title: Mode structure effects in optical resonance excitation.
abstract: 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 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-response model. With the aid of correlation analysis a resonance can be localized with a single-moderesolution. 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.
authors.forename: H M
authors.initials: HM
authors.lastname: Lauranto
authors.forename: T T
authors.initials: TT
authors.lastname: Kajava
authors.forename: M I
authors.initials: MI
authors.lastname: Santala
authors.forename: R R
authors.initials: RR
authors.lastname: Salomaa