Experiment and Theory of the Chemical Effects of Satellites on K X-ray Emission Spectra

IGhi satellite of l h x-ray fluorescence spectra were measured with two-crystal spectrometer. The intensity of the multivacancy satellite depends on the covalency of the orbital from which the electron(s) is(are) shaken off. However if a resonance condition holds, then the satellite is anomalously weak due to resonant electron transfer (RET) from the center atom to the ligands. Some examples of RET are presented from 2=17 to 2=22. It is also shown that the intensity of the 16Y satellite depends on the atomic number. The probability of M electron shake-off after photoionization of a Is electron is large: the probability becomes 35 % relative to that of the single photoionization of a Is electron for C1free ion. Therefore the IGY satellites in I(@ x-ray emission spectra can Electronegativity (Pauling-Allred) Fig.1 Line-width satellite (parasite) %sity, S/L, plotted as a function of the electronegativity of the neiahbour atoms. The definition of S/L-has been given in Ref. [l, 31 . The K Q x-ray fluorescence spectra of (a) KSCN (somewhat covalent) and (b) KZTiFs (quite ionic) [21. The spectra deconvoluted with the Lorentzian function of natural width are overlaied. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879128 C9-746 JOURNAL DE PHYSIQUE be measured easily, however it is difficult to discriminate multivacancy satellites from weak lines originated from molecular orbital splittings. Such I(M multivacancy satellites in 1(C( x-ray emission spectra were, however, not precisely measured till the reports of Kawai et a1.[1-31 who used a two-crystal spectrometer, because the 1(M satellites emerge quite close to the Ka1,e lines or inside of the parent lines. Thus the linewidth satellites are called parasites. There is however a problem concerning the chemical effect of satellite intensity: the experimental result of the ordering of the K I(cc' intensity [2] is the inverse of that of the C1 Ka' [l ,3] or the F Ka3,4 [4] satellite intensity with respect to the ordering of electronegativity of the neighbour atoms. That is to say, for C1 l(a' or F 1b.4 , the more covalent the bond, the weaker are the satellites (Fig.1). The result of K I(cr' indicates that the more ionic the bond, the weaker are the satellites (Fig.2): though KF is a quite ionic solid, the satellite intensity of K Ka is weak. We must note here that the Cl Ka satellites have some exceptions: the satellites of CsCl and RE1 are anomalously weak in spite that these solids are quite ionic (see Fig.1). ELmxm Wehave developped a theory [5,61 of satellite intensity to interpret the two contradicted experimental results descrived above (C1 Ka' [l ,3] and K I&' [2]). If the 3p electrons are localized in a compound when the bond is ionic, then the orbital relaxation is large. Thus, the overlap integral Il is small (the asterisk denotes the IS-' hole state). and therefore the shake-off probability, in the sudden approximation [6], is large. Therefore the intensity of 3p shake-off satellites (Ka') of ionic compounds is as strong as that of the free atom. On the other hand, if the bond is covalent, then the 3p electron is well delocalized, and thus the relaxation due to the core hole is small; therefore I 12*1 , and as a result, the intensity of the shake-off satellite ls-'3p-'-2p3p ' ! s weak. This has been quantitatively formulated [5] through the second-order perturbation, IS,~/I.~~~=~(AE)~, (2) where A& is the difference of 3p orbital .eigenvalues between the ground state and the IS-' -hole state, n the number of electrons in the 3p orbital. For CsCl and RbCl C1 Ka and KF K Ica, a resonance condition holds as proposed by Benka et al.[8] and Urch[S] for PIXE F Ka satellites. That is to say, though the 3p orbital is well localized in the ground state, it becomes delocalized in the presence of 1 s hole because of the orbital resonance: E(CL$~)-E(R~~~) , E(C~$)*E(C~~) . E(@~)LE(F*~) . Therefore the shake-off hole is resonantly transfered to the llgand orbitals. The satellite intensity when the resonance condition holds, is represented as follows from the two-configuration expansion[6], where c is the shake-off probability of a free atom and y' is the increment of the molecular orbital coefficients from the ground state to the core-hole state. Here the increase of the molecular orbital coefficient is due to the temporary covalence of the atomic orbitals between the hole-site atom and its ligand atoms. This is because a molecular orbital coefficient of a two-atom molecule AB is, <,n. lh lmn\ condition holds in the 1s" -hole state: E(CU ) >c(FzS) (ground state) , E(C~~~) , becomes I~::O.38/Ti>+O.> in the 1s-I hole state. Therefore the increment of the molecular orbital coefficient is 0.16, i.e. y' is large. On.the other hand, the molecular orbital coefficients for TiN and Ti0 do not increase due to core hole.