A study of charge exchange reactions in a time-of-flight mass spectrometer

Internaiionai Journal of ,Wass Spectrometry and ion Physics Elsevier Publishing Company, Amsterdam. Printed in the Netherlands.

A STUDY OF CHARGE MASS SPECTROMETER

MICHEL J. FROMOXI

EXCHANGE

REACTIONS

235

IN A TIME-OF-FLIGHT

AXD RUSSELL H. JOHNSEN

Radiation Chemistry Laboratory, Chemistry Depamnent, The Fiorida Stare Unirersity, Tallahassee, Fix (U.S.A.) (Received

December

23th.

1969; in revised form March

3nd, 1970)

ABSTRACT

Ion-molecule reactions occurring in the drift tube of a time-of-flight mass spectrometer were studied using a retarding potential device and a second gas inlet on the drift tube. LMetastable ion decomposition, induced dissociation and charge exchange reactions were observed for ions having an energy of 2800 eV. For the charge exchange process, the experimental determination of the ratio of the neutra1 and initial beam intensities permits calculation of ihe cross section. Cross sections obtained for nitrogen and argon a;e in good agreement with data from the literature. Cross sections for charge exchange between several organic ions and noble gases are reported.

I. INTRODUCTiOS

In the normal operation of a time-of-flight mass spectrometer, reactions which occur in the drift tube following acce!eration are not observable. However, it is possible to modify the instrument in such a way as to permit the study OF a variety of secondary reactions. AlI techniques are base< 3~1the same principle: the use of a variable retarding potential device inside the drift tube. Neutral fragments are obviously not affected by this device, but ions are seiectively retarded accordi@ :o their mass, charge and kinetic energy. Ferguon, McCulloh and Rosenstock’ have used the stack of 2 Bendix T.O.F. mass spectrometer as their retarding potential device. Charge exchange, collision induced dissociations and spontaneous ion decompositions were observed but only qualitative results were obtained. McLafferty, Gohlke and Golesivorthy’, as well as Dugger and Kiser” have used the same device and also developed quantitative expressions for metastable ion decompositions that have per-r&ted the determination of the mass of the fragment ion, and the shift of its peak with change in the potential of the retardIn:. J. Mass Spectrom. Ion Phys., 4 (1970) 235-249

236

M. J. FRO.MONT,

R. H. JOHNSEN

ing assembly. Hunt et aI.“-6, -taed a different retarding assembIy but their resuhs wzre similar. The main inovation in this work was that the retarding assembly could be moved to various positions in the drift tube which permitted a &t approach to the study of metastable ion lifetimes. The purpose of the present work was to study reactions occurring in the drift tube of a time-of-flight mass spectrometer. Charge exchange was the main phenomenon studied and a number of cross sections for this process were determined.

_3fodz~cations

of the mass

spectrometer

In order to study ion-molecule interactions, the Bendix _Model-14 mass spectrometer was modified as follows- As suggested by Ferguson et al.‘, the stack of the detector was utilized as a support for the retarding assembly_ Grids were

Fig. G,. a = e =

‘b 1. Detector with stack modified as retarding device. G, = drift tube voltage (-2800 V); Gj = variable vokage (--3ooO V to -200 V); G1 = reaccererating grid (-2600 V); shield; b = cathode No. 1 (--3COO V); c = cathode No. 2; d = field strip of the detector; dynode strip of the detector_ 0

i

h

f --------_-_---_--__

--_----

I(

~;:llil~~_~~.:~~____:i;;

a

Fig.

2_ Modified Bendix 14 nxus spectrometer. a = normal gac inlet; b = filament; c = beginning

of the drift tube;d = diaphragm;e = ionizationgaugeP7o.1; f = secondgasinlet; g = precision valve; h = ion&&ion gauge No. 2; i = accelerating grid. rnf. <. M-ass Spectrohl. lt?RPhys., 4 (1970) 235-249

CHARGE

EXCHANGE

REACTIONS

IN

A T.O.F.

MASS

SPECTROMETER

239

peaks

Parent

peak

2.4 -

Stack in

Voltage Volts

Fig. 4. Shift of the ion peaks with change in the potential of the retarding device.

points are experimental. At low voltages our results are similar to those of Eunt et aI.*, and the fragments of mass 42 and 43 are not separ::ted. However, in our experiments the retarding assembly is longer and when rhe retarding potential is greater than - 1400 V the separation of this peak into two is observed. Induced dissociation Tf a neutral target gas is introduced into the drift tube, the intensity of rhe fragment peak from a metastable ion decomposition is usualIy increased. In Fig. 5

I

0

4

8 P x

12

16

105

urgent

20

24)

Fig- 5. Induced clissxiation for the CcH,,’

ion from n-butzne, 1~. I. Afa.rs Specfrom. ion Phys., 4 (1970) 23-5249

hf. J. FROMCfiT,

240

P-. H. JOHNSEN

the ratio Jr/Ii versus pressure of the neutral gas is plotted. The stack voltage was - 1900 V, the neutral gas was argon, 1, = intensity of the fragment peak arising from the decomposition of the C,HFo peak of n-butane. 1; was the intensity of the peak of mass 59 of n-butane. ifs9 was used instead of Is, because Is, and I, were of comparabIe intensities and they could be recorded at the same time with the same sensitivity Two processes can be used to explain the increase in I,. One involves collision where the parent ion was net metainduced dissociation as defined by Melton”, stable and would not have been dissociated even with a longer time of fright. This process would be endothermic and require conversion of some kinetic ener_gy into dissociation enerm. The second process is the induced decomposition of a metastable ion whose lifetime for spontaneous dissociation would normaMy have been greater than its time of fright, but which has the necessary ener,T required for disscciation. Charge exchcnge Neutral species can aIso be produced by charge transfer collisions between energetic incident ions and thermal energy neutral target moIecules in the drift tube. In such collisions, momentum transfer is negligible and the velocity of the resulting particles remains about equal to that of the incident ion. Tf we consider the process

(4) measurement of the number of neutral X5 and of ions Xi that initially enter the drift tube, should permit us to calcuIate the cross section for t& process. The main purpose of this paper is to present the results obtained on this subject.

IV.

EXPERME?XAL

DETERMINATION

OF CHARGE

EXCWGE

CROSS SECTION

Experimental procedure For the process outiined in equation (4) the general cross section equation is N where: N+ N d Q L

..= Nfe-dQL = = = =

initial ion beam attemuated ion- beam density of neutral target crosssection

7

effective lei;gth of the coliision chamber.

ff the pressure Im. 3. Mass s-crk?z.

of neutral earget in the collision chamber is much less than Ion PIlys_, 4 (1970)

23-249

CHARGE

EXCHANGE

REACTIONS

IN

A T.O.F.

MASS

SPECTROMETER

237

fitted as shown in Fig_ 1. The potential of the connected grids G,, G, could be varied from -200 V to -3300 V. A second gas inlet was placed in the drift tube (Fig. 2). A precision metering valve permitted regulation of the flow of gas and maintenance of a constant pressure in the drift tube. A diaphragm was installed between the source and the drift tube to reduce diffusion into the source from the drift tube, and to permit differential pumping when required A second vacuum ionization gauge was connected to the drift tube (Fig. 3) The electrostatic lens was connected electricaliy to the drift tube, since its adjustment would only have improved the focusing of ions without a similar effect on the neutral particles. Pressure reading

As mentioned above, two ionization gauges were used for pressure reading: gauge (1) situated near the cold trap and vacuum pump; gauge (2) on the drift tube (Fig. 2). When the gas was introduced into the source, the pressure was about the same at these two points. When the inlet on the drift tube was used, pressure at gauge (2) was higher than at gauge (1). The ratio was about 10 to 4. No difference was observed experimentally for various gases as shown in Table 1. TABLE 1 PRESSURE GR4DD3T

Pressure in the &ift tube X IO-j (gauge So. -7) 5 lc: 15 20

m -

hLL\ssSPECfROMETER

Pressure 0~ gauge No. I Air

FOR VARIOUS X

GASES

lo-’

Ar

xi

Xf?

2.0

2.0

4.0 5.8 7.6

3.6 5.4 7.7

2-O 4.1 6.3 7.5

2.0 4.2 6.2 8.0

The gauges wyrzrecalibrated wi’th nitrogen. When an absolute determination of the pressur,. p was needed, the sensitivity factors determined by Dushman and Young’ were used.

Detector The standard Bendix iMagnetic Electron Multiplier detector described by Goodrich and Wiley*, was employed. The response level of this detector is a func-

tion of the mass, composition and kinetic energy of the particIe which reaches the cathode. A question very important to this study is whether the number of secondary e!ectrons per incident particle is the same for an ion and the corresponding neutral with the same kinetic energy. In a_mement with the work of Schackert” the assumption was made that the response of the detector is in fact about the same in these two cases. I.,ater the validity and consequences of this assumption wili by discussed. ht.

J. Mast _ Spectront. Ion Phys., 4 (i970) 235-249

M. J_ FROMONT,

238

R H. JOHNSEX

The fact that the terminai tinetic energy of the ions and the neutral fragments are slightly different may be mother source of error. The ions are reaccelerared to so00 eV just before rzaching rhe detector (Fig. I). The neutrals are not aEectzd by this electric field yZld they retain the kinetic energy they had at the time of their formation (28CB eV)_ ThereCzre a slightly smal!er response would be expected for the neutral sps;ies compared to the ion.

III.

-PROCESSES

IN THE MASS SPEZTROMEfER

OBSERVED

With the modified T.O.F. mass spectrometer three types of reactions occurring in the dri& tube were observed: metastable ion decompositicn. in-duced dissociation and charge exchange. These are discussed in turn. Mer~stabie

ion decompositiou

‘The ti_meof Aight of a.;rion in ihe drift tube is of the order of8 psec. During this time the ioa can undergo a spontaneous dissociation of the type

A+-+B++C

(1)

where A’ is the parent izm, B’ the fragment or daughter ion and C is the neutra1 fragment. This phenomenon is normally not observed since the fragments have the same velocity as thz parent ions, If however the voltage or^tk stack is increased to retard the ions, the neutrsi peak is unaffected and the ion peaks are shifted toward longer times of flight. The fragment ion which has the same initial speed but a lower mass than the parent ioi: fs reta_ *=rded even more. The result is shown schematically in Fig. 3_

_A__f. 0. F.

Fig. 3. Norma.l sjj

and sp,rctxun with retarding potential device for a metastable ion.

Figure 4 shows the results for two pathways for decomposition in n-butane.

C,H& ~.

--, C,Hf

C&O +

Tpe CUI&

(3

+CH,

C3H: + CH4

are cAxlated

using standard

e!ec:rostatic

considerations

(3) and the

CHARGE

EXCHANGE

30 x lo’&

REACK-IONS

IN A T-0-F. MASS SPECTROMETER

241

tcrr the crass sectirx can be calculated from:

p=zcL

(6)

iv’ dL

Q=--

N”

1 10 -I3

w

P 0.3275 s L

(7)

?-6X1O-5

Fig. 6. Nor,nal peak and neutral peak for the process: N=+ -LAr + N=+Ar+ of argon increases.

P

Fig. 7. W/N*

x

105in

tow

versuspressure for the process: N2 + fAr

+ N=+Ar+_

P-S*iO

-5

wheo the pressure

M. J. FROMONT,

242

where Q is in

cm’,

P

in lo-’

R. H. JOHNSEN

torr, L in cm and W is the neEtra1 beam produced

by

chzrge exchange. The gns X whose cross section is to be studied is introduced into the source, and the target gas M into the drift tube. The pressure of _M in the drift tube is increased step-wise tu 2 x IO-’ torr. For each value of the pressure two determinations are made. The first is made at a stack voltage = -2800 V which is equal to the drift tube voltage. The intensity of the parent peak under these conditions gives N’, the number of ions I;*. The second is made at a stack voltage of -200 Y. Under these circumstances the ions arc delayed and only N’ the number of neutrals X” is recorded (Fig. 6). hPiNG was then plotted versus P and a straight iine (Fig. 7) was obtained in the range of pressure used for most ions. The slope of this line gives (N”/N’)(ljP). Determination:of rhe eflecrit-elength (L-j of the coNision chamber An experimental determination of the effective length of the ion path was necessary in order to caIculate the absolute cross section. The profiie of pressure in the mass spectrometer is not known and no direct measurement of I. could be obtained. However, by using a charge exchange reaction for which the cross section is kno;lm it becomes possible to calculate the vaIue of L. Hasted’ 1 has presented a review of experiment& and theoretical results for symmetrical charge exchange for both argon and neon. The average cross sections reported were Q = 21.5 x IO-‘s cm’ for argon and Q = I1 x lo-l6 cm’ for neon_ Figure 8 depicts the values of W/N’ obtained for argon and neon in this laboratory. Using these results together with eqn. (S), which is obtained from eqn. (7), L can be obtained. lo-‘3 lv”03275 PQ

L =--_. Ni

09

P x IOS in ton Fig. 8. W/IV+ Ink .Y_ Mms

versus pressure for resonant

Specmm.

Ion Phys.,

4 (1970)

charge exhange 235249

processes

(Ar and Ne).

CHARGE

ESCHAYGE

REACTIONS

IN

A T.O.F.

MASS

243

SPECTROMETER

Thz value oft found using argon was 20.0 cm and 21.0 cm for neon. The average value of 20.5 cm was adopted. These values are the average of five experiments for each gas. The reproducibility of the results for this length determination was -t_5 OA_ Results In order to check the validity of the T.O.F. technique results were compared with cross sections reported in the literature. There are very few determinations of charge exchange cross sections at 2800 eV. Results obtained at lower and higher energies, however, indicate that cross sections, for both resonant and non-resonant charge transfer do not change rapidly with energy from 900 eV to 3000 eV. Therefore it seems valid to compare these results with others obtained at lower energy (Table 2). TABLE

2

CHARGE

TRASSFER

CROSS SECX-IONS (_k=)

iv= ‘-iv=

-900

2CWO

lVZ+-ilr

2800eV

--900

References

Ar’-N~

2Gm

2800 e V

9W

Zoo0

2600eV 12 13 14 15

26

30 29 23 15 26

16 17

17

7

23 29

27

23

21

22

21

‘. 1 13

18 19 20

15 14

14 17 IS

COLLLSIOS-INDUCED

23 24

14.5

21

22-5

V.

22 18 17

CHARGE

EXCHANGE

FOR ORGANIC

l%is work

IONS

Organic ions with noHe gases Cross sections have been determined for charge exchange reactions between ions belonging to the cracking pattern of an organic hydrocarbou, and a noble gas as the neutral target. The main advantage of this technique over the procedure of total charge collection ” is that the cross section for a series of ions is determined in a single experiment and the neutral products are measured directly. As shown in Fig. 9 the parent spectrum and the neutral spectrum are recorded for each pressure_ W/N- versus P is plotted for each mass as in Fig. 10. Table 3 gives results obtained with argon, krypton and xenon as neutral targets. For the same ion arising from Inf. J. Mass Spectrom.

Ion Phys., 4 (1970) 235-249

244

M. J. FROMONT,

R..H.

JOHNSEN

Fig. 9. Charge exchanp, bet\\ een ions of mass 25 to 30 from ethane and argon (P = 1.6 x 10m4). spectrum_ Neutral s~mm magnified 5 times.

NormaI

Fig_ 10. W/X+ versus pressure for charge exchange between ions from the cracking pattern of ethane and argon as neutral target. Ions are identiEed by their mass_

the same molecule, Ihe cross section always increases from Ar to Kr and Xe as shown in Tabfe 4. This variation is presumably the resuit of the decrease in ionization potential for this series of noble gases. It appears also that the highest cross sections are found for ions which. produce a stable mokcule by ctiarge exchange and not a radical Thus cross sections for ions C,H:, GHz and C2Hb are significantly higher than for <=fHz and C&f. This mzq also be relaked to the relative -values of the ionization porentiak for these two types of ions.

CHARGE

EXCHANGE

TABLE

3

CH4RGE

EXCHAXGE

CROSS

REACTIONS

SECI-IOXS

OF

IN

A T.O.F.

VARIOUS

MASS

HYDROC.L\RBON

245

SPECTROMETER

IOSS

WITH

R;\RE

GAS

Z4RGEfS

Cross sectims i? .%‘; incident ion energy: 2800 eV.

zon

Target

Propane

Butane

Ethylene Hexane

Hepfane

Ar Kr Xe

Ar Kr Xe-

Ar

Ar

Ar

5.2 8.9

2.6 2.7 4.4 3.0 3.4 8.9

2.2 3.2 6.7 2.9 4.9 10.8

2.1 3.4

1.8 3.2

5.6 9.7

2.4 2-Q 5.1

2.2 3.5

2.0

2.0

Erhane ArKr 2-6 3.7 13.7

C=H,+ CzH,+ C2HG+

2.3 3.8 2.7 5.7 2.6 1.2

C+HS+ C2Hs+

2.4 5.3

s-4 1.8 2.3

6.5

TABLE 4 CROSS

SECTIOSS

CiOSS

sections itl _A’_

Mass of peak from butane 27 28 29 -

FOR

Ctl4RGE

EXCHANGE

BETWEES

IOSS

FROM

BUTAhZ

A>i

VXRIOUS

lOR

Af

Kr

Xe

GH,’ C2H;+ CZH,+

_._ 77 2.9 2.2

3.2 4.9 3.5

6.7 10.8 6.5 -

PiOBLE

GASES

In Table 3 is displayed the cross sections for exchange with the noble gases of a series of ions arising from a variety of hydrocarbons_ A comparison of the cross section for the same ion arising from a number of difterent hydrocarbons reveals a variation in cross section which seems significant. For example, the mass 28 ion reacting with argon shows a variation in cross section from 2.3 to 3.4 A2 which is outside the iimits of experimental error for these measurements. It seems reasonably ciear from the studies of 0ccoIowitz2’ on metastable ion abundance ratios that these ions have different energy distributions. The resuIts listed in

Table 3 suggest therefore that the cross section for charge exchange depends in part at least on the internal energy of the ion undergoing the reaction. Charge exchange between organic ions and parent

organic molecules

A number of experiments involving charge exchange between organic ions and their parent 3as were also made. As the sensitivity factors of the ior,ization gauge for organic molecules were not known, no attempt was made to calculate absolute cross sections in these experiments. Table 5 gives results for ethane compared with those of Lindholm and von Koch”_ The data have been arbitrarily normalized to coincide with Lindholm’s value for mass 27. The trend in the cross section is seen to be the same as that observed by Lindholm but the magnitudes arc different, especially for the C2Hg ion. The uniformity of results in the present case may be due to the appreciably higher kinetic ener,q of the incident ion. Table 6 gives results obtained for a variety of organic compounds. The results have been

arbitrarily normaIized to peak 26. Int. J.

Mass Spectrom. Ion Phys., 4 (i970)

23,S249

H. J_ FROMONT,

246 -l-ABLE -‘nVE

R. H. JOHNSEN

5 CROSS

SEEcnC

t-6 FOR

ClikRGE

EXCHANGE

B I?TWEE.li

ETHAXT

ANI)

IC_XS

FROM

IT’S CRACKING

PAY-rERN

26

/on

Ma35 of zhe peak

formula

26 ?.7 28 29 :o

C2I-527 CAL C=Hr-’ C,Hs ,czJ&+

VI.

DISiZU~ION

0.85 0.30 0.63 0.33 0.69

5.3 0.3 0.9 0.3 I.5

C~HI’ C&za’ C~H.$’

27 2s 29 30

Edane

Pf opane

Surwie

Heprone

3-,w+dpenfcwe

I.00 0.36 0.73 0.45 0.82

1.M) 0.20 0.45 0.20

l-o@ 0.16 0.47 0.17

I.00 0.1s 0.5s 0.17

1.00 0.15 0.62 0.14

GF THE VALIDITY

OF THII RESULTS

The gQestions which require consideration are: (i) does the ratio WIN depend only on charge exc&ange? (ii) what is the influence on the peak intensities &processes such as ion scdtering and metast&le ion &composition? The problem of scattering is easily resolved s&e we are interested in the ratio of neutral to toti peak intensi%ies which incIudes neutral fragments. The scattering of ions from t&‘_hp bezm by neutral targets in indicated by the decrease in *&e parent peak intensity wiih pressure (Fig. 6) and the oniy assumption is that neutrals aad ions are scattered with equal e%ciency as suggested by Utterback a nci Mitie?. In addition, some of the ions observed may be metsstable aud a small proptiion of the ne%r_ralpeak m:+y be due to spontaneous decompositions. This contribution is however ne&g;ibly small in the cases studLd_ When the pressur: ofthe neutral target molecules increases, the phenomenon of induced dissociations described in Section III appears. However, in the range of masses studied here (26 td 30’~ the or!y known mctastable ion deccmpos%ons are of the type?

hr.

i. Mt~.rs Spectrom.

Ion Ph,vr., i (1970) 231-248

CHARGE

EXCHANGE

REACTIONS

IN A T-0-F.

MASS

SPECTRO>fETER

247

Ln these reactions, the neutra! particle (H,) probably does not have enough energy to activate the detector. It seems reasonable therefore to assume that interference from the induced dissociation of metastable ions can be ignored. Examination of Figs. 7 and 10 reveals that the curves for W/N+ versus P exhibit a positive intercept. Similarly a small peak corresponding to neutral (N2) in the reaction N;’ +Ar -+ N, -i-Art at zero argon pressure is seen in Fig. 6. This formation of neutrals in the absence of a target gas has also been reported by Ferguson et al.‘. If it is not due to metastable ion dissociation as was concluded above, whar is the origin of this small background signal? The hypothesis was made &at this signal originated from charge exchange between the ion in question and the parent gas in the P-ight t?Jbe between the accelerating grid (i) and the diaphragm (d) as shown in Fig. 2. This is gas which has diffused from the ionization chamber_ Due to the location of the pump, the pressure of parent gas here would be expected to be sign3cantIy higher than in the flight tube itself. Thus in the case of the experiment depicted in Fig. 6 the nelutral peak at zero argon pressure is due to the reaction Nf -tNz -+ N,-Hq. Severat observations justify this conclusion: (i) the intensity of the neutral peak is that which would be predicted using the cross sections for the reaction postulated. (ii) the intensity of this neutral peak could be reduced by the addition of a second pump located before the diaphragm (d). Since the slope of the N”/JV%Lversus P curves is used to cafcuiate the cross section, the presence of this intercept does not influence the rest&s.

Response of the detector In Section II the assumption was made that the coefficient y for secondary electron emission from the first dynode of the detector is the same for an ion and the corresponding neutral particle with the same kinetic energy. This assumption is consistent with the experimental determinations of Shackertg, Berry12, Ghosh and SheridanI and Potte?‘. It is corroborated by the results on argon, neon and nitrogen cited here. ‘The effective length of the collision chamber was obtained using Ar and the known value for its symmetrical cross section. If this assumption was wrong, the cross sections found for the charge transfer processes Ne* -_e, N; ---N&and N; - Ar would not correspond well with the data from the literature. T!e fact that our results are in good agreement with those of other authors justifies our assmmption at least for these gases. There are however no data avaIIable for the relative y values of organic ions and their neutrals. Even in the case of argon, results obtained on molybdenum by Arifov et aLzy, and Medved et aL3’ shoii a different y for the ion and the neutral species- If the y values are different for ion and neutral, the cross sections found for organic ions nre not absolute. This problem of secondary electron emission from the first dynode of our eiectron multiplier is of fundamental importance and is being studied further.

Inr. J_ Mass Spectrum.

Ion Whys-, 4 (1970) 235-249

Xf. 3. FROhIONT,

248

vn.

R. H. JOHNSEN

CONcLUSION

This paper describes an experimentai technique for measuring charge exchange cross sections by detection of the neutral beam formed in the drift tube of a T.O.F. mass spectroi+i~~ter. The firs: results obtained are shown and compared with data from the literature. Several aspects of this study require further investigation. The response of the detector should be known and the profiIe of the pressure in the machine has to be stcdied. It woutd also be very interesting to var: the energy of the incident ions.

ACICSOWLEDG.MFST

This work was supported in part by the U.S_A_E_C_ Contract

AT-(40-1)2001.

REFEREXCES 1 R. E. FERGUSOX,

-2 F_ W_

Mc~~.

K. E. MCCVLLOH R. S. GOHLKE

+XD

AND

H. M_ ROSESSTCX~;,J. Chem. Phyx, 42 (1965) lO& 12th AnnuaI AST,SI Committee

R. C. GOLESWORTHY,

E-14 Meeting, MonrreaI, Canada, 1964. AND R. W. KISEI;, Kansas State University. 3 D. L. DUGGER 4 W. W. Hux?, JR., R. E. HUFFX+.N AXD I(. E. MCGEE, Rec. Sci. Insrr., 35 (1964) 82. 5 W- W- Htimr, JR., R_ E. Hum~x, J_ SAARI, G. W.m A~ZV3. F. Bm, Rev. Sci. Insir., 35 (1964) 88. 6 W. W. Huxz, JR. ASD T-C.E. MCGEE, J_ Chew. Php_, 41 (1964) 2709. Ed. edited by J. M. L.XFER~, 7 S. DUSU~, Scientific Fou.&tions of Vacuum Technique, ‘nd _ W&Y, New York, 1962. 8 G. W. G~ODRICX XVD W. C. WRY, Rev. Sci. Instr., 32 (1961) 846. 9 P_ SC%XP;ERT, 2. Phydc. 1’97 (1966) 32. 10 C. E MELTOX, in F. W. MCLAFFFIITY (Editor), Mass Specrrometry of Organic Ions, Academic Press. New York, lP63, Chsp. 2, p. 70. II J. B. HXZ-ED, in D. R. B-4x-s (Editor), Aromic and ~UalecuiarProcesses, Vol. 13 in the Series Pure and Applied Ph_~Tcs~Academic Press, New York, 1962, pp_ 696-720. 12 H. W- BERRY, P&s. Rev., 74 (1949; 848. 13 R. C. AAT N. G. U-~~EFCR~CIC, Atomic ColIision Processes, North Holland PubI. CO., Amsterdam, I9&%,p. 847. 14 R. F. STEBBMGS,B_ R. TUXXER x+m A. C. H. Sxmx, J- Chem. Ph,-s-, 38 (1963) 2277. AXD S. N. GI~o~H, 3_ Chem. P&s., 23 (1955) 15 3. k DRLOS, W. F. S~RDXX, R. D. EDXV,XRDS 776. 16 S. N. GHOSH AE~D W. F_ SERIDAX, J. Chem. Phys., 26 (1957) 480. I7 E GL.ZZAFsmx AxD E LIXDHOLH, A&z-G Fy5* 18 (1960) 219. 18 3. B. HOMER, F% D. Thesis, Birmingham, 1963. 19 J. SCARBOROUGH, Ph. D. ,Ti,lesis,Birmingham, 1966. 20 R. S. LEEZR.LE, J. E. Pw J. C. ROBB AND I.SCAREZOROUGH, inr.J- Aims Spectrom. ion Pbx., 1 11968) 455. 21~J. L- Occmo%rrz, J. Am. Chew. Sot., 91 (1969) 5202. 22 R. C- AA-ZiDH. C_ HAYDN. J. Chem- Phys., 32 (1965) 2011. 23 J. B. Ho.* R. S. LEX~J. C_ Roan AXD D. W_ THOMAS, Trans. Faraday Sot.. 62 (1966) 6’19. 24 H, B_ G~DFAND 3_ p_ Fb, RX_ Roy_ Se- (~ILw&wz), A238 (1956) 334.

25 H. VON I.&n.

Arkfo Fysiiq. B

41%.j- Mass Specrrom. -.L

-.

(1965) 559_

Ion Pays., 4 (1970)

235-249

CHARGE

EXCHANGE

REACTIONS

IN

A T.O.F.

MASS

SPECTROMETER

249

26 N. G. Ummmx A&D G. E. MILLER, Rm. Sci. Instr., 32 (19Ci) 1101. 27 Muss Spectnzi Data, American Petroleum Institute, Resevch Project 44, Pittsburgh, 1960. 28 R. F. Porn, J. Chem. Phys., 22 (1954) 974. 29 V. A. ARIFOV, R. R. miov x%x EW. DZHURII~CULOV, Societ P&S_ DokL, 7 (1962) 209_ 30 D. B. Mmnq

P. M.uxmmti

.xa

J. K. La-rox,

P&s.

Reo.,

129 (1962)

im. 3. A4ass Specffom.

Ion Phys.,

2086. 4 (1970)

235-249