Radiation Effects and Defects in Solids

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Radiation Effects and Defects in Solids

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DRIFT STUDY OF THE PRODUCTS FORMED BY RADIOLYSIS AND PHOTOLYSIS OF ALKALINE NITRATES a

a

Sergei Bannov ; Mikhail Miklin a Kemerovo State University, Krasnaya 6, Kemerovo, 650043, Russia.

To cite this Article: Sergei Bannov and Mikhail Miklin , 'DRIFT STUDY OF THE PRODUCTS FORMED BY RADIOLYSIS AND PHOTOLYSIS OF ALKALINE NITRATES', Radiation Effects and Defects in Solids, 157:5, 509 - 514 To link to this article: DOI: 10.1080/10420150214609 URL: http://dx.doi.org/10.1080/10420150214609

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Radiation Effects & Defects in Solids, 2002, Vol. 157, pp. 509–514

DRIFT STUDY OF THE PRODUCTS FORMED BY RADIOLYSIS AND PHOTOLYSIS OF ALKALINE NITRATES SERGEI I. BANNOV* and MIKHAIL B. MIKLIN Kemerovo State University, Krasnaya 6, Kemerovo, 650043, Russia

(Received 2 April 2002; Revised 13 April 2002; In final form 2 May 2002) The products of the radiolysis and photolysis of crystalline sodium, potassium, rubidium, cesium nitrates have been investigated by the diffuse reflectance infrared Fourier transform spectroscopy. The bands in the 1260–1220 and 804– 809 cm1 regions observed after the g-irradiation and photolysis by a light with the wave length 253.7 nm of crystalline alkali nitrates were identified as the vibrational modes of NO 2 n3 and n2, respectively. The frequency of the n3 oscillation of nitrite ions decreases from 1260 cm1 up to 1220 cm1 with the increase of the atomic weight or polarizability of a cation. The detection limit of the nitrite ions (1 107 mol g1) for the diffuse reflection method has been determined. The bands observed in KNO3, RbNO3 and CsNO3 spectra in the 947– 940 and 722–737 cm1 regions appearing only after photolysis are due to stretch oscillations of the peroxide O and wagging oscillations of the ON¼¼O group of peroxynitrite accordingly. bond O Keywords: DRIFT; Peroxynitrite; Nitrite ions; Alkaline nitrates

1

INTRODUCTION

As is known [1–3] during the radiolysis or photolysis (l ¼ 253.7 nm) of crystalline nitrates at room temperature oxygen, nitrite ions and peroxynitrite are formed. Besides it is only during the radiolysis that the paramagnetic centres such as NO2, O 3 , O2 , O observed by the ESR method [1] are formed. After the dissolution of an irradiated or photolysed sample of nitrates the content of peroxynitrite ONOO and nitrite NO 2 is determined chemically. However the experimental data in Ref. 4 show that this method cannot be relied upon because of the interaction of decomposition products at dissolution. The use of UV spectrophotometry for the determination of NO 2 and ONOO directly in crystals solves only part of the problem, as peroxynitrite in alkali nitrate crystals has an intensive absorption band at 335–355 nm with the extinction coefficient e (130–550) m2 mol1 [5], completely overlapping a weak absorption band of the nitrite ion. The problem can be solved by the IR spectroscopy technique. The type and frequencies of characteristic oscillations of the molecules of the peroxynitrite ONOO [6] and NO 2 [7] ions in different media have been determined. The oscillation of the oxygen molecule is inactive * Corresponding author. E-mail: [email protected]

ISSN 1042-0150 print; ISSN 1029-4953 online # 2002 Taylor & Francis Ltd DOI: 10.1080=1042015021000051576


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S. I. BANNOV AND M. B. MIKLIN

in IR-spectra. On the other hand, the systematical investigation of these centers directly in the irradiated crystalline nitrates of alkali metals in the IR range has not been made so far. The IR- and Raman-spectra [8, 9] of potassium nitrate exposed to g-irradiation show the appearance of new bands at 1250 and 806 cm1, attributed to NO 2 . The same bands appear in the spectra of nitrates containing NO 2 as an impurity. The similar bands of the nitrite-ion appear in the KI:NO 3 crystals due to the photolysis by the unfiltered light of a Hg lamp of medium pressure [10]. In KBr:NO 3 crystals during photolysis by the 254 nm light the band appears at 630 cm1, which is attributed to deformation oscillations of the ON¼¼O peroxynitrite group [11]. The UV-irradiation of KNO3 results in bands at 721, 815 and 943 cm1 attributed to stretch oscillations of the O O and NO group, and after photobleaching the remains a small residual absorption at 835 cm1 assignable to the most intense NO 2 vibration [12]. In the present paper the FTIR-spectra of photolysis and radiolysis products of alkali nitrates are evaluated. The detection limit of the nitrite ions at radiolysis has been determined. The comparative analysis of the matrix-isolated products of the radiation and photo decomposition of nitrates-ions is made.

2

EXPERIMENTAL

Single crystals of NaNO3, NaNO3 :NO 2 , KNO3, KNO3 :NO2 , RbNO3, CsNO3 were grown by slow evaporation of saturated aqueous solutions at room temperature. Materials used were reagent-grade. The samples were irradiated at 77 K in a 60Co g-ray source at a dose rate 1.5 Gy s1. The grown crystals were crushed, the 56–63 mm fraction was sieved and then the IR-spectra were registered. The nitrite ions were introduced into a crystalline matrix by cocrystallization during vaporization of aqueous solutions of salts of the corresponding metals. The content of the NO 2 ions was determined by the spectrophotometric analysis using the compound formed as a result of diazotization of the sulfanilic acid followed by its condensation with phenol [13] (the detection limit 2 108 mol g1). The grown crystals were ground in the mortar, the 56–63 micron fraction was extracted and stored in a vacuum oven during 1 hr at T ¼ 323 K, then it was photolysed and the IR-spectra were registered. The samples were photolysed in argon by the light with the wave length 253.7 nm from an Hg lamp of low pressure, whereupon the sample was carefully stirred. The IR-spectra in the 5000–450 cm1 region were registered by the diffuse reflectance infrared Fourier transform spectrometry (DRIFT), ‘‘System-2000’’ (Perkin-Elmer Corp.).

3

RESULTS AND DISCUSSION

The comparative analysis of the IR- spectra of the initial, photolysed and g-irradiated samples shows that after radiolysis the halfwidth of the lines of the basic oscillations of the nitrate-ion decreases by approx. 1.3–1.5 times. The halfwidth reduction is observed when the absorbed dose is less than 30 kGy, whereupon the changes of the parameters of spectral lines are not observed. This dependence is characteristic for all the investigated alkali nitrates and is presumably explained as the radiation ordering of a crystalline structure. Both the radiolysis and photolysis of all the investigated nitrates induce new bands in the IR-spectra belong to the decomposition products of the nitrate ion (Tab. I). For a nonlinear 1 1 1 NO oscillations in the IR-spectra 2 ion the n1 1320 cm , n2 830 cm , n3 1260 cm


DRIFT STUDY OF IRRADIATED AND PHOTOLYZED ALKALINE NITRATES

511

TABLE I Oscillations Frequencies of Photolysis and g-radiolysis Products of Alkali Nitrates and their Identification. Photolysis Matrix

Wavelength, cm

1

Radiolysis

Identification of bands

Wavelength, cm

1

Identification of bands

NaNO3

1271

n3 ðNO 2Þ

1271 841

n3 ðNO 2Þ –

KNO3

1253 944 722

n3 ðNO 2Þ O O ON¼¼O

1253 669 806

n3 ðNO 2Þ n2 ðO 3 or O3 Þ n2 ðNO 2Þ

RbNO3

1230 947 735


1230 668 809


CsNO3

1222 940 725


1222 669 804


FIGURE 1 IR-spectra in the regions of (a) n3 valent vibrations and (b) n2 deformation vibrations of the nitrite ions: 1 – KNO3; 2 – UV photolysed KNO3; 3 – KNO3 doped with the nitrite ions; 4 – g-irradiated KNO3. Absorption in arbitrary units.


512


FIGURE 2 Frequency of n3 valent vibration of the nitrite ions in alkaline nitrates vs. atomic weight and polarizability of a cation.

are active [7]. When nitrite ions are incorporated into the nitrate matrix as an impurity, two new lines in the 1220–1260 and 804–809 cm1 region due to valent and deformation oscillations of n3 and n2 NO 2 , respectively, are observed. Both the photolysis and radiolysis of all the investigated nitrates result in the same bands which enables one to attribute them to the oscillations of NO 2 ions. Figure 1 shows the IR-spectra of KNO3 in the region of valence and deformation oscillations of NO 2 . The spectra of RbNO3 and CsNO3 are similar to those of KNO3 whereas the band due to deformation oscillations n2 in NaNO3 is not observed. The frequency of the vibrations n3 correlates with the atomic weight and the polarizability of a cation (see Fig. 2). It is found out that the intensity of the band of the vibration n3 of the nitrite ion is proportional to the g-irradiation dose (see Fig. 3). Using the initial radiation-induced yield of nitrite

FIGURE 3 Intensity (in arbitrary units) of band n3 valence vibration of the nitrite ions in rubidium nitrate (1230 cm1) vs. the absorbed dose D.


DRIFT STUDY OF IRRADIATED AND PHOTOLYZED ALKALINE NITRATES

513

TABLE II Dependence of Wavelength and Oscillation Frequency from the Bond Order of the O Group in Oxygen-containing Compounds [14]. O Parameter

[O+2 ]AsF6

O2

K[O+2 ]

Na2[O22 ]

ONOO *

˚ Wave-length, A n(O2), cm1

1.123 1858

1.207 1555

1.28 1108

1.49 760

1.39 947–940

*The data of the present paper.

ions in nitrates [1], the detection limit for the DRIFT –1 107 mol g1 is estimated to be somewhat higher than the limit (2 108 mol g1) for the chemical method [13]. The appearance of the line in the 947–940 cm1 region observed only at photolysis is connected with the formation of peroxynitrite having the O O group. It is shown [14] that the bond order, the bond length and oscillation frequencies essentially vary as the number of electrons on 2pp* antibonding orbitals of this group increases (Tab. II). The decrease of the O O bond order results in the increase of the bond length and the decrease of the oscillation frequency. Besides, a linear relationship between the bond length and the oscillation frequency is observed. On the basis of this assumption it can be stated that the O O bond ˚ . The quantum-chemilength with an oscillation frequency 947–940 cm1 is equal to 1.39 A ˚ (cis- and trans- conforcal calculations of the geometry of peroxynitrite give 1.361–1.430 A mation of ONOO) [6] for the O O bond length. According to the calculations the OO oscillation frequency in the peroxynitrous acid [15] is 946 cm1 and the measured IR-spectra of the peroxynitrous acid which was obtained by the photolysis of HNO3 under the conditions of matrix isolation give 960.5 cm1 (N2), 952.0 and 957.4 cm1 (Ar) [16]. The bands in the IR-spectra due to oscillations of peroxynitrite after the g-irradiation of nitrates are not observed. Therefore it can be assumed that either peroxynitrite at radiolysis is not formed at all or it is formed with a very small yield (below the detection limit by the diffuse reflection technique). Bands in the 700–725 cm1 region (see Fig. 4) appearing only at photolysis are connected with the wagging oscillation of the ON¼¼O group of the peroxynitrite molecule. The

FIGURE 4 IR-spectra of diffuse reflectance: 1 – KNO3; 2 – UV photolysed KNO3; 3 – CsNO3; 4 – UV photolysed CsNO3. Absorption in arbitrary units.


514


presence of the ON¼¼O group in HOONO isolated in the N2 and Ar matrix results in the oscillations at 793.6 and 772.8 cm1 [16]. The absence of similar bands in sodium nitrate is connected with the low quantum yield of ONOO. Bands in the 668–669 cm1 region appearing during the g-irradiation of potassium, rubidium and cesium nitrates can be presumably referred to n2 oscillation of the O 3 ion – radical or ozone O3. It was impossible to identify the 841 cm1 band in sodium nitrate. Thus additional investigations are necessary to determine its nature.

4

CONCLUSIONS

The experimental data show that it is possible to apply the IR-spectroscopy to investigate and quantitatively determine the decay products of the nitrate ion directly in the matrix. The pro ducts obtained under photolysis and radiolysis are found to be different (NO 2 , ONOO and O3 or O3 , and NO2 , respectively). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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