Mechanisms of ion-induced desorption of large molecules and clusters

International Journal of Mass Spectrometry and Zon Processes, 78 (1987) 341-356 Elsevier Science Publishers B.V., Amsterdam - Rrinted in The Netherlands

MECHANISMS OF ION-INDUCED MOLECULES AND CLUSTERS

DESORPTION

341

OF LARGE

B.V. KING Department of Physics; University of Newcastle, N.S. W. 2308 (Australia) I.S.T. TSONG Department of Physics, Arizona State Universi@, Tempe, AZ 85287 (U.S.A.) S.H. LIN Department of Chemistry, Arizona State University, Tempe, AZ 85287 (U.S.A.) (Received 25 November 1986)

ABSTRACT Our model, which describes the final states of molecules and clusters desorbed from ion beam-irradiated surfaces of molecular solids and dielectrics, is reviewed. The desorption is considered to be induced by a relaxation process in which the deposited energy is transferred to the internal vibration modes of the perturbed region. The model is used to describe the mass distribution of the insulin molecular ions desorbed by MeV primary ions, the mass distribution of various alkali halide cluster ions desorbed by keV primary ions, and the energy distribution of desorbed neutrals from frozen SF,. The agreement with theory for experimental mass and energy distributions of desorbed particles is reasonable. The energy deposited per normal mode calculated by our model suggests that the MeV primary ions are more efficient in promoting desorption than keV primary ions, in agreement with experimental observations.

INTRODUCTION

Since the early 197Os, particular interest has been paid to the interaction of energetic beams with insulators like frozen gases and organic solids. The reasons were twofold, to understand the beam-solid interaction and to analyse the irradiated surfaces. The interaction of energetic ions with conducting or semiconducting solids is described in terms of elastic ion-atom and atom-atom collisions, i.e. collision cascade theory [l]. The yields and energy distributions of particles desorbed from insulators could not, however, always be described by the collision cascade theory. An alternate 0168-1176/87/$03.50

0 1987 Elsevier Science Publishers B.V.

342

approach was to consider the region around the ion trajectory in the solid to be in local thermal equilibrium. These LTE sputtering models [2,3] are disputable because a macroscopic temperature cannot be properly assigned to such a microscopic system. The impetus for the analysis of insulating solids came with the realisation that energetic beams could be used to desorb large and thermally labile molecules and clusters. There are three main variants of the panicle-induced techniques for the production of macromolecules and giant clusters. The first technique is the californium-252 plasma desorption mass spectrometry ( 252Cf PDMS) introduced by Macfarlane and co-workers [4,5]. Heavy ions, with 100 MeV energy from the spontaneous fission of *“Cf , bombard samples of biomolecules to induce io~~tion and desorption of molecular ions which are then analyzed via a time-of-flight (TOF) mass spectrometer, A variation of the high-energy PDMS technique is the use of beams of MeV ions from accelerators introduced by workers from the University of Uppsala [6]. The second technique is the use of primary ions in the keV energy range. It was shown by Be~n~oven et al. [7] that molecular ions can be desorbed from amino acids by keV incident ions. The spectra were found to cover a very similar mass range to those obtained by PDMS. Benninghoven’s technique, of course, fits in with the generic description of secondary ion mass spectrometry (SIMS) except that it is applied to organic solids. A derivation of the SIMS technique is the use of neutral primaries by Barber et “fast atom bomb~~ent” or FAB. The al. [8] who call their m~fication most significant difference about the FAB technique is the liquid solution matrix, usually glycerol, from which desorption-ionization takes place. The third technique employs a pulsed laser as an excitation source. Posthumus et al. [9] demonstrated that desorption and ionization of large non-volat~e biomolec~es could be carried out with irradiation by short-duration ( < 10 ns) laser pulses. The most interesting aspect of these three techniques is that, while the mechanism of energy deposition and the ion yields in each of the three techniques is quite different, the mass spectrum of desorbed molecular and cluster ion species is essentially the same. This has led Macfarlane IlO] to suggest that, at some stage during the transfer of energy to the surface molecules leading to ejection, there is a sequence of events or processes common to all three techuiques. We have developed a theory which is based on the production of a vibrationally excited region in the irradiated surface. This is the process common to all techniques. The isolated region can then be treated as a ~crocano~c~ system for which exact mathematics formalism exists without the necessity to resort to thermal equilibrium arguments. In this report, we have selected experimental desorption data obtained by MeV or keV ion bombardment to test our theory.

344

We assume that particle ejection during the desorption process is similar to this chemical process. In sputtering of dielectrics or molecular solids, M* represents the energized molecule caused by energy deposition of the incident particle. The deposited energy is absorbed in the form of vibrational motion of the molecules. If a molecule is sufficiently energized to reach an excited vibrational level of the transition state M #, bonds can be broken and the molecule can be desorbed. In the insulin case, for example, the desorption of the molecular ion is characterized by the probability of localizing the available energy into breaking a number of intermolecular hydrogen bonds (0.1-0.3 eV/bond) while the two fragment ions, a-chain and P-chain, are desorbed by breaking the two S-S intramolecular bonds ( - 2.4 eV/bond) in addition to the hydrogen bonds. In applying RRK theory to desorption, a molecule in the insulating target is treated as a system of loosely coupled oscillators. These oscillators are conveniently regarded as equivalent to the normal modes of vibration of the molecule. The rate constant, k, for the ejection process increases with the energy possessed by the molecule in its various degrees of freedom. The larger the energy possessed by the excited molecule, the greater the chance of its ejection. To determine the mass distribution of desorbed molecules and clusters, we need to calculate the particular rate constant, k, for each individual molecule or cluster. Since the statistical weight of a system containing j quanta of vibrational energy distributed in n degrees of freedom is equal to the number of ways in which j objects can be divided among n boxes, calculation of the rate constant k is simply an exercise in statistical mechanics. The various observed desorbed molecules and clusters can be considered to result from a set of competing ejection channels

where M is the parent molecule, Pi the product molecular or cluster ion, and kj the rate constant for the jth channel. According to RRK theory, a molecule can be treated as a collection of N weakly coupled classical harmonic oscillators where N is determined by the number of normal

345

models of vibration of the molecule. The rate constant is then given by kj(Er)

(E*-E,#)

a 'j

E

[

r

1 iv

(1)

where E, is the energy deposited by the incident particle to energize the molecule, Ej# is the activation energy for the jth channel of desorption and Sj is the symmetry factor for the jth channel, i.e. the number of ways that the jth channel can occur. The fractional abundance of the desorbed jth molecule/cluster is simply given by Aj=&

(2)

Equation (2) forms the basis of our calculations of mass spectra of desorbed molecules and clusters. The kinetic energy distribution of desorbed ions can also be predicted from our theoretical treatment. If the process occurs without charge transfer, the kinetic energy distribution of the desorbed particles arises in RRK theory from the division of the excess energy E # = (E,. - Ejz) of the activated complex into vibrational energy and translational energy along the reaction coordinate. This translational energy of the activated complex along the reaction coordinate then becomes part of the kinetic energy of the dissociated product. We have shown [15] that the flux, I, of desorbed particles with kinetic energy E, is given by I( E,) = AE:/* exp( - PE,) (3) where fi is a saddle point value that depends on E, and Ej# . During irradiation of insulators, the sample surface may develop a potential of a few volts. This makes identification of the zero energy point in experimentally determined energy spectra difficult, although the FWHM of an energy spectrum is unaffected. For these non-charge transfer processes, the FWHM can be calculated from Eq. (3) to be 1.79//S. For a thermal distribution of the form E exp( -BE), the FWHM is 2.45/j?. For the same FWHM, the value of fl for a thermal distribution would then be 1.4 times that for a non-charge transfer process. An energy spectrum given by Eq. (3) would have a more gradual high-energy decay in comparison with a thermal distribution with the same FWHM. This is the behaviour which is seen in most energy spectra of particles desorbed from insulating surfaces. If the oscillators of the activated complex are harmonic, then the excess energy of the complex can be related to the energy of the individual oscillators, t?q , by Ef = zAw,/(exp pt2wi - 1) (4)

346

If the energy of all oscillators in the activated complex is assumed to be the same, as in the Einstein theory of specific heat, and if this energy is low compared with p-r, then Eq. (4) simplifies to

Thus, there is a qu~tative link in our theory between the energy and mass spectra of desorbed particles through their dependence on /3, E # , and N which are related by Eq. (5). Of course, a more exact treatment involving the use of the phonon density of states, would be required for quantitative comparison of energy and mass spectra. MASS DIST~BUTION

OF BOVINE INSULIN

IONS

We have tested the applicability of RRK theory to the desorption of large organic molecules and, in particular, bovine insulin [12]. Briefly, both positive and negative ion spectra [16] contain peaks corresponding to the mass of the intact molecular ion (I) and the masses of the a-chain and ,&chain. These two chains are two peptide strands joined by two disulfide links making up the insulin molecule. For the purpose of our calculations, we take the approximate molecular weights of both the positive and negative ions to be 5800 u for the whole molecule, 2400 u for the a-chain, and 3400 u for the p-chain. The relative abundances of these three ions calculated from the experimental mass spectra [16] are shown in Table 1. To compare these results with Eq. (2), Sj and Ej must be determined for each of the three ejection channels. Conveniently, Sj = 1 in Eq. (1) for insulin since there is only one way of breaking up the insulin molecule, i.e. into Q- and P-chains. We assume further that the activation energies, Ej, can be written as E,# = E; E,’ = 0.41E; + E,,+ (6) Ea’ = 0.59Ez + Eb+

TABLE 1 Relative abundances of the insulin molecular ion I ( - 5800 amu), the a-chain ( - 2400 amu), the p-chain ( - 3400 amu). Rel. abund~ce

Positive ions

Negative ions

AI A, A8

0.386 0.438 0.177

0.177 0.657 0.167

347

where Ej, is the “sublimation” or “lift-off” contribution, which we will assume to be proportional to the mass of the species leaving the surface and Eb is the energy required (4.78 eV molecule-‘) to break the two disulfide links to produce (r- and P-chains. For convenience, we have chosen a value of E, equal to the gas phase bond energy of two S-S bonds. However, this by no means implies that the (Y-and &chains are due to fragmentation of the insulin molecule after desorption from the surface. Equation (2) effectively consists of two independent equations in three unknowns, N, (Er/EIZ)and (EC/E,+).Only for one value of (Ec/Ef), 0.566, can the relative abundances A,, A,, A,, be matched to the experimental values for the positive ions in Table 1. We cannot find another value OfCZ = (Ez/Ef) which satisfies this condition. When, however, C: = 0.566, there is a set of values of N and E, which satisfy Eq. (2). As an example, Fig. 1 shows a plot of Aj as a function of cr for C: = 0.566 and N = 10, 50, and 200. We see in Fig. 1 that correct relative abundances of A,, A,, and A, are obtained when C, has values of 2.7, 9.8, and 36.4, for iV = 10, 50, and 200, respectively.

N:ZOO er= 36.4 0.4300.3860.177-

I I

Fig. 1. A plot of relative abundances of positive ions of insulin, Aj, where j = I, a, and /3, versus the dimensionless variable of the deposited energy E, for different choices of the number of modes N = 10, 50, and 200. A value of E,, = 0.566 is assumed for this plot. For each choice of N there is one value of E, which gives agreement between measured and theoretical values of the relative abundances.

348

0.3G/N _ 0.2-

Fig. 2. The c,/N values that provide agreement with experimental relative abundances are plotted as a function of N, the number of normal modes participating in the desorption process for both positive and negative insulin ions. 0, Positive ions; n, negative ions.

Since 6; = 0.566 and EC = 4.78 eV molecule-‘, the activation energies are EIZ = 8.435 eV molecule-i, E,f = 9.77 eV molecule-’ and EB# = 8.24 eV molecule- ‘. These values of activation energies are not unreasonable considering the number of bonds, mostly H-H (- 0.3 eV/bond), that have to be broken in order to lift one of these molecules off from the surface. Since our model is based upon energy transfer to the vibrational modes of the molecule, the energy deposited per normal mode, represented by the quantity cJN where er = E,/Epf , is of interest. In Fig. 2, we plot r,/N as a function of N. The plot indicates that the e/N value that fits the relative abundances given in Table 1 rapidly approaches an asymptotic value as N increases. This implies that, as the number of normal modes participating in the ejection process increases, the energy deposited into each of these modes decreases and finally reaches a constant value. From Fig. 2, e/N = 0.178 and remains constant when N is greater than about 100. In the experiment considered, the insulin was bombarded by 90 MeV 127120+ions, which deposit approximately 800 eV ion-’ into each molecule for a stopping power of 9 keV nm-’ [17]. Corresponding to this upper limit for E, is a value of 81 for cr and about 450 for N. This value of N is also reasonable considering that an insulin molecule has 3n - 6 normal modes where n is the total number of atoms in the molecules, i.e. 2358 normal modes. Obviously, not all the normal modes contribute to the ejection of the molecule. Our result implies that approximately 450 normal modes are necessary and sufficient for desorption to take place. The energy deposited per normal mode in each molecule is given by EJN, which is about 1.7 eV/mode for N = 450.

349

n Fig. 3. A fit of Eqs. (14a) and (14b) to the experimental data of n, Campana et al. [19] (4.7 keV Xe+ ion bombardment, sector field mass spectrometry) and 0, Ens et al. [24] (8.0 keV Cs’ ion bombardment, TOF mass spectrometry). The relative yields Y,/Y, of the (CsI),Cs+ cluster ions are normalized to the yield of the first cluster, n = 1. The theoretical fit () was obtained with x = 4.60 for 2-D cluster and x = 6.70 for the 3-D cluster. The two theoretical curves virtually coincide and are shown as one curve in the plot.

For the negative ions, a unique value of C: = 0.417 produces a value of E, for each choice of N such that the relative abundancies agree with experimental values shown in Table 1. For ~b+= 0.417 and Ez = 4.78 eV molecule-l, the activation energies for the molecular ion I, the a-chain and the P-chain are EI# = 11.46 eV molecule-‘, Ez = 11.54 eV molecule-‘, and Ep# = 9.48 eV molecule-‘. A plot of e,/N as a function of N is shown in Fig. 2. As in the case of positive ions, the C/N values for the negative ions decrease as N increases, finally reaching an equilibrium value of E/N = 0.134 when N is greater than about 100. As before, we can estimate the energy deposited per normal mode for the desorption of negative ions to be about 1.6 eV/mode. This is very similar to the estimate for the positive ion case. The fact that virtually the same number of normal modes participate in the desorption and the energies deposited in these modes are virtually identical for both positive and negative ions indicate that these ions are formed via the same mechanism. This can be considered as a justification of our assumption that the ions are preformed, e.g. through the attachment of H+ or H-, and no charge transfer processes have occurred. The implication of our result is that, if the mass distribution of the desorbed neutral species can be measured, then it should also fit the RRK theory. Time-of-flight (TOF) spectra of macromolecules always show a large continuum which must be subtracted to reveal the molecule and fragment

350

peaks. In addition, the peaks corresponding to the different species overlap in the TOF spectrum due to metastable fragmentation and subsequent loss of smaller neutral fragments [18]. This makes unambiguous determination of yields quite difficult but does not markedly affect the values of the energy/mode found from the non-cascade model. For example, the relative positive ion yields found from a more recent bovine insulin spectrum [18] are 0.778, 0.077, 0.051, 0.068, and 0.017 for the molecule (I), a-chain, P-chain, and the clusters (21) and (31), respectively. Our model gives relatively good agreement with these data since yields of 0.778, 0.079, 0.051, 0.068, and 0.006 are obtained for activation energies of the order of 6 eV and an energy per normal mode of 1.3 eV. MASS DISTRIBUTION

OF ALKALI HALIDE IONS

Large secondary (CsI),,Cs+ cluster ions, where 1 G n G 70, ejected from CsI surfaces by keV ion bombardment have been observed by the Naval Research Laboratory group using a sector-field mass spectrometer [19-231 and by the University of Manitoba group using a time-of-flight (TOF) mass spectrometer [24,25]. While, in both types of experiment, the secondary ion yield decreases rapidly with increasing n, the sector-field experimental data show distinctive features near n = 13,22, 37, and 62. In the TOF experimental data, however, such features were not observed and a smooth curve was obtained. Since an ion in a TOF spectrometer needs to survive only long enough to be fully accelerated in order to appear at its original mass number, the anomalous features in the sector-field data are due to metastable decays which were not observed in the TOF experiment due to the short acceleration times, - 0.17 ps, compared with the 750 p.s needed to traverse the sector-field instrument. In our treatment of the experimental data of the two groups, therefore, we will ignore the anomalous features and assume that the ion yield decreases smoothly with increasing n. As in our foregoing treatment of insulin ions, the desorption of (CsI),,Cs+ does not involve charge transfer and RRK theory can be invoked to describe the process. In the (CsI),Q+ experimental data, the plot of ion yield versus n is normalized at n = 1. Making use of Eqs. (1) and (2), the fractional abundance of the nth cluster can then be written as A, -=A,

k,

S,, E, - E,,+ N

k, = s, [ E,-

El+ I

(7)

Since the activation energy Ez can be considered as the energy required to lift the cluster ion off the surface, we make the assumption that En’ is proportional to the mass of the n th cluster, i.e.

E,Z a (mcs + nmcsi)

(8)

351

(9) where a = 0.338, b = 0.662, and a + b = 1. Next, we define a dimensionless energy parameter energy E, as cr =

for the deposited

4 El

(10)

7

Substituting Eqs. (9) and (10) into Eq. (7), we obtain lnWY,)

= m(WSJ

+

xb(l -

n)

where we have written the ratio of ion yields Y,/Y, abundance A,/& and where

(11) for the fractional

X’N

(12)

Er

Calculations of the term ln( SJS,) in Eq. (11) required counting the number of ways a connected structure of 2n + 1 sites, as in the case of (CsI),$s+, can be constructed on a regular lattice. This mathematical problem has been studied in detail for both two-dimensional and three-dimensional lattices [26-281. If we make the approximation that the (CsI),$s+ cluster ions are sputtered as two-dimensional connected square lattices, then the symmetry factor S,, is given by [28] S,, = 0.3(2n + l)-‘(4.06)2”+1

(13a)

If the (CsI),Cs+ ions are in the form of simple cubic lattices, then S,, is given by [28] S,, = O.l7(2n + l)-1.45(8.33)2”+1

(13b)

Substituting Eqs. (13a) and (13b) into Eq. (9), we obtain 2-D log g =&[xb(l-n)-ln(2n+1)+2.8n-1.71 0. 1

044

3-D log

[ xb(l

where b = 0.662.

- n) - 1.45 ln(2n + 1) + 4.24n - 2.651

(14b)

352

Using Eqs. (14a) and (14b), we plot log (Y/Y,) as a function of cluster size n for various values of x and compare with experimental results. Figure 3 shows the relative ion yields normalized to n = 1 from the experiments of Campana et al. [19] and Ens et al. [24]. The best theoretical fit is obtained with x = N/t:, = 4.60 for the 2-D clusters and x = 6.70 for the 3-D clusters. This value of x does not change signific~tly if other 3-D lattices, for example BCC, are chosen to calculate S,. The parameter x determines the amount of energy deposited in each normal mode of the (CsI). cluster in the localized region of the solid disturbed by the incident primary ion. Since, from Eq. (lo), E, = E,/EP ,we can estimate the energy deposited per mode, EJN, if we know E,, the activation energy of the first cluster ion (CsI)Cs+. Er” can be taken to be approximately equal to the sum of the sublimation energy of the CsI solid (2.0 eV) and the bond strength of the CsI molecule (3.4 ev), i.e. Ef = 5.4 eV. For the 2-D cluster case, we have r/N = l/x = 0.217, which gives E,/N = 5.4 X 0.217 = 1.17 eV/mode. For the 3-D cluster case l/x = 0.149, which gives EJN = 5.4X 0.149 = 0.81 eV/mode. To see if these values are realistic, we make the following estimation. The energy loss of the incident ion through nuclear collisions is given by the stopping power dE/dx, or the energy lost per unit path length. For 8 keV Cs+ bomb~dment, as in the U~versity of ~a~toba TOF experiment [24], Shueler et al. [29] estimated dE/dx = 600 eV run-r_ If the incoming Cs+ ion deposits its energy along its path length over a cross-sectional area of 4 nm diameter in the CsI solid, density 4.51 g cma3, then 600 eV is deposited into approximately 130 CsI molecules in the 12.6 nm3 volume near the surface. The number of normal modes is given by 3n - 6 where n in this case is the total number of atoms. Since n = 2 X 130 = 260, then 3n - 6 = 774. If all the energy goes into exciting all the normal modes, then we have E/N = 600/774= 0.78eV/mode. Obviously, in nuclear stopping, not all the energy lost by the incoming ion in traversing the solid is available for exciting the normal modes. For that matter, not all the normal modes necessarily partake in the desorption process. Nevertheless, the roughly estimated value of 0.78 eV/mode agrees reasonably well with the 0.81 eV/mode for the 3-D clusters and 1.17 eV/mode for the 2-D clusters obtained in our theoretical fit to the experimental data. The fits to the data presented in Fig. 3 are only used to illustrate that the general trends in the data can be explained by our theory. Closer agreement between theory and experiment can be achieved by making some more assumptions which, however, do not add a great deal to the overall discussion. For example, the decrease in log( Y/Y,) for large n does appear to be proportional to n as predicted by Eq. (14). The fit between data and theory

353

13

5

7

9

1113

n Fig. 4. Fits of Eq. (14a) to the experimental relative yields of (NaCl),Na+, (NaI),Na+, and (NaBr),Na+ from 4.0 keV Xe+ ion bombardment measured by Barlak et al. [22]. x = 4.60 for all three fits. The b values are 0.716 for NaCl, 0.777 for NaBr, and 0.867 for NaI. 0, NaCI; q, NaBr; A, NaI.

Fig. 3 could have been made better by choosing x to match the slopes of the data and theoretical curves at large n. This procedure only changes the value of x slightly, but the theoretical curve would then lie above the experimental for large n. This is reasonable since the instrument sensitivity would be expected to decrease at large n and the factor S,, would be expected to be lower than that given by Eq. (18) at large n. For example, hollow clusters, although allowed in the counting of connected structures on lattices, would not be expected to be desorbed. We also test the validity of Eq. (14) by applying it to other alkali halide systems. Figure 4 shows the experimental relative yields of three sodium halides as a function of cluster size n measured by Barlak et al. [22] using a magnetic sector-field instrument. Also shown are the theoretical fits by Eq. (14a) assuming the clusters are 2-D. The best fits are obtained with x = 4.60. The b values of each of the three sodium halides are, of course, different from the CsI case, as evident from Eq. (14). We have b = 0.716 for NaCl, 0.777 for NaBr, and 0.867 for NaI. in

ENERGY DISTRIBUTIONS

It is extremely difficult to measure the energy distribution of an individual macromolecule or giant cluster. In the time-of-flight (TOF) experiments,

o;.ool ENERGY (eV)

Fig. 5. Energy distribution of SF6 neutral molecules desorbed from frozen SF, with a 6 keV ArC ion beam. Experimental points are from Szymonski et al. [30] and the solid line is a theoretical fit from Eq. (3).

because of the high masses involved, isotope effects alone will broaden the mass peak. Additionally, dissociation or fragmentation along the flight path contributes to further broadening. If one tries to use an electrostatic or magnetic analyser, then one can never be certain whether the observed mass peak is due to cont~butions from an intact mol~ule/cluster, a fragment, or both. Consequently, no data on energy distributions of large molecules or clusters presently exist. We have therefore chosen to test our theory with the energy distribution measured for a small molecule, SF,. Figure 5 shows the energy spectrum of SF, neutral molecules desorbed from a frozen SF, solid target by 6 keV Arf ions. The experimental points are from Szymonski et al. 1301, while the solid line is a theoretical fit according to Eq. 3. The energy spectrum was measured using SF, ions which were formed by post-ionization of desorbed SF, neutrals. The desorption process then involves no charge transfer and RRK theory can again be used to model the process. The fit of RRK theory to the data is quite good and yields a value of /3= 7.9 eV- ’ for Ar. A stringent test of the theory is related to the presence of the quantities and in the expressions for the energy and mass spectra. For example, from Fig. 5, the FWHM of the spectra is 0.23 eV. Since the FWHM = 1.79/p, p is 7.7 eV_l and the value of (E #/N) should be 0.13 eV/mode according to Rq. (5). The mass spectra of SF6 clusters and fragments should then be able to be fitted with this value of E ‘,I%. Since SF, has 15 normal modes, the energy deposited per molecule is then about 1.5 eV. Interestingly, if not all the excited modes lead to desorption, say only 2 or 3, then E, is of the order of the sublimation energy, 0.2 eV molecule-‘. Since they are both related to

355

the energy deposited per mode, simultaneous measurement of mass and energy spectra of the desorbed particles should enable more quantitative analysis of this proposed relationship. DISCUSSION AND CONCLUSIONS

We have shown that the desorption of insulin biomolecules by MeV primary ions and alkali halide clusters and SF, molecules by keV primary ions can be adequately described by the RRK theory. The fit of theory to the mass spectra is largely governed by the p~ameter E&V, or energy deposited into each normal mode. In estimating the number of molecules (or normal modes) excited by the incoming ion we have used (a) a 10 nm diameter surface area which is the average diameter of a MeV ion track [31] for the insulin case, and (b) a 4 nm diameter surface area for the CsI case, which is the average lateral spread of a collision cascade for a keV heavy ion 1321. Both estimates produce values of E,/N indicating all the energy lost by the incoming ion is available for excitation of normal modes leading to desorption. This is evidently not possible in reality. Nevertheless, it leads to the interesting question of whether MeV ions or keV ions are more efficient in desorption yields. This question cannot be answered directly in our theoretical treatment since we are only calculating relative abundances rather than absolute abundances. However, examination of Fig. 2 tells us the energy required per mode tends to a minimum if a large number of modes are excited and suggests that this is the preferred manner of energy partition for desorption. In the case of large energy deposition, e.g. 8 keV nm-’ for the 90 MeVi2’12’+ ion, * enough energy is deposited into each mode to satisfy this condition for desorption. If the total energy deposition is small, e.g. in the case of keV primary ions, insufficient energy is available to excite a large number of modes per molecule. For desorption to occur, more energy per mode concentrated into a fewer number of modes is required. This corresponds to the steep rising parts of the curves shown in Fig. 2 and is energetically less favourable. Such a reasoning leads us to conclude that MeV ions are more efficient for desorption than keV ions as demonstrated experimentally by Kamensky et al. [33]. On the other hand, this advantage may, in practice, be neutralized since it is easier to produce higher ion flux with keV ion beams. Finally, we should mention the fact that the experimental data sets used in the present work are neither corrected for detector efficiency nor for spectrometer transmission as a function of the secondary ion mass. Complete agreement with theory over a large mass range is therefore not expected. Nevertheless, the present approach demonstrates that RRK theory can be used to describe the desorption process and that the derived equations for the mass and energy spectra have the correct functional form.

356 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

P. S&mund, Phys. Rev., 184 (1969) 383. CA. Andersen and J.R. Hinthome, Anal. Chem., 45 (1973) 1421. M. Szymonski and A. Poradzisz, Appl. Phys. A, 28 (1982) 175. D.F. Torgerson, R.P. Skowronski and R.D. Macfarlane, B&hem. Biophys. Res. Cornmun., 60 (1974) 616. R.D. Macfarlane and D.F. Torgerson, Science; 191 (1976) 920. P. H&ansson, A. Johansson, I. Kamensky, B. Sundqvist, J. Fohhnan and P. Peterson, IEEE Trans. Nucl. Sci., NS-28 (1981) 1776. A. Benninghoven, D. Jaspers and W. Sichtermann, Appl. Phys., 11 (1976) 35. M. Barber, R.S. Bordoh, R.D. Sedgwick and A.N. Tyler, J. Chem. Sot. Chem; ‘Commun., (1981) 325. M.A. Posthumus, P.G. Kistemaker, H.L.C. Meuzelaar and M.C. Ten Noever de Brauw, Anal. Chem., 50 (1978) 985. R.D. Macfarlane, Act. Chem. Res., 15 (1982) 268. A.R. Ziv, S.H. Lm, M. Skiff, B.P. N&am, M. Szymonski, CM. Loxton and I.S.T. Tsong, J. Mol. Sci., 1 (1983) 55. B.V. King, A.R. Ziv, S.H. Lin and I.S.T. Tsong, Surf. Sci., 167 (1986) 18. B.V. King, A.R. Ziv, S.H. Lin and I.S.T. Tsong, J. Chem. Phys., 82 (1985) 3641. See, for example, W. Forst, Theory of U~ol~~~ Reactions, Academic Press, New York, 1983; K-J. Laidler, Theories of Chemical Reaction Rates, Kriger, Huntington, NY, 1979. S.H. Lin, I.S.T. Tsong, A.R. Ziv, M. Szymonski and C.M. Loxton, Phys. Ser., T6 (1983) 106. P. H&kansson, I. Kamensky, B. Sundqvist, J. FohIman, P. Peterson, C.J. McNeal and R.D. Macfarlane, J. Am. Chem. Sot., 104 (1982) 2948. L.C. Northc~ffe and R.E. Schilling, Nucl. Data Tables, A7 (1970) 233. B.T. Chait and F.H. FieId, Int. J. Mass Spectrom. Ion Processes, 65 (1985) 169. J.E. Campana, T.M. Barlak, R.J. Coiton, J.J. DeCorpo, J.R. Wyatt and B.I. Dunlap, Phys. Rev. Lett., 47 (1981) 1046. T.M. Barlak, J.E. Campana, R.J. Colton, J.J. DeCorpo and J.R. Wyatt, J. Phys. Chem., 85 (1981) 3840. T.M. Bark& J.R. Wyatt, R.J. Colton, J.J. DeCorpo and J.E. Campana, J. Am. Chem. Sot., 104 (1982) 1212. T.M. Barlak, J.E. Campana, J.R. Wyatt and R.J. Colton, J. Phys. Chem., 87 (1983) 3441. T.M. Bark&, J.E. Campana, J.R. Wyatt, B.I. DunIap and R.J. Colton, Int. J. Mass. Spectrom. Ion Phys., 46 (1983) 523. W. Ens, R Beavis and K.G. Standing, Phys. Rev. Lett., 50 (1983) 27. K.G. Standing, R. Beavis, W. Ens and B. Schueler, Int. J. Mass. Spectrom. Ion Phys., 53 (1983) 125. W.F. Lunnon, in A.O.L. Atkin and B.J. Birch (Eds.), Computers in Number Theory, Academic Press, London, 1971. M.F. Sykes and M. Glen, J. Phys. A, 9 (1976) 87. A.J. Guttmann and D.S. Gaunt, J. Phys. A, 11 (1978) 949. B. Schueler, R. Beavis, W. Ens, D.E. Main and K.G. Standing, Surf. Sci., 160 (1985) 571. M. Szymonski, R. Pedrys, A. Haring, R. Hating and A.E. de Vries, ~publish~ work. A. Hedin, P. Hakansson, B. Sundqvist and R.E. Johnson, Phys. Rev. B, 31 (1985) 1780. T. Ishitani and R Shimizu, Phys. Lett. A, 46 (1974) 487. I. Kamensky, P. IHkansson, B. Sundqvist, C.J. McNeaI and R. Macfarlane, Nuci. Instrum. Methods, 198 (1982) 65.