On the formation of multiply charged ions in a field ionization mass spectrometer

92 In~cmationaI J ournul 9 Envier

of ni-

Spccrruncrr_r

Scientific Publishing

Company_

and hn

Ph_rsics. I7 (I 975 ) 92-95

5lnwcrdam

- Printed

in The Nerhcrlands

Short communication

On the Formation ofi 3&Itiply Charged Ions in a Fidd Ionization l%ss Sptitr+ meter

The formation

of multiply

charged

ions during

the field ionization

(FI)

of o~csubsbnces has already ken extensively discussed [II 2]_ Using thz M’* ions of benzene and the (rrM-0)” ions (II= I, 2) which appear in the spectra of aldchydes and ketones, it has been shown that these ions are generated via ionic field adsorption

states- The desorption

of the M’*

ions is cornposed

of at least

the two steps

(a) M -

*.M+ -

e and (b) *M*

d

M’+-e

(I)

where * indicates a bond to the surface_ In the primary step, (a), molecules arriving from the -phase are ionically bound to the surface-The desorption in (b) is induced by the second ionization process_ A contrasting model of these processes was used by Goldenfeld et aI_ [3] who interpreted the observed field strength dependence of the M’* ion currents of benzene, naphthakne pxxiuced

and anthracene on the assumption

by post-ionization

of desorbing

M*

that the M’+

ions are

ions in the gas phase- A calculation

using this model show& an agrccmcnt between the measured and calculated field strength at which the benzene M” lolls are produced with maximum probability_ According to Goldenfeld et al_ the probability of the formation of an Mi’ ion is theprodtzct of two partial probabilities;

(1) the probability

that the neutral mole-

cules will reach the surface, and (2) the probability that the dcsorbing hI+ ions will give up a sccomAelectron by field ionizaEion_ One has to consider, however, that independently of whatever model assumptions are made, the position of the maximum in the field strength dependence of the M ** ion current is mainly determined by the decrease of the probability that the molccuks will reach the surfa=_ Conscquently a similar field strength dependence of the M’* ion current is to be expect-

93 cd on the basis of both models cited_ Thus the result of the calculation

cannot be accepted as supportkg

evidence for a post-ionization

in ref_ 3

model.

The folloGng

are the experimental results which indicate that post-ionization cannot play an important part in the formation of multiply charged ions: (1) In most cases the intensity of multiply charged ions ir FI mass spectra is of the same order of magnitude or higher than rhat of the correspondingsingly charged ions_ In many cases no singly charged ions corresponding to those multiply charged are observed at all-Table 1 shows the triply charged ions formed in the olefine spectra-These ions resuk from polymerization reactions on the emitter surface_With a few exceptions the corresponding doubly and singly charged ions are TABLE

I

TRlPI_Y

CHARGE9

I‘ASGE

FRO,,

BE

GIVES

IO:5 IO-’

HERE

mie

TO

FOUSD -I-HE

BECAUSE

OF

IS

THE

:o-6

OF

THE

LACK

Ech_dcnc

FIELD

IOS

MOLECUI_AR OF

HIGH

Prop8-Icne

-MASS ION

SPECFRA

OF SEVF.RAL

I~XE!~I-FY

REZOLUFION

I- BUlCllC

(STRUCTURE

OLEFISES

IN

THE

19FORSLtllON

IXTESSITY CASSOT

DATA)

I-Pen1cne

31-3 (IF 31-6 32-3 (I) 326 35-6

36.3 36-6 40.3 (I) 44-3 (I) 35.3

45.6 46.3 (I) 49-6 SO.3(I) SO-6 51.3 59-6 60-3 (I) a (I)

=

possible

isotope

peak of the precursor

ion

absent or less intense than the triply charged ions. Further examples of this can be found in the mass spectra of the amines and of unsaturated compounds in general. Corresponding singly charged ions with high intensity are found in the cases of Mz* and (ki+H)**_ (2) The appearance of M 2 * ions in the mass spectrum of non-cyclic compounds such as par&ins, alcohols, olefines, etc. (with the exception of ethylene)is not necessarily to be expected even if a post-ionization model is assumed, since the MZf ions formed can decompose by simple bond rupture in times of the order of mag-

94 nitude of an oscillation period, because of the coulomb repulsion of the two charges_ However, the origin of such decomposition products of Mz* ions can be dercrmined if the life-time of the M’* ions is at Ieasf IO-” s_ fn this case a broadening on the high mass side of the peak of the heavier fragment ion results from the kinetics of the reaction [43_ With no fragment ions of aliphatic compounds such a peak broadening on the high mass side of the mk hzs been observed- In contrast to the aliphatic paraffins. doubly charged molecular ions of cycfohexane should not be able to decompose immediately, since this wouid require two bondrupturesHowever, no evidence of the formation of CeH,22* ions could be found in the mass spectrum of cyclohexane taken using F’t-tips, cyan at intensities down to abcut 1o-4 of the M* intensity and at high field strength (3) It was found that for the Fl of btnzene at Pt, the intensity ratio M’*/M’ is not only a function of the field strength, but also depends very sensitively on the surface conditions of the emitter- For many emitte.3 a ratio of M’+/M+ exater than 10mQcould not be obtained_ It was found that the ratio M’+/M’ incn%sed after contamination of the tip by thick or_eanicdeposits. for instance by those resuItin&from activation of the tip_ (4) The different dependences of the absolute M* and M’* ion cxrent intensities on the emitter temperature

12) rules out the possibility

of a direct con-

of the M2+ and that of the M+ ions_ (5) The ratio M**/M* is lower for mixtures in which the added substance reacts with benzene surface ions [I ]_ (6) The appearana potential of benzene M ’ * ions was measured by Goldenfeld et al. [5] with a retarding pctential energy analyzer. They obtained for the appcaraxcpotential (AP = AU + &, where AU is the measured energy deficit and & the work function of the retarding electrode) a value of IA0 &O-l eV_ Using a slightly modified mctbd [6] the appearance potential of the M** ions was found nection between the formation

to be 14.0 3_ 6.3 eV_ The two values agree within the limits of exptrimental error. Using the model of post-ionization, this figure results in a value for the second ionization potential of benzene of IS_8 eV [3 1, which is about 2 eV too high (IP, = 16-8 eV f7])_ The size of the discrepancy suggests a systematic error, i-e_ the incorrectness of the basis assumption_ The analysis of the measured appearance potentials on the basis of the model developed in refs. 1 and 2 shows that the&’ of M** equals the second ionization potential and gives evidence for the formation ions from field adsorbed benzene ions in an open structure [8]of the C,H,‘*

ACFiNOWLEDGEbtEXT

WE would like to thank Professor I-V- Goldenfeld for his clarifying commentsWe are also very grateful to Profmor H_ D_ Beckey for helpful discussions-

95 REERENCES I F- W- Rdlgcn and Z F- W- RZTlIgcu and 3 I. V- Goldcnfcld. Phjx. 11 (1973) 9_

H- D- Bcckcy. &r. Butzscrrgcs Phys. Chcm.. 75 (1971) 988. H. D. Bcckcy. 2. Phys C/km. Ah. B. 82 (1972) 161_ 1. Z Korosryshmky and V. A. Nazarenko, ht. 1. Mass Specrrom.

4 H_ D_ Ekxkey. M. D- Migahed

and E W_ RillIgen_ Int_ I_ Afa.u Specfrom_

ion

Ion Phys-_ 10 (1972)

471_ 5 I_ V_ Goldenfcld.

I- Z Korostyshevsky

and

B. G_ Mischanchuk,

Int_ J_ Mass

Spectrom.

Haj-s-, 13 (1974) 297. 6 H_ J_ Heinm. F_ W_ Rbllgcn and H_ D_ Bcckty. Z_ Narurforsch__ A. 29 (1974) 7 F_ He Dorm= zmd J_ D. Morrison. Cam 1. P&s__ 35 (1961) 57% 8 F_ W_ RBllgen and H- J- Hcincn_ Z_ Nar@%sc!r_. submitted.

773_

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