X-Ray Absorption Study of the Spectrum of Free Electronic States in a KNO_3 Crystal

摘要

The near-edge fine structure of K-absorption spectra of nitrogen and oxygen atoms and L_(2, 3) absorption spectrum of potassium atoms in KNO_3 ion-molecular crystals containing planar molecular anions NO_3~- as structural units is studied by the method of measurement of the quantum yield of the extrinsic X-ray photoeffect with the use of a RSM-500 X-ray spectrometer-monochromator and bremsstrahlung emission of an X-ray tube with a tungsten anode. Resonant features in the structure of nitrogen and oxygen spectra are identified within the framework of the quasi-molecular approach on the basis of the comparative analysis of K-absorption spectra of molecular anion NO_3~- in NaNO_3 and KNO_3 crystals, K-absorption spectrum of B, and electron-molecule scattering spectrum of BF_3 molecule-isoelectronic and isostructural analog of the anion. As a result, the main absorption bands in the initial regions of the spectra were related to transitions of inner electrons to free molecular states of π- and σ-symmetries of the anion. The appearance of an internal structure of the σ-resonance in the N K-absorption spectrum of a NO_3~- quasi-molecule, which is absent in the B K-spectrum of a free BF_3 molecule, is associated with a pyramidal deformation of planar anions in crystals and the appearance of free 3d-states of a potassium atom. Consideration is presented in favor of the fact that an inner vacancy in X-ray absorption probing of the conduction band of NaNO_3 and KNO_3 crystals does not result in radical changes of its structure. On the basis of the combined analysis of the spectra of KNO_3 crystal obtained, characteristic features of its conduction band and the role of cation and anion states in the formation of its individual subbands are analyzed.