Resonance absorption at theLedges of tantalum: The white line

The x-ray absorption spectrum at the $L$ edges of tantalum has been measured on two spectrometers that are equipped with: (i) a conventional bremsstrahlung source and a LiF(200) crystal, and (ii) a synchrotron source and a channel-cut Si(220) crystal. Our results are similar to those reported by earlier workers except that the apparatus (ii) yields a Ta $L$ spectrum of the highest resolution reported thus far. Of particular interest is the sharp spike (white line) occurring at the $L_{\mathrm{III}}$ and $L_{\mathrm{II}}$ but not at the $L_{\mathrm{I}}$ edge. We associated the white line with an atomiclike allowed transition from the $2p_{\frac{3}{2}}$ or $2p_{\frac{1}{2}}$ state to the vacant $5d$ states of a high density. However, a quantitative comparison with the calculated band structure can not be made because none of the existing calculations has included the effect of a core hole. In an attempt to understand the white lines, we present two least-squares analyses of the Ta $L_{\mathrm{III}}$ edge in terms of (a) a Lorentzian profile and (b) a Breit-Wigner-Fano-type formula. The latter, which was first suggested by Cauchois and Mott, appeared to provide a better fit to the asymmetric line shape.