Interaction of MEV atomic ions with molecular solids: Ion track structure and sputtering phenomena

Radiation Measurements, Vol. 28, Nos 1-6, pp. 101-110, 1997 © 1997 Elsevier Science Ltd Printed in Great Brit,,in. All rights reserved

Pergamon

1350-4487/97 $17.00 + 0.00 PII: S 1350-4487(97)00048-6

INTERACTION OF MEV ATOMIC IONS WITH MOLECULAR SOLIDS: ION TRACK STRUCTURE AND SPUTTERING PHENOMENA D.D.N.B. DAYA, t P. DEMIREV, 2 J. ERIKSSON, 2 A. HALLI~.N, 3 P. H/~,KANSSON, ~ R.E. JOHNSON, 4 J. KOPNICZKY, 2 R.M. PAPALlY.O,5 C.T. REIMANN, 2 J. RO'I~LER, 2 & B.U.R. SUNDQVIST 2 ~Department of Physics, University of Colombo, Colombo 3, Sri Lanka. ~'Department of Radiation Sciences, Division of Ion Physics, Uppsala University, Box 535, S-751 21 Uppsala, Sweden. 3Department of Solid State Electronics, Royal Institute of Technology, Box E229, S-164 40, Kista, Sweden. ~Engineering Physics, Thornton Hall, University of Virginia, Charlottesville, Virginia 22903, United States. ~lnstitute of Physics, Federal University of Rio Grande do Sul, C.P. 15051, 91501-970, Porto Alegre, RS, Brazil. ABSTRACT Recent results obtained in our research groups from studies of the interactions of swift, heavy atomic ions with molecular solids are concisely outlined. The focus is on material ejection (sputtering) and surface track formation. The experimental techniques employed include time-of-flight mass spectrometry, energy analysis, collection and analysis of sputtered material, and scanning force microscopy. Characteristics of the sputtering process probed include the sputtering yield, radial and axial velocity di.stributions, angular distributions, and surface track morphology. Besides reviewing and correlating experimental results, we also emphasize the common quasi-thermal origin of pressure-pulse/hydrodynamic and evaporative spike sputtering models.

KEYWORDS Sputtering, cratering, surface modifications, surface tracks, swirl heavy atomic ions, scanning force microscopy, thermal spike, pressure pulse, shock wave, radial velocity distributions, collector experiments.

INTRODUCTION The interaction of fast atomic ions with condensed matter is a topic of research that has underlain the entire development of atomic and nuclear physics. Our understanding of the complex physicochemical processes occurring upon penetration of an energetic (energy >25 keV/nucleon) ion in a molecular solid has evolved in parallel with theoretical and experimental means of addressing such phenomena. Recent renewed interest in this venerable subject (Fleischer et al., 1975) has been inspired by potential technological applications of MeV ion beams for material processing on the nanometer scale (Spohr, 1990) and by emerging methods for medical diagnostics and therapy (Castro, 1995). An MeV atomic ion deposits its energy in a solid target primarily via collisions with the target atom electrons, resulting in a trail of ionized and excited species - - an ion track - - in times of the order of 10)s s. If the energy relaxation rate of the electronic subsystem of the solid is lower than the energy deposition rate (e.g. for insulators) a notable effect is the displacement of atoms in the vicinity of the ion track. In the bulk, latent tracks result, whereas at the vacuum-solid interface, ejection of lOl

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charged and neutral species occurs, leaving surface "defects". Studies of ion-induced bulk modifications have provided the dominant body of information on MeV ion-solid interactions in the past (Fleischer et al. 1975). More recently, probing sputtering characteristics and surface defects has offered a complementary and unique capability to directly, and sometimes in situ, probe the temporal and spatial structure and evolution of the deposited energy density in an individual ion track (Papalto et al., 1996a). Surface erosion ("electronic sputtering") upon MeV atomic ion impact is the net result of translational energy transfer, described in continuum terms as either an evaporative (activated) or a hydrodynamic/pressure pulse (correlated) material response to an underlying quasi-thermal energy diffusion process. In this paper we summarize measurements of sputtering phenomena from a number of molecular solids, including industrial polymers and biomolecules (peptides and proteins). We have performed in situ high-resolution mass analysis of the sputtered ionic species. Only the nascent (i.e. most energetic) ionic intermediates and reaction products, ejected in an individual impact event, are sampled. Initial kinetic energies and damage cross sections furnish information on the temporal and spatial origin of these species relative to the point of MeV ion impact. The total yield and angular distributions of sputtered species have been studied by a collector technique. Finally, the morphology and dimensions of surface defects, both "craters" and "hillocks", measured by scanning probe microscopy, are discussed. A discussion of current electronic sputtering models, e.g. hydrodynamic (Johnson et al., 1989) and evaporative thermal activation (Johnson and Evatt, 1980), including molecular dynamics simulations (Feny6 and Johnson, 1992), and their interrelation, is also provided. The scaling of experimental characteristics on the volume energy density, deposited by an MeV atomic ion in the track, is a "common denominator" to connect sputtering models in a cogent manner to the experimental results. EXPERIMENTAL

TECHNIQUES

For fundamental studies of electronic sputtering and other phenomena related to swift heavy atomic ion impacts, a tandem van de Graaff accelerator is an excellent tool. It can supply a wide range of ions with sufficient energy, and the limited beam currents available are not an obstacle since the experiments concern mainly individual ion impacts. Three beamlines at the Uppsala accelerator have been utilized for the investigations described below. One beamline is equipped with a scanner for homogeneous irradiations over large areas (Halltn et al. 1989). This facility is used to obtain bulk damage cross sections by measuring the deterioration of samples as a function of fluence. It is also used for producing controlled areal densities of surface tracks on various targets. A second beamline hosts an ultra-high vacuum system with a plasma-desorption time-of-flight mass spectrometer (PDMS) featuring an electrostatic mirror (Brinkmalm et al. 1995). Here, the secondary ion sputtering yield is measured for various beam parameters. The high resolution obtainable with the mirror makes it possible to quantify axial velocity differences of sputtered ions and, with two pairs of orthogonal deflection plates, the radial velocity distributions can be recorded. Irradiations of samples are also performed in this chamber, after which sputter yields are measured in situ to obtain damage cross sections. At the third beamline another PDMS system is mounted for various experiments. Most recently it has been employed for determining total yields of sputtered particles (ions as well as neutrals) by means of collector plates positioned at different angles surrounding the ion impact point (Eriksson et al., 1996a). These collector plates are analyzed in situ by PDMS, or ex situ by amino acid analysis (AA). In AA, estimates of absolute amounts of sputtered material are possible. In many of the experiments primary ions of the same velocity have been used. The ions: 7.4 MeV-t2C, 9.9 MeV-ttO, 19.7 MeV-32S, 48.6 MeV-79Br, and 78.2 MeV-J27I, appear on a number of occasions. The importance of keeping the velocity constant is that the energy density in the ion tracks can be varied while the ~'ack size is kept constant, since the track radius is proportional to the velocity of the incoming ion. The ions are in most cases placed in charge state equilibrium by passage through a thin carbon foil before hitting the samples. The two most commonly employed methods for preparation of samples are the spin coating and electrospray techniques. Proteins and peptides under investigation are dissolved in a solution. Then, for the spin coating technique, the solution is dropped onto a rotating backing and a thin film with a smooth surface is obtained. The olectrospray method produces a rougher surface but is often more convenient for the formation of thicker films (>1 l~m). The backing is normally a low-resistivity

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polished silicon chip or metal plate. Films of commercially available polymers are also formed by the spin-coating technique or are self-supporting. Some of the polymer and protein targets can be crystallized on the backing surface. Sample film thickness and surface topography are studied by various techniques such as eilipsometry and scanning force microscopy (SFM). Several kinds of studies are performed on ion-bombarded targets ex situ. Surface tracks from swift heavy atomic ion impacts are studied under ambient condition SFM using both contact and tapping (oscillating intermittent contact) modes. In the tapping mode, lateral forces which lead to plastic surface distortion are sharply reduced. Resolution is limited by the size of the probe tip, =10 nm. Nonetheless, measured lateral dimensions of surface defects yield important information on the response of the material to an impinging ion. Bulk damage cross sections are estimated from infrared (IR) absorption and UV-VIS spectroscopy of irradiated samples as a function of ion fluence. Raman/ micro-Raman spectroscopy is also used to study irradiated samples. T O T A L S P U T T E R I N G YIELDS MEASURED BY C O L L E C T O R TECHNIQUES When a collector experiment is performed in order to determine the absolute total sputtering yield, a few general features of the technique must be considered before the actual value of the total yield can be obtained. If a collector with sticking probability P is placed in front of a target bombarded with a number N of incident ions, and an absolute amount ct of molecules is collected, the total net sputtering yield Yo can be estimated as o./N. However, P is typically unknown, and therefore o/N is a lower bound on Yo. The approximation becomes worse if the collector does not cover all possible sputtering angles and if the angular distribution of ejecta is unknown. Knowledge of the angular distribution of the ejecta is therefore also desirable. Besides depending on the sputtering angle, the sputtering yield is also an exponentially decreasing function of the fluence ~, i.e. the number of incident ions per target unit area, since molecules which are not sputtered in one ion impact event might be destroyed by the cascade of secondary electrons released from the infratrack. Characterization of ion-induced damage has been reported previously (Salehpour et aL, 1984; Hedin et al., 1987; Eriksson et al., 1996b). If the ¢~dependence of the yield is known, a more accurate determination of the absolute total yield can be carried out. The first data on the total yield of biomolecules sputtered by fast ions were presented for a target composed of the amino acid leucine (M=I31 u) bombarded by 90-MeV 127Iat an angle of 60 ° with respect to the target surface normal (Sahlepour et al., 1986). A total yield of 1200-Y.300 was obtained by employing AA and assuming a cosine angular distribution with no normalization to account for target damage. This result shows that the number of biomolecular ions detected per incident ion in a secondary ion mass spectrometer is only a fraction 10.4 of the total yield. Total yield and polar angle distributions of intact tri-leucine molecules (M=357 u) have recently been measured by the collector method (Eriksson et al., 1996a). 55-MeV 1:7I ions impinged on the target at an angle of 51 o with respect to the target surface normal. During bombardment the ejecta were collected along two perpendicular bands on a hemispherical surface. The collectors were subsequently analyzed both by PDMS and AA. By accounting for the damage induced during irradiation and the angular distribution, a total yield of (3.5:~'0.7)x103 sputtered tri-leucine molecules per ion incident on a pristine target is obtained. In the plane encompassing the incident ion direction and the target surface normal (the "plane of incidence", shown in Fig. 2b) the observed distribution of sputtered molecules is asymmetric and peaked away from the incident ion direction (Fig. la). In the plane perpendicular to the plane of incidence the distribution is normally peaked with a shape similar to a cos~O function, with n=1.5. The plane-of-incidence data show excellent agreement with the asymmetric angular distributions obtained by molecular dynamics simulations of a 6-12 Lennard-Jones solid excited by expansion of a narrow 45 ° track (Fig. la) (Feny6 et al., 1989). The results of such simulations agree well with the predictions of the analytic pressure pulse model (Johnson et al., 1989).

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100 (a)

250 200

o -

-

- •

-

PDMS -

LU (b) /

- PDMS

•

AA

-

MD-simulatlon

(dE/

i

150

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100

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0 40 80 -40 SPUTTERING ANGLE [°]

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•

10 1/p ( d F / d x ) e l [ M e V

m

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100 cm 2 mg "1]

Figure 1. (a) The angular distribution in the plane of ion incidence of tri-leucine molecules sputtered by 0.43-MeV/u ml ions, obtained by collector experiments. The collectors were analyzed both in situ (PDMS) and ex situ (AA). The MD result shown for comparison is for a 612 Lennard Jones solid with an expanded track. The experimental and simulated ejection peak at an angle away from the ion incidence direction of 51 ° and the 45° track angle respectively. (b) In situ collector experimental result for the total yield of the peptide LHRH as a function of the incident ion electronic stopping cross section with the ion velocity kept constant (1.11 cm/nsec). Figure adapted from Eriksson et al. (1996a; 1995). In situ PDMS analysis of the collectors has been applied for determining the stopping power (dE/dx) dependence of the total yield. Hedin et al. (1987) found a cubic dependence for the amino acid leucine. A similar scaling was recently measured by Eriksson et al. (1995) for the peptide LHRH (M=1182 u) (Fig. lb). A cubic scaling with dE/dx for the total yield is predicted by the pressure pulse model (Johnson et al., 1989) and by molecular dynamics simulations (Feny6 and Johnson, 1992).

SECONDARY ION E J E C T I O N IN E L E C T R O N I C SPUTTERING: P R O B I N G NASCENT E F F E C T S IN ION T R A C K S The study of the ionic intermediates and reaction products ejected from molecular solids provides an example of the application of electronic sputtering for the investigation of ion track structure and fast ion track chemistry. Results obtained from radial velocity distributions and disappearance cross sections of low mass secondary ions (SI) have been particularly enlightening (Papalto et al., 1996a,b). The two moments of the radial v, (the component of velocity parallel to the surface in the plane of incidence, Fig. 2b) velocity distribution ( and the full-width at half-maximum, FWHM, ) of positive Sis (CnHm+, C.HmF+, CnHmO*, and CnFm+) ejected from different molecular targets vary systematically with their degree of hydrogenation/fluorination (Fig. 2). For example, C," formed by heteroatom depletion has the highest < v / > values, while saturated (e.g. C,H~. ÷) species show the lowest < v / > values. < v / > is indicative of the mean deposited energy density in the track, ~(r), averaged over the ejection time, and is proportional to the momentum transferred to the ejecta along the plane of the target, The dependence of the moments of the vx distribution on heteroatom content reflects the spatial inhomogeneity of the "fast" chemistry induced in the tracks or, in other words, the chemical and thermal history of the SI in the track.

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PVDF, 450r

(a)

'A..

.,

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Figure 2. (a) Solid-state track structure expressed as energy density, derived from , plotted versus r,, the effective radius of origin obtained from signal disappearance cross sections a~, for PVDF. (b) Mean radial velocity plotted versus mass for a series of similar fragment Sis from PVDF. (c) plotted versus r,. (d) The r~ scale is mapped onto a crater, imaged by TM-SFM, in PMMA, taking into account the angle of incidence. Far right: artist's rendition, random walk raptation of PVDF and PMMA. These species are sufficiently similar for the different data sets to be compared. Adapted from Papal6o et al. (1996a). An effective ejection radius for SI species i, r i, can be inferred from the signal disappearance cross section of that species, ~: ri=(o,./tt) °5. The resulting radial profiles for and are given in Fig. 2. The following qualitative picture of the track structure emerges from these data. At small r~, up to 2.4 nm, ions have a positive , corresponding to ejection backwards along the track, as well as higher . Sis characterized by small r i tend to be more or less depleted of the heteroatoms present in pristine target. At large r~ (>3.5 nm), Sis tend to have negative , directed away from the line of incidence of the MeV ion. They also tend to retain (or reincorporate) heteroatoms. A relatively steep transition region between 2.5 and 3.5 nm is also seen, in rough agreement with hydrodynamic models of ejection (Johnson et al., 1989). The observed radial variation of the mean deposited energy density, r-/.2, is less steep than the r -2 dependence obtained for the initial energy

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density in the tracks of fast ions in a gas. However, powers from -1 to -2 are in acceptable agreement with gas-phase data considering scatter in the data and a correction factor applied to tr, values (Papal6o et al., 1996a). Alternatively, a less steep r dependence can also be seen as consistent with the occurrence of significant energy transport (Papal6o et al., 1996c) before particle ejection transpires. It is clear from Fig. 2 that the near-surface energy density in the track produces fragmentation and ejection processes that are closely intertwined. The local energy density triggers the fast "hot" chemistry responsible for bond rearrangement in the track. The observation that the mean radial velocities for positive Sis are different from zero shows the non-equilibrium character of their formation process. Furthermore, the dependence of their mean velocity on the place of emission indicates that ejection occurs in response to a volume force arising from the energy density gradient in the track, in qualitative agreement with hydrodynamic ejection models (Johnson et al., 1989). The radial velocity distributions of negative Sis are uncorrelated with their chemical composition (i.e. =0 for all r~). These data suggest a distinctly different picture for their ejection mechanism, possibly dominated by thermal evaporation from a region which is closer to equilibrium. S U R F A C E T R A C K STUDIES One important manifestation of the interaction between an energetic heavy atomic ion and a surface is the formation of a surface track: a defect which is the surface signature of the response of the material to the energy deposited by the incident ion. The surface track is the surface analog to the well-studied latent tracks in the bulk of ion-bombarded materials. We have reported extensively on surface tracks in single-crystal amino acid L-valine (Daya etal., 1995; Eriksson et al., 1995, 1996b; Kopniczky et al., 1995, 1994; Reimann et al., 1995; Fig. 3). Normal-incidence ions produce craters with low rims, while grazing-incidence ions produce wider craters accompanied by hillocks and tails on the surface (Fig. 3a). Reducing dE/dx but keeping ion velocity constant, crater widths, lengths and depths become smaller (Fig. 3b). Crater widths scale as ,~ (dE/dx) 0.5,and crater areas scale as =dE/dx (Fig. 3c). Presently we emphasize the wide variations in crater form, just in L-valine. We observe tailed craters (Figs. 3,4), craterless tails (Fig. 4c), tails which are sharp-edged on one side but which merge smoothly into the surface on the other side (Fig. 4d), and several examples of pinching-off or "necking" between the crater and the tail (Fig. 4d). For the same type of incident ion, LB craters are narrower and longer than craters in L-valine (Fig. 4b). Tails do not follow the craters on LB films although small hillocks may appear (Reimann et al., 1995). Finally, craters in spin-coated biopolymer surfaces have been investigated (Eriksson et al., in preparation). When the sample is composed of the immunoglobulin IgG (M=150,000 u) the craters are narrow and surrounded by tall rims, unlike the craters observed in L-valine (Fig. 4a). However, the overall lateral sizes of the surface defects on IgG and L-valine are very similar. The scaling of crater width as (dE/dx) o5 suggests the operation of any kind of activated material modification mechanism, including evaporation (Johnson and Evatt, 1980), combined with energy transport. However, the formation of hillocks, tails and crater rims can be consistent with correlated material rearrangement and suggests the operation of the pressure pulse mechanism (Johnson et al., 1989). The often-observed "necking" between crater and hillock may reflect a complete spatial separation between areas in which evaporation (crater) or pressure pulse (material distortion, hillocks) occur. Nonetheless, as discussed above (Fig. 2), there is strong evidence that some material from the crater is also ejected by the pressure pulse. One possible scenario is that, in instances where the boundary between crater and hillock is sharp (Fig. 3), chunks or clusters may have ejected from near the hillock via the pressure pulse (Reimann, 1995), although the width of the crater at its middle may be determined by evaporation. Finally, as discussed below theoretically, the pressure pulse and activated processes like evaporation occur on different timescales, again suggesting that the specifics of crater formation may be impacted by more than one sputtering mechanism. For large biopolymers like IgG deposited by spin-coating, the individual proteins are extended in size due to denaturation and do not fit into the crater. Raptation occurs, and the proteins are more or less tightly intertwined with each other. Therefore, for IgG the crater formation process must cut through polypeptide chains, and the effective binding energy is much higher than for solids composed of much smaller proteins or peptides. Consequently, craters in IgG become short and narrow. Obviously, plastic deformation of the IgG sample is much less energy intensive than sputtering.

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Thus, much of the material that would have been ejected for L-valine simply undergoes plastic deformation for IgG, leading to the formation of pronounced crater rims (Fig. 4a).

1271

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Figure 3. (a,b) TM-SFM images of surface tracks, induced under the indicated conditions, in single-crystal L-valine. In (b) the & value is the low-to-high scale. (c) Scaling of crater width and area with stopping cross section for 79 ° angle of incidence. 127. I, H = 7 9 ° 78.2 MeV

(b)

79Br, t ) = 8 4 °

127I

79Br, u = 4 5 °

, u=79 °

(u)

55 MeV

Y

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(a)

250 nm

Y IgG

,

L-Val

LB

/I

(c) \

/

L-Val

Figure 4. TM-SFM images of surface tracks on various materials, as indicated, and for vanous incident ions, angles, and kinetic energies. In (a), a Y-shaped lgG molecule is shown below the image at the same scale. For details of (a)-(d) see the text.

THEORETICAL

PERSPECTIVE

Ejection of molecules from a region of a solid around an ion track occurs in response both to the random (i.e. uncorrelated) motion of molecules on the surface and to the integrated impulse received by a volume of material at the surface. These two aspects of the ejection process, referred to as evaporative spike and pressure pulse sputtering respectively, were placed on a common footing using a simplified but instructive analytic model (Johnson et al., 1989) in which energy diffuses linearly in the lattice. Although the underlying diffusion process is only really of a quasi-thermal nature, Johnson et al. (1991) showed that the principal results of the "thermal" spike model are NOT dependent on the local velocity distribution being Maxwellian. In the analytic model for quasi-thermal energy diffusion, the local energy density in the cylindrically excited region of an ion track determines the random motion of the molecules, while the gradient in the energy density produces a net force oriented radially and out of the surface. The cumulative impulse received by a volume of material depends primarily on the energy distribution at early times when the gradients are steep, leading to a "prompt" pressure pulse process, whereas the activated evaporative loss of material from the surface persists as long as the region has a "temperature"

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much higher than the initial temperature. In real materials these temporal differences are exaggerated since the dispersal of the energy, described in the model by a diffusivity, is non-linear (Feny6 and Johnson, 1992). In addition, the contribution to the yield by each process is critically dependent on the material properties. For example, the pressure pulse produced can either act to dislodge material or can simply compress it, as seen in molecular dynamics simulations. The Uppsala group and co-workers studied ejection of large molecules in which the spacing characteristic of molecular boundaries is smaller than the molecular size, so the material is relatively incompressible AND the large molecules act as efficient momentum integrators. The results of the calculations (FenytJ, 1993; Feny6 and Johnson, 1992; Feny6 et al., 1990b; Johnson etal., 1989), as well as experiments (Ens et al., 1989; Eriksson et al., 1996a; Feny6 et al., 1990a) on the ejection of large molecules, agree remarkably well with the model description of a pressure pulse. In contrast, in van der Waals solids made up of atoms or small molecules, "thermal" spike aspects appear to dominate at low excitation densities (Johnson et al., 1991). Further, in the molecular dynamics calculation of Urbassek et al. (1994) for solid argon, the radial component of the pressure gradient at high excitation density primarily produces material compression, not ejection, so the loss of material acts like a one-dimensional pressurized jet. This effect has been pointed out by the Uppsala group seen in the angular distribution of the ejection of FRAGMENTS from films composed of large biomolecules (Papal6o et al., 1996a,b). In a similar manner, ejection of molecules from Langmuir-Blodgett films, which respond differently in the radial and surface directions owing to the elongated nature of the LB molecules, will be determined by a restricted pressure pulse. CONCLUSIONS The set of experiments described above demonstrates the great diversity of ion-impact-induced phenomena in organic materials. Yet a simple, quasi-thermal energy diffusion picture underlies all these phenomena. An overall scenario of the sputtering process looks to be as follows. 1) At early times, positive fragment ions, intact molecular ions and neutrals, and even chunks/clusters are ejected. 2) Radial velocity patterns indicate mostly a correlated, pressure-pulse material response in ejecting these species (note, chunks cannot be evaporated). Collector-based angular distribution measurements confirm that ejection patterns are non-equilibrium and directed away from the incident ion azimuth, again favoring pressure pulse ejection. 3) The sputtering yield of neutral sputtered species scales as (dE/dr) 3 consistent with the pressure pulse. 4) Point-of-origin information fingers the positive fragment ions as originating from within the crater region of surface defects which are imaged by scanning force microscopy (SFM). Therefore, taking into account point 1) above, at least some material from the crater is sputtered by the pressure pulse. Yet, 5) crater widths in L-valine, an amino acid, scale as (dE/dr) °5, a dispersive, thermally activated signature, possibly indicating an evaporative contribution to crater formation. Finally, 6) negative fragment ions do not behave like positive ones. In particular they show a thermal signature in their radial velocity distributions. Our main conclusion here is thus that each ion impact manifests both activated and correlated features of the underlying quasi-thermal energy diffusion. Results of a now classic experiment (Hedin et al., 1985) showed that the threshold dE/dr for observing significant molecular ion emission increased with the size of the molecular ion. In light of recently obtained data on the scaling of crater size with dE/dx, the threshold observed in the old work can be regarded as reflecting the point at which track size and molecule size are comparable. One exciting new linkage shown in the present paper is the first attempt made to map an independent physical observable (here the point-of-origin of fragment ions of various ) onto topographical images of surface tracks observed by SFM. The future holds both more careful and elaborate measurements of surface tracks for an extremely broad range of materials, and attempts to correlate other physical observables with each other and with the craters. In particular, it will be important to treat materials which display varying degrees of anisotropy in their structure, as anisotropy should affect the energy transport in relatively simple ways which will lead to further theoretical insight. The common basis for pressure pulse and evaporative sputtering has been implicitly displayed before (Johnson et al., 1989) although not stressed in depth. However, no model has yet appeared to

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treat material rearrangements due to simultaneously occurring pressure pulse and evaporation mechanisms. Presumably the prompt ejection of hot material would redw,e the amount of energy remaining to contribute to further processes. Another open area for theory is to find ways of modeling material modifications on much longer time scales after ion impact than is now feasible using molecular dynamics. In the summary presented above, only rudimentary consideration was given to the ionization state or ionization mechanism of sputtered biomolecules. We know that neutral particles dominate the sputtering yield of organic targets, but we do not know yet why positive and negative ions appear to be sputtered by different mechanisms. Both place of origin and ionization mechanism may strongly affect our perception of the behavior of sputtered ions. More detailed investigations of sputtered ions, for example, by careful axial kinetic energy measurements (Zubarev et al., 1996), may shed further light in this area.

Acknowledgments The Uppsala group thanks the Swedish Natural Sciences Research Council (NFR), the Swedish Technical Research Council (TFR), the Knut and Alice Wallenberg Foundation, the GlSran Gustafsson Foundation, the ,~,nstrOm and Cluster Consortia at Uppsala University, the Swedish N/lrings och Teknikutvecklingsverket (NUTEK),and the National Research Council of Brazil (CNPq). All the authors thank numerous colleagues with whom they have collaborated and discussed physics through the recent years.

REFERENCES Brinkmalm G., H/tkansson P., Kjellberg J. and Sundqvist B.U.R. (1995) DIPLOMA: An Experimental System to Study MeV and keV Particle- and Photon-Induced Desorption of Molecules. J. Vac. Sci. Technol. A 13, 2547-2552. Castro J. (1995) Results of Heavy Ion Radiotherapy. Rad. and Env. Biophysics 34, 45. Daya D.D.N.B. et al. (1995) Radiation Damage Features on Mica and L-Valine Probed by Scanning Force Microscopy. Nucl. lnstrum. Meth. Phys. Res. B 106, 38-42. Ens W., Sundqvist B.U.R., H/tkansson P., Hedin A. and Jonsson G. (1989) Directional Correlation Between the Primary Particle and Ejected Molecular Ions in Electronic Sputtering of Large Organic Molecules. Phys. Rev. B 39, 763-766. Eriksson J., Rottler J. and Sundqvist B.U.R. (in preparation) Craters in Spin-Coated Bioorganic Films. Eriksson J., Demirev P., H/tkansson P., Papal6o R.M. and Sundqvist B.U.R. (1996a) Total Yield and Polar Angle Distributions of Biomolecules Sputtered by Fast Heavy Ions. Phys. Rev. B 54, 15025-15033. Eriksson J., Kopniczky J., Demirev P., Papal~o R.M., Brinkmalm G., Reimann C.T., H/tkansson P. and Sundqvist B.U.R. (1996b) Damage Cross Sections and Surface Track Dimensions of Biomolecular Surfaces Bombarded by Swift-Heavy-Ions. Nucl. lnstrum. Meth. Phys. B 107, 281286. Eriksson J., Kopniczky J., Brinkmalm G., Papal6o R.M., Demirev P., Reimann C.T., H/tkansson P. and Sundqvist B.U.R. (1995) Heavy-ion-induced sputtering and cratering of biomolecular surfaces. Nucl. lnstrum. Meth. Phys. Res. B 101, 142-147. Feny6 D. (1993) Sputtering by a Sum of Impulses: The Effect of Finite Track Width. Phys. Rev. B 47, 8263-8264. Feny6 D. and Johnson R.E. (1992) Computer Experiments on Molecular Ejection from an Amorphous Solid: Comparison to an Analytic Continuum Mechanical Model. Phys. Rev. B 46, 5090-5099. Feny6 D., Hedin A., H~tkansson P. and Sundqvist B.U.R. (1990a) Radial Velocity Distributions of Secondary Ions Ejected in Electronic Sputtering of Organic Solids. Int. J. of Mass Spectrom. and Ion Proc. 100, 63-76. Feny6 D., Sundqvist B.U.R., Karlsson B.R. and Johnson R.E. (1990b) Molecular-Dynamics Study of Electronic Sputtering of Large Organic Molecules. Phys. Rev. B 42, 1895-1902.

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PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE

Feny6 D., Sundqvist B.U.R., Karlsson B., and Johnson R.E. (1989) A Molecular Dynamics Study of Electronic Sputtering of Large Organic Molecules. J. de Phys. (Paris) C 2, 33-35. Fleischer R.L., Price P.B. and Walker R.M. (1975) Nuclear Tracks in Solids: Applications. University of California Press, Berkely.

Principles and

Hall6n A., Ingemarsson P.A., H~kansson P., Sundqvist B.U.R., and Possnert G. (1989) An MeV-Ion Implanter for Large Area Applications. Nucl. Instrum. Meth. Phys. Res. B 36, 345-349. Hedin A., Hfikansson P., Salehpour M. and Sundqvist B.U.R. (1987) Fast-Ion-Induced Erosion of Leucine as a Function of the Electronic Stopping Power. Phys. Rev. B 35, 7377-7381. Hedin A., H~ikansson P., Sundqvist B., and Johnson R.E. (1985) Ion-Track Model for Fast-Ion-Induced Desorption of Molecules. Phys. Rev. B 31, 1780-1787. Johnson R.E., Pospieszalska M. and Brown W.L. (1991) Linear-to Quadratic Transition in Electronically Stimulated Sputtering of Solid N 2 and 02. Phys. Rev. B 44, 7263-7272. Johnson R.E., Sundqvist B.U.R., Hedin A. and Fenyi5 D. (1989) Sputtering by Fast Ions Based on a Sum of Impulses. Phys. Rev. B 40, 49-53. Johnson R.E. and Evatt R. (1980) Thermal Spikes and Sputtering Yields. Rad. Eft. 52, 187-190. Kopniczky J., Hall6n A., Keskitalo N., Reimann C.T. and Sundqvist B.U.R. (1995) MeV-Ion-Induced Defects in Organic Crystals. Rad. Meas. 25, 47-50. Kopniczky J., Reimann C.T., Hall6n A., Sundqvist B.U.R., Tengvall P. and Erlandsson R. (1994) Scanning-force-microscopy study of MeV-atomic-ion-induced surface tracks in organic crystals. Phys. Rev. B 49, 625-628. Papal6o R.M., Demirev P., Eriksson J., H~ikansson P., Sundqvist B.U.R. and Johnson R.E. (1996a) Measurement of MeV Ion Track Structure in Organic Solids. Phys. Rev. Lett. 77, 667-670. Papah~o R.M., Demirev P., Eriksson J., H/tkansson P. and Sundqvist B.U.R. (1996b) Low-Mass Secondary-Ion Ejection from Molecular Solids by MeV Heavy Ions: Radial Velocity Distributions. Phys. Rev. B 54, 3173-3183. Papal6o R.M., Hall6n A., Sundqvist B.U.R., Farenzena L., Livi R.P., de Araujo M.A., and Johnson R.E. (1996c) Chemical Damage in Poly(Phenylene Sulphide) from Fast Ions: Dependence on the Primary-Ion Stopping Power. Phys. Rev. B 53, 2303-2313. Reimann C.T. (1995) Some Comments on Pressure-Pulse/Shock-Wave Desorption in a Real Material. Nucl. lnstrum, and Meth. in Phys. Res. B 95, 181-191. Reimann C.T., Kopniczky J., Wistus E., Eriksson J., H~'tkansson P. and Sundqvist B.U.R. (1995) MeV-Atomic-Ion-Induced Surface Tracks in Langmuir-Blodgett Films and L-Valine Crystals Studied by Scanning Force Microscopy. Intl. J. of Mass Spectrom. and Ion Proc. 151, 147-158. Salehpour M., H~kansson P., Sundqvist B.U.R. and Widdiyasekera S. (1986) Total Molecular Yields for Fast Heavy Ion Induced Desorption of Biomolecules. Nucl. Instrum. Meth. Phys. Res. B 13, 278-282. Salehpour M., Hhkansson P. and Sundqvist B. (1984) Damage Cross Sections for Fast Heavy Ion Induced Desorption of Biomolecules. Nucl. lnstrum. Meth. Phys. Res. B 2, 752-756. Spohr R. (1990) Ion Tracks and Microtechnology. Vieweg, Braunschweig. Urbassek H.M., Kafemann H. and Johnson R.E. (1994) Atom Ejection from a Fast Ion Track: A Molecular-Dynamics Study. Phys. Rev. B 49, 786-795. Zubarev R.A., Abcywama U., Hltkansson P., Demirev P., and Sundqvist, B.U.R. (1996) Kinetic Energy Measurements of Secondary Ions in MeV and keV Particle-Induced Desorption. Rap. Comm. in Mass Spectrom, submitted.