Rydberg series in small molecules

International

Joarnal

of Mass Spectrometry

191

and Ion Physics

Zkevier Publishing Company, Amsterdam. Printed in th.e Netherlands

RYDBERG

SERIES IN SMALL

MOLECUI.ES

XII. PHOTOELECTRON SPECTROSCOPY STRUCTURE OF PYRROLE

P. J. DERRICK, L. &BRINK, Department

of Physics,

0. EWVIST,

The Royal

Institute

AND

ELECTRONIC

B.-G. JONSSON AND of Technology,

S-l00

E. LINDHOLM 44 Stockholm

70 (Sweden?

(Received November 23rd, 19% j

ABSTRACT

The photoelectron spectrum of pyrrole has been measured up to 25 eV and its charge exchange mass spectrum has been determined as a function of ener,oy. New Rydberg series have been identified in the ultraviolet spectrum. This information, together with quantum-chemical calculations, allows a description of the electronic structure of pyrrole to be given. THE THEORETICAL ELECTRONIC STRUCTURE OF PYRROLE

The electronic structure of the ground state of the pyrroie moIecuie can be described with eighteen molecular orbit&. The energies of those orbitais have been calculated, and results for the outermost orbitals are shown in Tables 1 and 2. The orbitals are numbered so as to correspond to the orbitals of furan’. The authors of this paper used an extended Hiickel method (EHM), Clementi, Clementi and Davis2 used an ab initio SCF method, and Clark3 and Dahi and Hansen” used semiempirical SCF methods_ LMolecular orbital calculations often offer a reliable guide to the forms of orbitals, and we have used the eigenfunctions from the extended Hiickel calculation and from the calculation of Cl _ l.enti, Ctementi and Davis’ to obtain an approximate description of the molecular orbitals of pyrrole in the manner devised by Jonsson and Lindholm’ for the study of benzene. The molecular orbitals are described in terms of their constituent atomic orbitals, and are then c!assified into four types: z, t (tangential), I (radial) and s (Table 3). The symbols used in Table 3 have been defined in earlier papers Is5 _ The nitrogen atom is numbzred 1 and the carbon atoms are numbered consecutively around the ring; a hydrogen atom is given the same number as the nearest heavy atom. Table 3 shows the vertical ionization potentials (IP’s) resultin g from this paper, and the m’s of the analogous pyridine orbitals6. hr.

J_ Mass Spectrom.

Ion Ph_w., 6 (1971)

191-202

192

ht.

J. Mass

P. JmDERRICK

Specmom. fan Yhyse., 6 (1971)

191402

et al.

RYDBERG

SERIES

IN

SMALL

MOLECULES.

193

XII.

T_4BLE 2 CALCULATED

ES~RGXES FOR THE 2 ORBiTALS

Orbital

(this paper) (Clementi, Clementi and Davisl) SCF (Clark3) SCF (Dahl and Hansen*)

i3w

OF PYRROLE

ial

.=,

- 12.5

- 13.7

-16.5

--10.5 -10.0

-11.6 -11.1

-17.2 - 19.4

-

-

-

lb,

SCF

TABLE

4.7

14.5

3 FCR!

APPROXLU%TE

Orbital

3.5

OF THE MOLECULAR

Vertical

Approximate

ORBITALS

form

OF PYRROLE

IP of analogous p_widine orbital6

Type

IF (eev) (rhis paper) Ia2

8.2

2bi lb,

9.2 13.0

7a1 601

12.6 14.8 13.7 14.3 17.5 18.8

4h=

18.1

9al 56, 662

Sal

362 Sa,

&I

(e VI -7 -7

Ft

r \

(la=)

9.6 12.5

(2&J

12.2 15.4 14.2 13.4 16.9

f ! t r r

22.3 23.8

9.3

(lb,)

(7b2)

f5bz) K&J

S

>

i9.2

(lOal) (8al) (7a,)

S

>

19.2

(462)

S S

-29

S

Approximate descriptions of the bonding effected by the different orbitals can be obtained from Table 3 and from the orbital overiap populations of the extended Hiickel calculation. It is found that: la2 is 2b, is lb1 is 9q is 5b2 is 6& is 8a, is 7a, is 6u, is 4b2 is 3b2 is 5a, is 4a, is

weakly C-C bonding, weakly C-C bonding, C-C and C-N bonding, strongly C-C bonding, C-N bonding, C-I-I bonding, C-H and LX-H bonding, C-H and N-H bonding, C-C and N-H bonding, strongly C-H bonding, strongly C-C bonding, strongly C-C bonding, C-N bonding...-;_

194 THE

P. 3. DERRICK

PHOTOELECTRON

SPEaRUM

et d.

OF PYRROLE

The photoelectron spectrometer has been described earlier’=*_ The photoelectron spectrum up to ionization potentials of about 18 eV has been measured previously by Elands; our measurements (Fi g. 1) extend the range of ionization potentials up to 25 eV and improve the resolution of vibrational structure. The structures of the 8.2 eV and the 9.2 eV bands have been published in an earlier paper’ O. Details of the photoelectron spectrum are given in Table 4. The very weak band at about 16 eV (Fig. 1) is not believed to be an IP of pyrrole, and is not included in Table 4. Th.e vibrations are numbered as in an earlier paper”.

Fig. l_ The photoelectron Table 4).

!i;les (cf.

spectrum

ofpyrrole:Isingthe(a) 584 A and (b) 3M A heliumresonance

The bands at 8.2 eV and 9.2 eV can be assigned to be st tirbitals la, and 2bI on the grcunds of comparison with pyridine (see Table 3 and ref. 6), benzene’, and furan’. The similarity*o of the vibrational structures with those of the la2 and 2bI bands in furan identifiesthe 8.2 eV band as laz and the 9.2 eV band as 2b, ; the furan bands were identified from Rydberg series’. The identifications of the pyrrole bands are supported by the Rydberg series discussed in this paper and by the quantum-chemical calculations (Table 2). The pyrrole photoelectron spectrum resembles those of pyridine (see Table 3 and ref. : l), benzene5*12, furanl, and thiophene’ 3 as regards the number and the distribution of bands in the 16-26 eV region. This similarity suggests that the bands in pyrrole between 17 and 20 eV represent the r-type orbital 7~zrand the s-type orbitals 6~2,and 4bz, and that those bands at 22-24 eV represent the s-type orbitals 3bz and 5a,. The Sa, pyridine orbital (Table 31, the 3aI, benzene orbita!, and the 7~2, orbitals in furan and t&iophene Euve IP’S ciose to 17 eV, and it hence is assumed that the ‘?a, orbiti in pyrrole 4as B similar IP. The 17.5 eV band in pyrrole is therefore interpreted as 7al. This band has a weak vibrational structure, which has not been interpreted with certainty due to a suspicion of impurities. The extended Hiickel calculation appears to give reliable values for the energies of the Int.3. MussSpecrront. Ion Phys., 6 (1971) 191-202

RYDBERG TABLE

SERIES

IT*’ SMALL

MOLECULES.

I95

XII.

4

THE PHOTOiLECTRON

Adiabatic IP

Vertical IP

(ev)

(eV)

8.209 (See ref. 10)

SPECXR~M

8.209

OF PYRROLE

Interpretation (this paper)

Ia2

(Cf.

Fig. 1) Vibrational structure

Szergy (me V)

9.20

(See ref. 10) -12.0 12.6 -13.0 ~12-6 -13.7 -13.3 -13-s -14.3 -14.3 -14.8 -17-5 -16.9 -18.1 ~18.8 -22.3

w-17 -1s -21 -22.5

-23.8

Inrerpretation

0

vs

0

10s

m

132 170 182

s

Y1

P&

m

95

m

216

W

;::,

240

mw mw

91 fS& 2114

264 305

9.20

Intensiry

W

Y*+Ys

315 352

w

+‘4+ a.6

W

YJ +

0

vs

0

108

S

1’1

Sharp maximum No interpretable

76

vibrational

No resolved vibrational

structure

structure

Overlapping bands with weak vibrational in the 16.9-17.3 region Overlapping structure

bands

without

resolved

structure vibrational

s-type orbitals (Table 11, and is thus used to distinguish 6a, from 3bz and 3b2 from 5al _ It further indicates that the ionization potential of the s-type orbital &zl is about 29 eV. The unidentisied bands between 12 eV and 16 eV must represent the orbitafs lb1 ,9a,, 5bz, 6b2, and 8a,, provided our previous assignments are valid. RYDBERG

SERIES OF PYRROLE

The far ultravioiet spectrum of pyrrole has been investigated by Scheibe Price and WalshI Pickett et al.’ ’ and Mullen Grieneisen 14, Milazzo”, Orloff 18. No Rydberg series have been report:d explicitly by these workers. have examined the results of Scheibe and Grieneisen, Milazzo, Pickett et al., Mullen and Orloff, and have identified three Rydberg series conforming to equations:

and and We and the

AE = s.209-R/(n-o.4q5, AE = 3.209 - R,$z ;0.03)‘, AE = 9.200-R/(n-0.80)‘. IJILJ. &fussSpecrronr.Ion Phys., 6 (1971) 191-202

196

P. J. DERRICK

et al.

Details of the series are presented in Tables 5-7. The vibrations are numbered according to the scheme” employed for the photoelectron spectrum in Table 4. The series will now be discussed separately. TABLE RYDBESG

5 SERIFS IN PYEIOLE:

hZz 3

8.209 c-“)

npb2 (-t

Overlapped peaks marked with the superscript* Principa2 quai?Jum mmzber (E) am3 the reference

Energy (ev)

Quantum

dcfecf(a

Vibrational structure Vibrationa!

Intensity

Interpretation

energy (me v>

3 (MulIen and 0rlofZ” and Pickdt et aLx6)

4 (Scheibe and Grieneisen*l and Pickett et al.‘“)

4 R&s.. 13 and I6

5 Ref. 17

6 Ref. 1’1

5.870

5.974 5.999 6.049 6.106 7.095 7.202 7.227 7.265 7.273 7305 7.337 7.X52* 7.116 7.222 7.247 7.285 7.296 7.317 7.352* 7.55.5* 7.672* 7.703 7.726 7_772* 7&W 7.956*

0.59

0.49

0.47

0.44

0.42

0 104 129 179 236 G 104 329 167 175 207 Z!39 254 0 :;06 131 169 i80 501 236 0 117 148 171 0 96 184

YS

m s S W s S S m W mS m m.s S m S

mw m rd mS vs m mw mw W VS mw

IL+ + npb, (-+ 8.209 eV) The quantum defects of this series (Table 5) closely resemble those of the analogous bctizcne seriesS leI, - npelU 5- lQ with quantum defects 0.59, 0.49, and 0-M fcr the n = 3, n = 4, a&i n = 5 members, respectively, and analogous series occur with simiku quantum defects in furan’, thiophene’ 3, and cyclopentadiene”. Then = 4 member appears to be a doublet with spacing 18 meV. We have therefore tabulated two memlxrs with n = 4 in Table 5, each with a clear vibrational structure which is very sknikxr to that of the la c - 1 band in the photoelectron Int- J. Mass

Spectrom.

Ion Phys., 6 (1971)

l%-Mz

RYDBERG TABLE

SERIES

Ipu’ SMALL

SERIES

IX

PYRROLE:

Overlapped peaks marked

la2

Energy @V)

3

6.778 et a1.16)

4

(Mullen and OrlofF’) 5

6 Ref.

17

+

fd.?2

(i

P.209

ev)

with the superscript*

Principal quanrurn number (n) and tke reference

Ref.

197

XII.

6

RYDBERG

Bickett

NIOLECULES.

Quantwn &fect (a)

-0.07

6.SS6 6.911 6.954 6.965 6.996 7.@20 7.042 7.396 7.485 7.5.55f 7.672L

Vibrationalstructure Vibration& intensity energy Cmev) 0 iO8 I33 176

YS

ms S

m m

I87

212 242 264 0 92 159 0 loo 134 0

-0.09 -*0.03

7.772* 7.806 7.837

--o_o_z

Interpretation

IIlW ms ms

mw mW n

m mw W mw

17

TABLE

7

R’i’DBERG

SERL!Z

Overlapped

LN

PYRROLE:

peaks marked

%,

+

nsal

(4

9.200

ev)

wit!~ the superscript*

Principal quantum number (n) and the reference

Energy (ev)

Quuntum Vibrational structure defect (a> Vibrationaf Intensity energy (me v)

3

6.234

0.86

0

S

0

0230

84 0

m vs

VI 0

88

mw

1’1

(Mullen 4 Ref.

and Orloff”)

17

spectrum Rydberg

6 318 7:868+ 7.956*

(Table 4). It is improbable

Interpretation

that this doublet structure

series, since there appears to be only one corresponding

represents

two

n = 3 member.

Then = 4 doublet system was not analysed by Pickett et a1.17, as only one of the two very strong peaks at 7.10-7.12 eV w&s observed due to the presence of nitrogen emission lines. The two peaks are obvious in the measurements of Scheibe and Grieneisen’ 4 and of Mullen and Orloff I*. Int. J. Muss Spectrom. Ion Phys, 6 (1971) l!?l-XC

P. J. DERRICK

198

et af.

Iaz + rtalzz (-+ 8.2OQ eV)

The high intensity and the pronounced vibrational structure of the n = 3 member at 6.778 eV (Table 6) indicate that this is an ailowed Rydberg series, and the striking similarity with the 8.209 eV photoelectron band (Table 4) indicates the convergence limit. The quantum defect, - 0.03, is reasonable for an nd Rydberg series, so the series will be identified as la, + “da,, although it could also be la, ---, ndb, or la, + nd& . The series cannot ‘be an nfseries due to the presence of an n = 3 member. 2b, + nsq

(4 9.200 eVj

The n = 3 and n = 4 members of this series can be observed (Table 7).

THE CHARGE EXCHASGE MASS SPECTRUM OF PYRROLE

Charge exchange rnas spectra have been measured with a tandem mass spectrometer5 in the manner described previously’ 1v22. The mass spectra are presented in Table 8. No allowance has been made for the 13C isotope. The relative cross-section Q has been defined as the sum of the intensities of the peaks in the mass spectrum divided by the intensity of the slow positive ions effecting ionization, and is expressed in arbitrary units. The mass spectrum as a function of the ener_gyabsorbed during ionization is displayed in Fig. 2. The cu~--vesare normalized so that the sum of their ordinates

4

2 0 _

11,1fi* 8 10

12

14

IG

18

20

22

eV

F&r.2 The mass spectnm of pyrraIe as a fuzxtion of the energy absorbed during charge exchange with incident positie ions of low kinetic ener,T_ Int. 3. Mass Spectrum, Ion P&s.,

:S (1971) 191-202

RYDBERG TABLE

IN

SMALL

MOLECULES.

XII.

199

8

-MASS SPECTRA Kli’i&l-IC

SERIES

OF PYRROLE

ENERGY

OBTAINi

IN CHARGE

EXCHANGE

WITH

INCIDEXr

Zncident

Recombi-

KE

iOn

nation


energy

mle

26

27

28

37

38

39

40

41

Il.2

1 3 10 30

Cd&+

11.4

1 3 10 30

2 1

2

2

9 7 6 7

1 10 30

3 4 4

1 2 4

12 13 13

40 42 40

1 3 10 30

4 4 4 3

1 1 2

23 22 22 21

45 41 41 40

13.8

1 3 10 30

IO 9 8 7

5 5 5 6

30 30 28 24

33 28 27 24

14.0

1 3 10 30

14 12 10 10

13 14 15

27 26 26 23

27 26 24

15.76 15.94

1 3 10 30

L6

52 53 52 54

2 2 4 5

16 17 15 14

21.56 21.66

30

31

12

2

N20’

CO’

Ar’

Net

IONS OF LOW

n

cos+

H.O+

POSITIVE

(KE)

12.4

12.7

1

13

24 23 20 2

2

14

7

23

23

52

66

67

loo 100 1 99 4 96

tl
1

90 93 92 87

2

44 39 37

2 2 ‘2

1

28 32 32 33

2 2 2 2

1 I 1 1

222 2 25 3 28 3 35

2 2 3 3

I 2 2 1

4 3 4 4

14 18 20 24

2 3 3 2

2 2 3 3

2 2 3 4

tl
7

Cl

1 2 2

I

at any point is equal to 100. The general methods fcr constructing the curves have been discussed in earlier papers”-J*23*2’ and only the 9 oA of m/e 41 with incident C2H2 + ions requires discussion here. The probable explanation is that the intensity of ‘&e pyrrole parent ions is low at the Iowest kinetic energy of the incident ions (1 eV), as the recombination energy of C,H,‘, 11.4 eV, falls in a gap (see Fig. I) where pyrrole ions cannot be formed. Consequently, the small number of fragment ions is exaggerated when expressed as a percentage in the mass spectrum. Znt. J. MISS

Specirom.

Zon Phys., 6 (1971) 191-202

200

P_ J. DERRICK

et al-

The mass spectrum will be discussed with a view to identifying bands in the photoelectron spectrum. It is clear that ionization of the orbitals at 8.2 eV and 9.2 eV does not lead to c%ssociation, as the lowest appearance potentials of fragments are several eV higher. This is consistent with the identification of these orbitals as the weakly bonding rr orbitals la, and 2bl. The ring breaks at about 12.0 eV, yielding the dissociation products C2HzNzYt, C2HzNf or CNH2’. It is assumed that these dissociation processes are associated with ionization of the orbital with adiabatic IP 12.0 eV, and., therefore, that this orbital has ring-bonding properties_ Tons of m_/e60 appear at about 13.8 eV indicating the loss of hydrogen from the parent ion. It is assumed that there is an orbital which bonds hydrogen atoms to the ring with adiabatic ZP close to 13.8 eV_

THE ELECTRONIC STRUCTURE OF PYRROLE

The orbital with adiabatic IP 12.0 eV effects ring-bonding as evidenced by the mass spectrum, and is therefore likely to be either the rc orbita lb1 or one of the t-type crbitals Bal and 54i. The calculations will be considered as reliable in indicating that the IP of 9a, is iower than that of 5& (Table 1): so that the choice lies between lb, aud 9q. The quantum-chemical calculations cannot be used

directly to identify these orbitals, so instead we shall perform a comparison between pyrrole and pyridine in order to locate t&e lb, orbital_ The extended Hiickel calculations give approximately the same value for the energies of the lb, orbitels in pyrroIe and pyridine (- 16.5 eV and - 16.4 eV, respectively); similarly Clementi, Clementi and Davis’ fmd - 17.2 eV for lb, in pyrrole and Clementi” finds - 16.9 eV for lb1 in pyridine. It seems to be certain that the adiabatic IP of 15, in pyridine is about 12.5 eV; it must be higher than *hat of benzene, 12.1 eV5s” (see also ref. 26) and lower” than that of s-triazine, 13.2 eV1l. Thus the calculations suggest that the adiabatic TP of lb, in pyrrole is also about 12.5 eV, and we therefore attribute the band with the adiabatic IP of 12.6 eV to ib, _ The band wi:ih adiabatic IP 12.0 eV must therefore be 9a, _ It thus seems probable that ionization of the 9a, orbital leads to the rupture of the C,-C, bond in the formation of C2H2NHt and C2H2Nf, and to the rupture of the C,-C, or C,C, bond in the formation of CNH2’. It is satisfactory to observe that ionization of the highly analogous furan orbital 9a, leads to the rupture of the corresponding bonds in furan r _The vertical IP’S of the two 9a, orbitais are also similar, 12.6 eV in pyrroIe and 13.0 eV in furan. The vaiue of 13.0 eV

for the vertical IP of lb1 in plyrrole is also satisfactory, as the values in cyclopentadiene* O and furan’ are 12.6 eV and 14.4 eV, respectively_ The 6b2 orbital in pyrrole is very similar in form to the 6bz orbital in furan’ and the contribution from the heteroatom is small in both orbitals, in which case 1~. 1. MUSSSpectrum.ZenPhys., 6 (1971) 191-202

RYDBERG

SERIES

IN

SMALL

,MOLECULES.

201

XII.

the energies of the orbita& should be similar. The vertical IP of 6bz in furan is beIieved to be 13.8 eV’, so the 13.7 eV band in pyrrcle is attributed to 6&. The extended Hiickel caicularion for p_yrrole, which is reliable in the case of the r-type 7a,, gives - 13.8 eV for the energy of the r-type 6bz. The orbital 6b2 is C-H bonding and its adiabatic IP is about 13.3 eV, so one wouid expect to observe loss of hydrogen after ionization of one electron at this energy. It seems from Fig. 2, however, that loss of hydrogen begins at a somewhat higher energy, about 13.8 eV. This indicates that loss of a hydrogen atom from a carbon atom is not an important process, in agreement with the very small probability for loss of hydrogen from furan ions’, and further that the hydrogen is lost from the nitrogen atom, in agreement with the high probability for loss of hydrogen from cyclopentadiene ions *’ _ We therefore identify the band with adiabatic IP 13.8 eV as the N-H bonding orbital 8a,. A study of the mass spectra of partially deuterated pyrroles would clarify the situation. The remaining unidentified band at 14.8 eV must be attributed to the r-type orbital 56,.

ACKNOWLEDGEMEhS

One of the authors (P.J.D.) wishes to thank the Royal Society, London, for the award of a Feliowship under their European Programme. A sant from Malmfonden - Swedish Foundation for Scientific Research and Industrial Development - has made possible the construction of the tandem mass spectrometer. The work has further been supported by The Swedish Natural Science Research Council. We wish to express our appreciation to Dr. Rolf Manne for heIp with the extended Hiickel calculation.

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Spec-

Phys_. 46 (1967) 4725.

3 D. T. CLARK, Tetrahedron, 24 (1968) 4689. 4 J. P. DAHL AFCPA. E. HAKSEN, Them. Chinz. Acta, I (1963) 199. 5 B.-t). Joxssos AXD E. LINDHOLM, Ark. Fys., 39 (1969) 65. 6 B.-O. JONSWN, E. LIXDHOLM XND A. SKERBELE, inr. J. Mass Specrrom. Ion Phys., 3 (1969) 7 0. EDQVIST, E. LIFGGHOLF.~, L. E. SEUN A&Z L. ~%SBRIXK, Physica Scripta, 1 (1970) 25.

385.

8 0. ED~VIST, E. LINDHOLM, L. E. SELIN, H. SJ&XEN AND L. &BRINK, Ark. Fys., 40 (1970) 439. ELASD, Inr. J. Mass Spectrom. Ion PhFs., 2 (1969) 471. 10 P. J. DERRICK, L. &SBE(INK, 0. EDQVIST mm E. LIS;DHOLM, Spectrochim. Acza, to be published. I1 E. LISDHOLM, to -be published. 12 L_ &BRINK. 0. EDQVIST. E. LIHDHOLM AKD L. E. SRIN, Chem. Phys. Lezt., S (1970) 192. 13 P. J. DERRICK, L. ~~SBKINK, 0. EDQ~IST, B.-o. Joxssos ASJ E. LIX~HOLII, Int. J_ Muss Specrrom. /on Phys., 6 (1971) 176.

9 J. H. D.

Inr. J. Mass Spectrom.

Ion Phys_, 6 (1971) 191-202

202

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et al.

H. G-PI-, Z. Phys. Chem., 25 (1934) 52. S?ecrrochim. Acra, Z-(1942) 17. 16 W. C. PRIDE AND A. D. WA=, _&UC. Roy. Sot. (Lundon), i79A

14 G- %HSBE

&m

15 G- Mu~uo,

(1941) 206. L- W. PICgEfi. M. E- Co-G, G. I% WIEDER, D_ A. S~OXV phi J. M. BUCK, Chem. Sue., 75 (19S3) 1618. 18 P- k MUAND M_ K_ ORLOFF, J. Chem. P/iys., 51 (1969) 2276. 19 O- mws-r, E- LI~DHOW A-Z L_ &musz. to be published_ I7

J.

Amer.

~RIXK, 0. Emvxsr, B.-h JOLSON A~‘D E. iIh?)HoLH, Int. 1. Muss Specnom. ion Pfzys., 6 (1971) 203. 21 E- LIXDHOUi, Adrnn Chem Ser.. 58 (1966) I_ 22 H. vo? KOCH _A?? E ~ILUiHOLM, Aric. Fys-, 19 (1961) 123. 23 E. LISDHOLM, in J.F-L~ (Editor), ion-&folecufe Reactions, Plenum Press, New York, in

Z@ P. J. DERRICK, L.

preparation. 24 E. Lm-mio~ss A&V G. !%HLS&‘l, Int. J_ :%fass Spectrom. Ion Phys__ 4 (1970) 25 I% am, J. Chem. Phys-, 46 (19677) 4731. 26 B. NARXY~V mm J_ N_ Murwu. Mol. i%ys_, 19 (197(i) 169_ Int_ J. Akss Spectrom. Ion Phys.. 6 (1971)

191-202

46S_