Penning ionization electron spectroscopy

International

EIsevier

JournaI of Mass

Spectrontetry

and Ion Physics

399

Publishing Company, knsterdam. Printed in the Netherlands

PENNING

IONIZATION

IONIZATiON

V. k2tdk* Laboratoire

ELECTRON

OF NOBLE

GASES,

SPECTROSCOPY

Hg, NO, C,H,,

C3H,

AND

C&l6

AND J. B. Ode Physicochimie

des Ra_yonnements, FacultP des Sciences, Orsay

(France)

(Received December Sth, 1970)

ABSTRACT

Energy distributions of electrons released in ionizing collisions of Ar, Kr and Xe with excited metastable atoms of He and +Je, and of Hg, NO, C,Hal C,H, and C6H, with excited metastable atoms of Ar have been measured. An apparatus with high energy resolution was used. Shifts of electron energy are discussed in terms of transformation of collision energy into electron energy and in terms of associative ionization as a parallel reaction to Penning ionization. The dissociation energy of ArHg’ ions is estimated. The results of ionization of Hg are used to determine the reIative population of the 3P0 and ‘P2 metastable states of Ar formed by ekctron impact, and to detect and measxure the energy of the long-l&d

excited states of N2 in a beam.

INTRODUCTION Penning ionization electron spectroscopy (PIES) is based on the measurement of the energy of electrons released in the ionization of atoms and molecuIes by means of excited long-lived particles (atoms or molecules)’ - lo*** e.g. X*+AB

= AB+i-X+e.

Only one electron

(1)

is released in reaction

J??, = E(X*)-_~~(AB)tEjy(AB’)]+~

(I),

its energy being given by: (a)

* AMaitrede Recherche for 1969-1970 at the Centre National de la Recherche Scientifique. On leave from the Institute of Physical Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia. ** While the present paper MS in preparation another paper by H. Hotop and k Niebaus appeared (Z_ I%ys_. 238(1970) 352) which contains information of interest in connection with major yam of the discussion in the present paper.

ht. J. Mzss Spectrom. Ion P&s.., 7 (1971) 399413

V. (SERMP;K,

J. B. OZENNE

where E(X’-) is the energy of excited metastable particies released in the ionization of AB, Ip(AB) is the first or higher ionization potential of AB, Ej,(AB’) is the rotational and vibrational energy of the AB’ ion and AE is the term, positive or negative, which accounts fur swific phenomena accompanying Penning ionization, e.g. conversion of a part of the internal et ergy of the reactants into kinetic energy of the productsZ3.10, conversion of the kinetic ener,oy of the reactants into the energy of electrons released’ and the change of electron energy resuhing from associative ionization1 1*4A X*+AB

= ABX’+e.

(3

PIES is anaiogous to the rapidly developing photoelectron spectroscopy as far as the data on 1P snd Ejv(AB*) are concerned. However, measurement of the term LL?Zoffers a means of determining small but specific interactions in collisions of excited or ionized particles with neutrals. According to the data obtained so far the characteristics of Penning ionization are the following: It is a Franck-Condon2S7*“-‘3 and electron exchange process’.The ionizing transitioizs occur at different particle separations’*“* ’ ’ and/or at the repulsive part of the potemial energy curve of the reactants’ and the collision complex, if formed, does not seem to influence strongly the vibrational state of the target m5!ecuIe’0_ In order to establish the range of validity of the above conclusions more work with higher electron ene:gy resolution is necessary. In this respect especially comparison of the energy distribution curves obtained in PIES with those obtained in photoelectron spectroscopy is of particular value_ The work reported here concerns the determination of the electron energy distribution curves by means of an apparatus with improved energy resolution as compared with that previously obtained3. it deais aiso with the use of PIES in the reverse sense, i.e. for the determination of the energy of long-lived excited neutral particles themselves5_ Attention was not concentrated on the determination of ionization potentials but on the possible differences between photoionization :and Penning ionization_

EXPERLME?iTAL

The apparatus has four parts: the source of metastables, the collision chamber, the ana&ser and the detector (Fig. 1). The source is of the same type as that used earlierI_“. A magnetic field of about 100 G is used fur collimation of the exciting electrons. The electron energy is 40 eV. The source is enclosed by two concentric cylinders of Mu metal of 0.9 mm Int. J. iuQ.TsSpeCfrom. ion Phys.,7 (1971) 399-413

PENNING

IONIZATION

ELECTRON

401

SPECTROSCOPY

5h

section A-A

r

-.B -

Sh

section B-B

Fig. I. Schematic drawing of the apparatus. J = multiple channel jet for gases to be excited; F = filament; T = trap; M = electron collimating magnet; I> = pair of electron deflecting plates; Sh = double-walled magnetic shield; C = secondary electron collector; CH = grounded collision chamber. Target molecules are admitted through the tube. A = electrostatic cylindrical analyser; EM = electron multiplier; S = e!ectron accelerating or decelerating plate; P1 , flz. = two pairs of trajectory correcting half-plates. In section A-A only the electrostatic analyser is shown.

wall thickness and 180 mm Iong. One side of the screening cylinders is open for efficient pumping- The metastables leave the source region through an orifice of 3 mm diameter in the other, closed side. The collision chamber is placed at g distance of 45 mm from the source. The me&stables enter it through a hole of 3 mm diameter. The ekctrons released in the collision region and diffusing freely out of it pass through the trajectory correcting field of two pairs of half plates. Before entering a 120” electrostatic cylindkal ener,oy analyserls (radius of the central ray, r = 17.5 mm) they are either decelerated or accelerated so that their final energy equals the constant transmission energy of the analyser (1.75 eV). Electron energy scanning is achieved by changing the electron energy adjusting voltage. After passmg the analyser the electrons are accelerated to 100 eV and enter the Mullard B419BL channeltron. The counting technique is used to measure the e!ectron current. Typical counting rate is about 200-1000 counts/set. Inr. J. iW7ssSpectrom.

Ion Phys.,

7 (1971)

399-413

402

V. CER_MP;K,

J. B. OZENNE

The collision chamber, the analyser and the channeltron are located in a doubie walled Mu metai box (0.9 mm wall thickness, 125 mm Iong). In order to minimize the pressure; inside the box, the front is open and the component of the earth maprtetic field along the box axis is compensated by a pair of Helmholtz coils. Upon admission of the gases into the collision region the pressure inside the box is of the order of lo-’ mm Hg. The source and collision regions are separately evacuated by two mercury 500 I/set pumps. The space between them is evacuated by a third I20 I/XC pump. The resohnion should be:

where ,IE is the full width ar hzlf maximum, and S1 and S, the slit widths (0.1 mm), R the nrdius of the mean path (i’Y.5 mm), and z the angular aperture of the electron beam (3 x lo-’ rad). Thus the theoretical resolution is 87. The experimental resolution obtained, measured at the half height of the peaks corresponding to ionization by photons penetrating fro;m the source into the collision region, was E

1.75 - = 7G. OE - 0.025 In order to controi the current of metastable ionizing particies a ring-shaped collector (15 mm diameter) is placed near the entrance hole into the collision region. The current of secondary electrons emitted by the me&stables upon hitting *Se collision chamber outside the hofe is usually of the order of IO- lo A. The ener,T of metas*able noble gas atoms in their different states is presented in Table 1. TABLE

1

E?ZiRCiYLEVELSOF

Azonr

Sfare

lie

2’S

METASTABLE

ATOMS AND

2’S

20.614 19.818

Ne

3p0 3pZ

16.7! 5 16.619

Ar

3p0 3pZ

11.722 11.548

2Pg ‘P%

15.936 15.759

Ar+ Kr‘

Xe*

=P&

14.665

‘P$

13939

=Pg

13.436 121129

‘P!,

Int. 1. Mass Spectrom

IOEiIZATlON

fan Phys., 7 (1971) 399-413

POTEXlT.kiS

USED IN THIS WORK

PENNING

IONIZATION

ELECTRON

SPECTROSCQPY

403

RESULTS AND DISCUSSION

He* -Ar The electron energy distribution curve is shown in Fig. 2. Peaks No. 1 and 2 correspond to ionization into the Ar’ ’ P* and “P+ states by the He* (Z’S) atoms, peaks No. 3 and No. 4 to ionization by the He* (23S) atoms, respectively. The two small peaks No. 1’ and No. 2’ are due to ionization by the 21.2 eV photons, penetrating into the collision region from the source. They can serve well for electron energy calibration. Peaks No. 3 and No. 4 are broadened on the higher electron energy side. This is caused by associative ionization NO.

He*(23S)+Ar

= HeAri fe,

(3)

the various mechanisms of which have been outlined and discussed previously”*‘. Peaks assigned to ionization by means of He*(ZlS) atoms are sharp, but are also shifted toward higher energy. This shift which amounts to about 0.05 eV was

L

-3;

-itI

I -25

I v

Fig. 2. Electron energy distribution ewes for ionization of argon by helium metastables. Peaks No. 1 and No. 2: ionization by He*(Z*S) into Ar+ (*Qj andAr+ (2Pt) states. Peaks No. 3 and No. 4: ionization by He*(Z3S) into Ar+ FQ) and Ar+ (2J’+) states. Peaks No. 1’ and No. 2’: ionization by 21.2 eV photons into Ari (‘Pa) and Ar+(2Ps) states. The vertical full Iines indicate the unshifted position of the peak maxima. Y: accelerating or decelerating voltage. Int:J. Mass Spectrom. Ion @ys.,

7 (1971) 399413

404

V. ~ERM.,~.K, J. B. O Z E . ' N N E

discussed b y H o t o p a n d N i e h a u s s. T h e w i d t h at the h a l f height o f b o t h peaks N o . 1 a n d N o . 2 is 45 m e V a n d represents their n a t u r a l p e a k width s. N e * - Mr, Kr, Xe T h e electron energy, distribution curves are presented in Fig. 3-5. Peaks No. 2 a n d N o . 5 c o r r e s p o n d t o ioDiTation into the A r + 2P~r a n d 2P~r states by m e a n s o f N e * a t o m s in the higher energy, a P o state. Peaks N o . 3 a n d No. 6 are due to ior~iT~tion by the Ne*(aP2) state. S h a r p peaks N o . 1 a n d No. 4 are caused by 16.84 eV p h o t o n s originating in the source. P e a k No. 2 is b r o a d e n ~ at the higher energy, side but its m a x i m u m _~ies at the predicted energy. P e a k s N o . 3 a n d N o . 6 are b o t h b r o a d e n e d a n d shifted towards higher energy. O n the or_her h a n d , p e a k No. 5 is shifted towards lower energ% merges into p e a k No. 6, a n d increases its height. Broadening a n d shifting t o w a r d s greater electron energy c a n again be a c c o u n t e d for by the f o r m a t i o n o f N e A r +, N e K r + a n d N e X e + ions by associative ionization a n d collision energy t r a n s f o r m a t i o n . T h e shifting o f the p e a k o f ionization by the Ne*(aPo) state into the 2p~ ion state is a peculiar p h e n o m e n o n . It can

f

)/3 : J

, ~ "

I

I

I

I

Fig. 3. Electron energy distribution curve for ionization of argon by neon mctastables. Peaks No. 2 and No. 5: ionization by Ne~(~Po) into Ar+(ZPi) and At+ (zP~) states. Peaks No. 3 and No. 6: ionization by Ne*(aP2) into Ar"(zP~) and Ar+(:Pt) stat~. Peak No. 1: ionization by 16.84 e V p h o t o n s i n t o t h e A r + ( z P t ) state. V e r t i c a l full lines i n d i c a t e t h e u n s h i f t e d p o s i t i o n o f t h e p e a k m a x / m a . V: a c c e l e r a t i n g o r d e c e l e r a t i n g voltage. Int. J'. M a ~ Spcctrom. Ion Phys., 7 (1971) 399--413

PENNING

IONIZATION

ELECTRON

SPECTROSCOPY

405

Fig. 4. Electron energy distribution curve for ionization of krypton by neon metastabIes. Peaks No. 2 and No. 5: ionization by Ne*(3Po) into Kr+(2Pi) and Krf(2P$ states. Peaks No. 3 and No. 6: ionization by Ne*(3P2) into YJ+(~P~) and Kr+(2Ps.) states. Peaks No. 1 and No. 4: ionization by 16.84 eV photons into Kr+(2Pg) and Kr+(2P$ states. V: accelerating or decelerating voltage. i

I

Fig. 5. Electron energy distribution curve for ionization of and No. 5: ionization by Ne*(Pd into Xe+ (‘Pi) and Xe+ ionization by Ne*(3P2) into Xei (‘J’s) and Xet(2P$ states. 16.84 eV photons into the ‘4 and ‘P* states of Xei voltage. Int. J. Mass

Xe by neon metastables. Peaks No. 2 (‘Pg) states. Peaks No. 3 and No. 6: Peaks No. 1 and No. 4: ionization by ions. V: accelerating or decelerating

Spectrom.

Ion

Phys_, 7 (1971)

399-413

V. CERMhC,

occur if the downward transitions from the potential Ne*(3PO)fAr, Kr, Xe, end up on the repulsive part for the products, Ar+, Kr+, Xe+(‘P..)+Ne. On the other reactants should end .in the shallow well and

J. B. OZENNE

energy curve of the reactants, of the potential energy curve other hand, transitions of the on the flat part of the curve.

Ionization by me&table argon atoms has a certain advantage over ionization by either he&m or neon metastables: the population of the 3P0 state in the beam leaving the source is small. Consequently. the interpretation of many-peak electron energy distribution curves is much easirr. The ionization energy available is limited but still it is high enough for ionization of some inorganic and most of the organic molecules.

In this system only tvro electron energy peaks appear because the EIg+ ion pro-und state is the single-valued ‘S+ state (Fir;. 6, peaks No. 1 and No. 2). Both

Fig. 6. Ektron energy distniution curve fcjr ionization of Hg by .4r*(3Po) (peak No. 1) and ArY3Pz) (peak No. 2: states. LIindicates the area under the symmetrired peak No. 2. b the area under the _mstof the curve, dimikhed by tht area c, and c the area under peak No. 1. The areas b and c are sqparated by the dashed line. Y: accelerating or decelerating voltage. Int. 3. .Mz.ss Spt?crrom. loll Phys.T 7 (197:)

3?3-4I3

PENNING

IONIZATION

ELECTRON

SPECTROSCOPY

407

peaks are very markedly broadened towards higher electron ener,T. The broadened portion of the curve is structureless and smooth. The broadening is undoubtedly due to associative ionization Ar*+Hg

= ArHg’i-e,

(4

as discussed previously15*4 and represented schematically in Fig. 7. In ionization the transition takes place vertically from the curve for the reactants, Ar* + Mg, to the curve for the products, Ar + Hg+ or ArHg*. Penning ionization results either if the collision distance, R, is greater than R, or if Ek+EL 2 d (see Fig. 7), assuming conservation of internuclear distance and of reIative momentum during the transition. If E’;;+ EL cc d associative ionization becomes possible and the electron energy E, becomes greater than Ep, E, > Ek i Ep_ In this case the peak form is asymmetric and broadening towards higher energy results. The smooth distribution .of the E, values indicates that the ionizing transitions occur within a certain range of particle separations, not only, as in some cases, at the repulsive part of the potential energy curve of the reactants’. The term d is related to the dissociation energy of the ArHg’ ions by the inequality

De2 A = E,+E;-E,

w

which is valid for collision distances R 3 Ro. Unfortunately, equation (b) is unsuitable for evaluating 0, because the vaIue of EL is unknown. However, at the

E

Fig. 7. Schematic potential energy curves for reactants, W+Hg, and products, Ar+Hg*_ Ep = energy of eiectrons reieased in Penning ionization; E, = measured electron energy; 0, = dissociation energy of the .4rHg* ions; Ek is the relative collision energy; EL is the kinetic energy resulting from attraction of the reactants during the collision. fnt. J_ Mass Specrrom. Jon Phys., 7 (1971) 395413

408

V. i:ERMiK,

J. B. OZENNE

ckssicai turning point another equation holds:

= AE-E,.

D, >/ Lit = Ee-Ep-Ek

W

If the supposition is made that at the turning point the electron energy E, attains its maximum, and if Ep is mea>ruredat the maximum ofpcak No. 2, then, under the conditions of the experiment, 0, b AE_ -I?; = 0.37-0.03 = 0.34 eV. From the cwve in Fig_ 6 one can determine the ratio of the cross sections for Penning ionization and for associative ionization. -It is the ratio of the areas under the symmetrized peak No. 2 (area a) and under the rest of the curve (area b). Under the actual experimental conditions the ratio is 1.6.

Ar* -NO The electron energy distribution curve (Pig. 8) clearly reveals icnization into various vibrational levels of the NOi ion in the ground XiCt state. The relative peak heights and level spacings are summarized in Table 2.

Fig. 8. Electron energy distribution curve for ionization of NO by Ar+(3Pz) ground state. V: accelerating or decelerating voltage. TABLE

state into the NO+

2

FRANCK-CO?DON

FACTORS

A?44

VIBRXTIONAL

SPACISGS

XX THE

NO*

GROWiD

s+AfE

Franck-Condon factsrs Y

0

I

2

3

4

5

Theory’6

0.478

I.0

0.917

0.484

0.163

0.04

This work

0.40

0.93

1.0

0.62

0.29

0.11

Vibrational

energyspacings

Spectroscopicai value” 0.291

3.286

0.233

0.278

This work

0286

0.281

0.280

0.289

Inr. J. Masz Spzcrrom. Ion ?/zp-,

7 (197i) 3-13

PENKING

IONIZATION

ELECTRON

409

SPECTROSCOPY

The small but reproducible difference of Franck-Condon factors in Penning ionization with respect to theory means that the N-O internuclear distance in the coilision compIex Are-NO is, at the moment of ionization, shghtly greater than that in the unperturbed molecuIelO, or in any case is increased shghtly more than in the Ar-NO+ complex*.

The ionization of ethylene into the C2Hsi ground state (Fig. 9) is accompanied by the excitation of at least five vibrational Ievels of the v2 mode’*. The separation of the first two levels is 0.163 eV which is greater than the photoelectron spectroscopic value of Baker ct ~1.” (0. I& eV) but smaher than the value given by Eland*O (0.173 eV). The two levels of the v, mode (peaks No. 7 and No. 8) are also resolved.

Fig. 9. Electron energy distribution curve for ionization of C,H; by Ar*(3PZ) state into the C2&+ ground state. Peaks No. l-51 vibrational levels of the v2 vibrationzi mode. Peaks No. 7 and No. 8: vibrational levels of the rt mode. Peak No. 1’: ionirarion by A?(3P0) state. I/: accelerating or decelerating vohage.

Upon ionization by Ar* in the 3P, state five cIearly resolved vibrational levels of the C,H, f ion in the ground state are excited (Fig. 10). The separation of the first two Ievels is 0.170 eV and corresponds to the v6 and vp vibrational modes of the neutral molecuIe’*_ The excited vibration is the symmetrical deformation vibration of the CH3 group and the v6 C=C stretching vibration which is shifted by about 270 cm-’ to lower frequency with respect to the neutral molecule. * The measured vdue of the first ionization

potential

of NO is probably

Itx. 3. Mass Speczrom.

also slightly altered.

Ion PJzys., 7 (1971)

399413

V. CERM_iK,

i

J. 3. OZENNE

A

Fig. IO. EIectron energy distribution curve for ionization of CsHs by A?(3P2) state into the ground sta-e. Peaks No. 1-5: vibrational levels of v, mode. Peak No. 1’: ionization by &He* Ar*t3P0) state. V: accelerating or decelerating voltage.

The electron energy distribution curfe is shown in Fig. 11. It resembles closely the curve obtained by photoelectron spectroscopy”. Interesting is the absence of any peaks in the retardin, = voltage range O-1 -4 V which couid correspond to iorkation potentials 9-6-l 1.0 V. No positive evidence can thus be given for :-he ionization processes in this energy region”‘-23_

Fig. 11. Electron energy distribution curve for ionization of C6H6 by Ar*CP?) state into the CeHo+ ground state_ 5’: accelerating or decelerating voltage. The monotonous increase of the eiectron current beyond 0.3 eV is caused by scattered eiectrons. Peak No. 1’: ionization by Ar* CPe; state.

Population:olttfie metastable 3PGand 3P, stat-2sof rare gas atoms excited by electron impact ‘Lnthe ionization of a target atom with skgle-valued ionic ground state each of the m&as-table particles present in the bexr manifests itself by a separate electron energy peak (equation (a)). If the supposition is made that the ionization 1nt.J. MaS

Spectrom.IonPiiys.,7 (1971) 399413

PENNING

IONIZATION

ELECTRON

411

SPECTROSCOPY

cross sections and the angular distribution of the released electrons are the same for all excited species in the beam, the Peak heights can be used for the measurement of tiei population. ‘These considerations were previously applied in determining the population of the two metastable states of helium’. Here the ionization of Hg and Hz!3 has been used for the measurement of the relative population of the metasfable atoms of argon. The ratio of the heights of Peaks No. 2 and No_ 1 (Fig. 6) is 7.5 at an energy of the exciting e!ectrons of 40 eV. Contrary to this, the ratio of the areas under peaks No_ 2 and No. 1, (afb)/c, is 10.7. Peak height measurement in ionization of HIS into the H2SC ground state, in which the broadening is absent, again gave the value 7.0. The discrepancy probab!y means that the associative ionization cross section is greater with Ar* in the lower lying 3P2 state. Determination

of the energy

of

excited long-hed molecular stares

The determination of the unknown energy of long-lived excited particles bzzomes possibie if target molecules AB are replaced by terget atoms, A, with known ionization potentiai (e.g. Hg, alkah atoms). In this case equation (a) changes to E(X*)

(4

= E,+IP(A).

The method has already been excited N, molecuIes’. The new results obtained Peak No. 2 is due to ionization for energy calibration. Peak No.

used for the determination

of the energy of the

with high energy resolution are shown i;i Fig- 12. of Hg by argon atoms in the 3P2 state and serves 1 corresponds to ionization by Nz particles whose

i

I

I

0

I

I

I

I

I

05

I

I

V

Fig. 12. Electron energydistribution curve for ionization of Hg by N1* (peak No. i) and Ar*(jP2) (peak No. 2) states. V: accelerating or decelerating voltage. ht. J_ Mass Spectrom. Ion Phys., 7 (1971) 399413

v. CERMAK,

412

3. B. OZENNE

energy is 0.34 FV higher than the energy of the argon 3P2 state. Thus, the excited fl state is the Iong-lived E3Xl state28-26 with an energy of 11.54+0.34 = 11.88 eV, which is very close to the spectroscopic value of 11.87 eV. The broadening at the higher energy side might probably be accounted for by the associative ionization in which lrrjHg* ions are formed15_ The results presented here make the previous interpretation of peak No. 2 as the Y = 1 vibrational level of the E3ZZz state’ invalid. Evidently the nitrogen used in the earlier measurement was contaminated by argon. The presence of the flE3Zc state in a molecular beam bombarded by 40 eV electrons having now been cIearIy demonstrated, one must stress its importance in such a beam and correct previous statements in this respect”.

CONCLUSIONS

Studies on PIES using an apparatus with high energy resolution disclose the speci5c futures of Penning and associative ionization in more detail, especially when t%e electron energy distribution curves are compared with those obtained by photoelectron spectroscopy: the differences in shape and peak positions reveal, amcngst other things,the interactions of heavy particles at thermai energies, which are sometimes very weak and would hardly be detectable by another method. In the case of ionization of NO, C&l4 and C6H6 the Franck-Condon character of the ionizing transition has again been demonstrated The values of the Franck-Condon fztors in ionization of NO indicate that the internuclear distance in the target mofecclle can be slightly perturbed on collision with metastable atoms. The ionization of Hg by argon metastables is a case suggesting that the ionizing transition does not occur in the repulsive part of the potential energy curve, as in the systems studied earlier ‘, but in a certain range of particle separations. On the other hand, if the energy of electrons reIeased in the transition at the turning point is measured, the dissociation energy of the products of associative ionization can be estimated_ PI’ES seems particularly useful when applied for relative population and energy measurements with more than one excited long-lived species present simultaneously in a beam. 1n this case no preliminary separation of the individua1 excited species is necessary.

AClCXOWLEDGMEiVl-S

The authors wish to express their gratitude to PI-ofessor hf. Magat for his steady support and interest in their work and to their colleagues in the Laboratoire de Pbysicochimie des Rayonncments for many helpful di:cussions. One of the ht. 3. MQSS Spectrom- Ion Phys_. 7 (137f)

399413

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authors, V. cerm6k, wishes to thank the C.N.R.S. for the position offered to him, as well as J. and M. Durup for their generous, invaluable and friendly support before the work was started and during all its phases. He is.also very grateful for the extremely skilful participation of B. Benali and assistance of A. L. Schmeltekopf in the development of this work.

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