RELAXATION EFFECTS IN FLUORINE K AUGER AND PHOTOELECTRON SPECTRA

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RELAXATION EFFECTS IN FLUORINE K AUGER AND PHOTOELECTRON SPECTRA

O. Benka, M. Uda

To cite this version:

O. Benka, M. Uda. RELAXATION EFFECTS IN FLUORINE K AUGER AND PHOTO- ELECTRON SPECTRA. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-247-C9-250.

�10.1051/jphyscol:1987941�. �jpa-00227359�

JOURNAL DE PHYSIQUE

Colloque C9, supplbment au n012, Tome 48, dbcembre 1987

RELAXATION EFFECTS IN FLUORINE K AUGER AND PHOTOELECTRON SPECTRA

0. BENKA and M. UDA*

Institut fiir Experimentalphysik, Universitdt Linz, A-4040 Linz-Auhof, Austria

he

Fluorine K Auger and photoelectron spectra excited by monochromatized A1 K a X-rays were measured for alkali metal and alkaline' earth fluorides. An anomalous stucture of the main Auger line (KL23L23) was recently found and explained by a resonant electron transfer (RET) relaxation mechanism. From the energy difference of the Auger and photoelectron lines from the solids compared to free ions, relaxation and Madelung energies were evaluated. The results were in good agreement with theoretical ones used in the calculations o f the RET energies confirming the RET relaxation mechanism.

Measurements [l] of the relative intensity of the first KLI satellite line in fluorine K X-ray spectra o f ionic fluorine compounds gave evidence for a resonant electron-transfer (RET) mechanism following ls2p double ionization o f the fluorine ion. When this RET is interpreted as a relaxation mechanism of the ionic crystal in response to the created double hole, RET might also be expected in the normal K Auger spectra of ionic fluorine compounds, because the final state is also a double hole state. Therefore recently in a systematic investigation the K Auger spectra of fluorine for the alkali metal and alkaline-earth fluorids were studied [ Z ] . The measured spectra are shown in Fig. 1. They can be described by five lines or line groups in terms of final state holes

3 I 1

in the n = 2 shell:~=pp(l~+'~), C=sp( P), D=sp( D) and E=ss( S). The peak B comes from an initial double hole (K+L) state. In addition to these lines there is in some spectra near to the A line another line indicated by M. The anomalous structure of the line (A+M) is then proposed to be caused by a RET relaxation between the outer np level of the metal ion and the 2p level of the fluorine ion.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987941

JOURNAL DE _PHYSIQUE

6 2 0 6 4 0 6 6 0 6 6 0 Kinetic E n

Fig. 1. Fluorine K Auger spectra induced by A1 Ka X-rays. The position of the M line is indicated by arrows. The energy is given with respect to the Fermi level of the spectrometer.

For proof of this RET it is important to calculate the relative energies which yield resonance. If the energy of a state I, M+(gs)+~+(2p-'), is EI compared to a state without holes and of a state 11, b12+(np-')+~(2p-'), is EII, where M

+

< n - I > +

ion in the ground state and F (2p-") represents a F- ion with n holes in the 2p shell, we get a resonance condition for E=E ,-Err =O.

Because of the level noncrossing rule a split into two states with different energies is then expected corresponding to bonding and antibonding states. For increasing I & I this energy splitting is expected to decrease. As can be seen in Fig. 1, in the KF spectrum lines A and M corresponding to the bonding and antibonding states, have almost the same intensity, and therefore & = O was taken, and the energy difference of 2.4 eV between the A and M line was assumed to be due to the level crossing energy splitting. In order to calculate EI and E r r , the binding energies of the M+ outer np electrons and F- 2p electrons and corresponding relaxation energies have to be known.

In the calculation of ref. 2 theoretical values were taken. We evaluated now from the measured complete fluorine electron spectra (photoelectrons and Auger electrons) and from free ion Auger and photoelectron energies fluorine relaxation and Madelung energies.

Considering an ionic fluorine compound M'F-, BF1should be the binding energy of a core electron in a free F- ion and B the corresponding binding energy in the solid. The energy difference

AB=B-B~' can then be approximated by [ 3 ]

where U/e is the potential of the electron in the solid and R the

relaxation energy for one fluorine hole in the solid. For an Auger

F I

transition with the energies A for the free ion and A for the ion in the solid, the correspnding energy difference AA=A-A~' is

where R' is for the relaxation energies. In a simple model assuming the relaxation energy proportional to the square o f the charge, R 7 = 3 R giving

R = (AB+AA) /2 U = (3AB+AA) /2

The free ion fluorine 2p binding energy of NaF was taken 3.9 eV.

This value is the F electron a,ffinity (3.4 eV), but corrected for the fact that the ionicity of N a F is a little smaller than 1. The ionicity of the other compounbs were considered to be the same as the electronegativity of t h e h e t a l s are all about the same. The free ion fluorine Auger energy (A line) was evaluated from NaF vapor-phase free molecu1,e Auger energy, the effect of ~ a + was also taken into account [ 4 ] , giving 655.6 eV. The measured Auger and photoelectron energies were taken with respect to the vacuum level normalizing the fluorine 2p binding energy to 11,9 eV, give by Pool et a1 [ 5 ] . The yhergy shifts o f the fluorine 1s line was then used to get the bidding energies for the other compounds. In the high ionic fluorine compounds the 2p electrons are very localized at the fluorine ion, s o they are like core states. We used then the 1s energy shifts because of lower experimental errors. In Table I the Auger energies are given, and the evaluated R and U values were compared to theoretical ones. The Auger energies were corrected for the energy splitting caused by the RET relaxation [ 2 ] . As can b e seen for R and U good agreement is found with the theoretical values. Note that this good agreement is found by considering the RET energy splitting.

Concluding, the evaluated relaxation energies are in agreement with the values used to calculate the energy matching conditions in ref. 2, confirming the found results. The normal and RET relaxation gives therefore a consistent model of the relaxation processes in the high ionic fluorine compounds.

C9-250 JOURNAL DE PHYSIQUE

TABLE I. Energies A of the fluorine KL L Auger transition with respect to the vacuum level, the evaluated relaxation energies R and potential energies U, and corresponding theoretical values Rt h, Ut h [5,6] (units are e V ) .

Compound NaF KF RbF CsF MgF2 CaF2 SrF2 BaF2

A 650.6 652.7 653.5 653.9 649.2 651.9 652.1 652.3

R 1.5 1.6 1.8 1.8 1.5 2.0 1.9 1.7

U 9.5 7.7 7.5 7.1 10.8

9.7 9.2 8.5

Rt h

1.6 1.8 1.9 2.1 2.1 1.6 1.4

u

References:

[1] Benka,0., Watson,R.L., Kenefick,R.A., Phys•Rev.Lett.47 (1981) 1202

[2] Benka.O., Uda.M., Phys.Rev.Lett. 56. (1986) 1667

[3] Thomas,T.D., J. Electron Spectrosc.Relat.Phenom. 20. (1980) 117 [4] Vayryen,J., Aksela,S., J. Electron Spectrosc.Relat.Phenom. 3_4

(1984) 141

[5] Poole,R.T., Liesegang,J., Leckey,R.C.G., Jenkin,J.G., Chem.Phys.Lett. 23_ (1973) 194

[6] Poole,R.T., Szajman,J., Leckey,R.C.G., Jenkin,J.G., Liesegang,J., Phys.Rev. B12 (1975) 5872