Excitation of luminescent materials by synchrotron radiation

Journal of Luminescence 24/25 (1981) 289—292 North-I Iolland Publishing Compass

289

EXCITATION OF LUMINESCENT MATERIALS BY SYNCHROTRON RADIATION

Philips

Th.J.A. Popma, W.F. van der Meg and K. Research Laboratories, 5600 MD Eindhoven,

*Physikalisches

Vacuum-PU. in order to

Thimm* The Netherlands

Institut der Universitit Bonn, 5300 Bonn

I, F.R.G.

excitation soectra examine deco lying

are of special importance for luminescent materiaja levels. Knowladge 0f the position and width of such levels is of relevance for the understanding of ener~ transfer and the mechanism of cathodoluminesconce. In this study we report excitation spectra of seri~ s of materials with common host lattice or with coojoon ty activator ions.cerami— the usual cal was done at 300 F in the prepared wavelength region 30—300 nm The technigues. spectra were Ixcitation obtained from powdered specimens, with synchrotron radiation of the 0.3 0eV synchrotron of the University of Eonn, by passing it through a 0.2 a monochromator with concave holographic grating coated~ with Al/MgF 2. Luminescence was collected by selecting the strongest emission through a filter. Wavelength dependent intensity calibration was performed with the measured excitation spectrum of Na—salicyThte. For wavelengths below 50 nm large uncertainties (up to a factor of t) in the spectral intensities arise because of zero—order straylight contributions and rapidly decreasing throughput efficiency of the monochromator,

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In Fig. 2 the well—knusrn internal Ce and Tb transitions are observed in the low energy region whereas at wavelengths below 80 nm core excilation occurs. The excitation maxima around 170 nm have to be attributed to the presence of covalent Al—C—groups that coordinate the big metal ions. We are dealing here most likely with charge—transfer transitions within the coordinating groups (we observed comparable in Al3~ berates and (3pr phosphates). Electron transfer transitions 3~towardsbands empty levels of the coordinating complex) could alsofrom be RE responsible, but we expect for this case lower intensities because of the smell BE-Al-over lap. In Fig. 3, the case of simple oxides, rather broad bands are observed; the Tb transitions in 1203 and the LB transitions in GdpO 0~ are still rather localiced. The bandgap of 0d 3 and Yp 203 and Yp03 is found at 280 and 230 nm respectively. Below 230 nm band—band excitations lead to a very efficient eaission from the activators. In Fig. 8 the more curalent oxisulfides show very broad excitation bands. Details of localiced transitions are not even observed for samples with the low activator concentrations shoninhere. We attribute this effect to curalency. Bome influence of disorder in coordination of the activators, however, cannot be excliaded. Bandedges occur at somewhat higher wavelengths than for the oxides. Bummarising, the features observed in the spectra given can be interpreted as due to i) transitions within the activator ion or electron Iransfer transitions towards it,ii) transitions of the host lattice, and iii) core excitations. The puantum efficiency of the latter transitions is comparable to values obtained with direct activator excitation. We thank prof. W. Paul and prcf. C. BPldeke for their interest and hospitality, E. Eiiffner, P. Baas and J. Karthaus for technical support and we acknowledge able measurements by 0,M.G. van Leaden and encouragement and support by A.T.Vink.

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