Positive ion-molecule reactions in NH] and SF6 gas were studied in a static ion drift-tube mass spectrometer. E/N (the ratio of electric field strength to gas number density) varied typically from 20-400 Td and pressure ranged from 0.1 to 0.8 Torr. The most important process was found to b

Saporoschenk, M

Talib, Zainal Abidin

Universiti Putra Malaysia Institutional Repository

Pertanika 15(1), 61-67 (1992)Mass Spectrometric Studies of PositiveIon/Molecule Reactions in NH3 and SF6 GasesZAINAL ABIDIN TALIBI & M. SAPOROSCHENK02lDepartment ofPhysicsFaculty ofScience & Environmental StudiesUniversiti Pertanian Malaysia43400 UPM, Serdang, Selangor DarulEhsan, Malaysia2Department ofPhysicsSouthern illinois UniversityCarbondale, II 62901, USAKeywords: Ion-molecule reaction, drift tube, mass spectrometer, SF6, NH3,ABSTRAKSatu kajian telah dibuat mengenai tindak balas ion-molekul positifke atas gas NH] dan SF6• E/N (nisbah medan elektrikke ketumputan bilangan gas) telah diubah dalam julat 20-400 Td manakala tekanannya dalam julat 0.1 to 0.8 Torr.Di antara proses-proses yang penting yang didapati berlaku ialah(1)(2)(3)ABSTRACTPositive ion-molecule reactions in NH] and SF6 gas were studied in a static ion drift-tube mass spectrometer. E/N (the ratioofelectric field strength to gas number density) varied typically from 20-400 Td and pressure ranged from 0.1 to 0.8 Torr.The most important process was found to be(1)(2)INTRODUCTIONThis paper will attempt to show the relationshipbetween mass spectroscopy and the reactions ofions with neutral molecules by using an apparatuscalled static ion drift tube mass spectrometer. Drift-tube experiments constitute a class of low energyion-molecule collision experiments, where the ki-netic energies of the ions are varied typically inthe range of 0.01 to 10eV by varying the ratio E/Nbetween an applied electrostatic field E and the(3)number density N of the gas molecules. In suchan experiment, a swarm of ions moves through aneutral gas under the influence of a uniform elec-trostatic field. The ions undergo elastic, inelastic,and reactive collisions with many atoms or mol-ecules during their passage through the drift-tubebut there are so few ions present that ion-ion in-teractions may be neglected. The motion of theswarm of ions through the neutral gas in the drift-tube is influenced by the gas temperature, T, bythe ratio, E/N, and by the details of the ion-neu-FOLR PORTS FORGAS INLET, ELECTRICALLEADS, AND PRESSUREMaNOMETERZAINAL ABIDIN TALIB & M. SAPOROSCHENKO-_............_.._--._-2-_.__.........._._-- 4 IDRIFT TUBETO ION3PUMP ~ 4 1ZiI~ I INSULAnONI ;;>" ooc:::::::::::: IIZI~l!22Ig t P QUADRUPOLESPECTROMETERTO ION ~~•PUMP TO ION PUMP ..CHANNELTRON1024 CHANNEL ANALYZERFig. 1: Schematic diagram ofdrift-tube mass spectrometertral collisions. In this paper, we report measure-ments of the reactions of positive ammonia ionsand positive sulphur hexafluoride ions in theirparent gas, respectively.Sulfur hexafluoride, SF6, is well-known as a gaswith exceptionally good electrical insulatingproperties due to its high dielectric strength. Ofinterest in this paper is the mechanism(s) by whichproducts are formed since only a relatively fewexperiments (Frees et at. 1981) have dealt with thestudy of the basic processes occurring in the elec-trical breakdown of SF6, and the majority of theseexperiments have been mainly directed to the re-actions of negative SF6 ions2,3 (McGeehan et at.1975, Fehsenfeld 1971) in SF6 gas.In contrast, much work has been done on theammonia system especially in the study of ion-molecule reactions (Dorfman and Noble 1959'Derwish et at. 1963; Dawson and Tickner 1965:Hogg and Kebarle 1965; Hogg et al. 1966; Searle~and Kebarle 1968; Long and Franklin 1973). Thusthis gas would provide an excellent source of ref-erence.MATERIALS AND METHODSThe static ion drift-tube mass spectrometer whichwas constructed by the authors at Southern IllinoisUniversity at Carbondale, U.S.A. is shown in Figure1. The main ultrahigh-vacuum system is con-structed of stainless steel and is evacuated by ionpumps (400 lis, 200 l/s and 100 l/s). The up-per part of Figure 1 shows the drift-tube with theion source and the electrical shutter (electrode 2-3) with the drift space defined as the distancebetween electrodes 3-4 and the lower part showsthe ion detection system with the quadrupole massspectrometer and channeltron electron multiplier.The drift-tube is separated from the massspectrometer by an insulated metal plate whichhas an ion exit aperture of diameter 0.013 em.The position of the ion source in the drift-tube isadjustable externally by means of a linear-mo-tion feedthrough. The gases used in this experi-ment are research grade with a stated purity of99.995%.62 PERTAN1KA VOL. 15 NO.1, 1992MASS SPECTROMETRIC STUDIES OF POSITIVE ION/MOLECULE REACTIONS IN NH3 AND SF. GASES100PRESSURE = 0.5 TORR • NH +4+~ • NH 4 .NH:3~.. NH:tH'j''-.-./ 80 ¢ NH/ NH:3 :3~ t>. NH/ NH:3 4(f) ¢ ¢Z ¢ ¢ ¢ ~w 60 0 0 A••I- AZ 0 A •0•Z* ••0 40 0 At>. 0 • •W> ¢ A I~ A20 t>. A I A •A •W t>. A A •0::: A it>. ~ • AAA A • •00 400Fig. 2: Variations of the relative ion intensities of ammonia ion clusters as a function ofE/N at p = 0.5 TorrRESULTS AND DISCUSSIONAmmoniaMass spectra were obtained by scanning the mo-lecular masses in the range 2-127 amu using aquadrupole analyzer. Thirteen positive ion massspecies in all were observed in the spectrum at14<D/N<480 Td (1 Td = 10-17 V cm2) and at pres-sures 0.2<p<0.5 Torr. The drift distance were heldat 1 em. The major ion peaks observed had massnumbers corresponding to the solvation of NH/.The ions identified were:NH4+. (NH3)n: mass number 18, 35, 52,69, 86 (1)Two other series ofion clusters were also observed.They areNH+.(NH3)n: mass number 15, 32, 49, 66, 83 (2)N H+ (NH ) : mass number 63, 80, 87 (3)2' 3 nThe cluster series in (2) and (3) appeared as asmall percent of the total ion intensity except forthe NH+ ion which had an ion intensity about thesame size as that of NH4+ion.Comparison of the present work was made tothat of Long and Franklin (1973) and Dawsonand Tickner (1965). Their results are in goodagreement with those obtained in this experiment.Dawson and Tickner obtained NH/. (NH3), ionsup to n=4. Small ions current at mass number 46and 63 were suggested to be due to N2H+.NH3 andN2H+. (NH3)2 or to impurities. Better agreementwas obtained when the present data were com-pared with those of Long and Franklin. Clusterreactions up to n=8 were observed for theNH4+(NH3)n. The two smaller series, NH+.(NH3)nand N 2H+(NH 3)n were observed in smallerintensities.Figure 2 shows the typical results of the varia-tions of the relative intensity of the NH/. (NH3) nions up to n=4 with E/N at constant pressure anddistance. As can be seen, the higher order clustersformed at lower E/N and as E/N is increased,detachment of ammonia molecules was observedwhich led to the NH4+. (NH3) n-l ions. At high E/N,only the NH4+ions were observed.Pressure dependence of relative abundanceof ammonia clusters up to n=4 is shown in Figure 3.From this plot one may consider a general clus-tering reaction sequence leading to the solvationof ammonium ions byPERTANIKA VOL. 15 NO.1, 1992 63ZAlNAL ABIDIN TALIB & M. SAPOROSCHENKO100E/N = 240 Td E/N = 57 Td~ .NH + o NH/~NH3~3~ 4+--..- 80~ NH4 (NH3) .NH/ NH3 4~ a aa(f) aZ aW 60 c af-aZ J:J. J:J. J:J.AZ0 40AW> J:J.~ 20 • •• •W A ••cr: • AJ:J.•A00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0PRESSURE (TORR)Fig. 3: Variations of the relative ion intensities of ammonia ion clusters as a function ofpressure atE/N=240 Td and E/N=57 Tddecrease in the intensity of SF3+ions led to an in-crease of the Sl/ ions. From the above observa-tion, it is then possible to match the reduction inthe intensity of the primary ions with the increasedintensity of the secondary ions sufficiently well toidentifY the reactions that took place. Therefore,a reasonable sequence of reactions that can bededuced from Figures 4 and 5 after the primaryionization, SF6 + e --> SF5++ F + 2e, isTypical plots of the variation of the relativeabundance of the ions with E/N at constant pres-sure are shown in Figure 6. AB can be seen, the onlyion present at all the energy ranges studied wasSF3+ ions. At lower E/N, the predominant ionswere found to be Sl/ while at very high E/N, SF5+ions dominate the mass-spectra.Examining the variation of relative intensitiesof the ions with respect to the distance used, shownin Figure 5, we observed that increasing the dis-tance at low E/N will favour the formation of S2F7+Sulphur HexafluorideMass-spectra were obtained by scanning the mo-lecular masses in the range 1-300 amu using aquadrupole analyzer. Nine positive ion mass spe-cies in all were obtained in the spectrum of sulphurhexafluoride at 40<E/N<282 Td and pressurerange 0.1<p<0.8 Torr. Temperatures measuredranged from 299 Kelvin to 344 Kelvin. The pre-dominant ions were found to be SF3+, SFs+' andSl7+' accounting for more than 80% of the spectraformed under all experimental conditions. Otherions observed in less abundance were SF2+, SF/,and Sl6+ ions while S+, SF+, and Sls+ ions con-tributed less than 1% of the mass-spectra.The plots of the relative ion intensities versuspressure as shown in Figure 4 for constant E/N anddistance L travelled by the ions demonstrated thepressure dependence of the ions. Measurementsof the relative ion intensities as a function of dis-tance travelled by the ions is shown in Figure 5.AB can be seen from these figures, the in-crease and decrease in the intensity of the SF3+affected the intensities of both SFs+and Sl7+ ions.Increase in the intensity of the SF3+ ions resultedin the decrease of the SFs+ions' intensity while theand as the pressure is increasedSF3++ SF6 --> Sl/ + F2(5)(6)64 PERTANIKA VOL. 15 NO.1, 1992MASS SPECTROMETRIC STUDIES OF POSITIVE ION/MOLECULE REACTIONS IN NH, AND SF. GASES3.51.5 2.5Distance (cm)OL.----I-_.L.-----!-_.L.-----!-_.J..--..L_.J..--..L_.J..---L_.l..---L_L---J0.5Fig. 5: Variations of the relative ion intensities of SF. ions as a function ofdistance at::'/N= 74 Td and E/N=243 Td with P= 0.5 TmTPERTANlKA. VOL. 15 NO.1, 1992 65ZAlNAL ABIDIN TALIB & M. SAPOROSCHENKO••+ SF/• SF/0 SF/• SF +5 +X S2F6.A S2F/50 150 300E/N (Td)PRESSURE = 0.5 TorrDISTANCE = 2.75 em6080co 40OJ>-+-'o 20OJ0:::Fig. 6: Variations of the relative ion intensities of SF6 ions as a junction ofE/N atp = 0.5 Torr and dist. =2.75 emions while at high E/N, it favours the formationof SF3+ions.Thus, from Figures 4 to 6, we can conclude thatat a higher pressure, lower E/N, and longer dis-tance between the repeller and the ion exit aper-ture will favour the reaction (6).CONCLUSIONThirteen positive ion mass species in all were ob-served in the spectrum of ammonia in the drift-tube mass spectrometer. All of the ions identifiedcan be classified into three groups: NH/. (NH3)n'NH+. (NH3)n and N2H+. (NH3)n with NH4+· (NH3)nforming the major and most abundant group ofions and this was found to occur through NHt(NH3)n.1 + NH3 H NH4+. (NH3)n. For SF6, ninepositive ion mass species in all were observed andthe most abundant ions were found to be SF3+, SF5+,and Sl/. The mcyor reactions that took place werefound to be reactions (5) and (6).REFERENCESDAWSON, P. H. and A W. TICKNER. 1965. Ion Clus-ters in Ammonia Glow Discharge.J Chern. Phys.40: 3745-3746.DERWISH, G. A. W., A. GALLI, A GIARDINI-GUIDONIand G.G. VOLPI. 1963. Ion-Molecule Reactionsin Gaseous Ammonia. J Chern. Phys. 39: 1599-1605.DORFMAN, L.M. and P.C. NOBLE. 1959. Reactions ofGaseous Ions: Ammonium Ion Formation inIonized Ammonia. J Phys. Chern. 63: 980-981.FEHSENFELD, F. C. 1971. Ion Chemistry of SF6• JChern. Phys. 54: 438-439.FREES, L.C, I. Squers, H. W. ELLIS and L. G.CHRISTOPHOROU. 1981. Positive Ions in SparkBreakdown of SF6. J Phys. D: Appl. Phys. 14:1629-1642.HOGG, A.M. and P. KEBARLE. 1965. Mass-Spectrometric Study of Ions at Near-Atmos-pheric Pressure. II: Ammonium Ions. J Chem.Phys. 43: 449-456.HOGG, AM., R.M. HAYNES and P. KEBARLE. 1966. Ion-Solvent Molecule Interaction Studied in theGas Phase. Heats and Entropies of IndividualSteps. NHt (n-1) NH3 + NH3 = NH/.nNH3• J Amer. Chem. Soc. 88: 28-31.LONG, j.w. and j.L. FRANKLIN. 1973. Inti. J of MassSpetrom. and Ion Phys. 12: 403-408.McGEEHAN,j.P., B.C. O'NEILL, AN. PRASAD andj.D.GAGG. 1975. Negative-Ion/Molecule Reactions66 PERTANlKA VOL. 15 NO.1, 1992MASS SPECTROMETRIC STUDIES OF POSITIVE ION/MOLECULE REACTIONS IN NH, AND SF. GASESin Sulphur Hexafluoride.] Phys. D: Appl. Phys.8: 153-160.SEARLES, S.K and P. KEBARLE. 1968. Ion-Solvent-Molecule Interactions in the Gas Phase.Enthalpies and Entropies for the ReactionsNH4+ (NH,)n_l NH, = NH/ (NH')4.] Phys.Chem 72: 742-743.(Received 14 May, 1991).PERTANlKA VOL. 15 NO.1, 1992 67

Mass Spectrometric Studies of Positive

Ion/Molecule Reactions in NH3 and SF6 Gases

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Mass Spectrometric Studies of Positive Ion/Molecule Reactions in NH3 and SF6 Gases

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