Simultaneous measurements of super-radiance at multiple wavelengths from helium excited states: I. Experiment

In this paper, we report the results of measurements of the intensities and delays of super-radiance decays from excited helium atoms at multiple wavelengths. The experiment was performed using extreme ultraviolet radiation produced by the free electron laser at the SPring-8 Compact SASE Source test accelerator facility as an excitation source. We observed super-radiant transitions on the 1s3p → 1s2s (λ = 502 nm), 1s3d → 1s2p (λ = 668 nm), and 1s3s → 1s2p (λ = 728 nm) transitions. The pulse energy of each transition and its delay time were measured as a function of the target helium gas density. Several interesting features of the data, some of which appear to contradict with the predictions of the simple two-level super-radiance theory, are pointed out.