ionization signals, the corresponding pulse energies, and the mode structures of the resonant step laser pulses are recorded. The large signal fluctuations, up to 120%, cannot be explained by the modest (1-3%) pulse-energy fluctuations of either the resonant or the photoionizing field. In the case of weak excitation, the fluctuations in the signal correlate strongly with the intensity in a narrow frequency band around the resonance. In this week-field region, the experimental correlation curves, i.e., the correlation between the signal and the spectral intensity versus the frequency, agree well with calculations based on a simple linear-response model. With the aid of correlation analysis a resonance can be localized with a single-mode resolution. As the resonant field is increased the correlation between the signal and single-mode intensity diminishes and almost disappears at full saturation. However, also in the saturation region, the correlation technique can be applied to localize a resonance with a resolution much better than that determined by the laser linewidth.