HERMES experiment

The photoelectric effect is well understood, since its interpretation by A. Einstein in 1905: it results from the ejection of a single electron by a quantum of light (photon). Less well understood are the photon induced, multi electron processes where the interaction of a single photon with a target results in the emission of several electrons. The double photoionization process in the simple Helium atom for instance has attracted much interest in the last two decades of the XXth century, but there are many other multi electron processes that have remained unexplored because of the absence of adapted tools to observe them. The situation changed drastically with the development in 2003 [J.H.D. Eland, O. Vieuxmaire, T. Kinugawa, P. Lablanquie, R. Hall and F. Penent. Complete two-electron spectra in double photoionization: the rare gases Ar, Kr and Xe. Phys. Rev. Lett. 90 (2003) 053003] of the appropriate instrument to observe multi electron processes: a magnetic bottle time of flight spectrometer. It has enabled a tremendous development in this field, as detailed below.

Our team was the first to implement a magnetic bottle time of flight spectrometer on synchrotron radiation sources. This tool is a very performant electron spectrometer, capable of detecting and resolving in energy all the electrons emitted in multi-electron processes. Basically it is a long time of flight spectrometer (2 to 5 m long), which allows a good energy resolution (DE/E ~ 1-2%) and long enough time separation between successive electrons. The magnetic configuration (with a strong permanent magnet located close to the ionization volume) is designed to collect all the electrons irrespective of their emission angles and energies; this has the advantage to produce a very luminous apparatus and the efficient detection of all the electrons emitted in a multiple ionization event. J. Eland developed this apparatus in his laboratory with pulsed lamps, and could reveal the complete double photoionization paths in rare gas atoms [J.H.D. Eland, O. Vieuxmaire, T. Kinugawa, P. Lablanquie, R. Hall and F. Penent. Complete two-electron spectra in double photoionization: the rare gases Ar, Kr and Xe. Phys. Rev. Lett. 90 (2003) 053003].

The Magnetic bottle time of flight installed on the SEXTANTS beamline at the SOLEIL Synchrotron.

 

Our team at LCP-MR has implemented this technique on synchrotrons in order to have access to all possible photon energies [Penent, J. Palaudoux, P. Lablanquie, L. Andric, R. Feifel and J.H.D Eland. Multielectron Spectroscopy: The Xenon 4d Hole Double Auger Decay. Phys. Rev. Lett. 95 (2005) 083002.]. Since then, we have used this technique to explore a variety of subjects in BESSY synchrotron (Berlin, Germany) (2005-2009) and in SOLEIL (Saclay, France) (since 2010). We have also participated actively to similar experiments developed by Professor K. Ito at Photon Factory (Tsukuba, Japan). The experiment installed on the PLEIADES beamline of the SOLEIL synchrotron. The 2m-long magnetic bottle time of flight spectrometer is clearly visible as the long tube in the center of the photograph.