| 研究実績の概要 |
In the final year of this project we successfully carried out an experiment using soft X-ray free-electron laser pulses to ionise a dense sample of helium gas and directly observe the time profiles of pulses of superfluorescence at wavelengths of 469 nm and 164 nm following excitation of the dense sample of ions to the 3p state (164 nm emission only) and 4p state (emission at 469 nm followed by emission at 164 nm). The pulses at a wavelength of 469 nm were detected using a bandpass filter and a fast photodiode, those at a wavelength of 164 nm using a grazing-incidence monochromator and a fast micro-channel plate detector with a response time of around 100 ps. We were successful in confirming for the first time directly that the emission at 164 nm is indeed superfluorescence, and studied the dependence of the timing of the pulses on helium number density. We also studied the correlation between the emitted pulses at the two different wavelengths following excitation of the 4p ion state, revealing the longer delay of the emission for the 4p-3s-2p cascade. Due to the difficulty of aligning the system and the limited beamtime we were unable to conduct the measurements for the shorter wavelength transitions.
In parallel to the superfluorescence from helium ions experiment we continued to develop an apparatus for creating a beam of superfluid helium droplets, which will provide a sample of helium at densities equivalent to the bulk. Successful operation of the source was demonstrated, and attempts to observe superfluorescence from the droplets will be made in future experiments.
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