The measurement of appearance potentials of gaseous ions

87 fnternafical Journnl of &fa.u Spectromnerr,-and Ion Physics, 1S (I 975) 87-99 4 EIscvicr Scientific Publishing Company, Amsterdam - Printed in The Netherlands

I’3-I~

MEASUREMENT

OF APPEARANCE

POTENTIALS

OF GASEOUS

IONS

J_ H.

BEYKON AXD R. G_ COOKS

Deparrmcnr

of Chemisrr_i;

K_ R_ JEXSISGS Dcpurnmmf

AND

Purdue

Unirersir_v,

IV_ Lufiyrre,

Indiama 17907

(USA.)

A_ J_ FERRER-CORREIX

of Mofccufar Sciences. Unircrsify of Wurwick, Cormfry CV4 7AL (Gr-

Brimin)

CReccivcd 19 December 1971: in _-vised form 16 April 1975)

Because of their importance in the understandin,o of the enersetics and kinetics of ion formation and reaction, considerable effort over many years has been expended in determinin, * ionization and appearance potentials of ions by mass spectrometric methods [I]_ The accuracy of much of the earlier data is uncertain and many methods have been suggested for extracting reliabIe ionization and appearance potential data from ionization eflkiency curves [2]_ Although the advent of photoelectron spectroscopy [3] allows one to reassess ionization potential data critically, there is no recourse to non-mass-spectrometric methods for appearance potential measurements_ As will be shown belaw, the most important sources of error in making these measurements are not unique to any particular method of ionization_ Increasing interest in the properties of metastabk ions has stimulated a new concentration of effort aimed at understanding the energetics of ionic reactions [4]_ The use of computers [S] and signal averaging techniques [6] allows measurements to be made at greater sensitivity than heretofore and quasi-monoenergetic electron sources [7, 8 1, photoionization sources 191, and deconvolution methods [IO, II] have been used in an attempt to reduce errors arising from the spread in electron energies_ Appearacce potential measure ments have been made on the reaction products of metastable ions with a view to obtaining information on “kinetic shifts” for fragmentation processes [12]. In view of the lack of agreement between rest&s obtained using different techniques, this paper aims to review the assumptions, errors and misconceptions in the methods of treating the data which are currently in use. After an introductory section dealing with ionization potentials, additional problems associated with

the measurement of appearance potentials of normal daughter ions and products of metastable ion transitions are consider& Although electron impact ionization is considered throughout, the generai conclwions reached will be seen to apply to both electron impact and photoionization, dzspite the different threshold laws_

ing:

These difEcul&~ may convenIet&y be classi%d under the foilowing head(ij inherent difficuities of a non-instrumental wture: (ii) di&ulties arising

from instrumentation and from methods of obtaining data; and (iii) difficulties &sing from methods of treatins the dataThe main difficuky under the Grst heading is the non-adiabatic nature of energy transfer- In general, one‘s aim is to obtain a precise vslIuc for the minimum energy necessary to effect the ionization process ix_ to mearure the adiabatk ionization potential_ According to the Fran&-Condon principleP electron impact ionization occurs by a vertical transition so that at the moment of formation, the geometry of the ion will be the same as that of the molecule_ If the equilibrium interatomic distances in the ion differ from those in the molecule, it may be im-

possible to produce the ion in its ground vibrational level and its vibrational energ!* content will therefore be uncertain at the tbreshoid energy_ In polyatomic molecules, the situation becomes increasingly complicated because of the diftIcu!ty in identifying and properly weighting the transitions, due to variations in the densities of states and energy_ Several difficulties arise because of the kinds of instrumentation employed and the methods used to obtain data, Most studies have been carried out using electrons having a large spread of energies: this is partly due to the potential drop across the electricaIly-heated Giament and to stray fields within the ion source and partly due to the thermal distribution of electron energies arising from a filament temperature In excess of 2000 K_ The power used to drive the fii!amentis such that the tv~~llsof the ionization chamber usually attain an equilibrium temperature of 433-500 K, and unless steps are taken to cool this chamber, the sample mole-

cules zre likely to acquire the vibrational energy content corresponding to this temperature. Various methods have been developed to produce monoenergetic tiectron beams IS] or to simulate them by techniques such as the retarding potential difference method [7Jt but in much of the earlier work the results have not been as refiabfe as was thought initially_ More recentJy, attempts have been made to deterrr,ine directly the electron enere distribution and to deconvolute this out of the ionization efficiency curve [I I I_ However, diRerent analytki treatments of similar data do not always yield similar rentits, possibly because electron energy distrl%utions vary both with the nominal electron enere and with ionization

chamber geometry_

89 Another rather more serious difficulty arises because in order to avoid effects such as those discussed above and also those due to variable contact potentiak, the measurement of an ionization potential is usually carried out by a method of comparison against a compound of known ionization potential, hl order for this method to have any real meanin,,* it is essentiat that the unknown and reference compounds behave as simiiarly as possible in the vicinity of the threshold re$on, thereby giving the same shape of ionization efficiency curve in this region_ This is

often impossible, as it has been shown 1131 that the thermal interna energy spread of the moIecu.Ies has influence on the shape of the ionization efficiency curve just above threshold_ As this influence varies, for example, with the number oFinternal dsrees of freedom, the reference compound should be as similar as possibie to tbc unknown in complexity and vibrational frequencies_ Then, assumin,o perfect sensitivity of detection, the difference in eiectron energy required to produce a vanishingiy small current of each molecular ion is rhe measuremenf of interest.

In practice, for the easiest comparison, the two ionization efficiency curves should be parallel in the vicinity of threshold_ In order to arrange this, both sample and reference can be introduced simultaneously and their retative partial pressures adjusted so as to compensate for differences in cross-section and detector sensitivity, This ensures that both species of ions are produced and detected under identical source conditions_ The detector sensitivity may be less than perfect and noise and drift may add to the difkulties of comparing the t\ko curses, but the above conditions will ensure the minimum error for extrapoiation to the true threshold as is shown in Fis_ I. Even if these conditiotzs can be achieved, errors arising from the factors discussed earlier may stiI1 be present. The ionization cross-section of a molecufc is a compiicated function of the number and species of atoms of which it is composed and the slope of the ioniza-

I

J T

Fig_

1_ Illusmation

en

I

(eV>

of the desirability

of

/

I

I I True Atp

parallel ionization

( eV) cfficicn~

curves

in the vicinity

of

thrcshoId. Fig_ l(a) shows that u-ith paralld CU~WS, m%xsurcment ofthc energy diRerewe at the ptactica1threshold IcAs to no errorwhen this is cxtrapoia~cd to zero currcnt. fig. 1(bj sho&x thax zn error would disc with differently sh;lped curves from sample and reference_ This error is minimized by measuring as close as possible to zero current.

90

tion efficiency ewe immediately above threshoId will depend on the relative probabilities of forming molecular iqns in various vibronic states. The greater this slope is, the closer to the true onset can the ions be detected. It is clear that for ionization efficiency curves of different slopes, the measured efectron energy difkence wilf be a function of detector sensitivity_ 7he above discussion iJh.tstrates that the choice ofztg inert gas. such as argon, as a reference complotmd cannot be justified except at very high detector sensitivities since it is most unlihrely to have an ionization effkiency curve in the threshold region which resembles that of a polyatomic molecufe. For many such molecules, an additionaf complication is the fmgmentation of molecuhr ions at energies but Iittle above threshold, thereby changin g the shape by reducing the slope of the ionization efkiency curve_ Since this effect will be different for every individual polyatomic molecular ion and since there wiJJbe an elect starting at each enew that corresponds to a fragmentation process and continuing at a11higher energies, it renders quite arbitrary attempts to choose other “better--* reference compounds in the hope that they will more cfosely parallel the behaviour of the unknownIn summary, rherefore, the dificultics under the seccnd heading may best be minim&d by working with monoenergetic electrons in a cold source at the highest detector sensitivity_ Under these conditions, a rare gas would probably be the most suitable reference substance, but as the electron enera spread, source temperature and moJecuJar complexity of the unknown substance increase and detector sensitivity dew-es, it becomes increasingly dificuit to justit) the use of any substance as an internal standard_ The third type of difficulty arks from methods of treating the data_ Although the above disclrssion indicates the desir;lbiJity of using the vanishing current method at very high detector sensitivities, this is difficult in practice and several exfrapolation procedures have been used to try to overcome this diJkuJty_ Three commonly used methods, the “linear extrapolation” method [J4& the “extrapoJated potent&J difference” method [JSj and the “semi-Jog pIot” method [J67, ail attempt to make use of data obtained at eiectron energies appreciably above threshold energies, essentially ignoring observations made in the threshold region- The Iinear extrapolation method makes use of simple ionization eficiency curves of unknown and reference substances made parallel over several eiectron voJts by suitable choke of sample pressures_ The “taiiim$* of the curves is ignored, and the “s&tight“ portions of the curves are extrapoJated to the energy axis to obtain the energy difference_ From what has been 4d above, it is ciear that there is no particcular virtue in obtaining parallel plots at energies well above threshold ener&s_ What is required is ;L method of comparing behaviour very close to threshold. The results obtained depend markedly on the choice of reference substance and detector sensitivity and are therefore quite unreliable. Similar remarks may be applied to the semi-tog plot method in which the two ion currents are normalized at 50 eV and tie Jog of the ratio of the ion current divided by the ion

91 current at 50 rV is plotted against electron energy_ This procedure frequently produces pairs of curves essentiaIIy parallel over three decades of intensity and covering zn energ range of up to IO eV, and it is customary to take the ener=y difference as that between the two curves at an ion current level equal to O-1 TL of the vaiue at 50 eV_ There is no justification for this and it is again a method which ignores behaviour in the threshoid region_ Attempts to normalize at lower electron energies, e.g., 20 eV, rarely produce parallel curves, precisely because the different behaviour of the unknown and reference substances in the threshold resions

becomes apparent at low intensities_ The extrapolated potential difference method simply plots the difference in electron ener$es required to give a particular intensity for the two species as a function of their intensity and assumes that if a linear piot is obtained ever the ranse of measurement, it ma>9simply be extrapolated to zero intensity. There is, of course, no justification for this procedure and cases have been documented in which an almost constant energy diIZerence changed sharp!y at very low intensities [17]_ However, the method does have the advantage over the other two methods that if observations are taken at very low intensities and high detector sensitivities, it takes cognisance of threshold behaviour and becomes equivalent to the vanishing current method. The above discussion shows that none of the extrapolation procedures as normally used is valid under most circumstances and the only method which can be used, under appropriate conditions, to obtain accurate ionization poten:iaIs is the vanishing current method-

COXSIDERATtOXS FORMED

1X

SIEASURIXG

APPMRASCE

POTESTIXLS

OF

DXLrCliTER

LOSS

IN -t-HE SOURCE

Because of the very different mechanism by which daughter ions, as opposed to molecular ions. are formed, the difkulties encountered in measuring appelrrance potentials accurately are rather difikent from those discussed in the previous section_ The difficulties may be classified under the same headins as were used previously, but in view of the remarks akeady made concerning the errors introduced by the use of extrzxpo!ation methods (which apply with increased force to appearance potential measurements), discussion will be limited to the first two types of diIEculty_ The appearance potentiai of a fragment ion may be defined as the sum of the adiabatic ionization potentiai of the molecule from which it is formed and the minimum internal energy of the molecular ion which can Iead to the production of the fmgment ion. Even if the mokcuiar ion can be produced with this minimum internal energy, the shapes of the potential energy surfaces involved in the fmg mentation may require that the fragment ion and neutrai products be formed with

92 excess internal or translational energy, In addition, as was the case when ionization

potentials were considered, the operation of the Fran&-Condon principle may make it impossible to produce mofedar ions with this criticaI internal energyV thereby leading to high measured values of appearance potentials_ The formation of a daughter ion is the result of two processes, the primary ionization step which is a ““vertical” process and which produces an excited molecuhrr ion of geometry identicaf to that of the neutral, folIowed by a much sfower process in which fiagmentation occurs at a rate which depends on the amount of excess ener7 contained in the ion above the minimum required for the fragmentation to occur. Ideally, it should be possible to form the mofecuhrr ion with this minimum energy and stifl detect fragmentation_ Tn practice, this might be a highly improbable process owing to the requirements of the Fran&-Condon principle and because of the kinetic considerations noted later, In the second category, dit’EcuJtiesassociated with the energy spread in the electron beam and with the source temperature remain, and the problems arising from the use of a reference compound are intensified. In view of the radically different nature of the processes kading to the formation of a daughter ion and the molecdar ion of the reference compound, behaviour in the threshold region will be very different and a comparison will only be valid at uanishin,ocurrents using very high detector sensitivity, Additiona! problems arise from the fact that daughter io,w are formed as the result of competing (and sometimes consecutke) unimolecuiar decompositions of the molecular ion, the rate constant for a given reaction being a function of the excess energy present in the molecular ion above the minimum required for that reaction to occur, The number of moiecular ions having sufficient energy to undergo a given fragmentation may be ,wtly reduced if there is another, frrster fragmentation pathway of lower appearance potential_ Con~uen~y~ it becomes increasingly difficult to measure meaningful appearance potentials as higher and higher energy processes are examined_ ConverselyV for the lower enesy processes, ionization efficiency curves in the energy region just above threshold will be compkated by competition from other fermentation reactions as the electron enere is increased_ If the ionization efkiency curye of AM?is used to provide an internal energy standard, then from the discussion presented previously, the energy difference can be measured accurately only if extremely high detector sensitivity is used or if the two ionization efhciency curves are parallel in the region of threshold. The fatter condition cannot be expected to obtain in practice since, in general, the relative abundance of M* ions at energies near threshold is much higher than that of Fe ions near their threshok& and there is a consequent tendency to over-estimate E, and so obtain a high value of A-P_(F*)-). The yield of F* ions in the region of thresh&d depends OXI several factorsAssume for simplicity that there are no competing fragmentations of MT ions

93 and no further fragmentations of F” ions. If one considers only those IM~ ions formed with a particular ictemal energy, E, where E > E,, the fragmentation reaction can be characterized by a single rate constant, k. Untier these circumstances, the yield of F’ ions after time t is given in terms of the initial yield of hilt ions by the equation [F’],

= [M*],,

(I -exp

(-kr))

Since F’ ions must be formed in the ion source if they are to be detected as daughter ions, f in this equation is s 10-‘%, the source residence time in a typical instrument- In practice, there will be a range of internal energies E which will increase and also change its distribution as the electron enew is increased, thus

giving rise to a changing distribution

of rate constants, k, as the efectron energy

changes- The observed ionization efficiency curve therefore depends both on the energy deposition function and on k(E), together with any effects arisin,o from competing or consecutive fragmentations, or variations of source residence times such as those arising from changes in the ion repeller potential- The net result of these various effects, in general, is to cause the yield of F” ions to increase quite slowly with electron energy in the threshold region, thereby leading to the possibility of a large error in the measured value of A_P_(F*)_

_AIMXTIOSAL STABLE

COXSIDER4TlOSS

IN

M3SURlXG

XPPWRXXCE

POTESTIAIS

OF

IrZETA-

PE.%Fci

In addition to the problems arising from the spread o?energy in the electron beam, source temperature effects and attempts to use reference substances as intemai standards, there are several diJEculties and advanta,ges which are peculiar to the measurement of appearance potentials of metasfabfe peaks. These may be divided conveniently into straightforward instrumental problems and those arising

from the eZxt of fiqmentation rates. As ~111be seen beIowz although metastabfe peaks are usua1Jyof much iower intensity tl-.an peaks due to ions formed in the source when electrons of SO-100 eV are used, their intensity increases relative to the intensities of other ions in the m&x spectrum as the electron energy is Iowered. They are therefore, at 50 eV, normally observed at detector sensitivitiesdifferent from those used for ions formed in the source* and this complicates the comparison of ionization effxiency curves in the threshoJd region_ A further sensitivity problem arises from the fact that the fngmentation of a metastabre ion is invariably accompanied by the conversion of some internal energy into translational energy of the products_ This results in a broadening of the me&stable peak Apart from discrimination eJ?kcts in the Zdirection (panlIe to the Ion g dimension of the slits) one can expect to collect ail ions formed in a field-free region, each making its contribution to some part

93 observed peak_ (This may not always be true for Iisht ions formed by fmgmentation of doubIy-charged ions in which up to 10 eV of energy may be released.) In order to obtain maximum sensitivity, all the fngment ions should be collected of the

simultaneously which requires that all slits after the field-free region in which the metastable transition occurs should be wide enou@ to transmit the entire beam of ions giving rise to the metastabfe peak. Typically, if 0. I eV of enerq is released, maximum sensitivity of detection may require that the mass resolution be reduced to about 100. In order to obtain a meaningful value for the threshoid energy required for fragmentation to occur, it is essential to ensure that moiecuiar ions do not gain additional internal enafter leaving the source by converting translational energy into internal energacby colliding with molecules of background ,oaspresent in the field-free r&on_ fi. collision-induced component in a metastable peak can be detected in many w,ys_ For example, if the Io~arithm of the inte.nsit_vof the metastable peak in t\e threshold region is plotted against the e!ectron energythe coliision-indz:cd contribution to the intensity varies only slowly with electron energy and so produces a marked change in slope in the plot at low energies, as shown in Fig_ 2

Born&r@ing ckctron

mergy

CeVl

fig- 2- Change in slope obscn-cd when the abundann

of a metstable

rilhmicAC Y ;I fknetion or bombarding clcetron energy- The Iowx

peak is plotted on ZItogasis due to the p~nr~

slope

of a collision-induced component_

sensitivity effects produced by three different hypothetical rates of unimolecu!ar decay of the molecufar ions on the intensities of the peaks given respectively by the moIecuiar ions, the fragment ions formed in the source and the products of the metastabie transition are shown schematically in Fig- 3- The mcleeular ions are assumed to possess a f&d amount of internal energy in each case which c ()I c ea d ing to corresponding decreases in the decreasesintheseries(a)<(b) The

95

(a)

5

TF

Fig 3. hbundancc of molccuhr ions 111having the same internal cncrgy and of fragment ions formed from them, both as a function of time The half-Iife for fr;lgmentation Increases from Figxl)-Xc)_

rate constant for fmgmentation_ The numbers of Fi ions formed in the source and in the field-free region are denoted respectively by the length s and _r in the @ttre_ For an instrument in which the source residence time is less than the time required LO traverse the field-free region, as is usually the case, xj_t- +z 1 for a high rate constant as in (a), becomes equal to unity for a particular intermediate rate constant in (b), and finally attains a vahte which is less than unity, for very low rate constants correspondin,Q to threshoid energies. In many instruments, the ratio of source residence time LO the time spent in the field-free region, ~skr is O_l-O_S, but it can never be zero, so that whenever any fngment ions are formed at all, some will be formed in the source_ In practice, molecular ions are formed with internal energies ranging from zero up to E,_, where E_ +I.P_(M) is the total energ of the bombarding electron. The relative probabilities that dilferent amounts of energy are transferred from theelectrons Lo the mokcxde are described by an energy deposition function, P(E), SO that the number of ions formed in the source and the field-free region respectively may be written as

96

IF’k

= 1” ‘3

IF’],=

1”’

[MtJP(E){I

-exp(-k(E)r,)dE

[iM*]P(E){exp(-k(E)r,)-exp(-k(E)rt)jdE

-0

correspond to the times at which the ions leave the source and region respectively, and where Tf is the time spent in the field-free regionwhere

T,,

zI

and

s:,

enter and leave the field-free

I

P(E)dE = I

-0

This implies that the number of fngment ions formed as a function of time will be a weighted combination of a series of curves similar to those in Fig 3_ Nevertheless, if one can assume equal detection sensitivizy, the mtio of the slopes of the ionization efficiency curves at threshold for fngment ions formed in the source and in the field-free resion shoufd approximate to T&_ lmmediateiy above threshohi, as the average internal energy of the ions increases, the ratio of the slopes will increase very rapidly as the situation more closely approximates Fig_ 3(a) rather than Fis_ 3(c)_ The resuhin, - ionization efficiency cul~es will clearly be

markedly dependent on the explicit form of both P(E) and k(E) but their general characteristics are expected to be as depicted in Fig_ 4(a)_ In accordance with the above discussion, both curves or&ate at the true threshoId energy E. and the (b)

Fig- 4_ The eFkct of norm&zing both the normal fragment ion abundance and the height of the mctastable peak at 50 eV_ Fig_ 4(b) shows the displxcmcnt that is thereby introduced when data arc plotted by the “3cmi-log*v method; Fig- 4(a) which is plotted on an absolure. linear suk shams how this displacement arises- E,- is the value found cxpcrimentally for A-P-(F*) at the detection threshold_&+ is the corrcsponding\aluc forthe mctastablc pcak.(E,-- ES*) is then the apparent kinetic shifr due to the finite dctmion threshold. (&I(Eg*)‘) is the apparent kinetic shift from the semi-Iog plot- (Et--- &*) cm be seen to fend tozerowhcn thedc:cction threshold is ZOWCEd.

97

appearance poten&&

of both F+ ions formed in the source and in metastable transitions are identical_ An apparent difference in appearance potentials would

resuit whenever the detection sensitivity is less than perfect, e-g_ as shown for the experimental detection threshoId in Fig_ 4, but this difference will be small at high sensitivities_ When the experimental results are dispIayed using the “semi-iog plot” method described earlier, the ar:ificiai threshoId at which a comparison is made between a normal fngment ion peak and the metastable peak is some arbitrarily fixed percentage (often O-1 %) of the heights of i-he two peaks at an electron enercgy of 50 eV_ Thus, as shown in Fi g. 4(a) the minimum current used is much higher for the normal peak than for the metastable p:ak_ Obviously, when starting from these two diKerent minimum currents, the corresponding energies E;

and (Eg)’ will, when the semi-log plot is used, lead to dispiacement of the curves by this amount as shown in Fig_ 4(b)_ Obviously, too, if the detector sensitivity couId be improved, say to give measurements down to 0.01 %, both E& and (~5:) would come cioser to the truevalue of the appearance potential E, and the differ-

ence E$- (Ez) would be reduced_ It has been assumed by several groups that an observed difference in appearance potentials of F’ ions formed in the source and field-free region gives a measure of part of the kinetic shift [l&16]. The faIlacy is that the kinetic shift is defined in terms of /IzL’Qn lifetimes and the statistical

nature of fi_agmentation makes it possible to study reactions having rate constants diKering appreciably from the reciproca1 of the observation times_ At sufkiently d high detector sensitivities using a linear plot, the di!i?x-encein measured appearance potentials of the normai fn_gment ion and the metastable peak will tend to zero as shown in Fig_ 4(a)_ The apparent difierence normally measured in the semi-Iog plot arises for the-reasons discussed above and it is clear that if the minimum observed current for the metastabIe peak Iies close to the ultimate threshold, then at sufficiently high detector sensitivity, the appearance potential of the metastable

peak will give a more correct vatue for the minimum energy necessary for fngmentation to occur_ The above analysis serves as a basis for explaining some recent experimental results [6(b)]. These results were obtained at a very high sensitivity and are in agreement with the simpfe kinetics presented above. Fci example, the appearance potential for the metastabIe peak in benzonitrile was measured to be the same as that of the corresponding normal mass peak at nl,“r 76_ The value obtained for the normal mass peak was much lower than the previously accepted value; the value for the metastable peak was unchanged as would be expected in view of the very high absolute sensitivity with which this peali had been measured in previous work The advent of laboratory computers has already stimulated attempts to determine accurate appearance potentials. Not only do they enable measurements to be made at very high sensitivity but they also aIIow the electron energy resolution

9s Lo be degraded in a controlled fshion so as LO enhance sensitivity still further_ Suppose, for example, that in an experiment to improve sensitivity by means of signal averaging using k computer, 9 series of IO00 ion currents aie sampled repetitively over an electron energy range of JOeV. (This is essentially the method employed in refs_ 6a and 6b_) Further increase of signal-to-noise ratio can then be obtained by averaging the signal in two or more adjacent channels_ If, for example, to enhance the signal an average is taken over the signals in channels l-5, 2-6, 3-7: etc- in the hypothetical example above, the energy resolution would be correspondingly reducedThis might still be considered a small price Lo pay for the greater confidence in the accuracy of an appearance potential at the resultant higher detector sensitivity. If, in retrospect, too much emphasis is found to have been placed on achieving sensitivity at the expense of resolution, the results from the computer averqing can be extmcted at an earlier stage of the calculations- Of course, the most sensitive detector possible must be used in conjunction with the computer method.

coxcLrJsIoxs

In this paper, we have sought LOpoint out some of the difficulties associated with the measurement and interpretation of appearance potentials. The uncritical application of many of the more commonly-used methods can lead LOserious errors, the magnitudes of which are diEcult LOestimate and which will usually vary with operating concF.itions_ These errors may be minimized, although not eliminated completelyY by workin g at the highest possible detection sensitivity and basin_e measuremenls on the region of the ionization efficiency curve immediately above threshold_ When this procedure is followed the appearance potentials of a normal fragment ion and of the corresponding peak arising from decomposition of the metastable ion bexzomeidentical and any displacement due Lo the so-called “kinetic shift” is not observed_ Since higher detection sensitivities are normally used in the determinarion of appearance potentials of metastable peaks than in the determination of appeamnce potent&Is of fngment ions, the former are likely to be the more accurate.

ACKXOWLEDGEMEN-LThe support of the Science Research Council (Warwick) Science Foundation (F%rdue) is acknowledged-

and the National

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