A versatile “monoenergetic” electron impact spectrometer for the study of inelastic collision processes

International Journal of Mass

Spectrometry

ar;d Ion P.‘rysics

23

Elsebier Publishing Company, Amsterdam - Printed ~IIthe Netherlands

A VERSATILE “MONOENERGETIC” SPECTROMETER FOR THE STUDY PROCESSES

ELECTRON IMPACT OF INELASTIC COLLISION

C. E. BRIOS AKD G. E. THOMAS Department of Chemistry, The Unicersity

(Received Febnnry

of British Columbia, Vancoucer (Canaaiz)

Ind, 1958)

Inelastic electron-atom or electron-molecule collisions are of significance in many phenomena such as gas discharges, radiation-induced decompositions and in the general field of aeronomy. The results of such studies are of impcrtance in the evaluation of theoretical models for electron-neutral interactions. In addition to the understanding of these phenomena studies of low-energy electron collisions are of fundamental importance in that the optical selection rules for transitions are no longer rigidly obeyed_ Indeed, at or near threshold it is possible to capture the incident electron into an excited orbital of the target molecule and a molecular electron is ejected_ This probability of electron exchange can be very high and the excitation of triplet states occurs for example in heliumiW3_ The importance of such studies has been recognised for many ycar~~~’ but much of the work has ‘been done using electrons with a Maxwell-Boltzmann energy distribution characteristic of a hot lament. A broad energy spread does not allow a detailed study of the exact form of ionization and excitation cross-sections nor does it permit the resolution of closely spaced quantum states. It is desirable to use electrons of narrow energy bandwidth_ Nottingham6 made an early attempt to achieve energy selection using a magnetic analyzer and was able to observe fine structure in the ionization efficiency curve of mercury. The pseudo-monoenergetic electron retarding potential difference (RPD) _oun’ has produced many remarkable results both for ionization in mass spectrometry and excitation using the trapped-electron techniques*g. However the method is indirect and often tedious. A considerable improvement has been experienced by Brongesma and Oosterhoff’ and also Burns? ’ by using double modulation techniques to display difference currents directly. In principle it would seem much more suitabIe to use an electrostatic electron energy selector to pr-ovide “monoenergetic” electron beams. Several types of elec’3ostatic selectors are in use. The parallel-plate analyser used by Hut&son” has the disadvan3. Mass Spectrometry

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tage of Iow eIectron intensity_ The 127” cylindrical analyzcrL2-‘5 and the hemispherical analyzcri6*” have proved to be more suitable. Attempts have also been made’ ’ to deconvolute conventional ionization efficiency curves. The procedure is complex and requires an exact knowledge of the electron energy distribution. An alternative semi-theoretical treatment of experimental data has been used by Collins et al-l9 In the study of ions, total ionization offers an incr~d epmitivity but lacks specificity due to other ions in the system. In general mass analysis is desirable and the high transmission of monopole and qualrupole mass filters makes them ideal for this type of work_ These devices have the additional advantLg+ of operating at very low ion energies compared with conventional sector instruments. This obviates the need Forfloating all source circuits at a high potential as well as greatly reducing the perturbing effects of draw o:tt potentials. Very small ion currents are produced and consequently the ion detection system must be carefully designed to minimize noise problems. The%: general considerations have been used in the design and construction of the instrument described in the next section.

Fig. 1 is a schematic diagram of the system. The electron selector and analyser are constructed of goId-plated brass Stainless steel (type 304) has been Found unsatisfactory due to magnetic effects which increase with age probably due to

Fig. 1. Schematic of efectron impact spectrometer.

temperature cycling_ The dimensions of the selector are the same as those described in an earlier publicationzO_The Focussinggrids are Formedby winding goldplated tungsten wires on suitably grooved frames. The resulting double grid (80 % transparent) has. been found necessary to prevent excessive field penetration from the catcher electrod’es. The use of double grids provides an improvement in both J. Mass Spectromefry

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resciution and intensity. The tungsten or rhenium Slamem is contained in a soft iron housing. The first slit is aiso of soft iron which improves performance by minimizing the magnetic field due to the filament current. We are indebted to Dr. P. Marmet for suggesting this modification. The ion chamber consists of a double brass box, the outer box shielding the inner from stray electrons. The electron energy is applied between the Giament center*tap and the l-cm3 inner box. The outer box (slits 5 x O-4 mm) is maintained at constant positive potential with respect to the inner box. This provides a constant fringing field at the inner box entrance and exit slits (8 x 3 mm) as the electron energy is varied. A movable plate, operable from outside the vacuum housing during operation can close the inner box exit slit_ The closed position permits a true measurement of the ionizing electron beam fIux within the inner ion chamber. It is found under suitable conditions that the electron flux can be maintained constant for energies above about 1 eV. This simplifies the presentation of cross-section data since it is generahy unnecessary to divide the ion current by the electron current- A capillary gas inlet tcrminates St th.=inner ion chamber. The electron collector at the exit slit of the analyser consists of a shielded cylinder with a longitudinal slit. It should be stressed that extreme care must be taken to prevent stray electrons from the filament from reaching the ionizing region, either analyser, or the final electron collector. In particu!ar the shielding and leads to the inner box and electron collector must be effectively “light tight” to achieve satisfactory operation_ The electron selector and ion chamber potentials are derived from mercury cells. Each potential is individuaUy metered and can be varied by a IO-turn helipot. The electron ener_q distribution is obtained by scanning the potential of the complete analyser with respect to that of the selector. This potential and also that of the inner box (the electron energy) can be varied by motor-driven helipots at a series of constant speeds. The electron energy is measured by a digital voltmeter. A variable bucking voltage may be apphed to the electron energy and is useful for setting the starting level of an electron ener,v scan particulariy in the study of negative ions whem a negative bucking voltage is needed to compensate for contact potentials which make attaining zero-ener,g electrons ditlicult- The Shunent is heated directly by means of a 6 V lead storage battery. Filament heating with alternating current is also provided to maintain the source temperature overnight while the batteries are being recharged. Continuous sonme heating is necessary for reliable and reproducible operation. However, a-c. heating has been found to be Iess satisfactory for operation due to ripple_Careful attention to minimize “hum” is required inthe whoie instrument to prevent broadening of the electron energy distribution. Magnetic shielding has not been found to improve performance. The electron flux (IO-‘IO-* A) is monitored by a Fluke Electronic Galvanometer and the final current through the analyser is measured by a Keithley Model 601 electrometer having a triaxial input compatible with this “off-ground” measurement_ Fig. 2 shows a typical electron distribution with a sharp cut-off on the high-energ$ side. The po.I. b1as.sSpecrromerry

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tentials for the ion lens system are derived from a single d-c. power supply. The surface condition of the ion gun has been found to affect critically the linearity of transmission and satisfactory operation has only been obtained with clean goldplated surfaces. The spaces between the ion source plates must be optically block-

Fig. 2. Electron

energy

distribution.

ed with shielded insulators to prevent detection of spurious ionization by stray electrons_ No Aquadag, eIectron velvet or soot is used in zny piace in the instrnment and clean gold-plated surfagive reliable and long-term reproducible results. Insulators are made of boron nitride, teflon and nylon. The off-axis injection of ions into the mass filter had been found to si,@ficantIy increase ion intensity N ithout affecting mass resolution. The mass Clter is a monopole device of identical dimensions to that reported by Von Zahn” and is constructed of gold-plated brass with ceramic insulators. The mass filter is operated either by fixed crystal oscillators or by a variable frequency self-excited oscillator similar to that used by Von Zahn22. The polarity of the d-c. component may be inverted to change from positive to negative ion transmission. The oscillator is powered by a Fluke (O-1000 V) voltage &ibrator which provides the required stability and high current. Ion energies of IO-50 V are used depending on the mass resolution requircd. Mass spectra may be obtained either by voltage scanning or by varying the frequency (1-2.5 MHz). The mass range can be varied by changing the d.c_ to r-f_ ratio. The high mass limit of the present instrument is approximately nl]e = 200 though this could easily be extended by r-f. amplification. Fig. 3 shows a typical residual mass spectrum at medium resolution. The differentially pumped mass titer chamber is connected to the ion source chamber only by the entrance aperture. This is necessary TG prevent unwanted ions or electrons from reaching the collector around the o&side of the mass filter. The transmitted ions are accelerared to the input dynode of a sixteen-stage EMI 9603 Venetian blind particle multiplier. To collect negative ions the complete ion detection and amplification system is designed for operation 30 V off ground. The multiplier gain is variable to a maximum of about 3 x 106. The dynode resistor chain (1 MS2 glass encapsulated resistors) J. &CzssSpecrrometry and Ion Physics, 1 (1968) 25-39

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Fig_ 3. Residual mass qectrum.

is mounted directly on the multiplier inside the vacuum system. This largely eliminates baseline noise. The multiplier output is measured by a mains isolated Cary Model 31 vibrating reed electrometer. For ground operation the output is measured directly by a recorder or via a voltage to frequency converter connected to a Nuclear Chicago 34-27 multichannel analyser system. For off ground operation the output of the Car-y ehxtrometer is passed through a 4 kV insulated d-c. to d.c. converter. The complete data collection and display system is shown in

I

ELECTRON MCSOOROhtAT~ toN

?!%?cE

MASSFILTER

DC. &DC coNvERTER

1

t VIDAR VOLTAGE TO

241 FR!FOUE?XY I

?i?z?zF NUCLEAf2 CHICAGO 34-27

Fig. 4. Electron impact spectrometer output and data retrieval system-

J. Mass Spectrometry and Ion Physics, 1 (1965) 25-39

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Fig. 4. The gunched *ape is interpreted by an IBM 7044computer and the results are plotted automatically. Gas samples are allowed to diffuse into the ion chamber from a double inlet system- It has been found that different gases change the operating characteristics of the fdament and electron selectors so that the system must be allowed to reach a steady state before recording any data_ The all-metal vacuum systems are pumped by liquid nitrogen trapped diffusion pumps using D.C. 704 silicone oil. An ultimate vacuum of 8 x lo-’ torr is obtained_ Sample pressures in the range 10-6-10-’ torr are used.

EXPERl_I’TAL.

RESULTS

He+ : Fig. 5 SJUWSthe observed ionization efficiency curve for helium reported in a preliminary communication23. The Iarge departure from linearity greatly exceeds any effect of electron energy distribution (AE* = 0.05 V). The exact r /

E? He+

+1X?? PONER -EXPERIMENT

/

b%‘f (4 SCANS)

/ /

/-

1 0123456789

10

VOLTS

ABOVE

II

12

THRESHOLi3

fig_ 5. Helium imization efkiency curve.

shape of fhis curve is reproducible and is used as a test for satisfactory operation. Any non-linearity in ion source transmission as referred to in the previous section is immediately apparent. On the same plot is displayed a curve calculated on the basis of Wannier’s predicted 1.127 power law2”. It can be seen that there is reasonI. Mass SpecmmumyandIcn Physics, 1 (1968) 25-39

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able agreement except for a small discrepancy in the first few volts where a somewhat higher power law could be fitted_ Apart from the choice of threshold channel number the complete data collection and graphical dispiay of Fig. 5 is automatic, minimizing subjective errors. Much attention has been given to the exact form of ionization cross-sections near threshold as a result of both electron impact and photoionization. It is fairly well established that the form of the photoabsorption cross-section of a single state is a maximum at threshold and thereafter generally decreases with increase in photon frequency. The electron impact ionization near threshold to a single state has until recently been assumed to depend linearly on the excess energy of the incident electron. The main experimental evidence for this has been the linear RPD ionization efficiency curve for He” formation reported by Fox et al.‘* This is supported by the theory of Geltmanzs. Recent experiments using directly produced “mono-energetic” electron beams have strongly suggested that for the atoms studied the cross-section depends on the excess energy raised to a power somewhat in excess of unity. McGowan et al.‘” have shown that near the ionization threshold of atomic hydrogen a 1.127 power law is operative as predicted by War&e?‘. The study by the present authors of helium ionization to 12 V above threshold where there is only a S, ground stat: also suggests a power law in excess of unity. Mainly on the basis of believed threshoid laws for photoionization and for electron impact ionization. attempts have heen made to compare the shapes of tist differential ekctron impact curves with the photo-ionization cross-section or alternativeLy to compare the electron impact curve with the integral of the photohave discussed qualitatively the ionization cun-e2s--32. (Collin and Delwiche” reservations that must in general be placed on such a comparison.: On the basis of this comparison it is tempting to ascribe any differences in shape to the high probability excitation of optically forbidden states by electron exchange in the case of electron impact. Attention should be drawn to the fact that in the case of helium, at least, the comparison outlined above would be invalid. Fig. 6 (a) shows the electron impact ionization efficiency curve obtained for helium (Fig. 5) and the experimental photoabsorption curve due to Samson3 3 and Lowry et al. 34. The photoabsorption for atoms should be identical with the photoionization cross-sectionJ5. The shape of this curve is well substantiated by the sophisticated theoretical calculations of Cooper36 and also of Stewart and Webb3’. In Fig. 6(b) the photoabsorption curve is compared with the fust differential electron impact curve. In Fig. 6(c) the integral photoabsorption curve is compared with the electron impact ionization efficiency curve. It can be seen that the comparison is poor irrespective of the energy at which the photon and electron curves are chosen to be cormalized. The comparison would only be reasonable in the first fraction of a volt above threshold. Since it is in this region t’hat any difference in energy spread of the electron and photon beams will produce the largest differences in the J. Mass Specrrornetryand Ion Htysics, 1 (1968) 25-39

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C. E. BRION, G. E. THOMAS

ELECTRON

ENERGY

(VOLTS

ABOVE

THRESHOLD)

Fig_ i. Comparison of electron impact and photoionization data for helium. ~4-0 curves such a comparison

remains questionable

unless all experimenti

param-

eters are included. This suggests that caution be exercised in comparing photon and electron impact data for larger atoms and molecuies where the cross-section dependences for electron impact and photoionization to six&e states are usually unknown_ On the basis of available helium data the extent of non-validity of the coftparison would increase as the energy separation of successive quantum states increases_ I@*: Since no electronic states of this species exist it is suitable for threshold law studies for doubly
corded by the multichanneI

anaiyser. The solid Iine in Fig. 7 has ken calculated

for a square-law dependence of ion current on excess electron energy. This is in agre!ment with the experimental work of Fox3’ and also with the semi-empirical predictions of Wannier40. 1. Mess

Spzcttvmerry

d

Ion P&=ia,

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uNcowiEclEo Fig. 7. ionization square law-

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ELECTRCN ErERGY kvl

efficiency curve for double ionization of helium. (3 ) Exptl.

(8 scans); (--_)

Nitrogen Nz+-: Fig. 8 shows the ionizationefficiencycurvefor molecularnitrogenover a range of 5 V above threshold. The spectroscopic xkzation potentials are indi-

ELECTRON Fig. 8. Ionization

ENERGY ABOVE -iHfiESHOW

efkiency

0

-

curve fbr nitrogen.

3. Mass Specfrometry

and Ion Physics,

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C.E.RRION,G.E.THOMAS

catcd by arrows. A considerabIe improvement in signal to noise ratio is obtained b:t aMing several runs on the muhi-channei analyser. The overall shape of the IE curve is similar to some of the results of the RPD technique4’, the energy distribution difference (mu) methodfg, deconvolution 43 and also the early electron selector work of Clarkea. Unlike many RPD results in the literature, straight lines separating the electronic states are not evident. The X2.YSi ground state of N,+ is the only onset clearly observed_ The A”&!_ onset is obscured by unresolved structure which may be identified with auto-ioniitng states. These may include the optically-aliowcd states observed in the w absorption work of Huffman et aL4’ and also those states forbidden opticaIIy but allowed in low-energy electron impact where electron exchange can occur ’ - 3. The “hump” in the electron impact cinve just above the exvcted position of the A’J7, state is close in energy to the large preionizing level obsetved by Huffman et al. at 725 A. However, caution must be exercised in any such comparison since the relative cross-sections for the excitation of singIet states and their associated terms of higher multiplicity are unknown for electron impact close to threshold. In Fig. 8 the region between the A’lI, and the B2EuT states is also presumably the envelope of a number of autoionizing states belonging to Rydberg series urhich converge at the upper level. The curve appears linear above the B’,T:,- state and it is of interest that Huffman et al.

I4

I5

6

I7

RECTFC% EkEFGYINWLTS

B

19202l22

(WCW?ECTEO)

Kg. 9_ Ionization efficiencycurve for o.xygen L MUSSSpectrometry

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observe no significant autoionization

structure in this higher region. and his co-workers have discussed the relative contribution of direct and autoionization below the A*&?, state4’. McGowan

Oxygen Oz’: The ionization efficiency curve for the oxygen molecule is shown in Fig. 9. Thresholds are observed corresponding to the various states of OzL in agreement with previous workzo*46*47_ The ‘C, state not previously detected by electron impact, but well known opticahy, is also apparent. The increase in ionization just above 21 eV, also previously observed”6**7, may result from autoioniz&ion of Rydberg states4s converging on the C*.?Th state of Oz+. Fig. 10 shows

Fig. IO. Fit-derivative

ionization &ciency

curve for oxygen.

the iower part of the curve in more detail. The addition of seven scans makes possible the first derivative curve shown in the lower part of the figure. The broad “bump” behveen the ‘LTg and “& states is shown to advantage on this curve. The optical absorption spectrum of oxygen 41 shows many excited states superimposed

on the direct ionization continuum and 1:hese are particularly intense and numerous between the ‘II, and “l7, states. The photoionization efficiency curve obJ. Mar; Spectrometry and Ion Physics, 1 (1968) 25-39

36

C. E. BRION,

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by Libeler and Walker4g also shows extensiveautoionization. The general remarks concerningthe comparison of optical and electron impact data outlined in the previoussection on nitrogenwill also apply here. Similarstructurehas been observed by Morrison43 and also by McGowan et a12.50.

tained

Negative

ions

SFG-: The molecular negative ion SE, - is formed and the sharp resonance capturepeak is shown in Fig. 13. Hickham and Fox” claim that this ion appears at 0 eV and that the profile of the -peakmay be used as a measureof the electron energy distribution.

ELECTRON

ERERCX (VOLTS)

Fig. 11. Electron

capture in su!phur hexafiuoric!e.

Carbon tetrachloride

CT: This formation of this ion has been studied by Fox and Curran” using the RPD method_An extensivebibliographyof other work on this ion, mostly usingwide energy-spreadeIect.ronbeams, is to be found in ref. 52. We have reexamined the dissociativecapture process leading to the formation of this ion and in addition looked for indicationsof ion pair formation_ Fig. 12 shows the observed ion yield curve as a function of electronenergy. In the firsttwo electron-volts,two peaks are observedin agreementwith Fox and Cm-ran.The firstappearsat 0 eV on a scale calibratedby SF,- formation. It is a very narrow resonanceof comparable width to *hat for SF,-. The position and magnitudeof the second broad peak at about 0.8 eV is independentof electroncurrentin the range 1O-g-1O-8 A. We do L MassSpectrometryand Ion Physics,

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g

6*Yotzno9816543210

-ELECTRON-

/VOLTSJ

Fig. 12. The formation of Cl- from carbon tetrachloridenot find the effects discussed by Fox and Cm-ran who were operating at much higher electron currents. Above 2.5 V in Fig. 12, the curve is shown at a sensitivity increased by a factor 300. On the “tail” of the broad peak at 0.8 eV we routinely find an even broader dissociative capture peak at about 6 V. At about 8 eV there is a stead) rise of ion current which levels off at about 16 eV. This must be due to an ionpair type process. The threshold is undoubtedly shifted by the broad tail of the dissociative capture peaks at lower energy.

O-: This ion has been studied by Frost46 using the RPD method, by Fite and Brackmam? 3, and also by Briglia and Rapps4 using a totai ionization RPD method. Our result shown in Fig. 13 shows both dissociative capture and ion-pair

0

5

lo

55

E&x&&N&z Fig. 13. Negative ion formation from oxygen_

40

45

50

has been calibrated relative to SFB- formation. The dissociative capture peak, indicating a widespread ion kinetic energy has a maximum at 5.62 eV. The appearance potential of O- arising from an ion pair process is approximately 17.3 eV. At higher energies the structure seems to indicate at least one further process.

processes_ The energy scale

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C. E. BRION,

G. E. THOMAS

ACKNOWLEDGEhfENTs

We wish to acknowledge helpfkl corre- =3ndence with Dt_ UK von Zahn and Dr. JAR. Samson. Financial suprxc WL xovided by the National Research Conncil of Canada.

SUhWARY

The construction and operation of a monoenergetic ekctron impact spectrometer empioying a monopole mass analyser is described. Results are given for a wide variety cf inelastic collision processes_ REFERENCES R. N. COMPTOS, R. H. HUE~XER. P. W. REIXHARDTASD L. G. CHRISTOPHOROU,J. Chem. Phys-. CO be published. C. E. Bruolsi AND C. R. EATON, fnrernufl. J_ AZuss Specfry. Ion Phys., 1 (1968) 102. G. 3. SCHULZ ASD R. E Fox, Phys. Rec., 106 (1957) 1179. H. M.NER-J~EIBWZ, 2. Phrs., 95 (1935) 489. H. D. Ssfnx, Rec. Alod_ Phys., 3 (193 1) 347. W. B. Nomscwr, Phys_ Rec., 55 (1939) 203. R. E Fox. W_ M. HICKHAM. D. J. GROVE AXD T. KJELDAAS. JR., Rec. Sci. Znstrum., 26 (1955) 1:01. 8 G. J. Scrwrz, Phys. Rec., 112 (19%) 150. 9 H. H. BROSG~U AL_ J. 0 o%ERHOpp, Chem. Pbys_ Letters, 1 (1967) 169. 10 J. F. BZIR’IS, Nurure, 192 (1961) 651. II D. A. H~TCHWX, A&m. A%.ss Specrry, 2 (1963) 527. 12 P. MAR.= A&D L. KERV~IN. Con. J. Phys.. 38 (1960) 787. 13 J. W. MCGOWAK A%D M. A. FISXXAK, Proceedings of 4rh International Conference on rhe Physics of Hertronic and Atomic Collisions, Quebec, It&S, Science Bookcrafters Inc.. New York, p. 429. I‘$ C. E BXION, D_ C. FROST A&P C. A. MCDOWELL, J. Chem. Phys_. 44 (1966) 1034. 15 G_ J. SL-HULZ, Phys. Rec., 125 (1962) 229. 16 C. E. J$IYA~~. J. A. SIMPSON AND S. R. MJELCZAREK. Phys. Rec., 138 (1965) 385. 17 V. D. MEYER, A. SKERBERLEAND E. N. LASSETIRE, J. Chem. Phys., 43 (1965) 805. IS J_ D. hfoRFusDN, _I. Chem. Phys., 39 (1963) 200. 19 R-E_ Wm-rrms, J_ H. Cors_ms AND W. L. (=OURCXENE, J. Chem. Phys., 45 (1966) 1931. 20 C. E. BRION, J. Chem. Phqs., 40 (1964) 2995. 21 U. VOX =HN, Reo_ Sci. Insfr_, 34 (1963) 1_ 22 U. VON 2x-m, pribate communication. 23 C. E. BFUON %ND G. E THOR, Phys- Rev. Lerrers. 20 (1965) 241. 24 W. M. HICKHAM, R. E. Fox AND T_ KJELDAAS. Phys. Rec., 96 (1954) 63. 25 S. GEL-N. Phys. Reo., 102 (1956) 171. 25 J. W. MCGOWAN, M. A. Fm-!umx, E. M. CLARKE AXB H. P. HANON, Phys. Reo., to be published. 27 G. H. WANNIER, Phys- Rec., 90 (1953) 817. 28 J. E. COLLIN AND J. DEL~KHE, CUK. J. Chem., 45 (1967) 1875. 29 F. H_ Do-x J_ D. Monnrso~ XEiD A_ J_ C_ Nr~~or_sox. J_ Chem. Phys_-. 32 (1960) 378_ 30 J. D. MORRISON, Rec. Pure /pp.?_ Chem., 12 (1962) 117. 31 J. D. MORRISON, Bull. Sot. Chim. Beiges, 73 (1964) 399. !

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