HAL Id: jpa-00216772
https://hal.archives-ouvertes.fr/jpa-00216772
Submitted on 1 Jan 1976HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
ARE MONOLAYERS DETECTABLE BY
CONVERSION ELECTRON MÖSSBAUER
SPECTROSCOPY (CEMS) ?
M. Petrera, U. Gonser, U. Hasmann, W. Keune, J. Lauer
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 37, De'cemb~e 1976, page C6-295
ARE
MONOLAYERS DETECTABLE BY CONVERSION
ELECTRON
MOSSBAUER SPECTROSCOPY (CEMS)
?
M. PETRERA (*), U. GONSER, U. HASMANN, W. KEUNE (**) and J. LAUER Fachbereich Angewandte Physik, Universitat des Saarlandes, 6600 Saarbriicken, Germany
R6sumB. - La sensibiliti: de la spectroscopie Mossbauer par Bectron de conversion a 6t6 6tudi6e et il a et6 demontre qu'une monocouche de Fe57 correspondant % 1,8 X 1015 atomes donne un
effet d'environ w 1,s % en supposant que le spectre consiste en une seule ligne de resonance ayant la largeur naturelle.
Abstract. - The sensitivity of conversion electron Mossbauer spectroscopy has been investi- gated and it is demonstrated that a monolayer of Fe57 corresponding to w 1.8 X 1015 atoms give
an effect of about 1.8 % assuming the spectrum consisting of only one resonance line with natural line width.
1. Introduction. - The question concerning the sensitivity of conversion electron Mijssbauer spectro- scopy (CEMS) has been asked at various occasions. The usefullness of this method in the field of adsorption and catalysis depends on the possibility to distinguish the spectral contribution of surface atoms.
Baverstam et al. [l] tried to separate in a CEM- spectrum the contribution of the various surface layers through computer deconvolution techniques and found that a depth selectivity of about 20 monolayers in the case of natural iron can be achieved with CEMS. In our approach to this problem thin Fe57 films, down to a few monolayers in average thickness, are evaporated onto several substrates and investigated by measuring the 7.3 keV conversion eiectrons and the 5.4 keV Auger electrons.
2. Experimental. - Three series of samples were prepared :
1) Fe57 onto 2 000
A
Cu films based on mylar substrates ;2) Fes7 onto natural bulk a-iron ; 3) natural iron onto bulk stainless steel.
The Fe5' films were produced by vapour deposition in a UHV system using an electron-beam heated fur- nace [2]. The vacuum was better than 10-7 torr during evaporation. The thickness with an error of about f 3 layers was measured by a quartz crystal monitor calibrated by the optical1 Tolanskij interference method. The evaporation rate was determined to be about 2
+
1 A/s.All samples had disc form with an area of 19.53 cm2.
After preparation the samples were removed from the vacuum system, and mounted in the gas filled (4
%
methane, 96%
heIium) electron detector [3]. Since the samples were exposed to air during this procedure, oxidation of the Fe films could not be prevented.3. Results and discussion. - All spectra of the thin oxidized Fe57 films up to 12 monolayers exhibit a typicaI quadrupole split Fe3+ pattern at 295 K. In figure 1 the spectra of approximately 12 (a), 8 (b)
- 8 -6 -L - 2 0 2 L 6 8 V e l o c i t y l r n r n l s l
FIG. 1.
-
CEM-spectra of Fe57 films deposited on 2 000 A Cu(*) Donegani Research Institute, Novara, Italy. substrates based on mylar foil and exposed to air (source :
(**) FB 6 - Angewandte Physik, Gesamthochschule, Cos'-Rh) : a)
-
12 atomic layers ; b) 8 atomic layers ;4100 Duisburg, Germany. c)
--
5 atomic layers (note the different scale).C6-296 M. PETRERA, U. GONSER, U. HASMANN, W. KEUNE AND J. LAUER
and 5 (c) monolayers Fe57 on Cu-substrate are shown. The isomer shifts resulting from computer fitting are 0.38 f 0.04, 0.321
+
0.04 and 0.341 f 0.06 mm/s (with respect to @-Fe at 295 K), and the qua- drupole splittings are 0.87 f 0.07, 1.01 f 0.07, 0.93 f 0.03 mrn/s, respectively. The resonance lines in the peak maximum for 12 Fe57 layers show an effect of about 5%,
and for 5 monolayers Fe57 an effect of about 1.8%.
In figure 2 the spectra of 2 (a) and 8 (b) mono- layers of Fe5' on natural iron and for comparison an unenriched iron film of about 100 layers on stainless steel (c) are shown. In figure 2a and b the quadrupole split line of deposited (oxidized) Fe57 is superimposed by the six line pattern of the substrate and in figure 2c
,
-8 -6 -L -2 0 2 L 6 8 V e l o c i t y Imrn I s l
FIG. 2. - CEM-spectra of iron films deposited on various
substrates and exposed to air (source : Co57-Rh) : a) 2 domic
layers on natural bulk a-Fe ; 6) N 8 atomic layers on natural
bulk a-Fe ; c)
-
100 atomic layers natural iron on stainless steel (note the different scale).the big central peak of stainless steel dominates the six- line pattern of the unoxidized fraction of the deposited natural iron film.
In figure 3 the effect for all spectra on the basis of a single resonance line with natural line width is plotted
FIG. 3.
-
Relative emission reduced to a single resonance ofnatural line width versus the number of Fe57 atomic layers.
Closed circles : Fe57 on copper. Open circle : nat. Fe on stainless steel. Asterisks : Fe57 on natural iron.
as function of the number of Fe57 layers. The value of the unenriched @-Fe film on stainless steel is in agree- ment with measurements by Thomas et al. [4]. A
straightforward extrapolation of all values indicates that on the basis of a single resonance line a monolayer of Fe57 (corresponding to w 1.8 X 1015Fe atoms)
gives an effect of about 1.8
%.
Thus, from less than 10i5 Fe57 atoms it is possible to obtain by CEMS a reasonable resonance signal. The high sensitivity of this method has been shown also by Stanek et al. [5] studying CEM-spectra of Fe5' implanted with doses of 1015 Fe57/cm2 into solids.Acknowledgment.
-
The support of the Deutsche t.orschungsgemeinschaft and IRSID is gratefully acknowledged.References
[l] BAVERSTAM, U., EKDAHL, T., Born, Ch., RINGSTROM, B., [4] THOMAS, J. M., TRICKER, M. J., WINTERBOTTOM, A. P., J.
STEFANSSON, V., LILJEQUIST, D., Nucl. Instrum. Methods Cfiem. Soc., Faraday Tvans. II,71(1975) 1708.
115 (1974) 373. [5] STANEK, J., SAWICKI, J., SAWICKA, B., Proc. Intern. Con$ on
[2]
