TOF-SIMS analysis of polymers

Beam Interactions with Materials & Atoms ELSEVIER

Nuclear Instruments and Methods in Physics Research B 131 ( 1997) 38-54

TOF-SIMS analysis of polymers Karl Wien * Institut f?ir Kerphysik,

Technische Hochschule. Schlossgartenstrasse 9, 64289 Darmstadt, Germany

Abstract When solid polymers are irradiated with heavy ions, atomic and molecular particles are ejected from the uppermost layers of the surface. A technique to determine the mass spectrum of the charged fraction of these particles is time-of-flight secondary-ion-mass spectrometry, TOF-SIMS. The present article describes, how the mass spectra measured with polymers are generally structured and under which conditions the various types of secondary ions like cationized oligomers, fragment ions and “fingerprint” ions are observable. The mechanisms leading to formation and ejection of the ions are not well understood. At bombarding energies of 10 keV, they are mainly based on atomic collision processes, at 100 MeV on the electronic excitation of the solid in the vicinity of the nuclear track. Processes, which are capable to desorb large organic molecules, seem not to work with oligomers of similar mass. Mass spectrometry of “real world” polymers, i.e. thick samples, depends mostly on the low-mass fingerprint spectrum, which can be produced by keV MeV SIMS. Modem TOF-SIMS instruments are equipped with a pulsed ion gun and an energy focussing ion mirror. They provide high mass resolution (m/Am N 10000) and high transmission (20-50%). Examples of applications are given, like the determination of mean molecular weights or investigations of radiation induced modifications of polymers. @ 1997 Elsevier Science B.V.

1. Introduction Mass spectra of secondary ions emitted from polymer surfaces by bombardment with primary ions supply information on the chemical structure and composition of the polymer and, under certain conditions, on the molecular weight distribution. The spectra can also be used to investigate modifications of polymers produced by the impacting primary ion itself or by prior ion irradiation. The method mostly employed to generate the mass spectra is time-of-flight secondary ion mass spectrometry, TOF-SIMS. The primary ion beam has typically an energy of 10 keV, but also high energetic beams in the MeV range have been used. Accordingly, the processes leading to ion ejection from

*Tel: 49 6151 162121. Fax: 49 6151 164321

polymers are sputtering by atomic collision cascades or by electronic excitation, respectively. TOF-SIMS has been particularly developed and established as a method of polymer analysis by the two collaborating laboratories of A. Benninghoven and D.M. Hercules [ l-141. During the past decade, they contributed to the instrumental part of the TOF method, but studied also systematically the fundamental problem of ion formation as well as the applicability of TOF-SIMS to a large variety of polymers. They showed that polymers, which are soluble and can be cast as a thin layer on a metal surface, provide up to 104u mass spectra of series of cationized oligomers and of fragment ions, which consist of multiple monomer units. In the low-mass range, these spectra exhibit usually characteristic fragment ions often build up by 1 or 2 monomer units. Ion irradiation of thick polymer films gives rise only to the character-

0168-583X/97/$17.00 @ 1997 Elsevier Science B.V. All tights reserved PII SO168-583X(97)00147-X

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istic fragment ions, i.e. the “fingerprint” spectrum. Today, TOF-SIMS with pulsed keV ion sources is on the way to become a widely used tool for polymer characterization. Successful applications concern molecular weight determination, blending of polymers with additives, chemically modified polymer surfaces, reactive mixing of immiscible polymers, surface diffusion on metals, adhesion of polymers to a substrate, et cetera. TOF-SIMS using MeV ion beams or 252Cf fission fragments was focussed, so far, more on fundamental aspects in context with radiation damage and nuclear-track structure. The samples of polymers to be investigated by TOF-SIMS are solid and, in a few cases, liquid. Mass spectrometry (MS) in former times required usually gaseous material as it is obtained with polymers by pyrolysis (PY-MS) . The polymer was identified by means of characteristic fragment ions employing magnetic and electric sector-field instruments of low transmission [ 15,161. Pyrolysis is a high energetic process producing preferentially low-mass fragments. A way of softer volatilization and ionization is field desorption (FD-MS), which has been used for the determination of oligomerdistributions [ 17,181. Limited control of the desorptionfionization process at the heated emitter and elaborate sample preparation affects the precision of ID-MS. An electrohydrodynamic method to generate multiply charged ions is electrospray ionization (ESI-MS) [ 191. The polymer has to be soluble in easy volatile solvents. Because the mean charge state of the desorbed polymers increases steadily with mass, the often very complex mass spectra are concentrated in a limited range between m/z = 500 and 20001~. Ions ejected from polymers are also obtained by irradiating polymer surfaces by laser light (LMS) [ 20,211. Generally, only fragment ions and small cationized clusters were observed. A more promising LMS technique for polymers is matrix-assisted laser desorption mass spectrometry (MALDI-MS) [ 22-241. The soluble polymer has to be embedded into crystalline matrix material absorbing effectively the pulsed laser light. The polymer ions, oligomers as well as fragment ions, are extracted from the plume of material evaporated by one laser pulse from the area of impact. Adding a cationizing agent to the matrix enhances the formation of oligomer ions [ 251.

39

Secondary Ion Mass Spectrometry (SIMS) has been used to study polymeric systems since almost 20 years [ 261. Review articles of Briggs [ 271 and Davies and Lynn [28] give a good survey about the applications of the SIMS technique to polymers. Often, quadrupole mass analysers are used, thus, the studies are focussed on the low-mass fingerprint region of the spectra below 500~. As shown, for instance, by Briggs et al. [ 29-361, in the static mode SIMS can provide highly structurally specific positive ion spectra and often even more informative negative ion spectra of surfaces of bulk polymeric material. A way to overcome the charging of insulating materials is to irradiate the samples by beams of neutral particles, i.e. to employ Fast Atom Bombardment (FAB) . Comparing charged and neutral primary particles, it turned out that degradation of the polymer samples is caused to a high extend by the electronic interaction between the bombarding particles and the surface [ 37,381. Regarding the reproducibility of the SIMS technique, at low doses (< 1013 particles/cm*) degradation of polymers can be considerably reduced by the use of primary neutral particles. In 1972, R.D. Macfarlane and coworkers rediscovered the advantages of the TOF technique for mass spectrometry (TOF-MS), when they investigated the ejection of large molecular ions from solid, involatile organic material by 252Cf fission fragments [39]. Soon after this impetus, the TOF technique was transferred to static SIMS using pulsed ion sources with keV energies and to LMS using laser pulses. Today, TOF-MS has become the modern method to generate the mass spectra of ions produced by the various ionization techniques mentioned above. Compared with other MS methods, TOF-MS provides a mass range extended up to lo%, high analyser transmission and a relatively high mass resolution (up to 15000). Coupling the ion source to a Fourier transform mass spectrometer (FTMS) extends the mass resolution even to lo6 [40]. The major problems concerning mass spectrometry of polymers are the preparation of the samples and the ionization/desorption technique. Oligomer mass distributions require very diluted solutions and submonolayer films of polymers deposited on metal surfaces. The analysis of thick polymer films, i.e. “real world” samples [ 121, depends on the availability of low-mass fingerprint ions.

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lnstr. und Meth. in Phy.y. Res. B 131 (1997) 38-54

detector 1)

Detector

1

/14 [

to start detector (version

2)

9 -..._: . _..__

-. SI? >

e-

t 0 stop

detector

Fig. I. Arrangement instrument. Version

of target

and *‘*Cf source

of an PDMS

5: ----= -

11

r

I can be used for thick samples, version 2

requires a sample thickness of less than 5 mg/cm2.

2. The time-of-flight method As described

in Ref. [ 391, Macfarlane

and cowork-

by 252Cf fission fragments and accelerated the secondary ions by means of an electric field between the sample and a grid. The sample was on a potential of typically + or - 10 kV. Passing the grid, the ions drifted a distance of 8 m to the stop detector, an assembly of planar micro-channel plates. The start signal for the TOF measurement was produced by the complementary fission fragment. The mass spectrum is then accumulated by measuring the time difference between the start and stop events being proportional to m '12.Since the number of stop events per 1 start event is usually > 1, a multistop time-to-digital converter (TDC) has to be employed. A modern TDC has a channel resolution of 0.5 ns, a time range up to about 100 ps and a pulse-resolving time of 20 ns [41]. Such a simple linear TOF instrument as used by Macfarlane et al. [ 391 can attain a mass resolution of about 2000. An improvement is achieved, when the velocity spread of the fission fragments is avoided as illustrated in Fig. 1. Here, the start signal is generated by means of secondary electrons ejected by the fission fragment from the target backside. With a 2 m long drift path, the linear TOF technique provides then a total detection efficiency of about 20% up to m = 10001.1 and a mass resolution of 2500 [ 421. The limiting factor is the initial energy distribution of the secondary ions. For organic molecular ions, a mean axial energy of about 2 eV (for a-cyclodextrine, m = 963~) has ers irradiated

[/I2 I I

thin targets

Pi I

drawing of the TOF-SIMS instrument constructed by Niehuis et al. [45]. Only the keV SIMS part is shown. Fig. 2. Schematic

,C,H,OH + 1500

/

43.018

i t

c

s

500

:2m

43.2

n-h (u)

Fig. 3. TOF spectrum of isobaric ions (~1 = 431~) ejected from PVDF by72.3 MeV ‘*‘I ions. The figure was taken from Ref. [ 48 1.

been reported [ 431. Concerning mass resolution, considerable has been achieved by installing an energy electrostatic ion mirror in the drift path, a reflectron [ 441. A modern TOF instrument

progress focussing so-called equipped

K. Wien/Nucl.

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41

m/z Fig. 4. TOF spectrum of neutral decay products originating from the decay of positive fragment ions Ck (see Fig. 15b). The parent ions were ejected from Krytox by 10 keV Ar+ ions. They are assigned by 1Ck 1 above a related mass line. The spectrum has been published in Ref.

[ I I I.

with such a reflectron has been constructed, for instance, by Niehuis and coworkers [ 451 for keV SIMS and for PDMS. It is sketched in Fig. 2. In the past decade, liquid-metal ion sources have been adapted as primary ion sources for keV TOFSIMS. A strong electric field extracts singly or multiply charged metal ions out of a tip of liquid metal by field evaporation ionization [46]. Part of the instrument build by Niehuis et al. is a chopped and mass analysed primary ion beam produced with help of a pulsed 90” deflector (see Fig. 2). Subsequent bunching and focussing delivers ion pulses of about 1 ns width and a minimum spot diameter at the sample surface of 30 pm. The ion energy is typically 10 keV, the ion intensity 5000 ions per pulse. For PDMS, a Cf-source-target arrangement is used similar to that sketched in Fig. I. The rate of fission fragments impacting the sample area is 4 or 5 orders of magnitude smaller than the rate delivered by pulsed keV ion sources. The secondary ions ejected from the sample surface are accelerated by an electrostatic extractor to energies of 3-10 keV. A gridless extractor working as a focussing and isochronic device has been developed, for instance, by Schmidt et al. [47]. After a drift path of’ about 1 m the ions penetrate the reflectron, where energy focussing is achieved by different penetration depths for ions of different energies. Depending on

the number of stages, the reflectron provides first or second-order energy focussing. The reflectron compensates changes of the sample potential and partially the time dispersion due to the initial velocity spread of the ions. The direct as well as reflected ions are detected by micro-channel plates. In order to increase the detection efficiency of heavy molecular ions - particularly above 1000~ - the ions are postaccelerated to an energy of 10 keV and then recorded by a channelplate-scintillation-countercombination. The mass resolution of a single stage reflector instrument is about 10000, the transmission 20 to 50% [ 451. Such a mass resolution allows one to resolve the isotopic pattern of oligomer peaks, but it separates also mass lines of isobaric molecular ions in the low-mass region. An example is shown in Fig. 3. This is essential for identification of fingerprint ions in a high background of ions released from surface contaminants. A particular problem with thick polymer samples is the charging of the surface due to the incident beam of positive ions or local charging of the nuclear track area [ 491. The latter causes a broadening of mass lines measured with a linear TOF instrument. This broadening seems to depend on the conductivity of the polymer (or of the damage zone along the nuclear track?). Partially carbonizing of the polymer by high dose irradiation with heavy ions increases the conductivity. Mass lines obtained with a 10 pm thick polyimide

42

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(Kapton) foil aluminized at the back-side were before irradiation 2.5 times broader than after. The foil was irradiated by a dose of 3 x 10” Pb ions per cm* having an energy of 11.4 MeV/n [ 501. As for PDMS [49], the beam induced surface charging by keV ions can prevent the appearance of useful mass lines completely. B. Hagenhoff et al. [2] developed a charge compensation system for TOF-SIMS instruments by bombarding the sample surface with low-energetic (10 eV) electrons. The electron source as well as the acceleration voltage for secondary ions was pulsed, in order to allow the electrons to reach the sample in-between two primary ion pulses. The TOF spectrum shown in Fig. 6 for a thick PMMA sample was taken with charge compensation. Without compensation no spectrum was observed. A TOF instrument as shown in Fig. 2 can be used in linear mode or reflectron mode. In the linear mode, a mass line can represent a parent ion and corresponding decay products, i.e. in case of a binary decay process the neutral and charged fragment of the parent ion. The neutral particles are detected by the stop detector behind the reflectron (see Fig. 2). Fig. 4 displays the mass spectrum of neutral decay products of the positive fragment ions Ck (in brackets) ejected from Krytox by 10 keV Ar+ [ 111. A section of the corresponding reflectron mass spectrum is given in Fig. 11b. It exhibits the Ck ions, which have survived the time before entering the second drift path, and the positively charged decay products. By measuring the yields of these three components as a function of flight time, it is possible to evaluate the half-live of the parent ions [ 5 I 1. Coincidence counting of events occurring simultaneously in both detectors allows investigation of individual decay path ways.

3. Mass spectra of polymers measured TOF-SIMS

by

In case of keV SIMS, the ions forming the TOF mass spectrum originate mostly from the first two monolayers of the sample. This means their spectrum is characteristic of the composition of the polymer surface. In Figs. 5-7, typical examples of TOF-SIMS spectra measured with 10 keV ions are shown. The spectra in Figs. 5 and 6 were obtained with monolayer samples of polysterene (PS) cast on an Ag substrate;

38-54

Fig. 5. TOF-SIMS spectrum of negative ions ejected from a polysterene (M, = 930~) monolayer sample cast on Ag contaminated with trifluoroacetic acid. The peaks of oligomers anionized by F- are marked with an asterisk. The spectrum has been published in Ref. [ 8 1.

Fig. 6. TOF-SIMS spectrum of negative secondary ions ejected from a thick PMMA film by 12 keV Ar+ ions. Dose 8.6x 10R/cm’. The insert explains the characteristic peaks generated by fragmentation of the polymer. The charging is intrinsic. The spectrum is published in Ref. 121.

the spectrum in Fig. 7 was recorded with a thick, multilayer sample of polymethylmethacrylate (PMMA) . Such spectra usually consist of three regions, which are well expressed in Fig. 5. First, a series of intact, cationized oligomers is seen representing the molecular weight distribution of the polymer sample. The mass difference between two oligomer peaks gives the mass of the repeat unit, mR. In case of PS, the repeat unit has the composition CH2CH(C6Hs) with mR = 104. Beside the series of singly charged oligomers also a series of doubly charged

K. Wien/Nucl. Instr. and Meth. in Phys. Rex B 131 (1997) 38-54

Polymethylmethacrylote.

25

50

75

43

neg.

100

125

150

175

200

225 m/z

Fig. 7. TOF-SIMS spectrum of positive secondary ions ejected from a polysterene (M, = 3770~) monolayer sample cast on Ag. Ia assignes the Ag+ cationized oligomer distribution, Ib the Agz+ cationized oligomer distribution, II the fragment ion region and 111 the fingerprint region. The spectrum was taken from Ref. [ 121. *

oligomers exists, which corresponds to cationization by Ag;+. A second region, in case of PS extended over the whole spectrum of positive ions, is formed by fragment ions. This series consists of multiple monomer units terminated or not by an endgroup. Fragmentation occurs statistically, for example, by scission of the main chain between repeat units, but also within repeat units. When the polymer is, in addition, terminated by two different endgroups, the periodic pattern of the fragment ion region can become relatively complex as demonstrated in Figs. 8 and 9 for PS and perfluorinated polyether (PFPE, trade name Krytox) samples. Also series of fragment ions can be used to determine the mass of the repeat unit. The pattern within the spacing of one repeat unit gives information about the endgroups, possible substituents and the cleavage within repeat units. The charging of the fragments does not require necessarily cationization via substrate constituents, but can occur also as an intrinsic process. An example is the negative fragment ions of Krytox. The third region assembles the mass lines below typically 300~ and is called the fingerprint region. An example is shown in Fig. 7. In this case, the secondary ions carry an intrinsic charge and originate mostly from one or two monomer units. The fingerprint region is often overloaded with numerous ions ejected

from surface contaminants or formed from nonspecific destruction products. Thick samples can be characterized usually by their fingerprint spectrum. In cases where intrinsic charging occurs, series of fragment ions are also available as illustrated in Fig. 9 for Krytox. Charged oligomers were not observed with thick polymer samples. Only a few examples of mass spectra obtained with MeV ions have been published. Because monolayer samples of polymers deposited on a metal surface and irradiated by MeV ions do not deliver specific ions with sufficient intensity, the measurements were performed with multilayer samples. Oligomer ion series were found only below lOOOu, for instance with a polyethylene-glycol (PEG 750 [ 521) film or with frozen samples of low-mass chain molecules like the alkanes [ 531. A comparison of the keV and the MeV technique, i.e. keV SIMS and PDMS (Plasma Desorption Mass Spectrometry using 252Cf fission fragments as projectiles) has been performed by Feld and coworkers [ 41 for polycarbonate. The two mass spectra presented in Fig. 10 show characteristic lines in the fingerprint region - apart from an intense pattern of ions due to surface contamination. In case of intrinsic charging, fragment ions also appear in the spectra as shown in Fig. 11, where a multilayer Krytox sample has been irradiated by 10 keV Ar+ ions and 252Cf fission fragments. A general problem of 252Cf sput-

K. Wien/Nucl. Instr. and Meth. in Phys. Rex B 131 (1997) 38-54

44

Fig. 9. Section of a negative ion mode TOF-SIMS spectrum of Krytox. Analog fragment ions in each group are assigned by the same letter. For explanations see Ref. [ 61, where this figure has been published. C&i,

-

CH,

-

/ C,H,

I T

CH

CH2

-

I

c

;

i

CH2

-

;

-

H

I

I

CHZ

I I 1

-1

Fig. 8. Section of the TOF-SIMS spectrum of polysterene showing groups of cationized fragments. The distance between neighbouring groups is given by the mass R (= 104”) of one repeat unit. Below the figure, cleavage marks at C-C bonds in the polymer chain indicate 7 possible fragmentation channels associated with one repeat unit: 4 with one endgroup included and 3 without endgroups. The most intense peak of each group corresponds to an integral number of repeat units. The figure and fragmentation scheme has been taken from Refs. [ 1,2].

tering of polymers is the comparatively very low rate of the fission fragments impacting the sample surface. PDMS spectra need measuring times up to 20 hours, keV SIMS spectra only a few minutes.

4. Formation ion impact

of secondary

ions from polymers by

4. I. Sputtering by keV ions The primary ions used for TOF-SIMS are, for instance, rare gas ions like Ar+ or ions extracted from a liquid metal source like Ga+. When such ions penetrate a solid with an energy of typically 10 keV, they distribute their kinetic energy to the atomic system

of the solid via recoil collision cascades. Intersecting the surface, the collision cascades lead to sputtering of preferentially atomic or small molecular species. In addition to atomic collisions, the charge of the incident primary ions induces electronic excitations, which can lead to further damage and subsequent sputtering. As shown by Leggett and Vickerman [ 371, degradation of polymers by ionic primary particles is considerably enhanced compared to degradation by neutral particles. This is also expressed in the pattern of the lowmass secondary ion spectrum. 4.1.1. Oligomer ions Oligomer ions have only been observed with monolayer samples deposited on metal substrates - particularly Ag. The fact that large oligomers are released from the surface without destruction is not well understood. But recent investigations by Delcorte and Bertrand [54] of the initial energy spectrum of an cationized oligomer seem to support the collisional character of the sputter mechanism. Fig. 12 shows the axial (normal to the sample surface) kinetic energy distributions of three secondary ions ejected from PS ( M, = 2180~) by 15 keV Ga+ Comparing the energies of the cationized oligomer [ lo9Ag + H(CgHg)t&Hs]+ with that of Ag+ and CgHt, the energies of the oligomer are closer to that of Ag’ known to be sputtered by atomic collisions. This means, the ejection of the large oligomer is a comparatively high energetic process. The collision cascades comprise up to several 1000 atoms, which are set

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Fig. 10. TOF spectra obtained with keV (SIMS) and MeV (PDMS) primary ions. The target was a polycarbonate foil. Numbers in circles assign the mass of characteristic fragment ions in the fingerprint region. PID = Primary Ion Dose (ions/cm2). The spectra are published in Ref. [4].

”

m/z

00

.-

1300

.

.

.

..-.-.

.... 1400

,. 1500

1600

1700

m/z Fig. 1I, Sections of TOF-SIMS spectra measured with a thick Krytox sample bombarded (a) by IO keV Ar+ ions and (b) by 2s2Cf fission fragments. The spectra show the region of fragments carrying an intrinsic negative charge. The letters indicate types of ions within the periodic groups of fragment ions, the index numbers give the number of repeat units per fragment. For further explanation see Ref. [ 111, where the figures have been published.

46

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J

Fig. 12. Axial kinetic energy distributions of three secondary ions ejected by 15 keV Ga+ ions from a monolayer PS sample cast on Ag. V’ is the corrected sample voltage in volt [ 541. The figure was presented by Delcorte at the Int. workshop on Inelastic Ion Surface Collisions (IISC-11) [ 541.

Fig. 14. TOF-SIMS spectrum of positive secondary ions ejected by I2 keV Ar+ ions from a monolayer polysterene( M, = 858 000~) sample cast on Ag. Primary ion dose 1.5 x 10” ions/cm’. The spectrum has been published in Ref. [ 3 1.

from the sample surface can be described section U(M) using the expression

by a cross

N(M, t) = N( M)e-“‘“)J~“‘. N( M, t) is the number of intact polymers covering the analyzed area of the sample after a time t of ion bombardment (current density J,, ) The count rate of an individual cationized oligomer X having the mass M + m,Q is then given by

R(X,t)

Fig. 13. Transformation probability P(M X) of cationized oligomers plotted as function of molecular weight M. P( M --t X) has been measured with PS standards of an average molecular weight between M, = 1770~ and 3000~ using 12 keV Ar+ ions and monolayer samples. The diagram was taken from Ref. I3 1.

into motion within about lo-‘* s. This could create a coherent impulse on the polymer stretched over a certain area of the metallic substrate. Related shockwave models have been developed by Yamamura et al. [ 5.51 and Bitenski et al. [ 561. Van Leyen et al. [ 31 observed that the count rate of detected oligomer ions ejected from monolayer PS samples decreases exponentially with time of ion bombardment due to ion induced destruction or desorption of the polymers. The disappearance of intact polymers

=e(X)P(M

+ X)

dN(M, t) dt

Here, E(X) is the detection efficiency of the oligomer X and P( M + X) the transformation probability [ 31, i.e. the probability that a polymer M disappearing from the surface is transformed into a cationized oligomer X. The disappearance cross section, derived from the decay curves, turned out to increase linearly with M and was independent of the substrate material and the surface coverage (in the monolayer range). Estimating N(M) , the original number of oligomers of mass M in the analysed area, and the detection efficiency e(X) , also P ( M + X) was evaluated. The result, a plot of P( M -+ X) versus M, is displayed in Fig. 13. Above M = 2OOOu, P( M + X) decreases much steeper than linearly with M. P(M + X) for M = 10000~ is about 3 orders of magnitude smaller than for M = 2000~. Since cationization occurs by attachment of an energized Ag+ ion to the oligomer, it seems unlikely that the ionization probability exhibits

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such a dramatic decrease with M. We conclude that larger oligomers tend to fragment with rising probability. This can have two reasons: Firstly, the larger the attacked oligomer, the more transferred energy is converted into internal excitation, increasing the fragmentation probability. Secondly, the chain length or size of large polymers extends over an area which is larger than the surface area around the point of primary ion impact, which is energized by collision cascades or a shock wave. Thus, only portions of the polymer are desorbed. This increasing fragmentation probability explains, why PS-oligomer distributions with average molecular weights above M, N 12000~ have not been observed so far by means of TOFSIMS. Such heavy polymer distributions generate in TOF-SIMS spectra only series of fragment ions as shown in Fig. 14. MALDI-MS being a softer desorption method is capable to detect oligomers up to 105u.

4. I .2. Fragment ions and metastable decay Fragment ions are ejected from monolayer as well as multilayer (thick) samples. Charging is established by cationization and also by intrinsic ionization. The various ideas and models to explain ion formation in the fingerprint region have been reviewed and replenished by Leggett and Vickerman [38]. The heavier fragment ions (in the fragment region, see Fig. 6) exhibit intrinsic charging only, when the polymer contains halogen constituents. Fragment and oligomer regions often overlap, fragmentation may include the loss of only one hydrogen atom. The complexity of the spectra increases considerably, when co-polymers or polymers with side chains are analysed. On the other hand, such a periodic diversity of mass lines (see Fig. 9) yields information on the polymer structure as it has been impressively demonstrated by the groups of Hercules and Benninghoven for perfluorinated polyethers [ 6,7]. The analytical feasibilities are limited, when only fingerprint spectra are available as it is the case for many thick polymer samples, i.e. “real world” samples. Such spectra can be used to identify certain polymers and to reveal modifications. The analysis sometimes impeding phenomenon is the me&stable decay of desorbed fragment ions in the drift path of TOF instruments. Feld and coworkers detected metastable fragment ions with the perfluorinated polyether Krytox using both a linear version of

41

their TOF instrument and a version equipped with an energy focussing mirror [ 111. The structure of Krytox is F

F

I -C-CF

I

The two most abundant series of fragment ions were Ck = (FRkCsF6)+ and Ek = (FRk) - with Rk = (F&O)k. They are formed both by the cleavage of the C-O bond, either at the left side of the oxygen atom or at the right side. It is interesting to note that the two corresponding ions of opposite charge come from the same (left) end of the chain. For a thick sample, Feld et al. [ 1 l] measured half-lives of Ck and

800

850

900

950

1000

1C50

1100

,150

1200

m/z X1037,’ ^

-I

0

m/z 15. Sections of TOF spectra obtained with Krytox and a linear TOF instrument using postacceleration. (a) negative SIMS spectrum, (b) positive SIMS spectrum. The lines of the ions Ek and ck show line broadening due to metastable decay. k gives the number of repeat units. An, Be, Dk. Fk assign other fragment ion series. Further explanations see Ref. [ 1I].

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Ek of 13 and 37 ,us. The half-lives were independent of the fragment mass. The effects of metastable decay on a linear TOF spectrum are illustrated in Fig. 15. Because the decay of a parent ion into a charged and a neutral particle occurs in the field-free drift path of the instrument, the flight time of both decay products should not change considerably. Only the recoil energy distributed to both particles can cause a small line broadening. The instrument of Feld et al. employed, however, postacceleration in front of the stop detector to increase detection efficiency. Therefore, the parent ion and the two decay products gain different velocities, what generates satellite peaks in the surrounding of the mass line of the parent ion. In part (a) of Fig. 15, the lines of the negative parent ions Ek exhibit small tails to shorter flight times indicating a charged decay product, which has a mass only slightly lower than the mass of the parent ion. Feld et al. have shown that the parent ions Ek loose only one repeat unit per metastable decay. Quite different is the situation for the positive parent ions Ck: The masses of the decay products are extended over the whole possible mass range. Beside the sharp parent ion line, a wide distribution of charged decay products forms at shorter flight times a broad peak. Regarding the half-lives of the two ions Ek and Ck. the negative ions are more stable than the positive ones. The authors suggest that the oxygen atom retaining the negative charge stabilizes the ions Ek. Stabilizing fragment ions by certain components of the polymer could be important for the detectability of polymers by SIMS. It turned out that the half-lives of the two fragment ions Ek and Ck become longer ( 17 and 79 /_LS[ II] ) , when the ions are ejected from monolayer samples of Krytox. This has the positive effect that the mass lines of heavy and therefore slowly moving parent ions yield relatively more counts as those measured with multilayer samples. The half-live of metastable ions is obviously an important parameter concerning the measurable number of these ions. Half-lives shorter than the flight time in the acceleration gap between target and drift path (N 100 ns) lead to the loss of parent ion lines, i.e. eventually of an essential fraction of the fragment ions. Delcorte and Bertrand [ 541 have shown that certain fragment ions represented by well resolved mass lines are actually products of a very fast decay close to the sample surface. As men-

c

0 1 T-

-

--.-

I!

:

0001

-15

-10

-5

0

5

16

V’ Fig. 16. Axial kinetic energy distributions of three secondary ions ejected by 15 keV Ga+ ions from monolayer PS and PIB samples cast on Ag. V’ is the corrected sample voltage in volt [ 521. The figure was presented by Delcorte at the Int. workshop on Inelastic Ion Surface Collisions (11X-I 1) [ 541.

tioned, they measured the initial axial kinetic energies of various ions. They found distributions with tails in direction of “negative” energies. The corresponding ions did not experience the full acceleration potential, because they are fragments of heavier molecular ions decaying in the acceleration section of the instrument. As seen in Fig. 16, a relatively large fraction of the fingerprint ions C7HT produced with polyisobutylene (PIP) has “negative” energies. Less pronounced is the negative-energy tail of the C7H,f being ejected from PS (M, = 2180~). In this case, most of the CyHT ions are produced on top of the surface. For further discussion see Ref. [ 541. Another remarkable result reported by Feld et al. [ 1 l] is that oligomers cationized by Agf did not exhibit any metastable decay. 4.2. Sputtering by MeV ions It has often been pointed out that the primary ion energy has only minor influence on the pattern of mass spectra measured with organic material (see, for instance, Ref. [ 571). Differences between keV and MeV sputtering of polymers concern mainly the yields of secondary ions being for 252Cf fission fragments about 10 times higher than for 10 keV Ar+ ions [ 111. Contrary to heavy keV ions, far most of the kinetic

K. WiedNucl. Instr. and Meth. in Phys. Res. B 131 (1997) 38-54

energy of a 100 MeV fission fragment is primarily transferred to the electronic system of the irradiated solid. The various concepts of electronic transport and relaxation processes in insulators following the primary excitation are discussed, for instance, in the review article of Johnson and Schou [ 581. Characteristic for the high energetic heavy ions penetrating an insulator like a polymer is the formation of an highly energized core around the nuclear track, called the infratrack (radius 5- 10 A), and of a wider less energized zone, where S-electrons react with polymers via secondary excitations inducing, for instance, bond breaking in polymer chains. Essential for desorption processes at the surface is the evolution of the infratrack, the “hot” core. Due to fast relaxation of the electronic excitations in conductive material, such a “hot” core is not or hardly [59] formed in metals. This is probably the reason, why 100 MeV ions are not capable to produce useful TOF-SIMS spectra with monolayers of polymers deposited on metals [ 111. The low-mass ions observed in the fingerprint region stem partly from contaminants and partly from destruction products of the polymer. We assume that individual excitations by secondary electrons are responsible for these unspecific ions.

4.2.1. Oligomer ions A desorption model based on electron “hits” of large molecules is the ion-track model of Hedin et al. [60,61]. The central idea of this multi-hit model is that, if a molecule near the surface interacts with at least a minimum number of secondary electrons, it is desorbed. The minimum number of hits depends on the mass or size of the molecule. But oligomers ejected from submonolayer samples on metals, where the polymers were well separated, have not been reported. Oligomers cationized by alkali ions have been found in TOF spectra only in the mass range below 100011 [ 52,531. The fact that oligomer ions were not observed at higher masses in PDMS spectra, is to a certain extend surprising, because desorption of large fragile organic molecules as intact species by MeV ion impact is a well-known phenomenon [ 391 successfully utilized for mass spectrometry since 20 years [ 621. Particularly angular distributions measured with these large molecular ions (see, for instance, Ref. [63])

49

have proven that the corresponding desorption process can be described as ablation caused by a pressure pulse (shock wave) propagating from the “hot” core into the surrounding material [ 64,561. A reason that shock-wave ablation does not work for thick polymer samples could be that strong intermolecular forces within the polymer bulk, or the entanglement of polymer chains hinder oligomer ejection [ 31. Van Leyen et al. have shown that the admixture of cationizing additives or a thin Ag layer on top of the polymer sample do not accomplish desorption of cationized oligomers by MeV ion impact [ 31.

4.2.2. Fragment ions and ions generated by reactions in the track plasma TOF-SIMS analysis of polymers using MeV ions depends on the fingerprint spectrum and in a few cases on the intrinsically charged fragment ions. Recent studies of initial velocities of these ions by Papaleo et al. [65,66] indicate that one part of the ions is ejected out of the “hot” inner core of the nuclear track and an other part more from the surrounding “cooler” areas. They measured the radial (perpendicular to the surface normal) velocity distributions of hydrocarbon and fluorcarbon ions ejected from polyvinylidene fluoride (PVDF), polysterene (PS) and polytetrafluoroethylene (PTFE) foils by 72.3 MeV 1271’3+primary ions. The accelerator beam penetrated the samples under 45’ against the surface normal (see Fig. 17). By measuring radial velocities, they were able to distinguish between ions leaving the surface more in direction of the beam “axis” ( ux positive) and those flying more perpendicular to the beam “axis” ( ux negative). As an example, results obtained with C,Fi ions emitted from PTFE are presented in Fig. 17a. C,Fi ions with m < 5 move obviously more in direction of the beam axis, i.e. the nuclear track axis, and those with m > 6 away from the track axis. In addition, also r, the full width half maximum of the radial velocity distributions, was determined (see Fig. 17b) and used to derive the local “temperature” at the point of ion generation. The authors concluded that the less fluorinated clusters moving in track direction come from the inner zone of the track, where the energy density, i.e. “temperature”, is highest. This picture is in fair agreement with a flow of gaseous destruction products out of a crater formed at the track

K. Wien/Nucl.

50

0

2

4 6 8 NumbcrofFamu

10

Instr. und Meth. in Phys. Res. B I31 (1997) 38-54

I2

0

1

4

6

0

IO

12

Number of F amns

Fig, 17. (a) Average mean radial velocities of a group of C,F;fi ions with a constant IN (n varying) plotted versus the number m of fluorine atoms. (b) Average of the Iv2 (c( M(LI~) - Mu) of radial velocity distributions measured for C,P$ ions with a constant m as a function of tn. M = secondary ion mass. For further details see Ref. 1651 and text.

exit. The clusters with highest fluorine content would come from the outer “cooler” regions of the track, but they move perpendicular to the track axis. This behaviour fits better in a pressure pulse model. Similar results were obtained with positive hydrocarbon ions ejected from PVDF and PS, but not for negative hydrocarbon ions [ 651. Craters produced by MeV ions in organic solids have been in fact observed by means of scanning force microscopy [67]. PDMS spectra measured with frozen samples of light alkanes like methane exhibit a rich diversity of reaction products indicating dissociative as well as associative reactions in an expanding gas [ 53, 681.

4.2.3. Meta stable decay Comparing keV and MeV sputtering from Krytox, van Leyen et al. [ 111 studied also the metastable decay of fragment ions produced by fission fragment impact. The half-lives of the Ck and Ek ions (see Fig. 11) were found to have larger values (27 and 96 /.G., respectively) than those measured with 10 keV Art ions. The spectra presented in Fig. 15 were recorded by means of a high-resolution reflectron-TOF instrument, which clearly separates parent ions from charged decay products. For instance, the line ET, corresponds to the decay product of the parent ion Eta. ET, possesses one repeat unit less than Et0 and is, therefore, located close to Eg. The longer half-live of MeV sputtered f& ions leads to relatively higher intensities and sharper mass lines in TOF spectra compared with keV sputtered ions (see Fig. 11) . When the differences of half-lives are taken into account, the actual yields of

I,,/ 400

820

I I r c* t I

,,,, 1240

1660

2080

2500

x .z 2 2 c Y

50

0 690

705

720

135

750

M/Z Fig. 18. Sections of a PDMS spectrum of positive carbon cluster ions ejected from PVDF by 72.3 MeV iodine ions. The spectra have been published in Ref. [ 701.

fission-fragment-produced ions turned out to be a factor 10 higher than those produced by 10 keV Arf ions.

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4.2.4. Carbon cluster ions The gas-flow concept has also been employed to explain the formation and ejection of positively charged carbon clusters out of the nuclear track [ 691. C& and other carbon cluster ions were found in TOF spectra of poly( vinylidenedifluoride) , PVDF, bombarded by 252Cf fission fragments [7] or 78.2 MeV ‘27I ions [ 70,711 (see Fig. 18), Measurements of the radial velocity distributions of C& proved that these fullerenes move preferentially backwards along the primary ion trajectory. This leads to the assumption that the fullerene C60 is formed as a reSUlt of cluster growth during expansion of the plasma in the “hot” core of the track.

5. Applications In principle, TOF-SIMS can be applied to a11 kinds of polymers, if their vapour pressure is sufficiently low for vacuum requirements. Till 1991, the group in Mi_inster/FRG had investigated already a dozen of different polymer materials [ 721. Today, the list of studied polymers is much longer in accordance to the increasing interest in TOF-SIMS for basic research as well as for technological applications. About 30 contributions to the SIMS-X conference in Miinster ( 1995) confirm the importance of this analytical method. In the following, two types of applications will be shortly expounded: 5.1.

Molecular weight distributions

The knowledge of the molecular weight distribution is essential for manufacturing polymers, for controlling their properties and performance. A TOF spectrum of mass resolved oligomer peaks offers the possibility to determine the absolute molecular weight distribution. A first example was given by Chait and coworkers [ 521, who measured PDMS spectra of polyethylene glycols (PEG) using a linear TOF instrument. Cationizing was accomplished by means of the additive 6LiC1. For the standard PEG 750 they determined a mean molecular weight M, of 647~. When the molecular weight distribution is directly derived from the measured mass spectra, the number average molecular weight M, js often smaller than measured with other methods (for instance, nuclear

in

Phys.Rex B 131 (1997) 38-54

51

magnetic resonance, NMR, or gel permeation chromatography, GPC) . This tendency of TOF-SIMS was studied systematically by Bletsos and coworkers irradiating a set of 12 polymers cast as monolayers on Ag by 12 keV Ar+ ions [9]. In order to determine the absolute molecular weight distribution, firstly, the relative yield of each oligomer ion ejected from the sample has to be determined. A reflectron instrument provided sufficient mass resolution to separate oligomer ions from fragment ions. In case of equal mass, the measured oligomer intensity was estimated with help of its isotopic pattern. Then, the detection efficiency, which decreases at 20 keV postacceleration between 1000~ and 7000~ by about a factor of 2 [ lo], has to be taken into account. The disappearance cross section is also mass dependent. M, values obtained with polysterene samples were found to decrease slightly (l-5%) due to sputtering during the time the mass spectra were recorded. The application of the corresponding two corrections provides the molecular weight distribution of the ejected cationized oligomers, which can be transferred into the actual distribution of polymers by means of the transformation probability, i.e. the probability to transform an attacked polymer into a desorbed cationized oligomer [ 111. This mass-dependent probability can hardly be calculated and should be determined with help of standard samples. Surprisingly, Bletsos et al. realized, that even without applying all the various corrections to the measured intensities, the agreement between molecular weight averages obtained by TOF-SIMS and other methods is remarkably good, the deviations are in the order of 10%. 5.2. TOF-SIMS analysis of irradiated polymers TOF-SIMS analysis of polymers is usually performed under “static” conditions, this means, the dose of primary ions is sufficiently low (< lOI primary ions/cm*) to guarantee that each primary ion hits an undamaged surface area. Therefore, static SIMS creates mass spectra, which reflect radiation effects produced by the primary ion itself as it is discussed in Section 4. SIMS under “dynamic” conditions delivers spectra, which are taken during continuous modification of the polymer by an high intensity beam of primary ions. A third way to study radiation effects is, to alternate between periods of high-dose bombardment and TOF-SIMS analysis. Such experiments

K. Wien/Nucl. lnstr. and Meth. in Phys. Rex B 131 (1997) 38-54

ions. Additional amounts of these ions are produced by the loss of hydrogen of the more saturated ions. Dehydrogenation leads finally to carbonization of the organic material, a well-known phenomenon.

Acknowledgements The author gratefully acknowledges A. Benninghoven and M. Deimel for providing the reprints on the polymer work of the group in Miinster/FRG. He also thanks them, A. Delcorte and P. Bertrand as well as R.M. Papaleo for the permission to present individual figures published by them and their coworkers. Fig. 19. SIMS intensities of positive hydrocarbon ions CsHz with n = 7, 9, 11, 13, 15, 17 as a function of the cumulated Xe+ dose. The sample was a polypropylene foil. The intensities are normalized to the initial intensity of a characteristic ion at m = 69~. The figure. was taken from Ref. [ 731.

have been performed, for instance, by Delcorte and coworkers [ 731. They measured TOF spectra with thick films of 3 saturated aliphatic polymers (polyethylene, polypropylene and polyisobutylene) as a function of primary ion dose up to 1014 ions/cm*. The primary ions were 4 keV Xef and 15 keV Ga+. A first result was that the intensity of characteristic fingerprint ions decreased with dose due to gradual degradation of the polymer. A similar behaviour has also been observed very recently by Trautmann and the author [ 501, who studied by means of PDMS polyethylene terephthalate, polycarbonate and polyimide foils irradiated by 11 MeV/n Pb with a dose of lO”/cm*. The dose needed to produce this degradation is lower than the dose of keV ions. This is consistent with the fact that the disappearance cross sections measured with molecules of similar mass are larger for PDMS [74] than for keV SIMS [3]. A second result of Delcorte et al. [73] was that the polymer surface is dehydrogenated. They concluded this from intensities of ions, which belong to the hydrocarbon group CsHT ejected from polypropylene by 4 keV Xe+ ions. Fig. 19 shows the relative intensities of 6 ions of the C8 group as a function of dose. It is seen that the degradation of the more saturated hydrocarbon ions is larger than that of the less saturated. Dehydrogenation is indicated by an increase of yields at low doses for those ions, which contain small numbers of hydrogen

References D. van Leyen and A. 111 IV. Bletsos, D.M. Hercules, Benninghoven, Macromolecules 20 ( 1987) 407. 121 B. Hagenhoff, D. van Leyen, E. Niehuis and A. Benninghoven, J. Vat. Sci. Technol. A 7 (1989) 3056. [31 D. van Leyen, B. Hagenhoff, E. Niehuis, A. Benninghoven, IV. Bletsos and D.M. Hercules, J. Vat. Sci. Technol. A 7 (1989) 1790. [41 H. Feld, R. Zurmtlhlen, A. Leute, B. Hagenhoff and A. Benninghoven, in: Secondary Ion Mass Spectrometry, SIMS VII, eds. A. Benninghoven, C.A. Evans, K.D. McKeegan, H. Storm and H.W. Werner (Wiley, New York 1990) p. 219. M. Deimel, B. Hagenhoff and A. r51 D. van Leyen, Benninghoven, in: Secondary Ion Mass Spectrometry, SIMS VII, eds. A. Benninghoven, C.A. Evans, K.D. McKeegan, H. Storm and H.W. Werner (Wiley, New York, 1990) p, 757. Lb1 I.V. Bletsos, D.M. Hercules, D. Fowler, D. van Leyen and A. Benninghoven, Anal. Chem. 62 ( 1990) 1275. H. Feld, R. Zurmtlhlen, A. Leute and A. Benninghoven, J. Phys. Chem. 94 ( 1990) 4595. IV. Bletsos, D.M. Hercules, D. van Leyen, A. Benninghoven, C.G. Karakatsanis and J.N. Rieck, Macromolecules 23 (1990) 4157. I.V. Bletsos, D.M. Hercules, D. van Leyen, B. Hagenhoff, E. Niehuis and A. Benninghoven, Anal. Chem. 63 ( 1991) 1953. 101 B. Hagenhoff, A. Benninghoven, H. Barthel and W. Zoller, Anal. Chem. 63 (1991) 2466. M. P 111 H. Feld, A. Leute, D. Reading, A. Benninghoven, Chiarelli and D.M. Hercules, Anal. Chem. 65 ( 1993) 1947. 121 A. Benninghoven and D. Reading, Macromol. Symp.. Vol. 83 (1994) 27. 1131 M. Deimel, B. Hagenhoff and A. Benninghoven, in: Secondary Ion Mass Spectrometry, SIMS X, ed. A. Benninghoven, C.A. Evans, K.D. McKeegan, H. Storm and H.W. Werner (Wiley, New York, 1996) p. 961. [ 141 M. Deimel, H. Rulle, V. Liebing and A. Benninghoven, in: Secondary Ion Mass Spectrometry, SIMS X, ed. A.

K. Wien/Nucl. Instr. and Meth. in Phys. Res. B 131 (1997) 38-54 Benninghoven, C.A. Evans, K.D. McKeegan, H. Storm and H.W. Werner (Wiley, New York, 1996). p. 755. [ 151 H.-R. Schulten, and R.P. Lattimer, Mass Spectrom. Rev. 3

1161

[ I7 ] [ 181 [ 191 [ 201 121

]

(1984) 231. G.T. Mol. R.J. Gritter and G.E. Adams, in: Applications of Polymer Spectroscopy, ed. E.G. Brame (Academic Press, New York 1978) p. 257. H.-R. Schulten, lnt. J. Mass Spectrom. Ion Phys. 32 ( 1979) 97. B. Uhr, 1. Ltidetwald, R. Miller and H.-R. Schulten, Angew. Makromol. Chem. 120 (1984) 163. J.B. Fenn, M. Mann, Chin Kai Meng, Shek Fu Wong and CM. Whitehouse, Science 246 (1989) 64. J. A. Gardella, D.M. Hercules and H.J. Heinen, Spectrosc. Lett. 13 (1980) 347. J.A. Gardella and D.M. Hercules, Fresenius’ 2. Anal. Chem.

308 (1981) 297. 1221 U. Bahr, A. Deppe, M. Karras and E Hillenkamp, Anal. Chem. 64 (1992) 2866. [ 231 P.O. Danis, D.E. Karr, E Mayer, A. Holle and C.H. Watson, OMS Lett. 27 (1992) 843. 1241 G. Montaudo, MS. Montaudo, C. Puglisi and E Samperi, Rap. Comm. Mass Spectrom. 9 (1995) 1158. 1251 PO. Danis, D.E. Karr, Yansan Xiong and KG. Owens, Rap. Comm. Mass Spectrom. 10 (1996) 862. 1261 J.A. Gardella and D.M. Hercules, Anal. Chem. 52 (1980) 226; 53 (1981) 1879. 127 ] D. Briggs, Br. Polym. J. 21 (1989) 3. 1281 MC. Davies and R.A.P. Lynn, Crit. Rev. Biocompat. 5 ( 1990) 297. 1291 D. Briggs and A.B. Wootton, Surf. Interface Anal. 4 ( 1982) 109. [30] D. Briggs, Surf. Interface Anal. 4 (1982) 151. [31 I D. Briggs, Surf. Interface Anal. 5 ( 1983) 113. [ 321 D. Briggs, M.J. Heam and B.D. Ratner, Surf. Interface Anal. 6 (1984) 184. [33] D. Briggs, Polymer 25 ( 1984) 1379. [34] D. Briggs and H.S. Munro, Polym. Commun. 28 ( 1987) 307. 1351 D. Briggs, Org. Mass Spectrom. 22 (1987) 91. [ 361 D. Briggs and M.J. Hearn, Vacuum 36 (1986) 1005. [37] G.J. Leggett and J.C. Vickerman, Anal. Chem. 63 ( 1981) 561. 1381 G.J. Leggett and J.C. Vickerman, Int. J. Mass Spectrom. Ion Proc. 122 (1992) 281. 1.391 R.D. Macfarlane and D.E Torgerson, Science 191 (1976) 920. [40] Long-Sheng Sheng, S.L. Shew, B.E. Winger and J.E. Campana, in: Hyphenated Techniques in Polymer Characterization, eds. T. Provder, M. Urban and H.G. Barth, American Chemical Society Series, American Chemical Series, Washington DC, 1994, Ch. 5, p. 55. [al] E. Festa, L. Tasson-Got and R. Sellem, Nucl. Instr. and Meth. A 234 ( 1985) 305. 1321 K. Wien, 0. Becker and W. Guthier, in: Proc. 3rd Int. Conf. on Radiation Effects in Insulators, eds. I.H. Wilson and R.P Webb (Gordon and Breach, New York, 1986) p. 751.

[43]

[44] [45] 1461 [47] 1481 [49]

[50] 1511 1521 [53] 1541

53

R. Moshammer, R. Matthaus, K. Wien and G. Bolbach, in: Proc. V. Int. Conf. on Ion Formation from Organic Solids (IFOS V), 1989 L&&ngar, Sweden, eds. A. Hedin, B.U.R. Sundqvist and A. Benninghoven (Wiley, New York, 1990) p. 17. B.A. Mamyrin, V.I. Karataev, D.V. Shmikk and V.A. Zagulin, Sov. Phys. JETP 37 (1973) 45. E. Niehuis, T. Heller, U. Jthgens and A. Benninghoven, J. Vat. Sci. Technol. A 7 ( 1989) 1823. J. Orloff, Rev. Sci. Instrum. 64 (1993) 1105. L. Schmidt, H. Jungclas, H.-W. Fritsch and I? Kohl, J. Am. Sot. Mass Spectrom. 4 (1993) 782. R.M. Papaleo, Thesis at the Uppsala University, Sweden, no. 184 (1996). K. Wien, I? Koczon and M. Weber, in: Proc. 4th Int. Conf. on Ion Formation from Organic Solids (IFOS IV). ed. A. Benninghoven, Mtlnster 1987 (Wiley, New York, 1989) p. 57. C. Trautmann, GSI Darmstadt/FRG, and K. Wien. 1996, private communication. B. Schueler, R. Beavis, W. Ens, D.E. Main, X. Tang, K.G. Standing, Int. J. Mass Spectrom. Ion Proc. 92 ( 1989) 185. B.T. Chait, J. Shpungin and EH. Field, Int. J. Mass Spectrom. Ion Proc. 58 (1984) 121. M. Wagner, K. Wien, B. Curdes and E.R. Hilf, Nucl. Instr. and Meth. B 82 (1993) 362. A. Delcorte and P Bertrand, poster contribution to the 11 th Int. workshop on Inelastic Ion Surface Collisions (IISC- I 1). Sept. 1996 Wangerooge/FRG; Nucl. Instr. and Meth. B 115 (1996) 246; B 117 (1996) 235.

[55] Y. Yamamura, Nucl. Instr. and Meth. 194 (1982) 515. [56] I.S. Bitenski and ES. Parilis, Nucl. Instr. and Meth. B 21 (1987) 26. [ 571 W. Ens, in: Fundamental Processes in Sputtering of Atoms and Molecules (SPUT92), ed. P. Sigmund, Mat.-Fys. Medd. 43 (1993) 155. [58] R.E. Johnson and J. Schou, in: Fundamental Processes in Sputtering of Atoms and Molecules (SPUT92). ed. I? Sigmund, Mat.-Fys. Medd. 43 ( 1993) 403. [59] A. Dunlop and D. Lesueur, Material Science Forum 97-99 ( 1992) 553. 1601 A. Hedin, P Hrlkansson, B.U.R. Sundqvist and R.E. Johnson, Phys. Rev. B 31 (1985) 1780. [ 61 I C.T. Reimann, in: Fundamental Processes in Sputtering of Atoms and Molecules (SPU’l92), ed. P Sigmund, Mat.-Fys. Medd. 43 (1993) 351. [62] B. Sundqvist and R.D. Macfarlane, Mass Spectrom. Rev. 4 (1985) 421. [ 631 R.E. Johnson, B.U.R. Sundqvist, A. Hedin and D. Fenyb, Phys. Rev. B 40 (1989) 48. [64] G. Betz and K. Wien, Int. J. Mass Spectrom. Ion Proc. 140 (1994) 1. 1651 R.M. Papaleo, G. Brinkmalm, D. Fenyo, J. Eriksson, H.-F. Kammer, P Demirev, P H&ansson and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 91 ( 1994) 667. [66] R.M. Papaleo, P. Demirev, J. Eriksson, P H&kansson and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 107 (1995) 308.

54

K. Wien/Nucl.

Instr. and Meth. in Phys. Res. B 131 (1997) 38-54

1671 J. Eriksson, J. Kopniczky, G. Brinkmalm, R.M. Papaleo, J? Demirev, C.T. Reimann, P H&ansson and B.U.R. Sundqvist, Nucl. Instr. and Meth., B 101 (1995) 142. [68] K. Wien and V.V. Obnorskii, in: Secondary Jon Mass Spectrometry, SIMS X, Mtinster 1996, eds. A. Benninghoven, C.A. Evans, K.D. Keegan, H. Storm and H.W. Wegner (Wiley, New York, 1996) p. 199. 1691 I. Bitensky, P Demirev and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 82 (1993) 356. 170) 1. Bitensky, G. Brinkmalm, P Demirev, J. Eriksson, P Hlkansson. R. Papaleo. B.U.R. Sundqvist and R. Zubarev. Int. J. Mass Spectrom. Ion Proc. 138 (1994) 159.

( 71 I G. Brinkmaim. P Demirev, D. Fenyo, P. HSkansson, J. Kopinczky and B.U.R. Sundqvist, Phys. Rev. B 47 ( 1993) 7560. 1721 H. Feld, A. Leute, R. Zurmtihlen and A. Benninghoven, Anal. Chem. 63 ( 1991) 903. 1731 A. Delcorte, L.T. Weng and P. Bertrand, Nucl. Instr. and Meth. B 100 (1995) 213. 1741 M. Salehpour, P H&ansson and B.U.R. Sundqvist. Nucl. Instr. and Meth. B 2 (1984) 752.