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EPR Of the SeO43−, SeO3−, SeO2− and NH3+ radicals in ultraviolet-irradiated crystals of ammonium compounds
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1. Phys. C&m. Soli&Vol. 34, No. 9, pp. 10154021, 1993 Printedin Grm Britain.
EPR OF THE SeO;-, SeO,, SeO,- AND NH; RADICALS IN ULT~VIOLET-IR~DIATED CRYSTALS OF AMMONIUM COMPOUNDS JIANG-TSU YIJ, CHING-JIUN Wut and SSU-HAO Lov Institute of Physics, National Taiwan Normal University, Taipei 11718, Taiwan, Republic of China (Received 10 December 1992; accepted in revised form 18 May 1993) Aktanct-Oxyradicals of selenium, such as the SetI$-, SOi, SeO, and Se& are ~nvention~ly uroduced via Y - or X-irradiation. The same is also true of the NH? radical. We report for the first time RPR observations of these radicals produced by ultraviolet (u.v.)-irradiation kr some ammonium compounds. We have investigated u.v.-irradiated crystals of (NH&SO,, LiNH,SO,, (NH,),H(SeO,)r and NH,HrPO,. The NH: radical has been detected by EPR in u.v.-irradiated (NH,)rSO, and LiNH,SO, crystals. The NH: radical is p&u&d at the first stage of the U.V. photo-dissociation of the NH: ion in solids. The oxyradicals of selenium detected by EPR in some ammonium compounds oan be regarded as the product of an U.V. photo-redox reaction between the NH: ion and the W,:- ion. I@words: EPR, u.v.-i~dia~on,
radical ions.
1. R’J’I’RODUCTION Oxyradicals of selenium, such as SeO; , SeO; , Se& and W,:- , are conventionally produced via y- or
X-irradiation and studied by the method of electron pammagnetic resonance (EPR). Irradiation with U.V. light had been used in the past for producing organic or inorganic radicals. But to our knowledge the production of the oxyradicals of selenium by means of U.V. irradiation had not been reported previously. The same is also true of the NH: radical which is also conventionally produced via y- or X-irradiation. We have found that oxyradicals of selenium and the NH: radical can be produced via U.V. irradiation. This EPR investigation was carried out to study the U.V. photochemical reaction of the NH: ion and the w,ion in solids and to see if the mechanism of the radical production by means of u.v.-irradiation is any different from that by means of y- or X-irradiation. There are many EPR reports on the production of the NH: radical and the selenium oxyradicals by means of y- or X-irradiation. These results indicate that as many chemically inequivalent NH; radicals are usually produced as there are chemically inequivalent NH; ions in solids. But the same cannot be said for the selenium oxyradicals; in fact, the number of chemically inequivalent species sometimes exceeded those of the %I$- groups in solids. It was hoped that the number of radical species produced would be tPermanent address: Department of Mathematics and Science Education, National Hualien Teachers College. Hualien, Taiwan 97055, Republic of China.
smaller in the case of the u.v.-radiolysis of the selenate ion. The selenium oxyradicals have been used as EPR probes to study the statics and dynamics of structural phase transitions. Therefore, it is desirable not to create too many radical species at the same time to give rise to a congested EPR spectrum. Finally, hydrogen passivation of defects on semiconductor surfaces is a current topic of interest. A study on the U.V. photo-dissociation of the NH: ion is relevant to this application. 2. EPR OF THE OXYRADICALS OF SELENIUM The radical ions SeO, , SeO; and SeO; produced by y-irradiation in potassium and sodium &mates and selenites were first reported by Atkins et al. [l]. The four radical ions SeO; , SeO; , SeOi and SeO:have been detected by EPR in K,SeO, crystals yirradiated at room temperature [2]. The SeO; radicals have been detected by EPR in X- or y-irradiated alkali t~hydrogen (or deute~um) selenites p-61. These four radicals exhibit distinctive anisotropies in the g-factor and the “Se (I = l/2) hypcrfine splitting, so that an unambiguous identification can usually be made. The EPR spectra of these radicals can be analyzed by an S = l/2 effective spin-Hamiltonian of the form,
.?L?= @SgH + SAI,
(1)
where the first term is the electronic Zeeman term and the second the Se hyperfine term. The Se hyperfme splitting observed for the oxyradicals of selenium is
1015
JIANGTSUYu et al.
1016
usually fairly large, such that the perturbation method is inadequate for analysis. Kawazoe et al. [7j have given a method for the evaluation of the eigenvalues of the Hamiltonian in eqn (1). We have adopted their method for the enumeration of the eigenvalues and resonance fields. We assume that the g-matrix and the hyperIme A -matrix share a common set of principal axes. We expanded the secular equation into an algebraic equation of the fourth order and wrote a Pascal program for the computation of the eigenvalues and resonance fields. In the most general case, the spin-Hamiltonian in eqn (1) includes nine independent parameters; the three principal values of the g-factor, the three principal values of the hype&me splitting,, and the three Euler angles required to bring the principal axes into complete coincidence with the chosen reference axes. This program allows simultaneous scannings of the nine parameters through chosen ranges, and at the same time compares the calculated resonance field values with some pre-chosen experimental resonance field values. We used a PC computer equipped with a 386 microprocessor and a 387 mathematical coprocessor for computation. For the most general case, in which all the nine spin-Hamiltonian parameters are unknown, it may take a couple of days of computer-running time to get a best fit. 3. EPR OF THE NH2 RADICAL
Since the tirst EPR observation of the NH: radical in y- or X-irradiated NH,ClO, crystals [&lo], there are quite a few reports of EPR observations of this radical in y- or X-irradiated solids in which the ammonium ion is either a constituent or a dopant. In solids, the NH: radical could be a very stable species as in LiKSO, crystals (11, 121,or a short-lived species as in (NH& SO., crystals. This radical is stabilized by the %#impurity in (NH&SO, [13], and by the C@,- impurity in LiNH4S04 and LiCsSO., crystals [14-161. Only a portion of the irradiated ammonium compounds exhibited the EPR spectrum of the NH: radical; others exhibited the N2H$ radical, as in NHICl [17]; and still others exhibited no radical species derived from the ammonium ion. The EPR spectrum of the NH: radical can be analyzed by the,following spin l/2 spin-Hamiltonian,
i=l
where the first term is the electronic Zeeman term, the second the nitrogen hype&e term, and the third the hydrogen hyperfme term. Rapid molecular reorientational motion of the radical at room temperature renders the three hydrogens magnetically equivalent,
resulting in a characteristic 1Zline hyperhne pattern. The nitrogen and the hydrogen hyperfine terms can be regarded as perturbations to the Zeeman term. Resonance fields which include terms up to the second-order perturbation terms have been used to fit the EPR spectrum of this radical [ 181. However, first-order perturbation theory is usually quite adequate for the analysis of its EPR spectrum. 4. EXPERIMENTAL
Sample crystal are grown by slow evaporation from aqueous solutions. (NH&SO4 and LiNH,SO, crystals doped with SeO]- were grown by adding about 5% of (NH,)2Se04 into the growth solutions. Pseudo-hexagonal LiNH,SO, crystals are usually twinned. We prepared, with the aid of a polarizing microscope, single-domain crystals for use in this experiment. Single crystals of (NH,),H(SeO,), were grown from aqueous solutions made up of stoichiometric proportions of (NH& Sea, and H, SeO,. Ultraviolet-irradiation was carried out at room temperature by using a Bruker 200 W U.V.irradiation system. The U.V. source is a mercury arc lamp. The crystals were irradiated in front of the lamp without the use of an optical filter. The intensity of the U.V. light given off by this source is adequate for this experiment, although a more powerful light source may be desirable. The X-band (cavity resonance frequency at about 9.42 GHz) EPR spectrometer has been described previously [19]. The radicals produced by u.v.- or y-irradiation were investigated by EPR at room temperature. 5. EXPERIMENTAL RESULTS (A)
N-0,
SQ
We experimented first with u.v.-irradiation of (NH& SO., : SeO:- crystals. (NH,), SO4 was selected as a host because our previous experiments have shown that the ammonium ion in this compound is an efficient reducing agent for the U.V. photoreduction of MnO; into Mn*+ [20], and for the thermal reduction of CrO]- into Cr’+ [21]. (NH,),SO, becomes ferroekctric at temperatures below about - 50°C [22]. The NH: radical produced via y- or X-irradiation has been used as an EPR probe for the investigation of the ferroelectric transition [13, 18,23,24]. Addition of (NH1)2Se04 into (NH4)*SOd increases (or stabilizes) the yield of the NH: radical [13]. Besides NH:, the SeO; radical was also observed [13]. The space group of the room temperature paraelectric phase of (NH,),SO., is the orthorhombic Puma [25]. There are two crystallographically inequivalent
EPR of the SO:-,
SeO; , !3eOFand NH: radicals
‘ina
f =0.42Qllz
3zou
336oQ
3aou
Fii. 1. The EPR spectra of the two NH: radicals and the SeO, radical produced by 40 h of u.v.-irradiation in (NH&SO,: W,-, for the &ld parallel to the a-axis. A stick diagram showing the decomposition of the hyperCne structures is also shown. The line marked with the letter X could be due to another SeO, radical.
NH: ions, but the SOi- ions are all crystallographitally equivalent. The two chemically inequivalent nitrogen atoms and the sulfur atom are located in the (010) mirror plane. The hydrogens of the NH: ion are hydrogen-bonded to the sulfate oxygens. EPR of y- or X-irradiated crystals revealed the presence of two chemically inequivalent NH: radicals, which is consistent with the fact that there are two chemically inequivalent NW ions.
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Ultraviolet-irradiated (NH&SO4 crystals is EPR silent. Ultraviolet-irradiated (NH4)2S04:SeO~- CQStals exhibited several radical species. These (NH&SO,: Sea’,- crystals were u.v.-irradiated at room temperature for 40-50 h. EPR measurements were performed at every 5”, for rotations about the three orthorhombic axes. EPR revealed the presence of two chemically inequivalent NH,+ species in u.v.-irradiated crystals (see Fig. 1). The yields of the NH: radicals were relatively large, such that the rotation patterns exhibited by these radicals can be analyzed (see Fig. 2). The determined g-factor, nitroFn and hydrogen splittings of these two radicals (see Table 1) are in agreement with those reported previously [13, 181. y-irradiated (NH,),SO,:SeO:crystals exhibited two chemically inequivalent SeO; species [13]. According to Shibata et al. [13], one of these two SeO; species is produced with one of the Se-O bonds of SeO:- is broken by the y-ray photon, whereas the other species is produced when another Se-O bond is broken. Both of these m; species exhibit axial symmetry. Besides the two NH,+ radicals, two of the radical species produced in u.v.-irradiated crystals can be analyzed. One exhibits gll = 2.0030 + 0.0002 and g, =2.0144 +O.OOOl, with the u-axis of the Pnma lattice as the symmetry axis. The g-values of this radical species fall within the ranges reported for the SeO; radical (see Table 2). The yield of this radical species is not large enough to allow observation of the Se hype&e structure. This species may be identified as the Se-O,(I) species reported
a
b
Angle
C
(deg)
Fig. 2. Observed and fined rotation patterns of NH: (I) (dashed curves) and NH: (II) (solid curves) produced by u.v.-irradiation in (NH,),SO,: w,-
crystals.
JIANGTSUYu et al.
1018
Tabk 1. The principal values of the g-factor, t&o n (A (N)) and hydrogen (A (H)) splittings (in unit of Gauss) of the NH?> radical in (NH.), 8 0. and LiNH.SeG. at room temnerature . T,.
Species NW (I)
Host (NH& SQ
NH; (II) NH:
LiNHl SO,
-
g 2.0065 2.0038 2.0033 2.0065 2.0038 2.0027 2.0026
.
.
A(N)
2.0069 2.6038 2.0038 2.0065 2.0038 2.0036 2.0027
2.0066 2.0038 2.0029 2.0065 2.0038 2.0031 2.0037
18.8 19.1 19.9 19.7 20.4 19.2 35.7
[13], although the g, value we have observed is slightly different from that previously reported [13]. The other species gives g, = 2.0066, g, = 2.0218 and g, = 2.0210, with the three orthorhombic axes as the principal axes. This species could be another SeO; species, but its g-values differ from those reported for the SeO; (II) species [13]. previously
(B) LiNH4 SO4 The space group of the pseudo-hexagonal LiNH.,SO, at room temperature is the orthorhombic P2, cn [26]. There is only one kind of NH*+ ions and one kind of SO:- ions in the room-temperature phase of LiNH,SO,. Pseudo-hexagonal LiNH, SO, is ferroelectric between 186S”C and 10°C [27j. The NH,+ radical cannot be detected by EPR in y-irradiated LiNH,SO, crystals, but can be detected in LiNH,SO,:CK$crystals [14]. The observation of a single NH: species [14] in the paraelectric phase is consistent with the fact that all the ammonium ions are chemically equivalent [26]. Ultraviolet-irradiation of LiNH,SO,:CrOjcrystals resulted in the EPR observation of the NH: radical. This corroborates the observation made in (NH&SO,: SeO:- . Ultraviolet-irradiation of LiNH,SO,: SeO:- crystals resulted in the EPR observation of the SeO; radical but not the NH: radical. A LiNH,SO,:CrO:crystal which was irra-
11.3 11.6 11.7 16.3 14.4 13.4 13.9
Irradiation
A (H) 26.9 27.0 26.2 21.3 22.6 21.9 5.6
24.6 24.7 24.3 24.7 24.6 24.5 26.6
23.7 23.6 23.3 23.1 24.6 24.5 23.9
25.2 25.3 25.3 25.1 24.6 24.8 24.2
References
X Y U.V.
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X Y U.V. X
t::; work
WI I131 This work 1141
diated with U.V. for 15 h exhibited the EPR spectrum of the NH: radical (See Fig. 3). The EPR signal intensity was larger for a crystal irradiated for 23 h. The absolute EPR signal intensity decreased after this same crystal was further u.v.-irradiated for a total of 49 h. This indicates a possible bleaching effect of the NH: radical which the U.V.light itself had produced. We have previously observed (to be published) this effect of U.V. bleaching of the NH: radical produced by y-irradiation in LiNH,SO,: Cacrystals. Irradiation with U.V. light for 6-7 days completely bleached the NH: radical produced by about 5 Mrad of y-irradiation. This same bleaching effect can also be observed in y-irradiated (NH,),SO,: CrO:crystals. The unstable NH: radical in (NH,)rSO, transforms into another radical species identified by Bailey [28] as the NS@species. Ultravioletirradiation, or thermal annealing [12], hastens this transformation process. Therefore, it can be concluded that the NH: ion was first photo-dissociated by U.V.light into the NH: radical, which was further photo-dissociated into NK$ in (NH&SO.,. The final product of the U.V. photo-dissociation of NH,+ in LiNH,SO, crystals cannot be detected by EPR. The yields of the NH: radical in u.v.-irradiated LiNH,SO,: Cacrystals are much smaller than those in y-irradiated crystals, such that it is not possible to completely determine the principal values
Table 2. The principal values of the g-factor and the “Se hyperhne splitting (in unit of MHz) of the oxyradicals of selenium produced in some ammonium compounds via u.v.-irradiation. Species produced via Y- or X-irradiation are included for comparison !bXiCS
Host
CsH,WAX LiNH, SO,
(!$ (III): Se% zI%o, (NH,)%‘, ~~(SeG,” Se@N&H,&& NH;H;FG;
(I): (II):
2.0261 2.0390 2.03 I 2.0295 2.0285 2.0286 2.0145 2.0150 2.0124 2.0144 2.0140 2.0087 2.0084 2.0095
Irradiation
A
g 1.9983 1.9984 1.995 1.9983 1.9880 2.0022 2.0156 2.0153 2.0140 2.0144 2.0140 2.0087 2.0084 2.0095
2.0063 2.0052 2.607 2.0079 1.9972 2.0068 2.003 1 2.6033 2.0036 2.0030 2.0029 2.0055 2.0041 2.0065
839.7 817 734
203.4 270 242
189.1 300 280
ii ”
U.V.
1211 1139 1560
1210 1127 1531
1728 1616 2092
1204 3178 2891 2911
1204 3178 2891 2911
1698 3389 3323 3325
Reference
[41 This work
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U.V.
U.V.
X x U.V.
This work This work [37l 1361 This work
EPR,of the SeOi- , SW?, ScO; and NH: radicals
1019
but exhibited EPR spectra of several radical species. The rotation patterns of three of these spin S = l/2 species can be completely analyzed (see Fig. 4). These EPR spectra did not show up in pure LiNH,SO, crystals irradiated with U.V. light. We designate these radical species as @es (I)--@), The principal values and anisotropy of the g-factor exhibited by these three radical species indicate that them are SeO, species, although the EPR signal intensity was not strong enough to exhibit the Se hyperflne structure.
Fig. 3. The EPR spectrum of the NH: radical in LiNH,SO,:C#,er@als produced by 49 h of u.v.-irradiation, for the field parallel to the c-axis. A stick diagram showing the decomposition of the hype&e structures of this radical is also shown. The two lines marked hy the letter CareduetoaC~-radicalw~hadbecnobsemdin y-irradiated crystals.
and principal axes for the g-factor, nitrogen and hydrogen hyperfme splittings. But the EPR spectrum exhibited by this radical for the field parallel to one of the three orthorhombic axes of the host lattice is identical to that exhibited by the NH: radical produced by y-irradiation. Therefore, the NH,+ radicals produced by u.v.- and ~-i~d~tion can be regarded as identical. Ultraviolet-irradiated LiNH,SO, : SeC#- crystals did not exhibit the EPR spectrum of the NHJ’ radical
(NH,)~H(SeO,), exhibits a sequence of phase transitions [29-331. It undergoes a ferroelastic transition at about 29°C [32], with the simultaneous appearance of twin domains. Above this transition, the crystal structure is trigonal with the R3m space group, while below this transition it is monclinic with the A2/a space group [32]. The twin domain strucSure in the monoclinic AZ/a phase has been studied by Kisbimoto et al. 1321and by Wu et al. 1341. y-irradiated (NH,),H(SeO& crystals exhibited the EPR spectrum of a SeO; radical but not the NH: radical [34]. This SeO; radical exhibits axial symmetry in the trigonal phase of this compound, with the symmetry axis parallel to the trigonal axis of the lattice. An identical SeO; species was detected by EPR in u.v.-irradiated crystals (see Fig. 5). The EPR spectrum is consisted of the main Z = 0 and Z = l/2 (“Se) transitions in the central region of the spectrum (see Fig. 5) and the two Se hype&e lines (see Fig. 6).
Fig. 4. Ohserved and Etted rotation patterns of the SeO;-(I)-&O,(III) radicah in u.v.-irradiated LiNH,SO,:wcrystals, for rotation of the Eeld about the three orthorhomhic axes.
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JIANGTSU
Yu et al. 4000
i
”
’
”
”
”
”
’ “”
’ i
I
,
I 33200
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Fig. 5. The central portion of the EPR spectrum exhibited by the !3eO, radical in u.v.-irradiated (64 h) (NH,),H(SeO,), crystals. The main transition and the two “‘Se hyperfine transitions am each flanked by a pair of satellite lines (marked by the letter H) which could be interpreted as due to hyperline interaction with a nearby proton.
The structure of NH,H,PO,
at room temperature similar to that of KH,PO,. The space group is V,,12 (Z42d). The ammonium ions and the phosphorus atoms occupy special positions, and the oxygens occupy general positions. All the ammonium ions are chemically equivalent and so are the phosphate groups. The SeO:- radical was first reported by Aiki in y-irradiated K2Se0, crystals [2]. This radical was subsequently reported by Kawano in y-irradiated NH,H,PO, (ADP): SeC$- crystals [35], and by Hukuda in y-irradiated KH,PO, (KDP) : we crytals [36]. The SeO:- radical has been used extensively as an EPR probe in the studies of the ferroelectric transitions in KDP-type crystals and in the antiferroelectric transitions in ADP-type crystals. A review on the EPR of SeO:- in KDP- and ADP-type crystals can be found in an article by Dalal [371. NH,H,PO,: SeOi- crystals irradiated with U.V. for about 20 h exhibited the EPR spectrum of a SeO:is tetragonal,
3700
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’ ”
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a,, , , , , , , , , , , , , , , a C a Fig. 7. Observed and fitted cu-plane rotation patterns of the mradical in u.v.-irradiated NH, H,PO,: SO- crystals. 2600
330OG
;
Fig. 6. Observed and fitted rotation patterns of the !3eO, radical in u.v.-irradiated (NH,),H(!SeO,), crystals, for rotation of the field about the a-axis.
radical. This SOradical exhibits axial symmetry, with the tetragonal c-axis as the symmetry axis. The u.v.-yield of this radii1 is fairly large such that the two “Se hype&e lines can be detected (see Fig. 7). The evaluated spin-Hamiltonian parameters (see Table 2) agree fairly well with those reported for the same radical produced via X-irradiation [35]. 6. DISCUSSION AND CONCLUSIONS (A) NH:
radical
The observation of the NH: radical in u.v.-irradiated (NH4)2S0.,: wcrystals is an unexpected result. This represents the first observation of the production of this radical in solids by means of u.v.-irradiation. In hindsight, it may be rationalized by noting that the addition of (NH,),SeO, into (NH4)2S0., stabilizes the NH: radical [13]. The production of this radical via y- and X-irradiation is assumed to be the result of the breaking of an N-H bond by the energetic photons. A similar process has been assumed by us to explain the thermal reduction CrO:- + e- + CrO- observed in (NH,),SnCl, and in NH:-doped K,SnCl, crystals [38]. The thermal reduction of MO@- into MoS+ observed in these two crystals can be similarly explained [39]. The drawback of this hypothesis was that neither the NH: radical nor the N2 Hz radical had ever been detected by EPR in thermally-treated or u.v.-irradiated crystals of the ammonium compounds. The observation of this radical in selenate-doped ammonium sulfate and chromate-doped lithium ammonium sulfate vindicates such a hypothesis. It can be concluded that the NH: radical is produced at the first step of the U.V. photodissociation of the NH: ion and that this radical is further photo-dissociated with the release of free electrons and protons (or free hydrogens), which become available for further chemical reactions. Because of this bleaching effect, the yield of stable
EPR of the Semi- , SCOT,!kO; and NH: radicals
NH: radicals by means of u.v.-irradiation is low, when compared to X- or y-irradiation.
(B) thyradicals
of selenium
y-irradiation of KrSeO, crystals has produced all the known oxyradicals of selenium [2]. However, we found that none of these radicals can be detected by EPR in u.v.-irradiated K,SeO, crystals, even after long durations (days) of irradiation. An oxyradical of selenium, identified as the SeO; radical [40], has been detected by EPR in y-irradiated LiKSO,:SeO]crystals. However, none of the oxyradicals of selenium can be detected by EPR in u.v.-irradiated crystals. The SeO:- radical and other oxyradicals of selenium have been detected by EPR in X-irradiated KDP:SetZ$- crystals [36], but none of these radicals can be detected in u.v.-irradiated crystals. Therefore, it appears that the radiolysis of the seo2,- ion via yor X-irradiation is a direct process, for it does not need a catalyzing agent. On the other hand, the production of the selenium oxyradicals via u.v.irradiation does need the catalyzing action of a reducing agent such as the ammonium ion. We do not know if u.v.-irradiation will result in a photodissociation of the SeO]- ion. But if the seof- ion is reducible via u.v.-i~diation, then the reduced product must be a non-paramagnetic species. The selenium oxyradical species produced via u.v.irradiation, in (NH&SO,: SeO:- , NH,H2P04: wand in (NH,), H(SeO& crystals, are identical to those produced via y- or X-irradiation. The difference is that the yields of the radicals are smaller in the case of u.v.-irradiation but which could possibly be increased by employing a stronger U.V. light source. However, this apparent similarity observed in the end-products of the radiolysis of the selenate ion in ammonium compounds may be accidental. The mechanism of radical production can be regarded as an u.v.-photochemical oxidation-reduction reaction between the NH,+ and Se@- ions in solids; but the details of this u.v.-stimulated reaction process are still not understood.
Acknowledgements-The authors gratefully thank the comments and a critical reading of the manuscript given by the referee and the support given by the National science Council of the Republic of China during the Period of this research. REFERENCES 1. Atkins P. W., Symons M. C. R. and Wardale H. W., J. Chem. Sot. 5215 (1964).
1021
2. Aiki K., J. Phys. Sot. Japan 29, 379 (1970). 3. Gade S., Johnson B., Halvorsen A. and Knispel R., J. Chem. Z’hys. 742671 (1981). 4. Machado da Silva J. C., Chaves A. S,, Gazzinelli R., Ribeiro G. M., Blinc R. and Zupancic L, Phys. Rev. B. u, 57 (1981). 5. Blinc R., Poberaj S., Schara M. and Stepisnik J., J. Phys. Ghem. Solti 27, 1391 (1966). 6. Ivanshin V. A., Kurkin I. N., Brunner W. and Volkel G., Phys. Slclr. Sol. fb) 151, KS3 (1989). I. Kawazoe H., Hosono H. and Nishii J., .I. Chem. Phys. 76, 3422 (1982). 8. Cole T., J. Ghem. Phys. 3S, 1169 (1961). 9. Hyde J. S. and Freeman E. S., J. Phys. Chem. 65,1636 (1961). 10. Fujimoto M. and Morton J. R., Can. J. Chem. 43,1012 (1965). 11. Yu J. T., Kou J. M., Huang S. J. and Co L. S., J. Phys. Chetn. Softi 47, 121 (1986). 12. Yu J. T., Solid Stare Commun. 71, 1159 (1989). 13. Shibata N., Abe R. and Suzuki I., J. Phys. Sot. Japan 41,2011 (1976). 14. Tsai S. F. and Yu J. T., J. Phys. Sot. Japm 37, 2540 (1988). 15. Yu J. T.. Chou S. Y. and Huang S. J., J. Phys. Chem. SOB& 49, 289 (1988). 16. Yu J. T. and Chou S. Y., Mol. Phys. 62, 971 (1987). 17. Marquardt C. L. and Patten F. W., Solid State commun. 2, 393 (1969). 18. Bailey C. E. and Cuesto-Barro R. C., J. Chem. Phys. 63, 4120 (1975). 19. Chou S. Y. and Yu J. T., J. Phys. Ghem. Solids 51,993 ww 20. Yu J. T., J. Ptrys. Chem. 96, 5746 (1992). 21. Yu J. T. and Chou S. Y., J. Phys. Ghem. So&.&51,1255 (199% 22. Mathias B. T. and Remeika J. P., Whys. Rev. 103,262 (1956). 23. Fujimoto M., Dressel L. A. and Yu T. J., J. Phys. Chem. Soli& 38, 97 (1976). 24. Rakvin B. and Dalal N. S., J. Ghem. Phys. 85, 6060 (1986). 25. Schlemper E. 0. and Hamilton W. C., J. Chem. Phys. er, 4489 (1966). 26. Do&se W., Acta Gryst~~og~. Bzs, 2298 (1969). 27. Mitsui T., Oka T., Shiroishi Y., Takashige M., Ito K. and Sawada S., J. Phys. Sot. Japan 39, 845 (1975). 28. Bailey C. E., J. Ghem. Phys. 74, 2740 (1981). 29. Gesi -K., J. Phys. Sot. Ja&n 42, 1785 (1977). 30. Gesi K.. J. Phvs. Sot. Jaw 43. 1949 (1977). 31. Osaka ‘I!., Ma&a Y. and’Gesi K., J. Phys. ioc. Jupan 46, 577 (1979). 32. Kishimoto T., Osaka T., Kamukae M. and Makita Y., .I. Phys. SW. Japm 56, 2070 (1987). A. V., Shuvatov L. A. and 33. Baranov A. I., T~~~he~o Shchaaina N. M., Sov. Phys.-Soiid State 29, 1448 (1988): 34. Wu C. J., Yu J. T., Tsai M. N. and Lou S. H., J. Phys.: Con&wed Matter 3, 3795 (1991). 35. Kawano T. and F&kata K., J: Phys. Sot. Japan 35, 1791 (1973). 36. Hukuda K., Hanafusa H. and Kawano T., J. Phvs. Sot. Jlqxm 36, 1043 (1974). 37. Dalal N. S.. Ada. h4asn. Reson. 10. 119 (1982). 38. Yu J. T., Wu C. J., L&I S. H. and Tsai M. N.,‘J. Solid State Ghem. 98, 159 (1992). 39. Yu J. T., J. Phys. Chem. SoMs53, 1131 (1992). 40. Maezawa K., Takeuchi H. and Ohi K., J. Phys. Sot. Japan 54, 3106 (1985).
1. Phys. C&m. Soli&Vol. 34, No. 9, pp. 10154021, 1993 Printedin Grm Britain.
EPR OF THE SeO;-, SeO,, SeO,- AND NH; RADICALS IN ULT~VIOLET-IR~DIATED CRYSTALS OF AMMONIUM COMPOUNDS JIANG-TSU YIJ, CHING-JIUN Wut and SSU-HAO Lov Institute of Physics, National Taiwan Normal University, Taipei 11718, Taiwan, Republic of China (Received 10 December 1992; accepted in revised form 18 May 1993) Aktanct-Oxyradicals of selenium, such as the SetI$-, SOi, SeO, and Se& are ~nvention~ly uroduced via Y - or X-irradiation. The same is also true of the NH? radical. We report for the first time RPR observations of these radicals produced by ultraviolet (u.v.)-irradiation kr some ammonium compounds. We have investigated u.v.-irradiated crystals of (NH&SO,, LiNH,SO,, (NH,),H(SeO,)r and NH,HrPO,. The NH: radical has been detected by EPR in u.v.-irradiated (NH,)rSO, and LiNH,SO, crystals. The NH: radical is p&u&d at the first stage of the U.V. photo-dissociation of the NH: ion in solids. The oxyradicals of selenium detected by EPR in some ammonium compounds oan be regarded as the product of an U.V. photo-redox reaction between the NH: ion and the W,:- ion. I@words: EPR, u.v.-i~dia~on,
radical ions.
1. R’J’I’RODUCTION Oxyradicals of selenium, such as SeO; , SeO; , Se& and W,:- , are conventionally produced via y- or
X-irradiation and studied by the method of electron pammagnetic resonance (EPR). Irradiation with U.V. light had been used in the past for producing organic or inorganic radicals. But to our knowledge the production of the oxyradicals of selenium by means of U.V. irradiation had not been reported previously. The same is also true of the NH: radical which is also conventionally produced via y- or X-irradiation. We have found that oxyradicals of selenium and the NH: radical can be produced via U.V. irradiation. This EPR investigation was carried out to study the U.V. photochemical reaction of the NH: ion and the w,ion in solids and to see if the mechanism of the radical production by means of u.v.-irradiation is any different from that by means of y- or X-irradiation. There are many EPR reports on the production of the NH: radical and the selenium oxyradicals by means of y- or X-irradiation. These results indicate that as many chemically inequivalent NH; radicals are usually produced as there are chemically inequivalent NH; ions in solids. But the same cannot be said for the selenium oxyradicals; in fact, the number of chemically inequivalent species sometimes exceeded those of the %I$- groups in solids. It was hoped that the number of radical species produced would be tPermanent address: Department of Mathematics and Science Education, National Hualien Teachers College. Hualien, Taiwan 97055, Republic of China.
smaller in the case of the u.v.-radiolysis of the selenate ion. The selenium oxyradicals have been used as EPR probes to study the statics and dynamics of structural phase transitions. Therefore, it is desirable not to create too many radical species at the same time to give rise to a congested EPR spectrum. Finally, hydrogen passivation of defects on semiconductor surfaces is a current topic of interest. A study on the U.V. photo-dissociation of the NH: ion is relevant to this application. 2. EPR OF THE OXYRADICALS OF SELENIUM The radical ions SeO, , SeO; and SeO; produced by y-irradiation in potassium and sodium &mates and selenites were first reported by Atkins et al. [l]. The four radical ions SeO; , SeO; , SeOi and SeO:have been detected by EPR in K,SeO, crystals yirradiated at room temperature [2]. The SeO; radicals have been detected by EPR in X- or y-irradiated alkali t~hydrogen (or deute~um) selenites p-61. These four radicals exhibit distinctive anisotropies in the g-factor and the “Se (I = l/2) hypcrfine splitting, so that an unambiguous identification can usually be made. The EPR spectra of these radicals can be analyzed by an S = l/2 effective spin-Hamiltonian of the form,
.?L?= @SgH + SAI,
(1)
where the first term is the electronic Zeeman term and the second the Se hyperfine term. The Se hyperfme splitting observed for the oxyradicals of selenium is
1015
JIANGTSUYu et al.
1016
usually fairly large, such that the perturbation method is inadequate for analysis. Kawazoe et al. [7j have given a method for the evaluation of the eigenvalues of the Hamiltonian in eqn (1). We have adopted their method for the enumeration of the eigenvalues and resonance fields. We assume that the g-matrix and the hyperIme A -matrix share a common set of principal axes. We expanded the secular equation into an algebraic equation of the fourth order and wrote a Pascal program for the computation of the eigenvalues and resonance fields. In the most general case, the spin-Hamiltonian in eqn (1) includes nine independent parameters; the three principal values of the g-factor, the three principal values of the hype&me splitting,, and the three Euler angles required to bring the principal axes into complete coincidence with the chosen reference axes. This program allows simultaneous scannings of the nine parameters through chosen ranges, and at the same time compares the calculated resonance field values with some pre-chosen experimental resonance field values. We used a PC computer equipped with a 386 microprocessor and a 387 mathematical coprocessor for computation. For the most general case, in which all the nine spin-Hamiltonian parameters are unknown, it may take a couple of days of computer-running time to get a best fit. 3. EPR OF THE NH2 RADICAL
Since the tirst EPR observation of the NH: radical in y- or X-irradiated NH,ClO, crystals [&lo], there are quite a few reports of EPR observations of this radical in y- or X-irradiated solids in which the ammonium ion is either a constituent or a dopant. In solids, the NH: radical could be a very stable species as in LiKSO, crystals (11, 121,or a short-lived species as in (NH& SO., crystals. This radical is stabilized by the %#impurity in (NH&SO, [13], and by the C@,- impurity in LiNH4S04 and LiCsSO., crystals [14-161. Only a portion of the irradiated ammonium compounds exhibited the EPR spectrum of the NH: radical; others exhibited the N2H$ radical, as in NHICl [17]; and still others exhibited no radical species derived from the ammonium ion. The EPR spectrum of the NH: radical can be analyzed by the,following spin l/2 spin-Hamiltonian,
i=l
where the first term is the electronic Zeeman term, the second the nitrogen hype&e term, and the third the hydrogen hyperfme term. Rapid molecular reorientational motion of the radical at room temperature renders the three hydrogens magnetically equivalent,
resulting in a characteristic 1Zline hyperhne pattern. The nitrogen and the hydrogen hyperfine terms can be regarded as perturbations to the Zeeman term. Resonance fields which include terms up to the second-order perturbation terms have been used to fit the EPR spectrum of this radical [ 181. However, first-order perturbation theory is usually quite adequate for the analysis of its EPR spectrum. 4. EXPERIMENTAL
Sample crystal are grown by slow evaporation from aqueous solutions. (NH&SO4 and LiNH,SO, crystals doped with SeO]- were grown by adding about 5% of (NH,)2Se04 into the growth solutions. Pseudo-hexagonal LiNH,SO, crystals are usually twinned. We prepared, with the aid of a polarizing microscope, single-domain crystals for use in this experiment. Single crystals of (NH,),H(SeO,), were grown from aqueous solutions made up of stoichiometric proportions of (NH& Sea, and H, SeO,. Ultraviolet-irradiation was carried out at room temperature by using a Bruker 200 W U.V.irradiation system. The U.V. source is a mercury arc lamp. The crystals were irradiated in front of the lamp without the use of an optical filter. The intensity of the U.V. light given off by this source is adequate for this experiment, although a more powerful light source may be desirable. The X-band (cavity resonance frequency at about 9.42 GHz) EPR spectrometer has been described previously [19]. The radicals produced by u.v.- or y-irradiation were investigated by EPR at room temperature. 5. EXPERIMENTAL RESULTS (A)
N-0,
SQ
We experimented first with u.v.-irradiation of (NH& SO., : SeO:- crystals. (NH,), SO4 was selected as a host because our previous experiments have shown that the ammonium ion in this compound is an efficient reducing agent for the U.V. photoreduction of MnO; into Mn*+ [20], and for the thermal reduction of CrO]- into Cr’+ [21]. (NH,),SO, becomes ferroekctric at temperatures below about - 50°C [22]. The NH: radical produced via y- or X-irradiation has been used as an EPR probe for the investigation of the ferroelectric transition [13, 18,23,24]. Addition of (NH1)2Se04 into (NH4)*SOd increases (or stabilizes) the yield of the NH: radical [13]. Besides NH:, the SeO; radical was also observed [13]. The space group of the room temperature paraelectric phase of (NH,),SO., is the orthorhombic Puma [25]. There are two crystallographically inequivalent
EPR of the SO:-,
SeO; , !3eOFand NH: radicals
‘ina
f =0.42Qllz
3zou
336oQ
3aou
Fii. 1. The EPR spectra of the two NH: radicals and the SeO, radical produced by 40 h of u.v.-irradiation in (NH&SO,: W,-, for the &ld parallel to the a-axis. A stick diagram showing the decomposition of the hyperCne structures is also shown. The line marked with the letter X could be due to another SeO, radical.
NH: ions, but the SOi- ions are all crystallographitally equivalent. The two chemically inequivalent nitrogen atoms and the sulfur atom are located in the (010) mirror plane. The hydrogens of the NH: ion are hydrogen-bonded to the sulfate oxygens. EPR of y- or X-irradiated crystals revealed the presence of two chemically inequivalent NH: radicals, which is consistent with the fact that there are two chemically inequivalent NW ions.
1017
Ultraviolet-irradiated (NH&SO4 crystals is EPR silent. Ultraviolet-irradiated (NH4)2S04:SeO~- CQStals exhibited several radical species. These (NH&SO,: Sea’,- crystals were u.v.-irradiated at room temperature for 40-50 h. EPR measurements were performed at every 5”, for rotations about the three orthorhombic axes. EPR revealed the presence of two chemically inequivalent NH,+ species in u.v.-irradiated crystals (see Fig. 1). The yields of the NH: radicals were relatively large, such that the rotation patterns exhibited by these radicals can be analyzed (see Fig. 2). The determined g-factor, nitroFn and hydrogen splittings of these two radicals (see Table 1) are in agreement with those reported previously [13, 181. y-irradiated (NH,),SO,:SeO:crystals exhibited two chemically inequivalent SeO; species [13]. According to Shibata et al. [13], one of these two SeO; species is produced with one of the Se-O bonds of SeO:- is broken by the y-ray photon, whereas the other species is produced when another Se-O bond is broken. Both of these m; species exhibit axial symmetry. Besides the two NH,+ radicals, two of the radical species produced in u.v.-irradiated crystals can be analyzed. One exhibits gll = 2.0030 + 0.0002 and g, =2.0144 +O.OOOl, with the u-axis of the Pnma lattice as the symmetry axis. The g-values of this radical species fall within the ranges reported for the SeO; radical (see Table 2). The yield of this radical species is not large enough to allow observation of the Se hype&e structure. This species may be identified as the Se-O,(I) species reported
a
b
Angle
C
(deg)
Fig. 2. Observed and fined rotation patterns of NH: (I) (dashed curves) and NH: (II) (solid curves) produced by u.v.-irradiation in (NH,),SO,: w,-
crystals.
JIANGTSUYu et al.
1018
Tabk 1. The principal values of the g-factor, t&o n (A (N)) and hydrogen (A (H)) splittings (in unit of Gauss) of the NH?> radical in (NH.), 8 0. and LiNH.SeG. at room temnerature . T,.
Species NW (I)
Host (NH& SQ
NH; (II) NH:
LiNHl SO,
-
g 2.0065 2.0038 2.0033 2.0065 2.0038 2.0027 2.0026
.
.
A(N)
2.0069 2.6038 2.0038 2.0065 2.0038 2.0036 2.0027
2.0066 2.0038 2.0029 2.0065 2.0038 2.0031 2.0037
18.8 19.1 19.9 19.7 20.4 19.2 35.7
[13], although the g, value we have observed is slightly different from that previously reported [13]. The other species gives g, = 2.0066, g, = 2.0218 and g, = 2.0210, with the three orthorhombic axes as the principal axes. This species could be another SeO; species, but its g-values differ from those reported for the SeO; (II) species [13]. previously
(B) LiNH4 SO4 The space group of the pseudo-hexagonal LiNH.,SO, at room temperature is the orthorhombic P2, cn [26]. There is only one kind of NH*+ ions and one kind of SO:- ions in the room-temperature phase of LiNH,SO,. Pseudo-hexagonal LiNH, SO, is ferroelectric between 186S”C and 10°C [27j. The NH,+ radical cannot be detected by EPR in y-irradiated LiNH,SO, crystals, but can be detected in LiNH,SO,:CK$crystals [14]. The observation of a single NH: species [14] in the paraelectric phase is consistent with the fact that all the ammonium ions are chemically equivalent [26]. Ultraviolet-irradiation of LiNH,SO,:CrOjcrystals resulted in the EPR observation of the NH: radical. This corroborates the observation made in (NH&SO,: SeO:- . Ultraviolet-irradiation of LiNH,SO,: SeO:- crystals resulted in the EPR observation of the SeO; radical but not the NH: radical. A LiNH,SO,:CrO:crystal which was irra-
11.3 11.6 11.7 16.3 14.4 13.4 13.9
Irradiation
A (H) 26.9 27.0 26.2 21.3 22.6 21.9 5.6
24.6 24.7 24.3 24.7 24.6 24.5 26.6
23.7 23.6 23.3 23.1 24.6 24.5 23.9
25.2 25.3 25.3 25.1 24.6 24.8 24.2
References
X Y U.V.
This
X Y U.V. X
t::; work
WI I131 This work 1141
diated with U.V. for 15 h exhibited the EPR spectrum of the NH: radical (See Fig. 3). The EPR signal intensity was larger for a crystal irradiated for 23 h. The absolute EPR signal intensity decreased after this same crystal was further u.v.-irradiated for a total of 49 h. This indicates a possible bleaching effect of the NH: radical which the U.V.light itself had produced. We have previously observed (to be published) this effect of U.V. bleaching of the NH: radical produced by y-irradiation in LiNH,SO,: Cacrystals. Irradiation with U.V. light for 6-7 days completely bleached the NH: radical produced by about 5 Mrad of y-irradiation. This same bleaching effect can also be observed in y-irradiated (NH,),SO,: CrO:crystals. The unstable NH: radical in (NH,)rSO, transforms into another radical species identified by Bailey [28] as the NS@species. Ultravioletirradiation, or thermal annealing [12], hastens this transformation process. Therefore, it can be concluded that the NH: ion was first photo-dissociated by U.V.light into the NH: radical, which was further photo-dissociated into NK$ in (NH&SO.,. The final product of the U.V. photo-dissociation of NH,+ in LiNH,SO, crystals cannot be detected by EPR. The yields of the NH: radical in u.v.-irradiated LiNH,SO,: Cacrystals are much smaller than those in y-irradiated crystals, such that it is not possible to completely determine the principal values
Table 2. The principal values of the g-factor and the “Se hyperhne splitting (in unit of MHz) of the oxyradicals of selenium produced in some ammonium compounds via u.v.-irradiation. Species produced via Y- or X-irradiation are included for comparison !bXiCS
Host
CsH,WAX LiNH, SO,
(!$ (III): Se% zI%o, (NH,)%‘, ~~(SeG,” Se@N&H,&& NH;H;FG;
(I): (II):
2.0261 2.0390 2.03 I 2.0295 2.0285 2.0286 2.0145 2.0150 2.0124 2.0144 2.0140 2.0087 2.0084 2.0095
Irradiation
A
g 1.9983 1.9984 1.995 1.9983 1.9880 2.0022 2.0156 2.0153 2.0140 2.0144 2.0140 2.0087 2.0084 2.0095
2.0063 2.0052 2.607 2.0079 1.9972 2.0068 2.003 1 2.6033 2.0036 2.0030 2.0029 2.0055 2.0041 2.0065
839.7 817 734
203.4 270 242
189.1 300 280
ii ”
U.V.
1211 1139 1560
1210 1127 1531
1728 1616 2092
1204 3178 2891 2911
1204 3178 2891 2911
1698 3389 3323 3325
Reference
[41 This work
Y
PI
”
I131
U.V.
U.V.
X x U.V.
This work This work [37l 1361 This work
EPR,of the SeOi- , SW?, ScO; and NH: radicals
1019
but exhibited EPR spectra of several radical species. The rotation patterns of three of these spin S = l/2 species can be completely analyzed (see Fig. 4). These EPR spectra did not show up in pure LiNH,SO, crystals irradiated with U.V. light. We designate these radical species as @es (I)--@), The principal values and anisotropy of the g-factor exhibited by these three radical species indicate that them are SeO, species, although the EPR signal intensity was not strong enough to exhibit the Se hyperflne structure.
Fig. 3. The EPR spectrum of the NH: radical in LiNH,SO,:C#,er@als produced by 49 h of u.v.-irradiation, for the field parallel to the c-axis. A stick diagram showing the decomposition of the hype&e structures of this radical is also shown. The two lines marked hy the letter CareduetoaC~-radicalw~hadbecnobsemdin y-irradiated crystals.
and principal axes for the g-factor, nitrogen and hydrogen hyperfme splittings. But the EPR spectrum exhibited by this radical for the field parallel to one of the three orthorhombic axes of the host lattice is identical to that exhibited by the NH: radical produced by y-irradiation. Therefore, the NH,+ radicals produced by u.v.- and ~-i~d~tion can be regarded as identical. Ultraviolet-irradiated LiNH,SO, : SeC#- crystals did not exhibit the EPR spectrum of the NHJ’ radical
(NH,)~H(SeO,), exhibits a sequence of phase transitions [29-331. It undergoes a ferroelastic transition at about 29°C [32], with the simultaneous appearance of twin domains. Above this transition, the crystal structure is trigonal with the R3m space group, while below this transition it is monclinic with the A2/a space group [32]. The twin domain strucSure in the monoclinic AZ/a phase has been studied by Kisbimoto et al. 1321and by Wu et al. 1341. y-irradiated (NH,),H(SeO& crystals exhibited the EPR spectrum of a SeO; radical but not the NH: radical [34]. This SeO; radical exhibits axial symmetry in the trigonal phase of this compound, with the symmetry axis parallel to the trigonal axis of the lattice. An identical SeO; species was detected by EPR in u.v.-irradiated crystals (see Fig. 5). The EPR spectrum is consisted of the main Z = 0 and Z = l/2 (“Se) transitions in the central region of the spectrum (see Fig. 5) and the two Se hype&e lines (see Fig. 6).
Fig. 4. Ohserved and Etted rotation patterns of the SeO;-(I)-&O,(III) radicah in u.v.-irradiated LiNH,SO,:wcrystals, for rotation of the Eeld about the three orthorhomhic axes.
IOU)
JIANGTSU
Yu et al. 4000
i
”
’
”
”
”
”
’ “”
’ i
I
,
I 33200
I
J
33600
Fig. 5. The central portion of the EPR spectrum exhibited by the !3eO, radical in u.v.-irradiated (64 h) (NH,),H(SeO,), crystals. The main transition and the two “‘Se hyperfine transitions am each flanked by a pair of satellite lines (marked by the letter H) which could be interpreted as due to hyperline interaction with a nearby proton.
The structure of NH,H,PO,
at room temperature similar to that of KH,PO,. The space group is V,,12 (Z42d). The ammonium ions and the phosphorus atoms occupy special positions, and the oxygens occupy general positions. All the ammonium ions are chemically equivalent and so are the phosphate groups. The SeO:- radical was first reported by Aiki in y-irradiated K2Se0, crystals [2]. This radical was subsequently reported by Kawano in y-irradiated NH,H,PO, (ADP): SeC$- crystals [35], and by Hukuda in y-irradiated KH,PO, (KDP) : we crytals [36]. The SeO:- radical has been used extensively as an EPR probe in the studies of the ferroelectric transitions in KDP-type crystals and in the antiferroelectric transitions in ADP-type crystals. A review on the EPR of SeO:- in KDP- and ADP-type crystals can be found in an article by Dalal [371. NH,H,PO,: SeOi- crystals irradiated with U.V. for about 20 h exhibited the EPR spectrum of a SeO:is tetragonal,
3700
; ”
’ ”
”
”
”
’ ”
’ ”
a,, , , , , , , , , , , , , , , a C a Fig. 7. Observed and fitted cu-plane rotation patterns of the mradical in u.v.-irradiated NH, H,PO,: SO- crystals. 2600
330OG
;
Fig. 6. Observed and fitted rotation patterns of the !3eO, radical in u.v.-irradiated (NH,),H(!SeO,), crystals, for rotation of the field about the a-axis.
radical. This SOradical exhibits axial symmetry, with the tetragonal c-axis as the symmetry axis. The u.v.-yield of this radii1 is fairly large such that the two “Se hype&e lines can be detected (see Fig. 7). The evaluated spin-Hamiltonian parameters (see Table 2) agree fairly well with those reported for the same radical produced via X-irradiation [35]. 6. DISCUSSION AND CONCLUSIONS (A) NH:
radical
The observation of the NH: radical in u.v.-irradiated (NH4)2S0.,: wcrystals is an unexpected result. This represents the first observation of the production of this radical in solids by means of u.v.-irradiation. In hindsight, it may be rationalized by noting that the addition of (NH,),SeO, into (NH4)2S0., stabilizes the NH: radical [13]. The production of this radical via y- and X-irradiation is assumed to be the result of the breaking of an N-H bond by the energetic photons. A similar process has been assumed by us to explain the thermal reduction CrO:- + e- + CrO- observed in (NH,),SnCl, and in NH:-doped K,SnCl, crystals [38]. The thermal reduction of MO@- into MoS+ observed in these two crystals can be similarly explained [39]. The drawback of this hypothesis was that neither the NH: radical nor the N2 Hz radical had ever been detected by EPR in thermally-treated or u.v.-irradiated crystals of the ammonium compounds. The observation of this radical in selenate-doped ammonium sulfate and chromate-doped lithium ammonium sulfate vindicates such a hypothesis. It can be concluded that the NH: radical is produced at the first step of the U.V. photodissociation of the NH: ion and that this radical is further photo-dissociated with the release of free electrons and protons (or free hydrogens), which become available for further chemical reactions. Because of this bleaching effect, the yield of stable
EPR of the Semi- , SCOT,!kO; and NH: radicals
NH: radicals by means of u.v.-irradiation is low, when compared to X- or y-irradiation.
(B) thyradicals
of selenium
y-irradiation of KrSeO, crystals has produced all the known oxyradicals of selenium [2]. However, we found that none of these radicals can be detected by EPR in u.v.-irradiated K,SeO, crystals, even after long durations (days) of irradiation. An oxyradical of selenium, identified as the SeO; radical [40], has been detected by EPR in y-irradiated LiKSO,:SeO]crystals. However, none of the oxyradicals of selenium can be detected by EPR in u.v.-irradiated crystals. The SeO:- radical and other oxyradicals of selenium have been detected by EPR in X-irradiated KDP:SetZ$- crystals [36], but none of these radicals can be detected in u.v.-irradiated crystals. Therefore, it appears that the radiolysis of the seo2,- ion via yor X-irradiation is a direct process, for it does not need a catalyzing agent. On the other hand, the production of the selenium oxyradicals via u.v.irradiation does need the catalyzing action of a reducing agent such as the ammonium ion. We do not know if u.v.-irradiation will result in a photodissociation of the SeO]- ion. But if the seof- ion is reducible via u.v.-i~diation, then the reduced product must be a non-paramagnetic species. The selenium oxyradical species produced via u.v.irradiation, in (NH&SO,: SeO:- , NH,H2P04: wand in (NH,), H(SeO& crystals, are identical to those produced via y- or X-irradiation. The difference is that the yields of the radicals are smaller in the case of u.v.-irradiation but which could possibly be increased by employing a stronger U.V. light source. However, this apparent similarity observed in the end-products of the radiolysis of the selenate ion in ammonium compounds may be accidental. The mechanism of radical production can be regarded as an u.v.-photochemical oxidation-reduction reaction between the NH,+ and Se@- ions in solids; but the details of this u.v.-stimulated reaction process are still not understood.
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