Measurement of ratios of Auger electrons emission probabilities and K–L shell vacancy transfer probability for bromine and iodine compounds

Chemical Physics Letters 579 (2013) 132–135

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Measurement of ratios of Auger electrons emission probabilities and K–L shell vacancy transfer probability for bromine and iodine compounds A. Kucukonder a, B.G. Durdu b,⇑ a b

Kahramanmaras Sütcü Imam University Faculty of Science and Letters, Department of Physics, 46100 K.Marasß, Turkey Kilis 7 Aralik University, Vacational Higher School of Health Services, Opticianry Program, 79000 Kilis, Turkey

a r t i c l e

i n f o

Article history: Received 15 February 2013 In final form 15 June 2013 Available online 21 June 2013

a b s t r a c t Ratios of emission probabilities of Auger electrons (u = p(KLX)/p(KLL), t = p(KXY)/p(KLL)) and the vacancy transfer probabilities from K to L shell, gKL for Br and I compounds were studied using Ki (i = a, b) X-rays line intensities. The samples were excited by c-rays 59.5 keV produced by an Am-241 radioisotope source. The K X-rays emitted from the samples were detected with a high resolution Si(Li) detector. Experimental results were compared with the other theoretical values of Br and I elements. The measured values of u, t and gKL for Br and I compounds are being reported here for the first time. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction When a single vacancy is created in an inner shell (e.g. the Kshell), the vacancy will be rapidly filled up (108 s) by an electron coming from some higher (sub)shell, so that in a radiative decay a photon and in a radiationless (Auger) decay an electron is emitted. Reliable accurate values of the decay probabilities are required in order to derive the vacancy creation from the observed photons or electrons. To estimate this contribution, knowledge of K to L shell vacancy transfer probabilities is required. However, knowledge of average vacancy distribution is also important for the study of processes such as nuclear electron capture, interval conversion of c-rays, photoelectric effect and generally, whenever primary vacancies produced in the shell must be distinguished from multiple ionization due to the decay of inner vacancy [1]. The vacancy transfer probabilities in the atomic shell have been studied by various workers [1–14]. Recently, alloying effect on K to L shell vacancy transfer probabilities for Fe, Ni, Cr elements have been investigated by Han and Demir [15]. Kup Aylikci et al. have studied on alloying effect for Al, Ni and Mo superalloys [16]. The K shell intensity ratios Kb/Ka and the K–L total vacancy transfer probabilities gKL for nine elements in the atomic range 40 6 Z 6 50 have been determined using a weak 133 Ba gamma source at excitation energy of 80.997 keV by Tursßucu et al. [17]. Vacancy transfer probabilities from K to L shell were measured using IKb =IKa intensity ratios for six elements in the atomic region 16 6 Z 6 22 at excitation energy of 5.96 keV by Ertug˘ral et al. [18]. Vacancy transfer probabilities from K to L shell for some elements in the atomic range 75 6 Z 6 92 have been determined

by Apaydın and Tırasßog˘lu [19]. The K–L total vacancy transfer probabilities (gKL) of selected elements in the atomic range 42 6 Z 6 82 have been determined using a weak gamma source by Bennal et al. [20]. Raj et al. [21–23] and Pawllowski et al. [24] investigated the changes of K X-ray intensity ratios with population of valance electrons by means of multiconfiguration Dirac-Fock (MCDF) method. We investigated the vacancy transfer probabilities from K to L shell for the elements in the range 19 6 Z 6 58 using K-shell fluorescence yields and Kb/Ka X-ray intensity ratios [25]. In addition to this, radiative vacancy transfer probabilities from L3 subshells to M, N, O shell and subshells in the atomic range 72 6 Z 6 92 have been measured by our research group [26]. In experimental studies, it was assumed that Br and I were in the pure elemental form. The results found in these experiments were compared with theoretical atomic values. However, Br and I are halogens and they normally occur in compound form in the nature. Although the vacancy transfer probabilities have been studied by some workers, there are not any studies addressing chemical effects on the vacancy transfer probabilities. In present study, ratios of emission probabilities of Auger electrons and the vacancy transfer probability from K to L shell for bromine and iodine compounds were experimentally determined using Ki (i = a, b) X-rays line intensity. These effects are interpreted in terms of valence electron distribution and chemical bonding. This is the first an alytical investigation of emission probabilities of Auger electrons and the vacancy transfer probabilities for Br and I compounds. 2. Experimental

⇑ Corresponding author. Address: Kilis 7 Aralık Üniversitesi, Sag˘lık Hizmetleri MYO Karatasß Kampüsü, 79000 Kilis, Turkey. Fax: +90 348 813 93 02. E-mail addresses: [email protected], [email protected] (B.G. Durdu). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.06.026

The geometry of the experimental set-up is shown in Figure 1. The purity of commercially obtained materials was better than

A. Kucukonder, B.G. Durdu / Chemical Physics Letters 579 (2013) 132–135

133

Figure 1. Experimental setup. Figure 2. (a) Characteristic K X-ray emission spectra of KI04. (b) Characteristic K Xray emission spectra of CaI2.

99%. To reduce the particle size effect, the powder samples were sieved with 400 mesh and supported on a mylar films at 34  103 g/cm2 mass thickness. The samples were excited by using heavily filtered 59.5 keV c photons emitted from a 75 mCi 241 Am radiative source and X-rays emitted from samples were detected by a Si(Li) (FWHM 155 eV at 5.9 keV) detector system. Two typical K X-ray spectra obtained from KI04 and CaI2 are given in Figure 2 for comparative purposes. Experimental Ki X-ray fluorescence cross-sections for Br and I compounds [27] were evaluated using the relation

rKi ¼

N Ki ði ¼ a; bÞ I0 GeKi bKi t

ð1Þ

I0 GeKi ¼

where NKi is the measured intensity (area under the photopeak) corresponding to the Ki group of X-rays, I0 is the intensity of exciting radiation falling on the sample, G is the geometry factor, eKi is the detector efficiency for K X-rays, t is the thickness of the sample in g/cm2 and bKi is the self-absorption correction factor of the target material. The self-absorption correction factor was calculated from the following equation

bKi ¼

1  exp½ð1Þðlinc = cos /1 þ lKi = cos /2 Þt ðlinc cos /1 þ lKi cos /2 Þt

to the normal at the surface of the sample, t (g/cm2) is the measured thickness of sample. I0 GeKi values in the present experimental geometry were determined in a separate experiment targets having areas of cross section that were similar to those used in the main experiment but with atomic number 30 6 Z 6 58, emitting fluorescent X-rays in the energy range 8.5–40 keV were irradiated in the same geometry and fluorescent X-rays were counted. The effective overall detection efficiency for the present geometry was determined by the following relation

ð2Þ

where linc and lKi are the mass absorption coefficients of incident photons and emitted Ki X-rays [28,29], respectively. /1 and /2 are the angles of the incident photons and emitted X-ray with respect

N Ka

rKa bKa t

ð3Þ

where, N Ka is the number of Ka X-rays recorded under the Ka peak,

rKa is the theoretical fluorescence cross-sections, bKa is the selfabsorption correction factor, t is the thickness of target. K-shell fluorescence yield xK was determined using the following equation semi-empirically [30]

xK ¼

rKa þ rKb rK ðEÞ

ð4Þ

where rKa and rKb are X-ray fluorescence cross-sections, rK(E) is the K-shell photoionization cross-section taken from the tables published by Scofield [31]. Ratios of emission probabilities of Auger electrons (u = p(KLX)/ p(KLL), t = p(KXY)/p(KLL)) were determined using following equations described in detail by Schönfeld and JanBen [32].

134

A. Kucukonder, B.G. Durdu / Chemical Physics Letters 579 (2013) 132–135

IKb rKb ¼ I Ka rKa pðKXYÞ ¼ x2 t¼ pðKLLÞ pðKLXÞ ¼ 2x u¼ pðKLLÞ x¼

ferences between experimental and theoretical values are in the range of 3–12% according to Schönfeld and JanBen [32], 13–26% according to Scofield [35]. The ratios of Auger electrons emission probabilities u and t for I compounds are given in Table 2. When it is considered experimental results for u, it is seen that the experimental result vary in the range of 1–18%. Also, those results vary in the range of 1–13% and 1–10 according to the theoretical values of Schönfeld and JanBen [32] and Scofield [35], respectively. In experimental t values for the same compounds, experimental results have a changing between 2% and 39%. On the other hand, differences between experimental results and theoretical values of Schönfeld and JanBen [32] and Scofield [35] are in the range of 1–26% and 1–21%, respectively. For the experimental results of Br vacancy transfer probabilities gKL given in Table 3, it can be seen that differences between experimental and theoretical values of Schönfeld and JanBen [32] and Ertug˘ral et al. [36] are 4–22% and 3–21%, respectively. From the experimental values of the gKL vacancy transfer probabilities for I compounds in Table 4, the changes of experimental values are in the range of 1–11% and 1–11% according to the theoretical value of Schönfeld and JanBen [32] and Ertug˘ral et al. [36], respectively. Ratios of emission probabilities of Auger electrons (u, t) and the vacancy transfer probabilities gKL measured for Br and I compounds were compared with theoretical values for Br and I. Br and I are halogens and, in nature, the halogens normally occur in compound form. The electronic configurations of Br and I are 3d10 4s2 4p5 and 4d10 5s2 5p5, respectively. Electron affinities and electronegativities of the halogens (Br and I) are larger than those of the other elements. It is a well-known fact that the orbital energy levels of L, M, N and O shells get closer to each other with increasing quantum number n and outer energy levels become sensitive to the chemical environment by this effect. Thus, outer energy levels are more strongly affected by ligands according to crystal field theory. In addition to this, valence electrons participating in the formation of a chemical bond are removed from the atom and this causes a change in the electronic screening and a change in the outer shell binding energies. These effects play an impor tant role in the X-ray transitions. Since Br has an unfilled 4p shell, and I has an unfilled 5p shell, cross-sections of Br and I are sensitive to these effects [27]. According to the results shown in Tables 1 and 2, there is a relation between the bond distance and Auger emission rate. Auger transition probability increases with decreasing interatomic distances of the other shell electrons. For compounds having high interatomic distances, X-ray emission becomes more probable. The electron density decreases or increases depending on the type of bonding with adjacent atoms in molecule or crystal. Furthermore, Auger transition probability depends on the type of chemical bond and interatomic distances. Therefore, chemical effect corre-

ð5Þ ð6Þ ð7Þ

This method was also used for the some elements in the atomic range 25 6 Z 6 42 by Öz [33]. In that study, Öz considered that a Kshell vacancy may be filled by an L electron or M electron by radiative transition (Ka = K–L2; L3; Kb = K–M) or by Auger transition (K– LL, or K–LX); X and Y denotes M–N etc. shell electrons. The aim here is to calculate the ratios of emission probabilities of Auger electrons and vacancy transfer coefficients gKL from these X-ray emission ratios [33]. The vacancy transfer coefficient gKL describes the mean number of vacancies produced in the L shell by one vacancy in the K shell. If all radiative and nonradiative processes and the production of two vacancies in the L shell by the ejection of the KLL Auger electrons are taken into account, the quantity gKL, is given by [32]

gKL ¼

2  xK 1þx

ð8Þ

3. Results and discussion The experimental and theoretical values for ratios of emission probabilities of Auger electrons (u = p(KLX)/p(KLL), t = p(KXY)/ p(KLL)) and the vacancy transfer probabilities gKL in Br and I compounds are given Tables 1–4. The overall error in present measurement is estimated to be 7%. This error is the quadrature sum of the uncertainties in the different parameters used to evaluate Xray product cross-sections, intensity ratios and average fluorescence yields [34], i.e. target thickness (2%), the evaluation of the peak area (2%), I0Ge product (2%) and the absorption correction factor (1%). To determine the chemical effect on ratios of Auger electrons emission probabilities (u, t) and vacancy transfer probabilities gKL of Br and I compounds, obtained experimental results were compared with the theoretical calculated values for pure elements by Schönfeld and JanBen [32], Scofield [35] and Ertug˘ral et al. [36] which is present below. We could not make any comparison of the results for Br and I compounds since there are no experimental and theoretical values for Br and I compounds in the literature. From the results in Table 1, while the experimental results for ratios of Auger electrons emission probabilities u for Br compounds show differences between 2% and 6%, same results are differ from theoretical values of Schönfeld and JanBen [32] and Scofield [35] within range 1–6% and 7–13%, respectively. Similarly, while experimental t values for Br compounds vary in the range of 4–12%, dif-

Table 1 Ratios of emission probabilities of Auger electrons (u = p(KLX/p(KLL), t = p(KXY)/p(KLL) for Br compounds. Compounds

Br KBrO3 C21H16Br2O5S NH4Br NaBr KBr C19H10Br4S C6H6BrN C7H5O2Br a

Interatomic distances (A0)

t[p(KXY/p(KLL)]

u[p(KLX/p(KLL)] Experimental

[32] 0.337

2.94

2.98 3.30

Calculated from values of Scofield.

0.334 ± 0.024 0.334 ± 0.024 0.334 ± 0.024 0.324 ± 0.022 0.324 ± 0.022 0.322 ± 0.021 0.322 ± 0.021 0.316 ± 0.020

[35]

Experimental a

0.2968

0.028 ± 0.002 0.028 ± 0.002 0.028 ± 0.002 0.026 ± 0.002 0.026 ± 0.002 0.026 ± 0.002 0.026 ± 0.002 0.025 ± 0.002

[32]

[35]

0.0283

0.0220a

135

A. Kucukonder, B.G. Durdu / Chemical Physics Letters 579 (2013) 132–135 Table 2 Ratios of emission probabilities of Auger electrons (u = p(KLX/p(KLL), t = p(KXY)/p(KLL) for I compounds. Compounds

Interatomic distances (Å)

I I2 KIO4 CaI2 NH4I NaIO3 Hg2I2 KI a

2.66 1.80 3.04 3.16 3.53

t[p(KXY/p(KLL)]

u[p(KLX/p(KLL)] Experimental

[32]

[35]

0.459

0.4336a

0.432 ± 0.030 0.476 ± 0.033 0.458 ± 0.032 0.434 ± 0.030 0.428 ± 0.030 0.410 ± 0.029 0.404 ± 0.028

Experimental

[32]

[35]

0.0527

0.0470a

0.047 ± 0.003 0.057 ± 0.004 0.052 ± 0.004 0.047 ± 0.003 0.046 ± 0.003 0.042 ± 0.003 0.041 ± 0.003

Calculated from values of Scofield.

Table 3 The vacancy transfer probabilities from K to L shell gKL for Br compounds. Compounds

Br KBrO3 C21H16Br2O5S NH4Br NaBr KBr C19H10Br4S C6H6BrN C7H5O2Br

Interatomic distances (Å)

2.94

2.98 3.30

Experimental

Theoretical [32]

[36]

1.174

1.190

1.254 ± 0.092 1.435 ± 0.099 0.958 ± 0.094 1.043 ± 0.089 1.098 ± 0.089 1.223 ± 0.090 1.229 ± 0.091 1.359 ± 0.094

References

Table 4 The vacancy transfer probabilities from K to L shell gKL for I compounds. Compounds

I I2 KIO4 CaI2 NH4I NaIO3 Hg2I2 KI

Interatomic distances (Å)

2.66 1.88 3.04 3.16 3.53

Experimental

difference appears to exist between the experimental and theoretical vacancy transfer probabilities gKL values of Br and I. The reason for this is that the theoretical calculations of I and Br were carried out in element form. Consequently, it is clear from the Tables that the ratios of of Auger electrons emission probabilities (u, t) and vacancy transfer probabilities gKL of Br and I compounds depend on chemical structure.

Theoretical [32]

[36]

0.909

0.909

0.916 ± 0.034 0.808 ± 0.039 0.838 ± 0.037 0.949 ± 0.036 0.879 ± 0.032 0.846 ± 0.035 0.846 ± 0.035

sponds to the difference in transition probabilities between X-ray and Auger transition. According to the Molecular Orbital Theory, molecular orbitals get closer to nucleus by increasing interatomic distances. That is, binding energies of outer shell electrons in molecules are higher than that of free atoms. This state causes a de crease in Auger effect and an increase in fluorescent X-rays. Thus, rKa and rKb fluorescence cross-sections change with increasing interatomic distances. The vacancy transfer probabilities of Br and I compounds depend on chemical structure. As seen from Tables 3 and 4, in general, vacancy transition probabilities increase with the increasing bond distance. The vacancy transfer probabilities strongly depend on the K X-ray cross-sections. Chemical bonding type (ionic, metalic, covalent) affects the vacancy transfer probabilities. The individual characteristic of the structure of molecules, complexes and crystals (polarity valency and electronegativity of atoms, coordination number, ionicities of covalent bond, etc.) mainly affect the vacancy transfer probabilities gKL. A change in chemical bond leads to a change in its valence electron density. Chemical effects on the vacancy transfer probabilities gKL for Br compounds are larger than I compounds because the ionic character of I compounds is less than that of Br com pounds. A significant

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