MODE STRUCTURE EFFECTS IN OPTICAL RESONANCE EXCITATION

The dependence of the excitation strength on the longitudinal mode str ucture of a pulsed, resonant excitation field is examined. As an exper imental test case, resonance ionization signal fluctuations are studie d with a two-step excitation of strontium. Successive ionization signa ls, the corresponding pulse energies, and the mode structures of the r esonant step laser pulses are recorded. The large signal fluctuations, up to 120%, cannot be explained by the modest (1-3%) pulse-energy flu ctuations of either the resonant or the photoionizing field. In the ca se of weak excitation, the fluctuations in the signal correlate strong ly with the intensity in a narrow frequency band around the resonance. In this weak-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-r esponse model. With the aid of correlation analysis a resonance can be localized with a single-mode resolution. As the resonant field is inc reased the correlation between the signal and single-mode intensity di minishes and almost disappears at full saturation. However, also in th e saturation region, the correlation technique can be applied to local ize a resonance with a resolution much better than that determined by the laser linewidth.