Mechanisms for the desorption of large organic molecules. Part 2

17

Internarional Journal of Mass Spectrometry and Ion Processes 126 ( 1993) 1l-24 0168-l 176/93/$06.00 0 1993 - Elsevier Science Publishers B.V., Amsterdam

Mechanisms for the desorption of large organic molecules. Part 2* R.E. Johnson Engineering

Physics,

Thornton

(Received 10 February,

Hall, University

of Virginia, Charlottesville,

VA 22903, USA

1992; accepted 15 November 1992)

Abstract Models are reviewed for ejection of intact biomolecules and biomolecular ions in response to the electronic energy deposited by a fast heavy ion. These models are compared with laboratory data and molecular dynamics calculations. Over a limited range of electronic energy deposited per unit path length, the scaling of the total yield with energy deposition is understood, and it has been shown that, except for Langmuir-Blodgett films of fatty acids, the large intact ions are ejected primarily from the surface or adsorbed sites. The principal outstanding problems are a detailed description of the electronic energy conversion into molecular motion and the ion formation-neutralization process. An understanding of these is needed in order to improve sensitivity at the higher masses in PDMS. Key wordx Plasma

desorption

mass spectrometry;

Langmuir-Blodgett

Introduction In Part 1 [l] the data and models for ejection of intact biomolecules from surfaces due to the energy deposited by an impacting fast heavy ion were reviewed. At that time the models available appeared to be inappropriate for describing the new data from the Uppsala group on ejection of neutral molecules from a sample of leucine [2] and the data obtained by a number of groups [3-51 for ion ejection from Langmuir-Blodgett (LB) films of fatty acids. Both types of data indicated that a volume of material was ejected, whereas most models at that time considered, in one way or another, the sublimation of surface layers [l]. Although this in itself was not necessarily contra’ Paper presented at the 6th Texas Spectrometry, Gasp&, Que., Canada,

Symposium IS-19 June,

on Mass 1992.

films; Biomolecules.

dictory, the small amount of data on the scaling of the yields with energy deposition also suggested the models were not correct. Subsequent to publication of Part 1 [l], W. Ens et al. [6], working with the Uppsala group, showed that the angular distribution of ejected intact large molecular ions exhibited a unique signature. This was a major breakthrough because it indicated the ejection process was impulsive, and this result has been used to discriminate between models for intact ion ejection. Later it was also shown that the angular distribution of the intact ions ejected from LB films [7] differed from that for films of other biomolecules, probably because of the elongated shape of the molecules and their order in LB films. The difference in the ejection angle distribution at first appeared to be consistent with the difference observed earlier between LB films and leucine samples [2-31: a difference in the scaling

18

of the apparent volumes ejected with electronic energy deposited. However, recent data by the Erlangen group [5] for the LB films suggests that the differences in scaling of the yields with the electronic stopping power, (dE/dx), were due in part to the fact that the ions were not incident normal to the surface in the experiments on LB films. Whereas non-normal incidence does not drastically change the scaling of the yield with energy deposition in PDMS for multi-layers of most molecular species [8,9] the elongated shape of the ordered fatty acid films aparently affects the material response at angular incidence. There now appears to be some agreement that the total volume of material ejected scales as (dE/dx)E with n x 3 over a limited range of (dE/dx),. This result should still be accepted with caution because the leucine data is limited and for LB films only ion ejection has been measured. However, it is presumed by most writers that the ejected intact ions from LB films of fatty acids roughly represent the total ejecta from an LB film of these peculiar molecules. A damaged volume due to fast heavy ion impact has also been seen experimentally for the track forming material [lo] mica using an atomic force microscope to observe the “crater” (hollow) formed. In additon, since Part 1 [l] a number of molecular dynamics (MD) simulations have been performed [7,1 l-l 51 to describe at the molecular level the process of ejection of material caused by exciting a cylindrical track of molecules. These were initiated by Hilf and Kammer at Oldenburg [14] and Fenyii et al. [7,1 l] at Uppsala; subsequently, such calculations were carried out by Urbassek et al. [15] and our group at Virginia [ 12,131. These calculations confirmed that the ejection of a volume of material occurs and that the scaling of the volume ejected with (dE/dx), is very similar to that found experimentally. Bitensky and Parilis [16] first suggested that both volume ejection and the ionization process could be understood in terms of the formation of a shock wave by the energy deposited by an incident ion and the intersection of this shock wave with a

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J. Mass Spectrom. Ion Processes 126 (1993) 17-24

surface. Whereas their analytic model gave reasonable agreement with the biomolecule ion yields, it did not describe the total yield. Johnson et al [17] modified their model, extending earlier ideas about the ejection of molecules from low temperature condensed gas solids. The modified model was referred to as the “pressure pulse” model because the net impulse produced by the transiently pressurized cylindrical track was used to calculate the volume ejection. This simple concept led to an approximate analytic model which has subsequently been tested against experiment and, in much greater detail, against the MD results [18]. Recently its relationship to the theory of weak shocks [ 191was discussed. Subsequently, Bitensky et al. [20] modified their model, also using momentum transport to inititate the ejection of neutrals. In this paper the diverse and as yet sparse set of experimental data and the more extensive set of MD calculations are briefly summarized and compared with the analytic expressions for the yield. Following Part 1 [l], two principal points of success and failure are pointed out. The latter primarily relate to the very important unsolved problems: the details of the conversion of electronic energy into energy of motion, which should not depend strongly on material properties, and the nature of the ionization process, which is very material-dependent. Readers are also referred to a number of excellent reviews [8,9,21,22] for many of the details. MD calculation

Molecular dynamics calculations of the evolution of a solid (t < lo-” s) in which a cylindrical track of material is “excited” (energized in some manner) have been performed on the following samples: atoms (40~) interacting by van der Waals pair potentials and excited by giving each atom in the track a kinetic energy in a random direction [15]; massive (M lOOO- 10 000 u) particles with hard cores bound together in the solid by van der Waals pair potentials and excited by expansion of the size of the core [7,11,18];

R.E. JohnsonlInt.

J. Mass Spectrom.

Ion Processes

126 (1993)

19

17-24

Fig. 1 a steep dependence is seen on (dE/dx),n. For a material with cohesive (sublimation) energy U

1.13 4, 0.56 /

4 2.82

1

I 10

100

1

1000

( dE/dx Jeff ( a/U ) l

Fig. 1. Yield in atoms (molecules) removed per “ion” incident plotted vs. (dE/dx),n scaled to the average atomic (molecular) size I and the cohesive energy per molecule U. The effect of the incident ion in each case is simulated by exciting a cylindrical region uniformly. In Fenyii et al. (7,l l] large “molecules” are expanded; in Banerjee et al. [12] diatomic molecules are vibrationally excited; in Urbassek et. al. 1151 (all other calculations) Ar atoms are given random kinetic energy; these curves are labeled by the radius of the cylindrical region Ro, scaled by the average lattice spacing I = n,-I” Note that in the latter results the excitations are different from those in Fenyii et al. and Banerjee et al., in which the molecules have physical size giving a less compressible material at high pressures. At low (dE/dx),n a cubic dependence is seen and for R,,/I = 2.82 or greater in Urbassek et al. [15], for higher (dE/dx),s and/or small Roll they find a linear dependence on (d.E/dx),s. (Taken from ref. 15.)

diatomic molecules [32 u] excited vibrationally having internal Morse potentials and with van der Waals pair potentials acting between atoms on neighboring molecules [ 12,131; linear molecules in two dimensions excited by expansion and intended to represent an excited LB film [14]; and a one dimensional chain of molecules [23]. The three dimensional calculations above all lead to “crater formation”. A summary of the yields for these models vs. (dE/dx),rf are given in Fig. 1. By (dE/d x )eff is meant the effective expansion energy per unit depth in the excited cylindrical region. Presumably this scales with (dE/dx),: where f is the fraction (dE/dx),n =f(dE/dx), of the initial electronic energy deposited that produces expansion. For molecular materials in

Where p and z are the average radial extent and depth of the volume ejected, n, is the molecular number density, and 1 = n,113 is the average molecular size. The dependence on (dE/dx),n to the third power can be roughly understood if each dimension of the ejected volume scales as (dE/dx),n. In the calculation for an atomic solid, it is seen that a steep onset occurs dependence at low (dE/dx),fr for each cylindrical radius assumed, which gives way to a much slower increase with (dE/dx),n at higher excitation densities (i.e. Y 0: (dE/dx),n). Using the first equation in Eq. (I), the quantity 2 appeared to change slowly at high (dE/dx),n, but the quantity G was still found to be approximately proportional to (dE/dx),. Those calculations were terminated, necessarily, at tx lo-” s, a time at which the crater had not relaxed and the walls of the craters were still highly compressed. In fact, the result at high (dE/dx),n may be consistent with standard “shock” models for ejecta production and crater formation in other impact phenomena. It differs from the other calculations in that the small atomic species can be compressed much further than the molecular species which have significant physical size [ 12,13,15]. Experimental

data

The data on total yields is extremely limited. The only direct measure of the total yields for biomolecules is that of the Uppsala group [2]. In that experiment the ejected intact leucine molecules were collected and counted by amino acid analysis and assumptions were made about the ejecta angular distribution. The total intact yield varies roughly as (dE/dx):, although this dependence must be accepted with caution because only four data points were obtained [8]. The measured total yields are compared to the ion ((M + H)+ and (M - H)-) yields in Fig. 2, clearly showing that

20

R.E. Jbhnsonjlnt. J. Mass Spectrom. Ion Processes 126 (1993) 17-24

l__A_.--

--

IO' ( dE/dx )e

( MeVimgicm*

102 )

I 0

Fig. 2. The yields vs. (dE/dx), for intact leucine molecules molecular ions. (Taken from ref. 2).

and

ion and total intact ejecta yields can scale differently with (dE/dx),. This point is often made but equally often ignored in developing models. The slower dependence of the ion yields for such samples is due, in part, to ion ejection being predominantly a surface process, as discussed below. In contrast to this, a remarkable result is that bombardment of a LB film of fatty acids results in ions being ejected from depth, as determined by marker layers [3-S]. The Erlangen group [5] recently showed that for fast heavy ions at normal incidence the average depth of origin x is roughly proportional to (dE/dx), consistent with the above discussion of the scaling of the total yields. The radial scaling is less certain, although conical shapes have been proposed [3,5,21]. The nature of the depth scaling was obscured earlier because nonnormal incidence was used in most LB experiments, with z varying as a low power of (dE/dx), (n < l/2 at 45°C) [3]. The “crater” (hollow) produced in a mica film by fast heavy ion bombardment has also been measured and is of interest, although mica is a rather different material [lo], and the crater defines a softened region. The actual measured dimensions were found to vary with tip force, but the dimensions of the craters observed scaled (roughly) with (dE/dx), at the higher energy depositions studied. This is shown in Fig. 3 for the crater diameter measured with a given force on the tip. What is seen is a region at the highest values of (dE/dx), in which the crater diameter is nearly linear in

10

20

(dE/dx

)e

a

/

L I

(

;

30

40

50

60

[ MeV

.mg-’ .cm”]

Fig. 3. Diameters of hollows (craters) produced in mica by heavy fast ions vs. (dE/dx),. The size is affectd by the force on the tip of the atomic force microscopic. At largest (dE/dx), the diameter varies roughly linearly with (dE/dx),. A threshold is seen at low (dE/dx),.

(dE/dx), (the scaling observed for the biomolecule yields). However, at low (dE/dx), there is a steeper “threshold” region for this refractory solid. A steep “threshold” behavior is also seen for the total ejection yield from low temperature solid Ss [24] with light fast ions. The existence of a “threshold” region for intact molecular ejection was clearly established by Hakannson et al. [25] for ion ejection. The Erlangen group [26] has found good fits to measured ion yields using an excitation density “threshold” (dE/dx)thres. That is, they write Y c( [(dE/dx),(dE/dx),hr,,]“. It is important to remember that both the ejection process for large intact molecules and the energy deposition process are necessarily statistical, as discussed in Hedin et al. [27] and in Part 1 [l], so there is no cut-off in (dE/dx), for the total yield. Further, the yields in Ref. 26 are ion yields and not total yields. Analytic models

For the cylindrical excitation geometry produced by a fast heavy ion, the various models can be grouped into three classes of dependencies on (dE/dx),n where (dE/dx),* is some fraction of (dE/dx), acting to cause expansion, as discussed above. First, “thermal spike” models, for which (dE/dx),r is converted to a “temperature”, give

R.E. JohnsonlInt. J. Mass Spectrom. Ion Processes 126 (1993)

21

17-24

the following scaling in Eq. (1): (~2) cx [(dE / dx)/n,U], and z 0: ,[l(dE/dx),&], except near where the yield decreases more “threshold”, rapidly with (dE/dx),n [l]. Allowing an evolving surface (i.e. crater formation) may alter this dependence. Such models were discussed extensively in Part 1 [l] and all had the property that the “depth” increased faster than the radial size of the ejecta region. The second type of dependence is given by the “shock wave” models of Yamamura [28], Carter [29], and Bitensky and Parilis [16], in which (dE/dx),M represents the kinetic energy left behind by the passing shock. In such models the radial scale of the ejected volume is found by assuming that this energy can be spread over an area determined by the sublimation energy U i.e. wp2 K [(dE/dx),R/n,U], identical to the spike model above. In addition, it is assumed in these models that z also scales as the radial extent of the region [(dE/dx),n/n, U]‘j2, giving a hemispherical crater. Finally, the “pressure pulse” model [17,30,31], in which (dE/dx),ff produces a radial and out-of-the surface expansion, and the revised “shock” model of Bitensky et al. [20] both consider the transport of momentum to the ejected volume. In these models each dimension of the ejected volume roughly scales as I[Z(dE/dx),n/U]. The “pressure pulse” model has been shown to describe extremely well the results of the MD simulations [18]. For a van der Waals solid of large structureless particles, the yield is written

(2) where 1= n-,‘13 (the molecular size) and c is a model-dependent constant. In Johnson et al. [17] c x 4 x 10m5 for structureless point particles, which is roughly consistent with the MD results [11,18]. (Note that it was stated in Ref. 11 that the agreement in the size of the yield between the analytic and MD calculations was quite close when the constant p in the analytic model (related to specific heat) was that for a gas of structureless particles; in fact a value of about twice that is needed.)

Based on the MD calculations, the lattice expansion energy required to produce the leucine yields (dE/dx),R =f(dE/dx), gives f= 0.01 [ll]. (Note that the MD results were for more massive particles, but the yields appeared to be independent of mass over the limited range tested and so were applied to leucine.) In Part 1 [l] it was pointed out that in such materials excited by fast heavy ions most of the electronic energy is indeed available to be converted into expansion, either by repulsive relaxation [32] or by internal excitations [33]. Therefore, because (dE/dx),n is a lattice energy the increase off divided by the lattice specific heat gives an estimate of the effective internal “heat capactiy” [22] at the time of ejection, including electronic and vibrational modes. That this is much larger than the specific heat of leucine means that a large fraction of the energy is still in electronic excitation at the time of ejection. Before proceeding it is important to note that the models above all exhibit “threshold” regions. The “threshold” dependence for “thermal spike” models is described in Part 1 [l]. For the “pressure pulse” models an aspect of this region has been discussed recently [34]. It is clear that the radial size of the ejected volume (the volume receiving the momentum) must exceed the larger of the molecular size 1 or the radial size of the highly fragmented core [27]. Applying this, the radial size of the ejected volume now scales as ,{[/(dE/dx)& U] - l}, because at low (dE/dx),R the molecular size is the important quantity [25,27]. This gives an effective “threshold” energy density determined by the cohesive energy U as (dE/dx),h,,

a (u/l)f

(3)

a form used earlier [l]. Although the threshold values of Brand1 et al. [26] only apply to ion ejection, the values for valine are equivalent to f= 0.03, roughly consistent with the value above derived by Fenyii et al [l 11. General consideration: ion formation

A point of disagreement

between the analytic

22

model [17], in which the molecules do not have size, and the MD simulations of FenyG et al. [ 11,181is in the angle of ejection of intact ions [32]. The MD results first gave the correct sense of the angular distribution [7,11] but not the correct average angle. Quite remarkably, for intact large biomolecules Johnson et al. [17] showed that the analytic model predicted quite accurately the average angle of ejection for intact ions measured by Ens et al. [6]. This was only the case if the biomolecule was an absorbed species [35]. Bitensky et al. [20] subsequently showed the full angular distribution compared well with the full angular distribution measured by Wein and co-workers [36] for dimers of valine. Again, this agreement is only obtained if these ejected ions are assumed to be primarily formed from adsorbed or surface molecules, a point not made clear in their paper. Based on this it is clear why the MD calculations of the “ion” yield do not describe the angular signature of the intact ions very accurately: the intact ions ejected are rare events and the MD calculations describe the dominant ejecta. In the analytic model only the ejection angles of the surface (absorbed) molecules can be predicted because these species interact weakly with neighbors, whereas even species leaving from the first layer are deflected from the direction given by the initial impulse [7] because of the physical volume they occupy in the surface layer. That a significant fraction of the intact ions of large biomolecules are derived from adsorbed sites was suggested experimentally by the fact that adsorption of biomolecules on nitrocellulose gave ejection angle dependences nearly identical to those for ejection from multilayers for large biomolecules [7,8]. Conversely, small biomolecular ions (monomers of valine and fragments) do not exhibit the same ejection angle signature [31,35,36] and, therefore, in addition to absorbed sites, these ions may be thermally ejected or be derived from molecules within the surface layer and, possibly, sublayers. As pointed out in Part 1 [l], the total yield for a given distribution of energy deposition can be

R.E. JohnsonlInt. J. Mass Spectrom. Ion Processes 126 (1993) 17-24

written as YZZ

ss

@,(p’,4 d2pdt

(4)

where (p, is the flux of sputtered material as a function of time t and of the radial distance p from the incident ion track. In order to understand the scaling with (dE/dx), this expression was approximated [l] above the “threshold” region as in Eq. (1). It is now tempting to write the ion yield as Yi M Pi Y, as is often done. But it is seen in Fig. 2 that this is not necessarily correct. Therefore, it was suggested by Hedin et al. [27] and in Part 1 [l] that Yi M Pi(.rrp2)iaZi

(5)

where Pi is the probability of formation of the ion and its survival to detection, and the quantities Zzi and 7rz for the ion yield can differ from the quantities in Eq. (1) for the total yield. Although Bitensky et al. [16,20] claim to use an expression like Yi z Pi Y to match both the scaling of the ion and neutral yields with (dE/dx),ff they in fact use different “shock” criteria [20], (as discussed above) for ion and neutral ejection. This essentially is equivalent to saying that (~~)i and (Az)i in Eq. 5 differ from ~7 and % in Eq. (1). For LB films of fatty acids ionic species clearly come from depth and, therefore, it may be acceptable to use Yi M pi Y. There is however, no direct experimental confirmation of this assumption. Hedin et al. [27] use an expression like that in Eq. (5) and calculate Pi vs. (dE/dx),. They show that the molecular size determines the “threshold” behavior. Although their statements on the ejection mechanisms have not been confirmed, it is clear from the data in Fig. 2 that a large volume is ejected and the ions come from rapidly ejected surface species within the ionization region calculated in that paper. Therefore, the description of pi in Hedin et al. [27] is still valid. Other descriptions of Pi [14,20] also need further consideration. Neutralization by electron capture from the surface is an efficient process for quenching ions, so it is reasonable to assume that the intact biomolecular ions come predominantly from the sur-

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J. Mass Spectrom. Ion Processes 126 (1993) 17-24

face. When this is the case, it is appropriate to use Azi M I ~ nm113[1,17,27], the size of an average monolayer in Eq. (5). Therefore, any dependence of Yi on (dE/dx)e comes about via Pi or (rF)i. Hedin et al. [27] have shown that, above the intact ion “threshold” region, the area from which intact positive ions derive is proportional to (dE/dx),. The dependence they calculate is due to the initial distribution of ionization produced by the incident ion, but they point out that the thermal spike scaling behaves similarly as discussed above and as also noted by Luchese [23]. In contrast, the dependence seen in Fig. 2 for negative ions is not yet explained. Negative ions involve attachment [8,22,37] (electron or H-) so these ions may also predominantly come from the surface region for most organic films. If this is the case, then the quadratic dependence in Fig. 2 would imply (nT)i 0: (dE/dx)z assuming that 4 is nearly independent of (dE/dx)e well above threshold, i.e. Pi constant. Such a dependence would suggest that these ions can form and be ejected uniformly from the surface area associated with the total ejecta. Experimental data is needed to test such a conclusion. Conclusions

Although data useful for determining ejection mechanisms is surprisingly sparse (considering that Mcfarlane and Torgerson [38] discovered PDMS over 15 years ago) the scaling of the total yields with (dE/dx), appears to be roughly cubic over a limited range of (dE/dx), due to an impulsive ejection process. The lack of an appropriate angular signature for ionic monomers of relatively small molecules (e.g. valine) probably implies that “thermal” ejection [22] occurs for such species. However, with the exception of LB films of fatty acids, the ion yields scale very differently with (dE/dx),. This is apparently the case as the data of Ens et al [6]. and Wein and co-workers [36] clearly show that the large intact ions are created primarily from adsorbed or surface species. It has also been shown that the calculation of the probability of positive ion formation and survival to

detection is determined, not surprisingly, by the initial ionization density produced by the incident ion. Although this gives the correct dependence on and molecular size, a quantitative (dE/dx), description of the size of the positive ion yield is lacking and the negative ion yield is not yet understood. The magnitude of the impulsive energy needed for ejection, as determined by the cohesive (sublimation) energy is now roughly known, as described above. Although, it was pointed out in Part 1 [l] that most of the electronic energy deposited by a fast heavy ion is likely to be converted into energy which leads to expansion, the details and, in particular, the time scale of the conversion of the initially deposited energy into expansion energy, are not understood. Improvement in our understanding of this conversion and an understanding of the ionization process is needed to suggest new substrates and/or sample preparation techniques in order to further improve PDMS.

Acknowledgments

The authors acknowledge the support of the NSF via grant AST-91-20078, an NSF travel grant, and travel funds from the Swedish National Science Foundation.

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15 16 17 18 19 20

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B.U.R. Sundqvist, in R. Behrischt and K. Wittmack (Eds.), Sputtering by Particle Bombardment, Vol. III, SpringerVerlag, Berlin, 1991 p. 257. B.U.R. Sundqvist and R.D. Macfarlane, Mass Spectrom. Rev., 4 (1985) 421. Thibandau, Phys. Rev. Lett. 67 (1991) 1582. D. Fenyo, B.U.R. Sundqvist, B.R. Karlsson and R.E. Johnson, J. Phys. (Paris) C, 2, 50 (1989) 33; Phys. Rev. B, 42 (1991) 1895. S. Banerjee, R.E. Johnson and S.T. Cui, Phys. Rev. B, 43 (1991) 12707. S.T. Cui and R.E. Johnson, Int. J. Quantum Chem., 23 (1989) 575; (errata) 41 (1992) 383. E.R. Hilf and H.F. Kammer J. Phys. (Paris) C, 2,50 (1989) 245. H.F. Kammer, PhD. Thesis, University of Oldenburg (1991). H. Urbassek, H. Kafemann and R.E. Johnson, Phys. Rev. B, (1992) in press. I. Bitensky and E. Parilis, Nucl. Instrum. Methods B, 21 (1987) 26. R.E. Johnson, B.U.R. Sundqvist, A. Hedin and D. FenyB, Phys. Rev. B, 40 (1989) 49. D. Fenyii and R.E. Johnson, Phys. Rev. B, 46 (1992) 5090. R.E. Johnson, Phys. Rev. B. submitted. I. Bitensky, A.H. Goldenberg and E. Parilis, in A. Hedin, B.U.R. Sundqvist and A. Benninghoven. (Eds.), Ion Formation from Organic Solids, Vol. V, Wiley, New York, 1990 p. 205; J. Phys. (Paris) C, 2, 50 (1989) 213. K. Wien, Radiat. Eff. Def. Solids, 109 (1989) 137; Nucl. Instrum. Methods B, 65 (1992) 149.

22 R.D. Macfarlane, Nucl. Instrum. Methods, 198 (1982) 1. 23 R. Luchesse, J. Chem. Phys., 85 (1987) 443. 24 L. Torrisi, S. Cotta, G. Foti, R.E. Johnson, D.B. Chrisey and J.W. Boring, Phys. Rev. B, 38 (1988) 1516. 25 P. Hakannson, I. Kamensky, M. Salehpour, B.U.R. Sundqvist, and S. Widdiyasekera, Radiat. Eff., 80 (1984) 141. 26 D. Brandl, R. Schmidt, Ch. Schoppmann, A. Ostrowski and H. Voit, Phys. Rev. B, 43 (1991) 5253. 27 A. Hedin, P. Hakansson, B.U.R. Sundqvist and R.E. Johnson, Phys. Rev. B, 31 (1985) 1780. 28 Y. Yamamura, Nucl. Instrum. Methods, 194 (1982) 515. 29 G. Carter, Nucl. Instrum. Methods, 209/210 (1983) 1. 30 R.E. Johnson and B.U.R. Sundqvist, Rapid. Commun. Mass Spectrom., 8 (1991) 574. 31 R.E. Johnson and B.U.R. Sundqvist, Physics Today, March (1992) 28. 32 R.E. Johnson and B.U.R. Sundqvist, Int. J. Mass. Spectrom. Ion Phys., 53 (1983) 337. 33 P. Williams and B.U.R. Sundqvist, Phys. Rev. Lett., 58 (1987) 1031. 34 D. Fenyo, Phys. Rev. B. (1993) in press. 35 B.U.R. Sundqvist, Int. J. Mass Spectrom. Ion Processes, 126 (1993) 1. 36 A. Moshammer, A. Matthaus, K. Wien and G. Bobach, in A. Hedin et al. (Eds.), Ion formation from Organic Solids, Vol. V, Wiley, New York, 1990 p. 17. 37 H.F. Kammer, in E.R. Hilf and W. Tusinsky (Eds.), Mass Spectrometry of Large Non-Volatile Molecules for Marine Organic Chemistry, 1991, pp. 61-88. 38 R.D. Macfarlane and D.F. Torgerson, Int. J. Mass Spectrom. Ion Phys. 21 (1976) 81.