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Reactivity of plasma-treated SiO2 against O2: An EPR study
Volume65. number 2
REACTIVITY
CHEMICAL
OF PLASMA-TREATED
PHYSICS LEZTTERS
I April
I979
SiOz AGAINST 02: AN EPR STUDY
H.-J_ TILLER and G_ RUDAKOFF Sekrion
Chemie.
Fries-cam-ScJliller-lir~t
Jetza. 69-Jetza.
GDR
Recci%ed5 December 1978
During p&ma tratment of SiOl (deh)dm:ed at 700°C in high *actturn) electrons become trapped at the surface and in the bulk. The captured electrons from the ~4 known Ei centre, an F-like electron centre with the unpaired electron &nIy in an s-t) oe orbit& and COT which comes irom carbon impurities. Adsorption of 02 (partial pressure 5 X low3 torr
I_ Introduction In many oxides (e-s- MgO, SiO,, ZnO, Ti02 and SnOz) the eIectrons released during thermal activation or during X- or W-irradiation are trapped in the bulk or near the surface- These electrons can react with adsorbed electron acceptors which may be able to initiate reactions with other adsorbed species_ The adsorbate of main interest is oxygen because of its importxxe in heterogneous oxidation of catalysts and beCXIS~Z it is an important component of the residual gas in high-vacuum experiments. A large number of papers deaI with the reaction of thermaity activated MgO with Oz which leads to OzLNotali of the species found can be identified in an unambiguous way- A critical review can be found in ref. IllO- can be produced by the reacticn of N20 with acthated blg0 121. O- reacts with 0, forming 05 [3] _ The reactivity of O- against C2H, [415] and several &oh& [Z] has been investigated and proves to be rather high. A fiequentIy investigated adsorbant is silicagel as a special form of SiOz, xvhere the electron transfer is initiated by y-irradiation in the presence of many adsorbates j6-91. O- and 0% can aiso be produced by covering (doping) SiO, with MOO, [IO], V205 [I I, 121 or Ag [Is] and subsequent adsorption of 0,. Electron transfer between oxide and adsorbate also occurs during 312
mechanical treatment of SiOZ in a CO2 atmosphere, where the CO_ is formed [14,ISJ
Volume
62, number
2
CHEMICAL
PHYSICS
2. Experimental Highly disperse SiO, (Aerosil300, Degussa) has been used as substrate in the form of repowdered pellets_ First it was heated in open air at about 700°C for 3 h to remove hydrocarbon impurities_ The sample was then filled into special sample tubn [21] and heated under high-vacuum conditions (3 h, 700°C, p d 5 X 10M6 torr). After this treatment the sample did not show an EPR signal. After filling the sample tube with pure argon up to pAr = 1 torr an rf discharge was maintained for about 30 min. To ensure a homogeneous plasma-surface interaction the sample tube was kept rotating all the time. Admixture of argon to the sampie has been done by carefully avoiding any traces of other gases. This care proved to be very important for a reproducible detection of the electron centres described below_ EPR spectra have been recorded in the X-band with an ERS 210 spectrometer of ZwG Berlin_
LETTERS
1
April
1979
The spectrum shown can be separated into three different signals with different saturation behaviour. The relative intensities of the three signals change as the temperature increases from 77 to 300 K_ The three separated signals are shown in fig. 2. For the following discussion the three centres are called E; , E, and Eg_ The following interpretation is the probable one. ,$ ce~zr~ [22): Following a model of Yip and Fowler [23] the centre consists of an electron trapped in an oxygen vacancy in the SiO, network- According to the model, the unpaired electron is localized in an sp3-hybrid orbital mainly at one of the two silicon atoms. This asymmetric model explains the large hfs (423 G) which shouId arise from 3gSi_ This interpretation is somewhat doubtful, because this hfs was recently expltined by Shendrik and Yudin (241 in a \ery convincing manner as splitting due to protons in silica. In the case of Aerosol the E; centres generated by plasma treatment are localized at the surface or nearby and are strongIy influenced by O-, adsorption (dipoledipole interaction) [25] _A line-shape analysis of the E; signal exhibits exchange narrowing of the line.
3. Results and discussion
Fig. 1 shows the EPR signal of the defects produced by the plasma treatment (sample temperatures 300 and ‘77 K). The defects are very sensitive to traces of 07_ To avoid irreproducible changes in signal intensity
during pumping
off the discharge gas. 0, was added
to the plasma (residuai) gas.
Fig. 1. EPR (Aerosil300)
spectra of Ar-plasma-tre;lted high-disperseSiOz at 300 K and 77 I;.
Fig- 2. Analysis of the EPR spectrum shown in fig. I_
313
The
reiaxation times c’dnbe estimatedto be
They are considerably smaller than those of E; centres in cryst&i& quartz {26j probablydue to a formation of defect clusters as describedin the case of alkaIinehatides[~??1_Defect cfusterformationcan be expI&nedby an attack of the piasma on ck%ttiRspots of the surface (Ieading to a sort "‘hole burning”). The temperaturedependence of Tt and Tz can -Jso be ercplainedby the assumption defect clusters; rl an& T2 increase with temperature instead of the expected decrease according to T, a: T-a (n > 0). This may be caused by ;r temperature-dependentchange in the average distance of the defectsin the dusters which reducesthe exchangeprobabiJity and increases the relaxationtime.5at low temperatures. EA C~ZZZZK The isotropic signa cw be saturated very easily 3%low microwavfz power(i-5 mW). l&gvalueg s 2.0019 is very near that of the free electron. Ihe signal resembJesthat of F-ten tres [281- GE-~hdp@ and g-v&e can be explainedby z~um&g rhe electron to be nwidy in an s-type orbital- me EA centre is probabJy caused by eJectrunsofreJativeIy high nob& ity in the SO2 network.
of
of
Es cenire: This signal with gr. = ZOOW * 0.0005 andg, = f-9979 2 0.0005 can be interpreted as CO,by comparison with the Iiterature (61. Together with the Co,-, another signal has been detected which can rf s,ZY j_ Both sign& come be interpreted as CUT L from impurities in the Aerosol and from hydrocarbon impurities in the high-vacuumapparatusAfter admiXture of 0, at 77 K (partial pressure0.1 torr) the three signalschange as shown in fig- 3a. EA , EB and the part of E; centres Jocaked at the surface aJJvanish_At the same time the 0% signal appears &t = Z.O39;& = 2.007;g3 = 2_002)_ At high gain some further s&nais of unknowll origin appear. One of them is a species with g-L= 2.025 andg, = 2.0015 already found on MgO by Ben Taarit et al. [2Of (fig3b). The detectability of 0~ depends strongly on the Oz-coverage (fig- 4). The signal first appears at po2 S 5x w-3 torr and vsnhher in the notie at ~0~ 2= OS ta due ta dipoiar broadening. An exact determination af the dipobr broadening with increasing Oz caverage is &f&uJt bemnse the overlappingf?i signalbecomes more intense at the same time ES?], By redut;ingthe 0, partial pressure the initial spectrum does not reap pear. Therefore a very tight binding of the Cl2 in the
Volume 62, number 2
CHEMICAL PHYSICS LE.-lXXRS
1
il
9.
5593
Fig. 4. Change of 0; sure.
spectrum at 77 I: with 02 partial pres-
region of 0_
or a reactive process has to be assumed_ After warming up to 300 K and retooling the sample to 77 K another adsorbed form of 05 is foundIts Sl = 3.069 is significantly higher than that of 0% immediately after adsorption at 77 K (fig_ 5). ?&ii form of 0% can also be obtained by 0, adsorption at 300 K and subsequent cooling to 77 K. Zt is stable
during repeated cycling of the temperature between 300 and 77 I(, even after intetiediate adsorption and pumping off of oxygen (300 torr) at 300 K. Obviously the stability of OF depends on the 02-
coverage, which is higher after Qz adsorption at 77 K with poz = 0.1 torr than at 300 K with poz = 300 torr.
The results of the 0, adsorption on plasma-activated SO, are nearly identical to those of Ben Taarit et al. [ZO), obtained after 0, adsorption on thermally activated MgO. In view of the carbon impurities on
1 April 1979
Fig_ 5. Influence of adsorption temperature on the 0; spectrum at 77 K Aerosil their interpretation
of the Or signal as a COT
complex (adsorption of 0, on preadsorbed CO, [20]) cannot be ruled out. The structure of this CO; complex, however, cannot depend critically on substrate structure. because adsorption on MgO and SiO, leads to identical spectra. Coadsoxption of O2 and H,O does not chatqe the spectra significantly (fig_ 3b). So
the independence of the CO: spectra on substrate structure may be explained tentatively by the assamption of an interaction with hydroxyl groups of th=: surface or lvith preadsorhed $0. Comparing the results obtained here on plasmaactivated SiO, and those obtained on y-irradiated silicage1 [3 11 with respect to the creation of surface defects and reactivity, the anaIogy between the actRation
methods is obvious. 315
VoIume 62. number 2
CHEh¶ICAL PHYSICS LETTERS
References [11 J-H- Lunsford. Gtzdysis Rev_ 8 (1973) 13~ 121 A-J- Tenth. T. Lzrwson and J.FJ. Kihblewhite. J. Chem_ Sot- i%rzi&y Trans. I 68 (1972) 1169s [3] A-J- Tenth. T- Lawson and J.FJ. Krbblewbite, J_ C&m_ Sot- Fy;tday Trans- I 68 (1972) 1181_ [4] C- Naccache. Chum_ Phys- Letters I1 (1971) 323. 151 Y. Ben Tatit, M.C.R- Symons and A-I. Tenth. J. Chem. Sot- Faraday T-m I 73 (1977) 1149. [6] 0. EdIund. J. Sohnu and K Sogabe. BuII. Chem. Sot_ Jq=n 47 (1974) 3163[7] T- Shiga. A- Lund and P--O_ Knell. hrtern- J_ E&&t_ Phys.Chem_ 3 (1972) 131_ f8l G. >i. S. Ohnhhi and 1. Nitta, BuII. Chem. SocJapan 45 (1972) I934191 H- Yoshioka. H- .\htsumoto, S- Uno and F_ Higgbide. J_ Polymer Sci_ 14 (1976) I331 _ [lOI A-K. Kolosov. V-A. Shvets and U-B. Kazmsky. Chem. Phys. Letters 34 (1975) 360. II I J V-A. Shrew. U-M. Vorot)ntsevand U-B. Kazansky. Kinet. Catal. USSR 10 (1969) 356. [I 21 B-N- Shetimov and Bl. Che. J_ Catalysis 51 (1978) 143_ [ I31 R-B. CIxkson and S. B1cClelJ.a. J. Phya Chem_ 82 (1978) 294_ [ 141 G. Hochstrasser and J-F- Antopini. Surface Sci. 32 (1972) 644_ [ISI I. JZbertand H--P. Henning. Z_ Physik. Chem. (Leipzig) 255 (1974) 812_
316
1 April 1979
[16] S_ Kryzanowski, BuU_ Acad- Poion- Sci- Ser- Sci. Chim. 24 (I 976) 165. [ 171 N--B_ Wang and J.H. Lunsford. J- Chem. Phys- 55 (1971) 3007_ [ISI J-C. Vedrine and C. Wccache. J. Phys- Chem. 77 (1973) 1606. [I91 Y- Ben Tsarit and J_H_ Lunsfbrd. J. Phys- Chem. 77 (1973) 780[20] Y. Ben Taarit. J-C. Vedrine, C. Naccache. Ph. de hlontgoolfier and P. hferiaudeau, J. Chem- Phys. 67 (1977) 2880_ [211 H--J_ Tier, Ch. ApfeI and G. Rudakoff. Wii. 2. FSU 27 (1978) 621[23] R-A. Weeks, J_ Appl- Phys- 27 (1956) 1376. [23] K-L. Yip and W.B. FowIer, Phys. Rev. B II (1975) 2327. [24j A-V_ Shendrik and D-M. Yudin, Phys. Stat. Sol. 85b (I9781 343. f25] H.-J_ Tier and C_ RucIakoff_ KristaU Technik. to be published. [26J J.G. C&tie. D.W. Feldnxmn and P.G. KIemens, Phys. Rev. 130 (1963) 577. 11-71hl- Schxxoerer rend H-C_ Wolf. 2_ PhysiIi 175 (1963) 457. 1281 R-L. N&on and AJ_ Tenth. J. Chem. Phys. 40 (1964) 2736. [ZSI P- hlerhudeau, Y. Ben Tarit. J-C. Vedrine and C- Naccache. J. Chem. Sot. Faraday Trans. II 73 (1977) 76. (301 \V-Wintruff, R. Herrling and H--J. liller, Chem. Phys. Letters 38 (1976) 524_ [31I P-K;. Wong and J-E. Wiird. J. Phjs. Chem. 73 (1969) 2226.
REACTIVITY
CHEMICAL
OF PLASMA-TREATED
PHYSICS LEZTTERS
I April
I979
SiOz AGAINST 02: AN EPR STUDY
H.-J_ TILLER and G_ RUDAKOFF Sekrion
Chemie.
Fries-cam-ScJliller-lir~t
Jetza. 69-Jetza.
GDR
Recci%ed5 December 1978
During p&ma tratment of SiOl (deh)dm:ed at 700°C in high *actturn) electrons become trapped at the surface and in the bulk. The captured electrons from the ~4 known Ei centre, an F-like electron centre with the unpaired electron &nIy in an s-t) oe orbit& and COT which comes irom carbon impurities. Adsorption of 02 (partial pressure 5 X low3 torr
I_ Introduction In many oxides (e-s- MgO, SiO,, ZnO, Ti02 and SnOz) the eIectrons released during thermal activation or during X- or W-irradiation are trapped in the bulk or near the surface- These electrons can react with adsorbed electron acceptors which may be able to initiate reactions with other adsorbed species_ The adsorbate of main interest is oxygen because of its importxxe in heterogneous oxidation of catalysts and beCXIS~Z it is an important component of the residual gas in high-vacuum experiments. A large number of papers deaI with the reaction of thermaity activated MgO with Oz which leads to OzLNotali of the species found can be identified in an unambiguous way- A critical review can be found in ref. IllO- can be produced by the reacticn of N20 with acthated blg0 121. O- reacts with 0, forming 05 [3] _ The reactivity of O- against C2H, [415] and several &oh& [Z] has been investigated and proves to be rather high. A fiequentIy investigated adsorbant is silicagel as a special form of SiOz, xvhere the electron transfer is initiated by y-irradiation in the presence of many adsorbates j6-91. O- and 0% can aiso be produced by covering (doping) SiO, with MOO, [IO], V205 [I I, 121 or Ag [Is] and subsequent adsorption of 0,. Electron transfer between oxide and adsorbate also occurs during 312
mechanical treatment of SiOZ in a CO2 atmosphere, where the CO_ is formed [14,ISJ
Volume
62, number
2
CHEMICAL
PHYSICS
2. Experimental Highly disperse SiO, (Aerosil300, Degussa) has been used as substrate in the form of repowdered pellets_ First it was heated in open air at about 700°C for 3 h to remove hydrocarbon impurities_ The sample was then filled into special sample tubn [21] and heated under high-vacuum conditions (3 h, 700°C, p d 5 X 10M6 torr). After this treatment the sample did not show an EPR signal. After filling the sample tube with pure argon up to pAr = 1 torr an rf discharge was maintained for about 30 min. To ensure a homogeneous plasma-surface interaction the sample tube was kept rotating all the time. Admixture of argon to the sampie has been done by carefully avoiding any traces of other gases. This care proved to be very important for a reproducible detection of the electron centres described below_ EPR spectra have been recorded in the X-band with an ERS 210 spectrometer of ZwG Berlin_
LETTERS
1
April
1979
The spectrum shown can be separated into three different signals with different saturation behaviour. The relative intensities of the three signals change as the temperature increases from 77 to 300 K_ The three separated signals are shown in fig. 2. For the following discussion the three centres are called E; , E, and Eg_ The following interpretation is the probable one. ,$ ce~zr~ [22): Following a model of Yip and Fowler [23] the centre consists of an electron trapped in an oxygen vacancy in the SiO, network- According to the model, the unpaired electron is localized in an sp3-hybrid orbital mainly at one of the two silicon atoms. This asymmetric model explains the large hfs (423 G) which shouId arise from 3gSi_ This interpretation is somewhat doubtful, because this hfs was recently expltined by Shendrik and Yudin (241 in a \ery convincing manner as splitting due to protons in silica. In the case of Aerosol the E; centres generated by plasma treatment are localized at the surface or nearby and are strongIy influenced by O-, adsorption (dipoledipole interaction) [25] _A line-shape analysis of the E; signal exhibits exchange narrowing of the line.
3. Results and discussion
Fig. 1 shows the EPR signal of the defects produced by the plasma treatment (sample temperatures 300 and ‘77 K). The defects are very sensitive to traces of 07_ To avoid irreproducible changes in signal intensity
during pumping
off the discharge gas. 0, was added
to the plasma (residuai) gas.
Fig. 1. EPR (Aerosil300)
spectra of Ar-plasma-tre;lted high-disperseSiOz at 300 K and 77 I;.
Fig- 2. Analysis of the EPR spectrum shown in fig. I_
313
The
reiaxation times c’dnbe estimatedto be
They are considerably smaller than those of E; centres in cryst&i& quartz {26j probablydue to a formation of defect clusters as describedin the case of alkaIinehatides[~??1_Defect cfusterformationcan be expI&nedby an attack of the piasma on ck%ttiRspots of the surface (Ieading to a sort "‘hole burning”). The temperaturedependence of Tt and Tz can -Jso be ercplainedby the assumption defect clusters; rl an& T2 increase with temperature instead of the expected decrease according to T, a: T-a (n > 0). This may be caused by ;r temperature-dependentchange in the average distance of the defectsin the dusters which reducesthe exchangeprobabiJity and increases the relaxationtime.5at low temperatures. EA C~ZZZZK The isotropic signa cw be saturated very easily 3%low microwavfz power(i-5 mW). l&gvalueg s 2.0019 is very near that of the free electron. Ihe signal resembJesthat of F-ten tres [281- GE-~hdp@ and g-v&e can be explainedby z~um&g rhe electron to be nwidy in an s-type orbital- me EA centre is probabJy caused by eJectrunsofreJativeIy high nob& ity in the SO2 network.
of
of
Es cenire: This signal with gr. = ZOOW * 0.0005 andg, = f-9979 2 0.0005 can be interpreted as CO,by comparison with the Iiterature (61. Together with the Co,-, another signal has been detected which can rf s,ZY j_ Both sign& come be interpreted as CUT L from impurities in the Aerosol and from hydrocarbon impurities in the high-vacuumapparatusAfter admiXture of 0, at 77 K (partial pressure0.1 torr) the three signalschange as shown in fig- 3a. EA , EB and the part of E; centres Jocaked at the surface aJJvanish_At the same time the 0% signal appears &t = Z.O39;& = 2.007;g3 = 2_002)_ At high gain some further s&nais of unknowll origin appear. One of them is a species with g-L= 2.025 andg, = 2.0015 already found on MgO by Ben Taarit et al. [2Of (fig3b). The detectability of 0~ depends strongly on the Oz-coverage (fig- 4). The signal first appears at po2 S 5x w-3 torr and vsnhher in the notie at ~0~ 2= OS ta due ta dipoiar broadening. An exact determination af the dipobr broadening with increasing Oz caverage is &f&uJt bemnse the overlappingf?i signalbecomes more intense at the same time ES?], By redut;ingthe 0, partial pressure the initial spectrum does not reap pear. Therefore a very tight binding of the Cl2 in the
Volume 62, number 2
CHEMICAL PHYSICS LE.-lXXRS
1
il
9.
5593
Fig. 4. Change of 0; sure.
spectrum at 77 I: with 02 partial pres-
region of 0_
or a reactive process has to be assumed_ After warming up to 300 K and retooling the sample to 77 K another adsorbed form of 05 is foundIts Sl = 3.069 is significantly higher than that of 0% immediately after adsorption at 77 K (fig_ 5). ?&ii form of 0% can also be obtained by 0, adsorption at 300 K and subsequent cooling to 77 K. Zt is stable
during repeated cycling of the temperature between 300 and 77 I(, even after intetiediate adsorption and pumping off of oxygen (300 torr) at 300 K. Obviously the stability of OF depends on the 02-
coverage, which is higher after Qz adsorption at 77 K with poz = 0.1 torr than at 300 K with poz = 300 torr.
The results of the 0, adsorption on plasma-activated SO, are nearly identical to those of Ben Taarit et al. [ZO), obtained after 0, adsorption on thermally activated MgO. In view of the carbon impurities on
1 April 1979
Fig_ 5. Influence of adsorption temperature on the 0; spectrum at 77 K Aerosil their interpretation
of the Or signal as a COT
complex (adsorption of 0, on preadsorbed CO, [20]) cannot be ruled out. The structure of this CO; complex, however, cannot depend critically on substrate structure. because adsorption on MgO and SiO, leads to identical spectra. Coadsoxption of O2 and H,O does not chatqe the spectra significantly (fig_ 3b). So
the independence of the CO: spectra on substrate structure may be explained tentatively by the assamption of an interaction with hydroxyl groups of th=: surface or lvith preadsorhed $0. Comparing the results obtained here on plasmaactivated SiO, and those obtained on y-irradiated silicage1 [3 11 with respect to the creation of surface defects and reactivity, the anaIogy between the actRation
methods is obvious. 315
VoIume 62. number 2
CHEh¶ICAL PHYSICS LETTERS
References [11 J-H- Lunsford. Gtzdysis Rev_ 8 (1973) 13~ 121 A-J- Tenth. T. Lzrwson and J.FJ. Kihblewhite. J. Chem_ Sot- i%rzi&y Trans. I 68 (1972) 1169s [3] A-J- Tenth. T- Lawson and J.FJ. Krbblewbite, J_ C&m_ Sot- Fy;tday Trans- I 68 (1972) 1181_ [4] C- Naccache. Chum_ Phys- Letters I1 (1971) 323. 151 Y. Ben Tatit, M.C.R- Symons and A-I. Tenth. J. Chem. Sot- Faraday T-m I 73 (1977) 1149. [6] 0. EdIund. J. Sohnu and K Sogabe. BuII. Chem. Sot_ Jq=n 47 (1974) 3163[7] T- Shiga. A- Lund and P--O_ Knell. hrtern- J_ E&&t_ Phys.Chem_ 3 (1972) 131_ f8l G. >i. S. Ohnhhi and 1. Nitta, BuII. Chem. SocJapan 45 (1972) I934191 H- Yoshioka. H- .\htsumoto, S- Uno and F_ Higgbide. J_ Polymer Sci_ 14 (1976) I331 _ [lOI A-K. Kolosov. V-A. Shvets and U-B. Kazmsky. Chem. Phys. Letters 34 (1975) 360. II I J V-A. Shrew. U-M. Vorot)ntsevand U-B. Kazansky. Kinet. Catal. USSR 10 (1969) 356. [I 21 B-N- Shetimov and Bl. Che. J_ Catalysis 51 (1978) 143_ [ I31 R-B. CIxkson and S. B1cClelJ.a. J. Phya Chem_ 82 (1978) 294_ [ 141 G. Hochstrasser and J-F- Antopini. Surface Sci. 32 (1972) 644_ [ISI I. JZbertand H--P. Henning. Z_ Physik. Chem. (Leipzig) 255 (1974) 812_
316
1 April 1979
[16] S_ Kryzanowski, BuU_ Acad- Poion- Sci- Ser- Sci. Chim. 24 (I 976) 165. [ 171 N--B_ Wang and J.H. Lunsford. J- Chem. Phys- 55 (1971) 3007_ [ISI J-C. Vedrine and C. Wccache. J. Phys- Chem. 77 (1973) 1606. [I91 Y- Ben Tsarit and J_H_ Lunsfbrd. J. Phys- Chem. 77 (1973) 780[20] Y. Ben Taarit. J-C. Vedrine, C. Naccache. Ph. de hlontgoolfier and P. hferiaudeau, J. Chem- Phys. 67 (1977) 2880_ [211 H--J_ Tier, Ch. ApfeI and G. Rudakoff. Wii. 2. FSU 27 (1978) 621[23] R-A. Weeks, J_ Appl- Phys- 27 (1956) 1376. [23] K-L. Yip and W.B. FowIer, Phys. Rev. B II (1975) 2327. [24j A-V_ Shendrik and D-M. Yudin, Phys. Stat. Sol. 85b (I9781 343. f25] H.-J_ Tier and C_ RucIakoff_ KristaU Technik. to be published. [26J J.G. C&tie. D.W. Feldnxmn and P.G. KIemens, Phys. Rev. 130 (1963) 577. 11-71hl- Schxxoerer rend H-C_ Wolf. 2_ PhysiIi 175 (1963) 457. 1281 R-L. N&on and AJ_ Tenth. J. Chem. Phys. 40 (1964) 2736. [ZSI P- hlerhudeau, Y. Ben Tarit. J-C. Vedrine and C- Naccache. J. Chem. Sot. Faraday Trans. II 73 (1977) 76. (301 \V-Wintruff, R. Herrling and H--J. liller, Chem. Phys. Letters 38 (1976) 524_ [31I P-K;. Wong and J-E. Wiird. J. Phjs. Chem. 73 (1969) 2226.