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Esca Studies of Core and Valence Electrons in Gases and Solids C ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS C. Nordling To cite this version: C. Nordling. ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS. Journal de Physique Colloques, 1971, 32 (C4), pp.C4-254-C4-263. 10.1051/jphyscol:1971447. jpa- 00214648 HAL Id: jpa-00214648 https://hal.archives-ouvertes.fr/jpa-00214648 Submitted on 1 Jan 1971 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 10, Tome 32, Octobre 1971, page C4-254 ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS C. NORDLING (*) Institute of Physics, Box 530, S-751 21 Uppsala 1, Sweden Rbsumb. - Les processus electroniques dans les systkmes atomiques sont habituellement associes a l'kmission ou l'absorption de photons ou a remission d'electrons. L'ktude spectrosco- pique peut par consequent s'effectuer par l'analyse du rayonnement electromagnetique ou par la mesure de 1'6nergie cinetique des klectrons. Alors que la spectroscopie electromagn8tique dans le domaine optique se pratique depuis des siecles et depuis plusieurs decades pour les rayons X, l'electron lui-m&men'a pas ete tres utilise pour explorer la structure Blectronique et les processus electroniques. Cependant en raison du developpement ces dernikres annees de moyens experimen- taux pour I'analyse precise des spectres d'klectrons, ce type de spectroscopie a produit des rksultats tres encourageants qui montrent que la spectroscopie basee sur l'observation directe des electrons est une methode efficace d'etude des systkmes atomiques et mol6culaires. La spectroscopie des electrons fournit egalement des renseignements qu'on ne peut obtenir par d'autres types de mesure et il y aun grand nombre d'applications de ce nouveau type de spectroscopie. Noys exposerons brikvement le travail effectue par notre groupe k Uppsala dans le domaine de la spectroscopie des electrons pour les atomes et les mol6cules. Un expose plus etendu peut 6tre trouv6 dans les rkfkrences [I] et [2]. Abstract. - Electronic processes in atomic systems are usually associated with theemission or absorption of photons or the emission of electrons. The spectroscopic study of these processes can therefole be made by the analysis of the electromagnetic radiation or by a measurement of the kinetic energies of electrons. While electromagnetic spectroscopy in the optical region has been made for centuries and in the X-ray region for many decades the electron itself has not been used very much to probe the electronic structure and the electronic processes. However, following the development in recent years of experimental devices for the exact analysis of electron spectra this type of spectroscopy has now produced some very encouraging results which indicate that the spectroscopy based on the direct observation of the electrons is a powerful method for the study of atomic and molecular systems. Electron spectroscopy also produces information which cannot be obtained by other types of measurement and there is a multiplicity of applications for this new type of spectroscopy. A brief account will be given of the work which has been done by our group at Uppsala in the field of electron spectroscopy for atoms and molecules. A more comprehensive account until the present year is given in references [I] and [2]. 1. EIectron binding energies and photoionization dynamics in the noble gases. - Different modes have been used>to excite the electron spectra, viz X-rays, UV-radiation, and electron impact, and the energy (or momentum) analysis of the spectra is made in double focussing electron spectrometers of electro- static or magnetic type, see figure 1. When photons are used for the excitation the kinetic energies of the SPECTROMETER expelled electron are where Eb is the electron binding energy (ionization FIG. 1. - Different modes of excitation of electron spectra. energy). With X-ray quanta one can liberate electrons from all parts of the electronic structure, i. e. one can widths can be obtained but the technique is limited study both the atomic core and the valence electrons to the outermost parts of the electronic structure. in molecules and solids. This is the mode of excitation X-ray induced electron spectra from the noble that we have used in most cases. UV excitation has the gases are shown in figure 2. The spectra map out in advantage that electron lines with smaller inherent some detail the electronic structure of the noble gas Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971447 ESCA STUDIES OF CORE AND VALENCE ELECTRONS IN GASES AND SOLIDS C4-255 c/120~, atoms, from the outer shells and as far into the atomic - 400- He Mg Ka core as attainable with the quantum energy of the magnesium Ka radiation, 1 253.6 eV. Electron emission from deeper lying shells was induced by harder radia- loo- tion, for example Cu Kcc. The electron binding energies 30 20 ' ' ev obtained for the noble gases are given in Table I (see 1s 2m0' Ne also ref. [2]). One can conveniently study the entire electronic structure by use of one and the same ins- trument and at a resolution close to the limit set by 1000 inherent width of the atomic levels. For example, the observed width at half maximum intensity of the neon o- 1 s line is 0.80 eV which is near the natural width 1. I 880k 870 860 " &I ' Lo ' 30 ' 20 ' 'ev of the exciting X-radiation. The spectrometer window was in this case approximately 0.2 eV. This high resolution is valuable for a more detailed study of e. g. C I>Z the dynamics of the K photo emission. Figure 3 8 100- 100 8 1 KINETIC ENERGY 100 FIG. 3. - Neon 1 s electron spectrum excited by Mg K radia- 690 680 670 " 210" 140" 70 tion at a pressure of 0.5 torr. The main peak (0)is the Ne l s line BINDING ENERGY excited by Mg Kal,z. The MgKX-ray satellites and KB radiation give the peaks with higher kinetic energy. The peaks with lower FIG. 2. - ESCA spectra from the noble gases excited energies (numbers 1-12) are due to shakeup, shake-off and by Mg Ka X-radiation. inelastic scattering. Binding energies for the noble gases (eV) (*) Reference value (from optical spectra). C4-256 C. NOR shows the electron spectrum from neon over an energy is an order of magnitude smaller, i. e. E,,,, E 1.8 eV, range of 130 eV around the Ne 1 s line. On both sides which is a reasonable numerical value. of the main line a number of satellite lines are observed. The intensities of these lines are less than 10 per cent 2. Autoionization and Auger electron spectra. - of the main line which in the figure has been reduced Electrons are used to excite autoionization and Auger in intensity by a factor of 20. electron spectra from gases. (Auger electron spectra The satellite lines have essentially three different are also excited by photons.) The electron energies are origins. The high energy lines are due to the high then independent of the energy of the bombarding energy satellite lines in the incoming X-radiation. particles : Photoelectrons from the neon 1 s shell induced by more energetic X-radiation will consequently have higher kinetic energy. The low energy part of the spectrum contains a number of fairly sharp peaks E" denotes the energy (above the first ionization energy) superposed on a broad continuum starting sharply of the excited atom in the autoionization process. E+ is around 362 eV and extending some 50 eV toward the energy of an atomic or molecular ion with an lower electron kinetic energies. The intensity of the inner (Auger) or outer (autoionization) shell vacancy, continuum and the distinct features 1 to 4 in figure 3, EC+ is the energy of the doubly ionized atom or measured relative to the main line, is found to be molecule in the Auger process. E,(i) denotes the bin- pressure dependent. These features are therefore ding energy of electron i (i = 1 is initial state vacancy interpreted as due to secondary collisions between in the Auger process, i = 2, 3 are the final state vacan- ejected photoelectrons and neutral atoms. The remai- cies). Thus from autoionization and Auger electron ning features 5 to 12in the spectrum are independent of spectra one obtains complementary information to pressure and are due to ionization processes at which that obtained from photoelectron spectra : from the a valence electron is simultaneously ejected or excited. autoionization spectra one obtains information on The former process is usually called shake-off and for highly excited states of the neutral atom or molecule the latter we have used the term shake-up. It turns out and from the Auger spectra one obtains information that lines 7-1 1 are due to shake-up processes to states on doubly ionized atoms and molecules. of the type 1 s 2 s2 2 p5 np2S and line 12 to a shake- As an example of an autoionization electron spec- up process of type 1 s 2 s 2 p6 ns2S. trum of a noble gas figure 4 shows a high resolution Lines 5 and 6 are interpreted as the Ka,,, satellite lines of 7 and 8 + 9.
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