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 $1s3p \to 1s2s$ ($\lambda=$502 nm), $1s3d \to 1s2p$ ($\lambda=$668 nm), and $1s3s \to 1s2p$ ($\lambda=$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.