KLL and LMM Auger peak intensity ratio dependence on bond ionicity

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena70 (1995) 193-196

K L L and L M M Auger peak intensity ratio dependence on

bond ionicity T. Vdovenkova Semiconductor Department, RadiophysicalFaculty, Kiev Taras Shevchenko University,64 Vladimirskaya,252017Kiev-17, Ukraine First received 19 April 1994; in final form 4 July 1994

Abstract A study of the influence of element bond ionicity on ratios shows that a decrease of the ratios of the KL23L23 to KLIL23M23M23t o L23M1M23,and L23M23M23t o L3M23M45peak intensities occurs with an increase in the effective negative charge of the atom. These dependences are explained by decreasing spatial separation of the 2s and 2p, 3s and 3p, and 3p and 3d electrons. The decreasing spatial separation of the 2s and 2p and the 3p and 3d electrons with increase of the atom effective negative charge becomes less abrupt with increasing element atomic number. L23 ,

Keywords: AES; Auger; Bond ionicity

1. Introduction It is known that the identification of chemical bond change by Auger electron kinetic energy change is a complicated problem, as Auger electron kinetic energy depends on the binding energies of electrons in three atomic energy shells [1]. However, the application of Auger electron spectroscopy (AES) for identification of chemical bond change is of great interest because of the better lateral resolution for AES than for X-ray photoelectron spectroscopy, which is normally used for chemical bond change identification. The dependence of Auger peak intensity ratio on bond ionicity has been studied [2,3] for transitions involving the valence shell, and was explained by variations in the atomic charge of the L23 valence shell. The present work is a study of the influence of element bond ionicity i on the ratios of KL23L23 to

KL1L23 for Na, Mg and Si and of L23M23M23 to L23MIM23 for Ca, Cr and Fe peak intensities for transitions which do not involve valence shells; and on ratios of L23M23M23 to L3M23M45 for Ti, Cr, Mn and Fe and of L23MnsM45 to L3M23M45 for Cr, Mn and Fe peak intensities for transitions involving the M45 valence shell (the strongest peaks of KLL and L M M Auger spectra).

2. Experimental Bond ionicities of elements in compounds were calculated as in Ref. [4] i = 1 - exp [-0.25(Xa - Xb) z]

(1)

where Xa and Xb are the electronegativities of elements A and B. The identification of KLL and L M M Auger spectra was achieved as described in Ref. [5]. The ratios of peak-to-peak intensities were

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T. Vdovenkova/Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 193-196

194

calculated using data published in Refs. [5-7]. These KLL and LMM spectra were obtained at various primary electron energies (Ep) and modulating amplitudes (U). It should be noted that Auger transitions for the studied ratios occur on ionization of the K shell only or L 2 and L 3 shells with similar electron binding energies. Therefore the dependence of intensity ratio on Ep was neglected. As is known, the intensity of the negative peak part (H) for a single peak obtained in the d[EN(E)]/dE regime is described by the expression H

=

KU/W

Rk

I

..

.

i

i

i,,

I

I

!

,1

r

0.5

0

1.0

i

(2)

where K = 0.9 for peak width W > 1.25U [8]. Hence the ratios of intensities do not depend on the modulating amplitude and are in inverse ratio to the ratios of peak widths. Therefore the dependence of intensity ratio on U was also neglected. It should be noted that peak-topeak intensity ratios, but not H intensity ratios, were studied, since the background line cannot be determined for each item of data published in Refs. [5-71.

Fig. 1. Dependences of ratios of KL23L23 to KLIL23 peak intensities R k on bond ionicity i in correspondence with data published in refs. [5] and [6] for: ©, Na in NaI, NaBr; x, Mg, Mg in MgO, MgF2; ,, Si, Si in SiO2.

Fe (atomic numbers 24, 25 and 26) do not depend on i. Hence the dependence of R'1 on i becomes less marked with increasing element atomic number.

•1" 10

3. Results and discussion

Fig. 1 shows the dependences of the ratios of

KL23L23 to KL1L23 (Rk) peak intensities on i for Na, Mg and Si. For these elements the 2p electrons are not valence electrons. The values of Rk decrease with decreasing i (as the effective negative charge of the atom increases). The dependence of Rk value on i for Si (atomic number 14) is less marked than for Na (atomic number 11). Hence this dependence becomes smaller with increasing element atomic number. Fig. 2 shows the dependences of the ratios of L23M23M23 t o L23M1M23 (Rl) peak intensities for Ca, Cr and Fe on i and the ratios o f L23M23M23 t o L3M23M45 (RI) peak intensities for Ti, Cr, Mn and Fe on i. For these elements the 3p electrons are not valence electrons, but the 3d electrons of Ti, Cr, Mn and Fe may be involved in chemical bond formation. The increase of effective negative charge leads to a decreasing RI value for Ti (atomic number 22) and decreasing R1 values for Ca, Cr and Fe. Values of R'I for Cr, Mn and

~Tt20 3 -~- T10 X

i

0

i

|

0.5

I

i

¢

i

1.0

i Fig. 2. Dependences of ratios of L23M23M23 to L23M l M23 peak intensities (Rl) on i for: V, Ca, Ca in CaO; m, Cr, Cr in CrNx; e, Fe, Fe in FeO, Fe203. Dependences of ratios of L23M23M23 to L3M23M45 peak intensities (R~) on i for: *, Ti, Ti in TiSi 2, TiB2, TiC, TiO, Ti203; +, Cr, Cr in CrNx; x, Mn, Mn in MnO2; ©, Fe, Fe in FeO, Fe203. In correspondence with data published in Refs. [51 and [71.

T. Vdovenkova/Journalof Electron Spectroscopyand Related Phenomena 70 (1995) 193 196 R]''

?F'eO

1.0

~Fe 0 2 8

0.5

I

0

0.5

1.0 t

Fig. 3. Dependencesof ratios of L23M45M45to L3M23M45peak intensities (R~r) on i in correspondencewith data published in Ref. [5]for: ,, Cr, Cr in CrNx; ×, Mn, Mn in MnO2; ©, Fe, Fe in FeO, Fe203. Fig. 3 shows the dependences of the ratios of L23M45M45 to L3M23M45 (RII) peak intensities on i for Cr, Mn and Fe. The value of RI / increases with increasing effective negative charge of the atom. In order to explain these dependences, an expression for C X Y Auger transition intensity is given [2,9]

I .~ q( Y)q(X)P~Yf

(3)

where q(Y), q(X) are the atomic charges of the Y and X shells xr 27r[I ei~f = T x~(rl)ff3~(r2)

e2 Irl

× xi(rl)~i(r2)drldr2 2

-

r2-~--[ (4)

where xi(rl) and x~(rl) are initial and finite bound orbitals of the first electron and ~bi(r2) and ~ ( r 2 ) are an initial bound orbital and a finite wave function of the continuous spectrum

195

for the second electron which are involved in the Auger process, and r 1 and r 2 are the coordinates of the electrons involved in the Auger process. Expression (4) shows that the Auger transition probability increases with decreasing spatial separation of the electrons involved in the Auger process. Hence the dependences of Rk on i for Na, Mg and Si, the dependences of Rl on i for Ca, Cr and Fe and the dependence of R'l on i for Ti can be caused by decreasing spatial separation of 2s and 2p electrons for Na, Mg and Si, 3s and 3p electrons for Ca, Cr and Fe and 3p and 3d electrons for Ti with growth of the effective negative charge of the atom. It should be noted that the decreasing spatial separation of 2s and 2p electrons and of 3p and 3d electrons with increase of the atomic effective negative charge becomes less marked with increase in the atomic number of the element. Expression (3) shows that the increase of RI / with increasing atomic effective negative charge may be explained by an increase of atomic charge of the M4s shell (M45 shell occupation) under transition from an oxidized metal (MnO2, Fe20 3 or FeO) to elementary Mn or Fe. It should be noted that the increase in Fe valency by transition from FeO to Fe20 3 leads to decreasing Fe RI'; the increase in Ti valency by transition from TiO to Ti203 leads to increasing Ti R I. These dependences can be caused by decreases in occupation of the Fe M45 and Ti M45 shells.

4. Conclusions The data show that decreases in the ratios of KL23L23 to KLIL23 for Na, Mg and Si, of L23M23M23 t o L23M1M23 for Ca, Cr and Fe and of L23M23M23 to L3M23M45 for Ti peak intensities occur with increase in the effective negative charge on the atom. These dependences are explained by the decreasing spatial separation of the 2s and 2p, 3s and 3p, and 3p and 3d electrons. The decrease in spatial separation of 2s and 2p and of 3p and 3d electrons with increase of atomic effective negative charge becomes less marked with increasing element atomic number.

196

T. Vdovenkova/Journal of Electron Spectroscopy and Related Phenomena 70 (1995) 193-196

Acknowledgement I thank my referee for helpful comments.

References [1] A.W. Czanderna (Ed.), Methods of Surface Analysis, Elsevier, Amsterdam, 1975. [2] R. Weissmann, Solid State Commun., 31 (1979) 347. [3] R. Weissmann and K. Muller, Surf. Sci. Rep., 1 (1981) 251. [4] L. Pauling, The Nature of the Chemical Bond, 2nd edn., New York, 1945.

[5] V.S. Ivanov, I.A. Brytov, V.V. Korablev, N.A. Kozyreva, T.V. Kuznetsova, E.A. Tsukerman and I.I. Kiseleva, Handbook of Chemical Element and Compound Auger Spectra (in Russian), M.K.T.I., Moscow, 1986. [6] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and R.E. Weber, Handbook of Auger Electron Spectroscopy, P.E.I. Inc., MN, 1976. [7] A.P. Dement'ev, T.M. Djibuty and V.I. Rakhovsky, Surface (Rus.) 3 (1987) 96. [8] M.P. Seah, in D. Briggs and M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (in Russian), Mir, Moscow, 1987, Ch. 5, pp. 203-243. [9] T.A. Carlson, Photoelectron and Auger Spectroscopy, Plenum Press, New York, 1976.