Band structure and the shakeup photoelectron spectra of copper and nickel halides and oxides

Solid State Communications, Vol. 9, pp. 1975—1979, 1971. Pergamon Press.

Printed in Great Britain

BAND STRUCTURE AND THE SHAKEUP PHOTOELECTRON SPECTRA OF COPPER AND NICKEL HALIDES AND OXIDES T. Novakov and R. Prins* Shell Development Company, Emeryville, California 94608

(Received 1 September 1971 by N.B. Hannay)

New measurements of the photoelectron spectra of Cu

20, CuC1, CuBr and NiO suggest that the satellites seen adjacent to the (Cu) 2p photoelectron lines reveal the excited states of the copper and nickel ions in these solids. The probability of the satellite emission is dramatically dependent on the presence of excess oxygen in the surface region of these materials.

IT WAS recently shown that X-ray photoelectron spectra of some solid copper compounds show complex satellites adjacent to the (copper) 2p, 2 and 2p3~2 photoelectron lines. A multiple excitation (‘shakeup’) mechanism was proposed to explain these satellites: whenever the emission of a 2p photoelectron takes place in parallel with a valence-to-conduction band excitation, satellites are observed. The satellite photoelectrons are then emitted with kinetic energies lowered by the amounts of the allowed shake-up transitions.’ In this paper we describe some new experiments on Cu2O, CuC1, CuBr and NiO, which support the idea that the satellite spectra reveal the excited states of Ni and Cu ions in these solids, Furthermore, we will show that thedependent probabilityonof satellite emission is dramatically

The spectra marked ‘A’ are taken with Cu20 ‘from the shelf’. The principal photoelectron lines of copper and the satellite structure are seen. The oxygen spectrum reveals that two kinds of oxygen are present in the sample. The one with lower binding energy is identified as the oxide oxygen, while the one with higher binding energy (i.e. less negative charge) is associated with excess or adsorbed 02 (the possibility that it is associated with water is not excluded, however.) After the sample is heated in situ for about 20 mm up to 200°C the spectra marked ‘B’ are recorded. Immediately evident is the drastic reduction of the satellite intensity and the concurrent increase in the peak the prin2p) height lines. of The total cipal remains photoelectron (Cu however. The amount of area the same, excess oxygen decreases significantly, while the amount of oxide oxygen remains practically the same. We have observed a similar behavior with CuC1 and Cu 504.

the presence of excess oxygen in these materials, The experiments described in this work have been performed with the Varian IEE-15 electrostatic spectrometer, utilizing Al K a radiation.

The process of decreasing the satellite In Fig. 1 the 2p, 2 and 2p3,2 photoelectron lines of copper from Cu20 are shown together with the oxygen is photoelectron lines (insert), *

intensity by heating the sample is reversible. For example, to induce satellites in a fresh, satellite free, CuCI sample it was sufficient to expose the sample to air for a few days or to expose it to

On temporary assignment from Koninklijke/ Shell-Laboratorium, Amsterdam, The Netherlands. 1975

water vapor at room temperature. In all cases the satellite intensity is related to the amount of

1976

SHAKEUP PHOTOELECTRON SPECTRA OF Cu

2 0, CuCI, CuBr AND NiO Vol. 9, No.22

excess oxygen as is evident from the oxygen in the photoelectron spectrum. In the case of CuCI the oxygen is binding energy is the same as the higher of the two oxygen is binding energies of Cu20 (see Table i). Similar experiments have been performed with CuBr with identical results, although the relative satellite intensity as cornpared to the p,,2 and p3,2 peaks was considerably smaller than for CuC1. —

______________________________________

I~ ~

540

530

520

0

states of both copper and oxygen. The following arguments then prove our case: (1) in CuCJ the oxygen is line is not characteristic of oxide oxygen; (2) in Cu20 the oxide 0 is photoelectron line does not decrease in intensity upon heating; (3) the CuC1 spectra of fresh, satellite free, samples are identical with heated CuCI samples, which originally showed high excess oxygen lines and intense satellites; (4) the values of the 2p~, copper 2binding energies reflect the correct oxidation states as seen from Table 1. Table 1. Copper 2p3~2and oxygen is binding energies (eV)

Co mpound Excess Oxygen 0 Oxide is 0 Copper 2p0

0

B~d~E~

Befwe He,hng

*

Cu20 CuCI

532.9 532.5

CuBr

532.9

CuSO4

532.8*

CuO

532.7

531.0 —

933.0 932.5

—

932.5

—

9337

530.7

The sulfate oxygen is indistinguishable from the excess oxygen is photoelectron line.

(C~ 2p

CuO appears to contain significant amounts of Cu02 in the surface region. The copper satellites can be divided into photoelectron line and the broad second group is two groups. The first group is located at about 3eV higher in binding energy than the principal

Af,~’ H~~’~g’. ~ 20-., 200’C~

970

9~0

950 B~d~g ~

940

930

(V

2P~2 and ~ photoelectron lines FIG. 1. The of copper and is photoelectron lines of oxygen from Cu 2O (taken with Al Ka radiation). The spectra marked ‘.4’ are taken with Cu2O ‘from the shelf’. After the sample has been gradually heated in situ for about 20 mon spectra marked ‘B’ were obtained, To prove that no reduction takes place when heating the oxide and chloride and that no oxidation takes place when exposing these materials to air or water vapor, we utilized the phenomenon of the chemical shift to identify the oxidation

centered at about 10eV higher in binding energy than the principal line. Regardless of the total satellite intensity, the relative intensities of these two groups stay approximately constant when heating the low-energy satellite couldthe be sample. mistakenEven for aifhigher oxidation state of copper, the second high-energy group would require extraordinarily high oxidation states to account for the apparent chemical shift of more than 10 eV. The different cuprous compounds that we studied show differences only in the highenergy satellite group, while the low-energy group always appears at the same energy. Differences in structure are, however, seen between a cupric compound such as CuSO4 and the cuprous compound (see reference 1).

Vol. 9, No. 22

SHAKEUP PHOTOELECTRON SPECTRA OF Cu

20, CuCl, CuBr AND NiO

~5

,0

1977

5

:~../

~I —

890

68~

~ 8~C

.

850

2Ps 2 and 2p~2 photoelectron lines from NiO (measured with Al Ka radiation). FIG. energy 2. Theranges for the d6 .- d8’, d5 — d7s and °2r— Ni 4s transitions, as given in reference 3 are The indicated. This spectrum was measured with high resolution setting.

Cu 20 is a semiconductor with a band gap of 2.17 eV and the diffe,ence in energy 94s between excited the Cu~3d’~ground andeV. the This 3d energy differstate is known to state be 2.75 ence is quite compatible with the energy separation between the low-energy satellite and the principal photoelectron line (cf Fig. 1). We note this similarity in energies as a possible clue to the understar~dingof the satellites phenomenon. For NiO far u.v. spectra have been measured and therefore we decided to measure the nickel and 2p 32 photoelectron spectrum of green NiO. The measured spectrum which shows a complicated satellite structure is shown in Fig. 2. Also for NiO two groups of satellites are observed: a low-energy group with structure at approximately 1, 2 and 3.3eV from the principal line and a broad high-energy group with energies between 4 and 14eV from the principal line. Heating of NiO to 200°Cdid not influence the relative intensities of the different photoelectron lines. The oxygen is spectrum again revealed

the presence of excess oxygen (or water) in addition to the oxide oxygen. The energy separation between the nickel 2p satellites and the main photoelectron line agree remarkably well with the energies obtained from u.v. absorption measurements.2 For instance, the three inflections in the low-energy group are in rather good agreement with the A 3T 3T,,. °T 8 -‘ d8’ absorption bands 2~-~ (cf. 2~ Fig. 2) 1Q d and the high-energy group agrees reasonably well with the first high-intensity band in the u.v. absorption spectrum. Unfortunately, however, no unambiguous assignment of the high-intensity u.v. absorption bands has been made and different authors favor different assignments. In Fig. we 9 -~2 d~s have indicated the energy ranges for the d and O2p -, Ni 4s transitions, as given by Adler and Feinleib.3 There are three factors which complicate a comparison between the energy separations of satellites and main line in the photoelectron

1978

SHAKEUP PHOTOELECTRON SPECTRA OF Cu

2O, CuCl, CuBr AND NiO Vol. 9, No. 22

spectrum and the energies of the u.v. absorption bands. In the first place, although both in the photoelectron and in the u.v. case one studies electron excitations from valence levels to empty (conduction band) levels, in the photoelectron

atom and for the Na+, K+, F and Cl ions. Unfortunately, no calculations for copper or nickel ions are available, nor has the effect of the incorporation of ions in solids been studied. According to reference 6, a treatment for solids is in prin-

case there is an additional hole in the transitionmetal ion 2p shell. This additional hole may perturb the properties of the valence and con-2~ duction state band has levels. Secondly, shell, since the the coupling Ni ground a non-closed

ciple possible by proceeding from the general probability formula, although it seems preferable to do calculations for the free ionseffects. first andThe then attempt to incorporate solid state

of this open shell with the 2p hole may give rise to a complicated multiplet structure. From estimates made by Fadley and Shirley4 for Mn2, however, one may conclude that these 2p multiplet splittings will be smaller than .~.2eV and will therefore manifest themselves only in some additional line broadenings. The third and most important factor that complicates a comparison between photoelectron satellite and u.v. data is the fact that different selection rules apply in these two cases. Optical excitatiohs follow dipole selection rules whereas the shake-up excitations must follow monopole selection rules. The reason for this is that after the sudden removal of an electron from an N particle state, the resulting N — 1 state is not an eigen state of the new hamiltonian but a mixture of such states. The probability that the N — 1 state is in a state (which is an eigenstate of the N — 1 system) after annihilation of an electron in state i of the N system is: P.. =

a~, =

We believe on the basis of the evidence presented that the agreement between the excitation energies as deduced from the photoelectron satellites and from optical excitation is not accidental. In relation to the first complicating factor mentioned above, it seems that the presence of a 2p vacancy has about the same effect on both the initial and the final level involved in the shakeup excitation. The dramatic influence of the amount of excess oxygen (or water) on the probability of the shakeup excitation suggests that this oxygen is responsible for the apparent violation of the monopole selection rules. Namely, when the surface region which is probed by X-ray photoelectron spectroscopy (in this case probably about 20 A deep) is stochiometric and free of excess oxygen (water), the shakeup excitation

2 0(N)>~ where a~is the operator annihilating an electron in state z. In order for the two wavefunctions b~(N — 1) and a 0 ,L,0 (N) to have a nonzero overlap they must be of the same symmetry, e.g., the selection rules are of the monopole type. 5,6

from the narrow and almost atomic like d levels is largely forbidden. When the presence of adsorbed or other excess oxygen changes the local symmetry of the metal ion sites, the character of

The shakeup process has been 5’6 studied for the neon experimentally and theoretically

metry conditions in itself sufficient for the (nonstoichiometry) shakeup excitation are to occur.

<

i.O,(N — 1)ta? Ri

relative satellite intensity should depend very simply upon the radial functions of the outermost shell spin orbitals which are most sensitive to the crystal environment.

the formerly almost pure d levels is changed so that a monopole component in excitation is possible. In the case of NiO the intrinsic sym-

REFERENCES 1.

NOVAKOV T., Ploys. Rev. B3, 2693 (1971).

2.

POWELL R.J. and SPICER W.E., Ploys. Rev. BZ 2182 (1970).

3.

ADLER D. and FEINLEIB

4.

FADLEY C.S. and SHIRLEY D.A., Ploys. Rev. AZ 1109 (1970).

J.,

Ploys. Rev. B2, 3112 (1970).

Vol. 9, No. 22

SHAKEUP PHOTOELECTRON SPECTRA OF Cu

20, CuCl, CuBr AND NiO

1979

5.

See, for example, KRAUSE M.O., CARLSON T.A. and DISMUKES R.D., Ploys. Rev. 170, 37 (1968);

6.

ABERG T., Phys. Rev. 156, 35 (1964).

SIEGBAHN K., NORDLING C., JOHANSSON G., HEDMAN J., HEDEN P.F., HAMRIN K., GELIUS U., BERGMARK T., WERME L.O., MANNE R. and BAER Y., ESCA Applied to Free Molecules, p. 30, North-Holland Publishing Company, Amsterdam (1969).

Neuere Messungen an den Photoelektronenspektren von Cu20, CuCl, CuBr und 2p NiO lassen vermuten, dass die Satellitenlinien, neben Photoelektronlinien erscheinen, den angeregtendieZustand von und Nickel in diesen Feststoffen anzeigen. Die Wahrden Kupfer (Cu) scheinlichkeit einer Satellitenemission hãngt weitgehend von der Gegenwart eines Sauerstoffüberschusses in den Oberflächenbereichen dieser Stoffe ab.