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Excitation density effects have a pronounced influence on relaxation processes in solids. They come into play in scintillating
and dosimetric materials exposed to ionizing radiation or in laser materials operating in intense ultraviolet light
fields. The scientific understanding of the underlying process is poor, mainly because most of the studies of light emitting
materials under short wavelength excitation have been performed at weak and moderate excitation intensities due to
limited availability of powerful light sources. Disembodied data on excitation density effects have been reported for
wide-gap dielectrics studied by luminescence spectroscopy, by using such excitation sources as powerful ion beams,1,2pulsed electron beams,2,3 and wide-band hard X-ray synchrotron radiation.4 It is obvious that such non-selective excitation
is a good tool for revealing density-related phenomena in these materials in general, but for investigating specific
features of relaxation processes in insulators, light sources with well defined parameters are necessary. Since the shortwavelength
free electron laser (FEL) technology has been devised by an international consortium at HASYLAB of
DESY, resulting in the development of TESLA Test facility (TTF)5 and later in the construction of a dedicated FEL
source FLASH in Hamburg,6 more advanced studies became possible. The range of interests towards this light source
covers the fields from material science and various other branches of physics to structural biology. The pioneering luminescence
study revealed excitation density effects in the decay of Ce3+ 5d-4f luminescence in Y3Al5O12 crystals and luminescence
of BaF2 crystals in UV-visible range.7 These results motivated systematic investigations of excitation density
effects in wide gap crystals using FEL8,9 and high-harmonic-generated VUV radiation,10 and, at lower energies, femtosecond
laser pulses in the UV.11,12 The main goal of the present work is to analyze the same phenomenon in wide-band
gap BaF2 crystals, where luminescence centres of different origin (self-trapped excitons and cross-luminescence) are
present. Using models developed for explaining the non-linear behaviour of luminescence and exciton-exciton interaction
effects causing scintillator non-proportionalities,10,13 simulations of luminescence decay curves are performed. Possible
quenching effects in the cross-luminescence decay of BaF2 under XUV excitation have been analyzed by Terekhin
et al.14
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