A two-stage double-hemispherical electron energy selector

lnternati~~l JOIUMI 01 &fw

F%evier Publishing Company.

A TWO-STAGE

Spectrametry and Ian Physics Amsterdam - Printed in the Netherlands

DOUBLE-HEMISPHERICAL

395

ELECTRON ENERGY

SELECTOR*

K, MAEDA**,

G. P. SIXELUKi

AND F. P. LCESING

Division of Pure Chemistry, iVationcZResearch Council of Canada, Ottawa (Canada)

(ReceivedJune 7th. 1968)

IN-iRODUCTION

Although the electron impact method for the study of enerm levels in atoms and moiecules has been widely used for several decades, it has been ‘Emited in scope and accuracy by a number of expetimental shortcomings. One of the principal difEculties is that the energy spread in the bombarding electron beam, produced most conveniently by emission from a heated filament, is so large that detailed structural features in the efficiency curves for production of excited states and ions are largely obscured_ Several attempts to produce electron beams sufficiently monoenergetic for this purpose have been made, starting with Lawrence in 1926’, but the first practical improvement in the design of mass-spectrometer ion sources for impact studies was the “r-Zarding-potential-difference” method of Fox and co-workers’. This method has been used successfully in a number of laboratories, and an apparent energy resolution down to about 0.1 eV can be achieved, but it is subject to a number of difEculfies_ Another approach to overcoming the energy-width problem has been the demonstration by Morrison that considerable structural detail could be recovered from ionization efficiency curves obtained with a conventional mass spectrometer ion source, either by the use of the derivative of the curve3, or by treatment of the data by a deconvolutional Fourier anaiysis4_ Further developments of this method appear promising5_ The use of steady magnetic or electric fields to produce beams of nearly

monoenergetic electrons appears in principle to be a more direct approach.

Al-

though a magnetic selector was used succcssfuhy by Not&&am6 many years ago, the low magnetic fields f- 10 gauss) required to give convenient orbits for electrons of a few volts energy, and the difScuhies in maintaining a uniform field of this strength in the presence of stray magnetic fields, have inclined later workers to the use of electrostatic selectors. The first successful selecttir of this we l l

t

National Research Council of Canada Contriiutioh No. 10357. * N.RC

Postdoctorate Fellow. 1964-66. from Univers&of

Visiting Scientist1967-68

New B runswick, Fredericton, Canada,

L Mass Spectromhy

and Ion PhysZcs, I (1368) 395-407

K_ MAEDA,

396

G,

P. SEMELUK.,

F. P. LOSSING

was that of Clarke’, later improved by Marmet8~g and co-workers to give electron beams of about lo-’ A with an energy half-width of 0.06 eV. This selector has been used successfully in a number of laboratories”, An alternative parallel-plate electrostatic design has been described by Foner and Nall” and by Hutcbison’*_ Thd factors concerned in the optimization of electrostatic nonochromator design have been covered in detail in a recent publicationX3, and it is necessary here ody to pint out some of the shortcomings of currem. designs with regard to their use in electron impact experiments_ To achieve an e iergy half-width of 0-l eV or Iess in the 12T electrostatic selector, the operatiny electron energy has to be 3 eV or Ies~ For instance, Mar-met and Kerwin used an operating energy of 1 eV or less to achieve an energy spread of about 0.05 eV*. At these low energies the electron beam is easily deflected by stray magnetic fields; even the earth’s magnetic field (- 0.3 gauss) is enough to cause the eIectrons to spiral with radii of a few centimeters. Helmholz neutralizing coils and p-metal shielding provided some improvement in performance’* when used with a mass spectrometer, but at condiderable inconvenience. A further shortcoming of this type of electrostatic selector is the inherently poor collimation of the electron beam leaving the exit. This causes at least two unfavorabIe conditions, electron refiection and surface ionization effects. The coefhcient of reffection for a slow electron at a metal surface can be surprisingly high, 50-70 % in some cases*. The introduction of a poorly collimated beam into an ionization chamber Ieads therefore to multiple reflection of electrons with consequent broadening of the energy 4stribution. Moreover, the formation of ions on the wall and subsequent chargeexchange can lead to the detection of spurious features in the ionization eEciency curves for even a mass-resolved ion beam. The use of “electron-velvet” to reduce the reflectivity can cause complex pressure- and time-dependent secondary signals to appear, as a result of increased wall reactionsg. This effect is undoubtedly worse than a deterioration of the energy spread. It seems clear that improved electron energy selectors for ionization studies with mass spectrometry should have two main characteristics: the ability to produce a reasonably narrow energy spread (c 0.1 eV) from electron beams of higher energy where the necessity for magnetic shielding is minimized, and good collimation of the emerging monoenergetic beam so that it can be passed through a reaction chamber with a minimum of wall bombardment.

A HEMSPHERICXL

SELECTOR

A third alternative to the paralleLplate and 127”~sector types is the doublehemispherical seiector. The focusing action of a spherical condenser was analyzed some years ago by Purcell’s and otherz?. An electron source with a hemispherical selector was used by Skerbele and Lassettre” for velocity selection of an electron L MizsssJX%zrWzeny curdrorlP&sic& 1 (1968) 395407

A TWO-STAGE

ELECTRON

ENERGY

SELECTOR

397

beam in a study of the electron impact spectrum of helium. In later work on carbon monoxide, they used two hemispherical fields, one for selection and the other for anaIysis of the electron beam after scattering18. With careful consideration of optimum design features, Simpson 1g_2o described a double-hemispherical selector and identical analyzer with which he achieved a remarkabIy homogeneous electron beam, having a half-width of only 0.005 eV. The conce,ntric hemispherical selector is advantageous in several respects. It has a resolving power ahnost twice that of a cyIindrica1selector of the same radius. This means that, for the same energy width, a hemispherical selector can employ an operating voltage nearIy twice that of the cyI.indricaIsdector. A further advantage is that two-dimensional focusing is inherent to the spherical fields, and is compatible with a cohimating lens system of circular symmetry, which is much more convenient for production of weII-coIIimated beams than are lenses of rectangular aperture. The final exit aperture in such a design can be a smaII or&e rather than a slit, providing favorable conditions for good differential pumping of the seIector to prevent surface contamination by gases from the ionization chamber. Prac:ical construction A two-stage double-hemispherical electron energy selector has been constructed after the design of Amstrong 21_ The electrode configuration is shown schematicahy in Fig. 1. The components were fabricated from Nichrome and AL300 refractory*. The assembly consists of three parts: an electron gun, a two-stage selector, and a coIIimator_ The electron beam from a standard triode gun with an uncoated tungsten fiIament is launched through an intermediate ekxtrode at a high voItagelg*2 O_It is then decelerated at a 0.36 mm diameter aperture leading to the selector_ The latter consists of two pairs of hemispherica electrodes, of radii 15.6mm and 31.2 mm, supported on a base-plate of 12.7 cm diameter by insulators of AL-300. Details of the mounting are shown in Fig. 1. The two pairs of hemispheres making up the two stages of selection are positioned so that the electron trajectories in the two stages are mutally perpendicuIar22_ In the first stage the energy dispersion of the beam causes the image of the entrance slit to be spread along a line_ The part of the beam which falls on the interstage aperture is focused in the second stage to an image roughly triangular in shape22_ A small fraction of the electrons in this image is selected by a 0.36 mm-diameter aperture Ieading to the coIIimator_ The potentiais of the hemispheres can be adjusted to give the maximum electron beam. Since the electron beam Ieaving the selector IS already roughly cohimated, conditions are favorable for focusing it to a cyhndrical beam in the cohimator. Tbe latter has the configuration of a unipotential Iens system * Obtained from Western Gold & Platinum Co., Del Mont, Califl

i. Mass Spectromerry and Ion Pizysics, 1 (1968)

395407

398

K, MAEDA,

-

G. P. SEMELUK,

F. P_ LOSSING

ti

quadmplate de&&or, and km% with an ez$t aperture of 0.64 mm diameter_ The sekctor is contained in a cylindrical vacuum housing (12.7 x 15.2 cm diameter) joined to a reaction chamber (IO_2 x 10.2 em diameter) by a short pipe ~-

sECnan A-A

-8 --A

Fig_ L_Schematicdiagram of doublohemkpherical ekctron energyselector.

(3-6 cm diameter) into which the collimator projects_ An O-ring serves to insulate the collimator from the housing, and provides a partial vacuum seal between the two housings, which are evacuated separately by two 60 1 see-’ mercury diffusion pumps- A residual vacuum of 5 x IO-* torr is easily reached after baking the housings at 250”_ In-the expen’rmentsdescribed below no magnetic shielding or Helrnholz coils were employed.

Operuriod

tests

The ekctrode vokges and the emission-regulated fiiament current for the selector were suppIied from transistorized circuits with 0.1 % stability. The electron beam curxent was meanxed with a battexy-operateclelectrometer or a vibrating-teed electrometer, depending on the intensity employed. For optimum electron beam intensity, the first electrode was held at -0-5 V. with respect to the center of the iilament, and the Second at +40 to 140 V. With these values, the third electrode, which determines the operating energy of-the sekctor, could be varied from 7-28 V. For a given operating potential, all other potentials were 3- Mass speeironlelrya&Ion Pkysics. 1 (1968) 395401

A TWO-STAGE

ELECTRON

ENERGY

399

SELECTOR

adjusted to give a maximum output curtent. In typical operation, a potential of 24 V on the third electrode required the potentials of the inner hemispheres to be

about 10.7 V and the outer hemispheres about 48.4 V. Variations in these potentials, resulting from contact potential changes, were relatively minor, and the beam intensity remained constant within a few percent over periods of several hours. The energy spread of the electron beam from the selector was measured by retarding potential analysis, using a gold-plated Faraday cup. Owhg to point-topoint di!Terencesin surface contact potential, which may be relatively large*-, the energy half-width measured by this method will be iarger thau the true halfwi&&_ To minimize this effect, the interior of the cup was coated with a deposit of

IO

OPERATING E;;RGY (VOLTS)

30

'-0

1.0

Fig. 2. Electron energy half-width as a function of the operating energy of the eiectrostatic sectors. The values were obtained from graphs of current to the Faraday cup as a function of retarding potential, as shown at the lower right. The electron currents in the collimated beam at different operating energies are indicated on the upper graph. In later work the beam current was about 10 times more intense (see text)-

carbon black formed by combustion of benzene from a hypodermic needIe. A typical haif-width measurement with the selector operating at 11.93 V is shown in Fig. 2. The variation in half-width as a fimction of the operating energy of the selector is also shown in Fig. 2. A linear relationship, in agreement with theory, was found except at the lowest operating energy. It is not known whethkr the departure from linearity at a half-width of 0.06 V results from an over-estimate by I. Mass Spectromem and 10~1Physics, L (1968) 39%40’S!

K.

MAEDA,

G. P. SEMELUK,

F.

P.

LOSSING

_________-_ I-- y_“___n~v

005

*-

t

.

, IO

.

;o .

ELEiX-RON

ENERGY

OPERATlNC ENERGY

t

30

1

.

40

WCJLTS1

Fii 3- Plot of the erxrgy halfkvidth in the collimated electron beam after passing through the am-gy wntrolkingekctrode (shown at upper right). lk5uks are given for two valuesof the operating ccxcrgyof the &ctrosa tic stctors

the retarding potential method, or from some disturbance by stray magnetic fields or contact potentials in the selector- The beam currents obtained for the various half-widths are indicated on the graph in Fig. 2. These were logarithmically linear with the operating energy, as reported by Simpsonzo. In later work, it was found that the resolved electron current could be increased to nearly 10 times that shown in Fig. 2 by reducing the dimensions of the filament and aligning it carefully. The next question of interest was wbether the electrons in the beam could be accelerated and decelerated to a useful range from 5-30 V, by means of an REACTION CHAMBER 8 COLLIMATION EFFICIENCY EiEcn?oN

TRAP a

COLLECTOR CNERGY Rum-m CHAEdER

CONTRULLER

COWUATOR

Fis 4. Schematic represention of the cylindricalmesh reaction cbmber, with electron trap and ion cokctor- The collimation effkienciesIJIO (75-80 %) and IJ& (70-90 *A) are indicated. The walk and exit septum of the chamber arc of gold-platedplatinum mesh with a tra~~~parency ._-* -_ ofabeut90$& 3. Mars speuromchy-a& rou P&sics, 2 (2968) 395-4u7

A TWO-STAGE

ELECTRON

ENERGY

SELECTOR

401

aruliiary ekctrode, without seriousdeteriorationof the half-width.The con&u-

rationof the auxiliarygold-platedelectrodeis shownin Fig. 3. While maintaining constantconditionsin the selector,the mean energyof the beam was varied over the range AAC\V by means of this auxiliaryelectrode,and the energyhalf-width was measuredat four valuesof the beam energy.This was done for two vaiae of the selectoroperatingenergy-As can be seenfrom the graphin Fig. 3, the changes in half-widthwith accelerationand decelerationof thebeam wererelativelyminor, and possiblyresul’&zd from variationsin the conditionsof the retardationanalysis. Although no disturbanceof the energy half-width should be expected on the basis of ideal behaviour, this constancyis so important that the experimental check in the presentapparatuswas made_ Finally, a cylindricalreactionchamberwas constructedof gold-platedplatinum mesh=,incorporatingthe energy-controllingelectrode and a gold-plated electrontrapas shovnzin Fig. 4. Tests of the efficiencyof collimationin thischamber showedthat 75-80 % of the beam coming from the selectorcould be brought into the reactionchamber,and of this 70-N % could be collectedon the electron trap- This currentwas nearly independentof the eiectron energy in the range C-30 V_ It would appear that the collimationof the electronbeam is suflicientiy good to minim2e spuriouseffectsresultingfrom wall bombardment.

Fig. 5. Colkctor currentfor helium as a function of eke&on energy below the ionization pobential. The currentarises from metassble atom formation as follows: EkIowionization potential Sample Afe--f A* feA*+wall+ A-i-wall <@)-i-e:_ Above ionization potential Afe+ Af+e-+e_ I. .Mass Specfromehy and Ion Physks, 1 (1968) 39-7

K. MAEDA,

G. P. SEMELUK,

F. P. LOSSING

In preJjminary experiments with rare gases, using the reaction chamber

shown in Fig. 4, a large positive current was found at the collector at ekctron energies much below the ionization threshold. This current resulted from reIease of electrons from the collector by metastabfe atoms formed in the electron beam. NEON

_-__--t IONIZATION

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15 Ik I7 I8 19 20 21 22 SPECTROSCOPIC VALUE

Fig. 6. Collector current below and above the ionization potential of neon, showing formation of metastabIe Ne atoms.

A plot of collector current against electron energy is shown in Fig. 5 for spectralgrade helium at a pressure of IO-’ torr. A comparison of the upward breaks in current with the known, transitions in helium shows that the nretastable states 23S and 2lS of the helium atom were being formed in abundance. The curve in Fig. 5 is essentially the same as that observed by Schulz and Fox”~. The curve for neon (Fig. 6) shows breaks correspondiig to formation of metastable Ne aroms below the ionization threshold. For both He and Ne, a contact potentiaf shift of about 0.6 V was observed. A further distinct upward break, shown at the right in Fig. 6 corresponds to the onset of Ne ionization. Experiments with mass-resoled

iors

It was evident from the prehminary experiments that a separation of icm from uncharged metastable -es must be made before any useful ionization 3. Mrz~3 Spctrome~

and fan Physks.

l(1968)

395-407

A TWO-STAGE

ELECTRON

ENERGY

403

SELECTOR

potentid data could be obtained_ The reaction chamber in Fig. 4 was therefore modified to allow extraction of ions while maintaining, in as far as practicabIe, the cylindrical symmetry of the field in the reaction chamber. The ion collector was replaced by an extractor electrode as shown in Fig. 7, which was maintained at -6.0 V with respect to the ionization chamber. This electrode had a slit 2 x 5 mm, with the long axis parallel to the electron beam. A second, field-adjustor electrode, maintained at about -70 V with respect to the chamber and containing a slit of the same dimensions, served to flatten the equipotential lines at the extractor slit. A third eIectrode containing a circular hole 5 mm in diameter, and maintained IONZATION ENERGY ELECTRON

CONTROLLER

TRAP 1

=-

0

I-

FIELD

0

n-

FOCUS

-

--

ION

EXTRACTOR ADJUSTER PLATE

GROUND PLATE OF QUAORUPOLE FILTER

1

Fig. 7_ Schematic diagram of the ionization chamber employed with the:quadrupoIe mass 6lte.r Dotted tines indicate goId-pIated pIati.num mesh.

at 0 f 10 V with respect to the ionization chamber provided focusing of the ion beam into the entrance aperture of a quadrupole mass analyzer. This was an Uhek Quad-250 with the ion source removed except for the grounded entrance plate. By suitable insulation of all circuits supplying the potentials to the selector, 2 constant ion acceleration of 26 V could be maintained between the iouization chamber and the grounded aperture plate of the Quad. The ion current delivered to the quadrupole mass selector increased with incre~ing negative potential of the extraction electrode from 0 to more than - 20 V. Examiuation of the effect of this potential on the energy half-width showed that a signScant broadening began at - - 10 V. Au extraction potential of -6 V was ultimately found to be a suitable compromise between increasing extraction efficiency and a minimi;ration of this effect. The operation of the combined monochromator and quadrupoIe mass Giter appeared to be quite satisfactory, and free from any mutual interaction effects. At the maximum ionization chamber pressures employed, 2 x 10” torr, the ion current was just sticient to allow a mass spectrum to be scanned and displayed on the oscihoscope at a repetition frequency of about one scan per second. For measurement of the ion current the siguai from the multiplier (gain 2.5 x 105) was fed to a Victoreen “Femtometer” vibrating reed amplifier. The easily adjuJ. MISSSpecrrometry

and Ion Physics,

1 (1968) 395407

_&_

-_

:

K. MAEDA,

I

G.

P. SEMELUK,

F. P. LOSSING

mass ~lution of the qua&pole ~~t& proved to be a valuable feature, since resoWion -couldbe Aged for in& sensitivity where there was no danger of infxxfgren~ by c~wtitsd ions. To_~usfrhti tkcapabilhies of the combined monochromator-Q&d ass&z bly two ionization efE&im curves are given here._Although these results do not . contain any new features they show the Low noise level, freedom from wall effects, a&i relative freedom from other spurious feat&s which can be achieved with this &$rument. Fig. 8 shows a portion of the ionization efficiency curve for kryp &He

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.

.

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curve for Jcrypton, at an ionizationchamberpressureof about1 x Iod torr.l.l.zefeatpresshownin thiscurvewereindependent of presnue,contactpotentiaiand

Fig 8 Ionization eS5cncy

cxatronbeam currcr& and appeared in all cxpclimmts with xr. Tzlc position of intel-saxion of the two linear portions VaKiedby about f0.02 v.

ton. These data are from a single experiment, without any averaging of multiple sets-of data. The-curvature at the onset indicates an energy dispersion of about _O.O?V. There follow+ a nearIy linear portion over about 0.28 V, followed by a reprc@Cily ‘fbumpy” s&on nearly p.8 V in extent, followed in turn by an

_ .-

.._ - -_ --. .-

_ _

A TWO-STAGE

SFLECTOR

405

which continues up to

a break i-98 V above @e onset.

ELECTRON

essentially linear

portion

ENERGY

.

4w

7

z c!

'0

x

4 6 0 = E g z z 0

300

l

/’

.

.

2Do

100

1v-.*.*-A--+ ; ; I ! ! 15.8 160

2

.4

ELECTRON

i 1 I 1 i { I I ; ! I 6

ENERGY

8

17.0

2

A

tVOLTS)

Fig. 9. Ionization &ciency curve for argon, at an ionization chamber pressure of about 1 x 1O-s tom The “breaks” indicated by the arrows appeared in aU fxperiments with Ar, although their positions varied by about &C-02 V. I. Mass S@ecmomeny amilon Physfcr, 1 <1968) 395407

_

K.

MAEDA,

G.

P.

SEMELUK,

F.

P.

LOSSING

The extrapolatedintersection of the two linear portions is at a point 0.69 V above the onset, correspondi.zg approximately to the threshoId energy (O-67 V) for the 2p+ state of Kr*_ mfes showing this feature have been reported from RPD measuremer~ts~~-~~, from energy selector measurements’ ‘.l’, second differential pxots28, and piots of energy-distribution difference’, although the point of departure from linearity above the onset appears to depend to some extent on ix&t.mental factors. This non-linear portion presumabIy results from the occurrence of autoionization2p. The break about 2.0 V above onset has also been observed by Brion et al.“, who pointed out there is no correlation of this energy with known spectroscopic states or enerm levels in the neutral excited atom. The ionization efficiency curve for argon is shown in Fig. 9. Significant discontinuities in slope occur at 0.17, 0.47, 0.81,0.96 and 1.26 V above the extrapolated onset. These positions are in good agreement with earlier velocity selector data10*30 and with the energy-distribution data of Winters et al_‘. Thereproducibility of the Ar+ data in different instruments indicates that these breaks are real, but no explanation in terms of known spectroscopic states has been presented, except for the Grst break, which corresponds closely to the energy difference between the L?P+and 2P+ states of Arf. The results in Fig. 9 appear to consist of straight-line sections separated in some cases by shorter non-linear portions. Two of the latter can be seen, one between the breaks at 0.17 and 0.47 V above the threshold, and a second betwl%l the breaks at 0.81 and 0.96 V above the threshold- The reality of this structure in the Arf curve has been discussed by Brion et al.“_ In the present work the good collimation of the electron beam, and the relatively low pressures empioyed (1.5 x lo- ’ torr) tend to support their conclusion that wall reactions or other secondary processes are unlikely to be the cause.

S-Y

The construction and performance of a two-stage electron energy selector of the double-hemispherical type are described. Energy half-widths of 0.06-O. 12 V in the f,iectron beam have been obtained with operating voltages of 10-30 V in the her;lisphericaI stages. No magnetic shielding was employed Resolved electron currents of 2 x lo-’ A at a half-width of 0.07 V were obtained, in the form of a well-collimated beam which was passed axially through a cylindrical reaction chamber_ Preliminary results without mass resolution on me&stable-atom and positive-ion formation are reported, together with later results on formation of Ar+ and Kr+ using a quadrupole mass titer, in rare gases.

A TWO-STAGE

ELECTRON

ENERGY

SELECTOR

401’

REFERENCES 1 E-o.LA WREN~E, Phys Rec., 28 (1926) 947. 2 R_ E_ Fox, W. M. HICKAM, T. KJELDAAS AND D_ J. GROVE, Phys. Ren.. 84 (1951) 859. 3 J. D. MORRISON, 3. Chem. Phys-, 21 (1954) 2090; F. H. Dowa AND J. D_ MORRISON,J. Chem. Phys.,

34 (1961)

578.

4 J. D. MORRISON, 3. Chem. Phys_, 39 (1963) 200.

5 R. E. WINTERS, J. H. COLLINS AND W. L. COURCHENE,J. Chem. Phys., 45 (1966) 1931. 6 W. B. NOTTINGHAM, Phys. Reo., 55 (1939) 203. 7 E M. CLARKE, Can. J. Phys., 32 (1954) 764. 8 P. MARAET AND L,_KERWIN, Can. J. Phys., 38 (1960) 787. 9 P. MARMET AI- J_ D. MO-N, 3. Chem. Phys-, 36 (1962) 1238. 10 C E. BRION, D_ C. FROST AND C. A. MCDOWELL, J_ Chem- Phys-, 44 (1966) 1034. See also severa! papers in the Report of the 13th Annuai Conference on Mass Speetrometry and Allied Topics, May 16-21, St_ Louis. ASTM Committee E-14. 11 S. N. FONER AND B. H_ NALL, Phys- Rec., 122 (1961) 512. 12 D. A_ HIJTCHWN, Ad.cmz.Afms Spectry., 2 (1963) 517. 13 C. E. KWJAT~ AND J. A_ SWPSON. Rec. Sci. Znstr., 38 (1967) 103. 14 P. MARWET, J. D. MORRISON AND D. L. SWINGLER, Reo. Sci. Znsrr., 33 (1962) 239. 15 E_ M. PUR~ELI_, Phys- Reo., 34 (1938) 818. 16 For example, H. EWALD ohm H. LIEXE, Z_ Xaturfirsch., 12a (1957) 28. 17 A_ M. SKERBEE AX-- E. N. LASXTIRE, J_ Chem. Phys., 40 (!964) 1271. 18 V. D. MEY?% A_ SKERBELEAND E. N. LASSJ_ Chem. Ph)s_. 43 (1965) 805 19 J. A. SIMPSONAND C. E. KUYAIT, Rev. Sci. Znstr., 34 (1963) 265. 20 J. A. SIMPSON, Reo. Sci. Znstr., 35 (1964) 1698. 21 R. A_ A RMslRONG, Can. J. Phys., 44 (1966) 1753. 22 R. W. E&DINGTON, G. A. SAU,C~AND P. J. VAN HEERDEN, IRE Trans. Electron Devices, 6 (1959) 297. 22a J. H. Pm JR. AX- R. W_ WARREN, Rec. Sci_ insrr., 33 (1962) 948. 23 G. J. SCHULZ Ahm R. E. Fox, PItys. Rec., 106 (1957) 1179. 24 R. E. Fox, W. M. HICICAM AND T. KJELDAAS, JR.,Phys. Reo., 89 (1953) 555. 25 C. E. MELTON A~u?)W. H. HAMILL, J. Chem. Pi?ys., 41 (1964) 1469. 26 Y. KANEKO, J_ Phys. Sot- Japan, 16 (1961) 1587. 27 D. C_ FROST A~I C_ A_ McDowa~ Proc. Roy. Sot. (London), A232 (1955) 227. 28 J. D. MORKISON, J. Chem. Phys., 21 (1953) 1767. 29 J. D. MORRISON, Bd. Sot. Chim. Beiges, 73 (1964) 399. 30 L. KERWIN, P_ MAR= AND E. M. CLARKE, Adcan. Mass S’ecrry., 2 (1963) 522. I. Mass

Spectrometry

and Zon Physics,

1 (1968) 395-%07