ELECTRONIC SPUTTERING:FROM ATOMIC PHYSICSTO CONTINUUM MECHANICS

Ejection of simple and complexmolecules from surfaces probes

the response of condensed matterto electronic excitations and has

applications in fields as diverse asastrophysics and biomolecular

mass spectrometry.

Robert E. Johnsonand Bo U. R. Sundqvisr

Robert Johnson holds the John Lloyd Newcomb Chair inMaterial Science and Engineering Physics at the University ofVirginia, Charlottesville. Bo Sundqvist holds the Chair in Ion

Physics at the University of Uppsala in Sweden.

The surprising fact that even very complex molecules canbe ejected intact into the vapor phase when a material iselectronically excited by incident particles provides a newprobe of the behavior of condensed matter at highexcitation densities. The physics of the conversion of theelectronic excitation energy into mechanical and chemicalenergy links atomic physics in a solid at low excitationdensities to nanometer-scale continuum mechanics athigh excitation densities.

Conventional sputtering is the ejection of atoms froma solid surface as a result of direct momentum transferfrom energetic ions to atoms of the solid. Such sputteringis a standard laboratory and industrial means for cleaninga surface, determining surface composition or ejectingmaterial from a solid for deposition on an adjacent target.In this article we describe a very different process—sputtering in response to electronic excitation of con-densed matter.

Photons, electrons and fast ions incident on solidsdeposit their energy in electronic excitations of the atomsor molecules of the material. When this energy isdispersed slowly, as in an electric insulator, or theexcitation densities are high, the deposited energy maycause atomic motion and chemical alterations. This isknown to produce radiation damage in organic solids andtracks in insulators.1 Chemical alterations have been putto use, for instance, in lithography for electronic devices.2

Counting of solar-flare ion tracks was used in determiningexposure ages of lunar grains and interplanetary dustcollected by the Apollo program and high-flying aircraft,respectively. We are interested here in how the depositedelectronic energy leads to ejection of surface species, aprocess referred to as electronic sputtering or desorption.We will use these terms interchangeably, although"sputtering" generally implies the removal of predomi-nantly neutral material (a process often called ablationwhen laser pulses are used), and "desorption" usuallyrefers to ejection of a specific adsorbed species, often as areadily detectable ion.

Recent historyIn studying the ion bombardment of ice at 77 K, Walter L.Brown, Louis J. Lanzerotti and coworkers at AT&T BellLaboratories found using Rutherford backscattering in1978 that the electronic energy deposited by protons of afew MeV was surprisingly efficient at sputtering water ice.They used their results to show that sputtering of the icy

2 8 PHYSICS TODAY MARCH 1992

Plasma torus of Jupiter's moon lo is composed of ions (primarily S+ , S2 + and O + ) derived from volatilemolecular species condensed on lo's surface. The molecules are ejected from lo's surface by sublimation,volcanism and sputtering, ionized by the plasma and picked up by the rotating magnetic field attached toJupiter. The ions also diffuse outward and sputter the water ice of Europa. This figure is a composite of aphoto of Jupiter and a false-color image of a piece of the torus; the measured relative densities of S+ and S2 +

are shown in green and purple, respectively. Voyagers I and II discovered a similar torus of primarily H + andO + formed by electronic sputtering of H2O from the icy moons of Saturn. (From J. T. Trauger,Science 226, 337, 1984.) Figure 1

moons of the giant planets by energetic ions trapped in theplanets' magnetospheres was one source of the localplasmas found by the Voyager spacecraft during theirtours of the outer solar system.3 For example, sputteringof the predominantly water-ice surfaces of the moons ofSaturn is a source of ambient neutral atoms and mole-cules. These are subsequently ionized and picked up bythe planet's rotating magnetic field, forming the trapped,toroidal plasma in Saturn's inner magnetosphere. Thesimilar plasma torus around Jupiter's moon Io is shown infigure 1.

The high-resolution plasma instrument on the pro-posed Cassini mission to Saturn will take advantage of thisprocess to determine the surface composition of the icymoons. It will be able to detect ion fragments not only ofthe dominant water molecules but also of trace moleculesthat have been ejected from the surfaces of the moons bythe local plasma bombardment. Similarly, electronicsputtering by ultraviolet photons and cosmic-ray ions ofboth small and large molecules adsorbed on interstellargrains plays an important role in determining theobserved balance between solid and gas-phase species indense clouds in the interstellar medium.

In 1976 Peter K. Haff pointed out that fast ions shouldcause electronic sputtering from materials in which theyalso produce tracks, and Thomas A. Tombrello andcoworkers at Caltech verified this a few years later for anumber of insulators. In 1974, Ronald D. Macfarlane andcoworkers had discovered the remarkable fact that fissionfragments from a californium-252 source (that is, fastheavy ions) could desorb intact large organic moleculesthat are thermally labile—that is, prone to decompose onheating. Figure 2 depicts an example of this process.Desorption induced by fast heavy ions4 has rapidly becomea practical technique, known as plasma-desorption massspectrometry, for mass analysis of biological macromole-cules that have masses up to about 4 X 104 atomic mass

units (or u). Figure 3 illustrates an experiment using thistechnique, and the results shown in figure 4 exemplify theenormous advance in the ability to study biomolecules thetechnique provides. More recently, techniques usingpulses of laser radiation5 have extended the mass range toabout3xl05u.

Figure 5 shows the sputtering yield Y, that is, thenumber of molecules ejected per incident particle, for ionsbombarding low-temperature H2O. The two very distinctregimes exhibited in figure 5a directly reflect two types ofenergy loss per unit path length, dE/dx, for an ionpenetrating a solid. At low velocities direct collisionalmomentum transfer to the atoms in the solid dominates,whereas the large peak at high velocities is due to the lossof energy to electronic excitations in the solid. (For evenfaster ions with many MeV per atomic mass unit, energyloss by nuclear reactions becomes important.)

Figure 5b shows the total yields produced by fast ionsfor a number of species versus (dE/dx)e, the electronicenergy loss per unit path length, which has beennormalized to a dimensionless quantity by division by themolecule's sublimation energy U and multiplication bythe molecular size l = n~1/3, where n is the molecularnumber density. It is seen that large numbers of bothsmall and large molecules can be ejected by a single ion,with the yields exhibiting a number of dependencies on(dE/djc)e. Recently even "buckyballs" and other fuller-enes were shown6 to be formed in and ejected from the hotcores of fast heavy ion tracks in the polymer (C2H2F2)n.

We now turn to the molecular physics that occurs inresponse to individual excitations, after which we willdescribe how these act collectively at high excitationdensities.

Low excitation densityA fast proton or electron (u>2xlO8 cm/sec, the Bohrvelocity) produces excitations and ionizations that on the

PHYSICS TODAY MARCH 1992 2 9

Fast heavy ions can eject an intactbovine insulin molecule

(C254H377N65O75S6) from a solid sampleof bovine insulin and eject a

"buckyball" (C60) from (C2H2F2)n .Buckyballs and other molecules are

formed in the highly ionized region a,which has a radius of about 0.4 nm for

a 1-MeV/u incident ion. Large intactions are ejected from region b (outerradius, 1-2 nm); large intact neutrals,from region c (about 4 nm). Damage

from the impact extends out to region d(about 10 nm). Figure 2

Bovine insulin

average are widely spaced along its path, as indicatedschematically in figure 6. Consequently the excitations donot interact but act as if produced by separate photons. Afast ion or electron produces excitations and ionizations byclose Coulomb collisions with target electrons as well as bydipole excitations out to a distance from the ion track of or-der v/co, where fuo is the bandgap energy in a solid or theionization energy in a gas. The electron-hole pairs, orelectron-ion pairs in a gas, are accompanied by secondaryelectrons with mean energies typically of order 50 eV.Such electrons produced in a near-surface event can exitfrom a solid immediately, a process often used as a startsignal in the time-of-flight mass spectrometry shown infigure 3. These electrons can also produce ionization whilelosing their energy to internal molecular vibrations andlattice heating (that is, to phonons) over an extendedradial region.

The average energy JP deposited per fast ion in an ion-electron pair is on the order of twice the ionization orbandgap energy. It is estimated that about 40% of theelectronic energy deposited by a fast ion in a molecularinsulator is initially in electron-hole pairs, and theremainder is predominantly in vibrational excitations ofthe molecules, with much smaller fractions directly inelectronic excitations (excitons) and lattice heating.

Figure 6 also shows the fate of the holes and excitonsin an insulator. After diffusing, they are trapped, that is,they become localized, at defects, grain boundaries and thesurface. In a very good crystal that lacks defects, "self-trapping" can occur; that is, the lattice adjusts to localizean exciton or a hole.7 Following localization the excited

species can relax to the ground state individually or reactwith a neighbor to form an excited pair. Bonds are brokenand kinetic energy is released by relaxation processes such

(AB)+ + e" - A + B* + KE(AB)* - A + B + ftv + K

(AB)2+ - A+ + B+ + KE

(1)(2)(3)

where AB represents either an intrinsic molecule orinteracting neighbors, hv represents a photon and theasterisk indicates an excitation other than ionization.

When the kinetic energy associated with one of theserelaxation events is larger than the binding energy to thesolid, sputtering or desorption can occur at the surface ordefects can be created in the interior. In addition, in thelow-temperature ices that are held together by weak vander Waals forces, the localized excited species can have anet repulsive interaction with the material sufficientlystrong to "pop" the excited species off the surface.7 Rapidrelaxation of vibrational excitations can also lead toejection in molecular solids.

The correlation between sputtering and lumines-cence, which is well studied for alkali halides and low-temperature, rare gas solids, forms a basis for understand-ing the ejection processes at low excitation densities. Insolid argon, for instance, an important interaction is thatof a localized excited species with a neighbor to form adimer,78 as happens in many of the solids of interest.Luminescence is seen from atoms and vibrationallyexcited dimers ejected during the localization process as

3 0 PHYSICS TODAY MARCH 1992

well as from vibrationally relaxed dimers on the surfaceand in the bulk. Emission of a photon from a dimer is fol-lowed by dissociation (process 2 above), producing energet-ic atomic argon and so causing sputtering.8

In molecular condensed-gas solids the localizedexcited molecules also can either relax individually orreact with neighbors. Since processes 1-3 lead to frag-ments with kinetic energies on the order of a few eV, solidswith low sublimation energies can be sputtered efficiently.Because the kinetic energy is primarily in the lightestfragment, single-molecule relaxation events eject H, H2and less often H+ from surfaces of solid H2O, NH3 andCH4, whereas N and O are ejected from solid N2 and O2.When the dissociation event occurs in a layer just belowthe surface, N2 or O2 molecules (with sublimation energies£7—0.08 eV) can also be ejected from the surface, butdissociation products such as H, H2 or H+ are not asefficient at ejecting, for example, H2O from water ice(f/=:0.5eV).

Since (dE/dx)e is proportional to the number of near-surface excitations produced by fast light ions, theproportionality of the N2 and O2 total yields to (dE/dx)e atlow excitation densities implies that sputtering occurs on aper excitation basis. That is, the total yield can be writtenas y = A z / i e , where Ae is the mean spacing betweenexcitations and Az is the mean depth from which anexcitation can lead to ejection, weighted over possibleejection processes.9 For fast ions and electrons Ae isroughly W/(dE/dx)e, and for photons i e

1 is roughly themolecular number density n times the absorption crosssection. When diffusion to and trapping at the surface isimportant, as it is for all alkali halides and rare gas solids,Az is determined by the diffusion length. When diffusionand trapping is not important, and the kinetic energyrelease AE is much greater than the bulk cohesive energyU of the solid, then Az/l~cAE/U, where c~0.1-0.2, as

shown by our classical dynamics simulations. In thosesimulations we assume nonradiative relaxation proceedsby repulsive dissociation, depositing energies10 of a few eVin solid N2 or O2. Writing a = AE/Wfor the fraction of theelectronic energy deposited in the solid that is effective atproducing sputtering, we have

Y~ca(dE/dx)J/U (4)

From the simulations and the measured yields a isestimated to be on the order of 0.1-0.3 for these solids.

In contrast to N2 and O2, the yield for solid H2O is seenin figure 5b to be quadratic in (dE/dx)e down to lowexcitation densities, indicating that at least two excita-tions are involved in the dominant sputtering process.Similarly, the sharp decline in the S8 yield at low (dE/dx)esuggests that sputtering of S8 requires multiple excita-tions. The sputtering of more refractory insulating solids(those with high sublimation energies) requires evenhigher excitation densities or inner-shell excitations.Therefore the behavior of the yield at low excitationdensity is material specific, being determined by theoccurrence of relaxation processes that are energetic withrespect to the material's cohesive energy. Conversely,detecting the ejected species provides a probe of thesenonradiative relaxation processes.

High excitation densityAt high electronic excitation densities a volume ofmaterial is excited, so that the character of the ejectionchanges." In addition to any changes in the electronicrelaxation processes, ejection of molecules can now bedescribed by solid and liquid continuum mechanics of thematerial response. This has two principal aspects.12

First, the relaxation energy from closely spacedelectronic events acts to "heat" the excited region, andmolecules are ejected due to their net random motion, a

Electrostatic mirror

Sample

Start detector

In time-of-flight mass spectrometryof biomolecules, a fast heavy ion hitsthe surface of a sample held at about10 kV. The sample consists of amonolayer of adsorbed molecules. Theimpact ejects secondary ions thataccelerate toward a grounded grid.These ions drift in a field-free region,are reflected in an electrostatic mirrorand strike a detector, giving a stopsignal for the time-of-flightmeasurement. The start signal can beproduced by the detection of secondaryelectrons ejected when the incident ionpenetrates the foil. The mass iscomputed from the time offlight. Figure 3

PHYSICS TODAY MARCH 1992 31

process like sublimation. The dispersal of this energy isusually described diffusively via the heat equation.However, for large, thermally labile molecules hightemperatures are detrimental to intact ejection, producingfragmentation if significant equilibration occurs betweeninternal vibrations and lattice motion.

Second, because of the large gradients in the depositedenergy density and the presence of a vacuum interface,large pressure gradients and material stresses also occur.These can give an impulse to a volume of material, asdescribed by the momentum equations of fluid dynamics(the Navier-Stokes or Euler equations), and so cause theejection of material prior to thermal equilibration. Thisprocess appears to be important for the ejection of largemolecules with internal excitation energies low enoughthat a sufficient number do not fragment before detection.

As the excitation density increases, the yields for thecondensed gases in figure 5b all become roughly quadraticin (dE/dx)e. This dependence is a result of the cylindricalexcitation geometry for fast light ions. That is, theexcitation density along the track becomes high enoughthat the kinetic energy from the relaxation eventsproduces a transiently heated cylindrical region, leadingto sublimation of condensed species and their weaklyattached products—for example, H2O, H2 and O2 fromwater ice.

The quadratic dependence can be understood by using

LU

5LUDC

1(M + 2H)2 +

LJi

II

2000 4000 6000MASS (atomic mass units)

b

flEL

D<\

LIZ

ED

>N

OR

M,

1.2

1.0

0.8

0.6

0.4

0.2

0.0

i

/

- /

• <

- / \

Normal \

. f . l i

the thermal-spike model11: One writes Y~<t>AAt, where <t>is an averaged ejecta flux (the number of molecules ejectedper unit area per unit time), A is the area over which theenergy can be spread and still cause sublimation, and At isthe time for the heat to dissipate. Using M~A/K, where Kis the thermal diffusivity, one has Y~$>A2/K. For ejectionto occur, the energy density available in the form of latticemotion, a(dE/dx)e/A, must be on the order of thebinding energy per unit volume nU. This leads toA~a{AE/Ax)JnU, and the quadratic dependence is ob-tained:

(5)nU

-400 -200 0 200 400 600 800

DEFLECTING VOLTAGE (volts)

The dependence of the measured yield on the incident ionangle 0 has also been shown to agree very closely with thatcalculated analytically using a cylindrical thermal spikethat is truncated at the surface.

From the measured yields for the condensed gases infigure 5b and equation 5, the size of a is found to be similarto that derived in the linear regime for solid N2 and O2,indicating that to first order the nonradiative relaxationprocesses do not change. However, at very high excitationdensities a appears to increase and the luminescence yieldappears to decrease, although the relation between thesetwo effects is not yet understood quantitatively. Equation5, combined with the angular dependence, has been usedin figure 5 to interpolate the laboratory data for H2O. Wehave also used such yields and the measured energydistributions of the ejected molecules to calculate, forinstance, the rate of erosion of ice particles in Saturn'sdiffuse E ring and the supply of plasma from the icysatellites to Saturn's inner magnetosphere.3

Diomolecule ejectionFigure 5b also shows that at very high excitation densitiesthe yield for ejection of whole molecules of the amino acidleucine depends more steeply on (d£7djc)e than the yieldsfor the ices do. In addition to typically having lowvolatility, organic materials exhibit certain other impor-tant differences from the molecular condensed gases wehave discussed. Whereas in H2O the loss of H leads toformation of another volatile, O2, in an organic solid such

Mass spectrum and ejection angle forexperiments using the spectrometer shownschematically in figure 3. a: Mass spectrum ofpositive ions from a sample of the proteinbovine insulin (m — 5734 u) showingmolecular ion peaks of protonated [(M + H) + ]and doubly protonated [(M + 2H)2 + ]molecules. The mass determination in thiscase can be made with a precision of betterthan 1 u. b: The dependence of secondary-ion yields on the voltage on the deflectorplates provides information on the radialvelocity distributions of the secondary ions.The arrow indicates the voltage correspondingto ejection normal to the surface of thesubstrate, which consists of the peptide renin(m = 1801 u). The small fragment ion C2H3

+

(blue) is predominantly ejected along thenormal, unlike the intact renin molecule(green), which is ejected in a non-normaldirection. Figure 4

3 2 PHYSICS TODAY MARCH 1992

Energy dependence of total yields (molecules ejected perincident ion) for different cases of desorption. a: Yields of

H2O from ice at 10 K struck with protons (black) and heavierions (blue). At low energies direct momentum transfer toatoms dominates; higher energies (peaks at right) involve

electronic excitations, b: Yields of small molecules at lowtemperatures (about 10 K) increase linearly (blue) or

quadratically (green) with increasing electronic energy loss perunit path length. Yields of intact leucine molecules increase

approximately cubically (red). S8 shows a very sharp decline(dashed black line) at low energies. The curves in b,compiled from data from a number of experiments,correspond to energies to the right of the peak in a.

(Adapted from ref. 3.) Figure 5

loss leads to the formation of more C-C bonds. Thisgenerally produces a more refractory material (that is, onewith a larger cohesive energy U), so that even whenelectronic sputtering is initially efficient (as with low-temperature condensed CH4), it is found that after asignificant irradiation time the material will not sputterefficiently.13 Therefore, in organic materials molecularejection competes with damage to the molecules, andejection of intact large molecules appears to requireimpulsive ejection of a volume of material. Anotherdifference from the case of small molecules is that theenergy from individual electronic excitations in largeorganic molecules or biomolecules can be shared amongthe large number of internal vibrational modes, so beforeejection a larger fraction of the energy is in vibrationalexcitation of those molecules that are in the energizedregion. This leads to the rapid expansion of the excitedmolecules, which Peter Williams has compared to poppingpopcorn.

Rapid production of high excitation densities can beachieved using laser pulses over areas on the order ofsquare millimeters and using individual fast heavy ionsover areas on the order of a few square nanometers. Thetwo methods have very different excitation geometriesand time scales: An incident ion produces a thin cylinderwith a hot core, as discussed above, in less than 10 ~14 sec,whereas a typical laser used in desorption takes 10~9-10~8

sec to heat a planar region with its highest excitationdensity at the surface. These differences suggest differentstrategies for intact molecule ejection.

In laser desorption one tries to avoid destroying thebiomolecules of interest by imbedding them dilutely in avolatile matrix material that preferentially absorbs theincident photons. The rapidly sublimating and expandingmatrix material can then carry the imbedded bio-molecules into the gas phase without significant fragmen-tation. Direct rapid heating using a laser pulse was firstdemonstrated by the groups of Koichi Tanaka and FranzHillenkamp in 1988. Although the nature of the ioniza-tion process and the properties of a useful matrix are stillnot clear, laser desportion has increased the mass limit forperforming mass spectrometric studies to more than100 000 u for biomolecules such as proteins.

In the cylindrical geometry associated with a fastheavy ion, although the molecules in the hot track aredestroyed, molecules are ejected intact from outside of thisregion. This has the advantage that no matrix is requiredand the very rapid excitation and ejection can produce gas-phase biomolecules with low enough levels of internalexcitation that the molecules can remain intact untildetection. Since the area excited is much smaller thanthat for lasers, the upper mass limit with presenttechniques is about 40 000 u. We will focus below on

10 102 103 104 105

INCIDENT ION ENERGY/MASS (eV/u)

1010 102 103 10"

SCALED ELECTRONIC ENERGY LOSS n - 1 / 3 (dE/dx)e/U

ejection induced by fast heavy ions.Following the discovery by Macfarlane and coworkers

of the ejection of intact biomolecules by fission fragments,a number of nuclear physicists (Yvon LeBeyec, Kenneth G.Standing, Helmut Voit, Karl Wien and Sundqvist) devotedaccelerator time and their knowledge of fast timingtechniques to the systematic study of this curious andpotentially useful process. Studies were made of thedependence of the intact ion yield on the incident ionenergy, type and angle. Although the nature of the radialdependence of the excitation density has been understoodfor some time, the understanding of biomolecule ejectionproceeded slowly at first. The data were generallyinterpreted in terms of thermal-spike models, repulsivedesorption (based on analogy with the small-moleculedesorption processes of equations 1-3) or Coulomb explo-sion models in which Coulomb repulsion in the positivelycharged (electron-depleted) track dominates.1 These mod-els were relatively unsuccessful; progress was limitedprimarily because laboratory studies detected onlydesorbed ions, the same limitation that hindered the studyof condensed gases prior to Brown and Lanzerotti'smeasurements on water ice. However, for fixed initialtrack diameter (that is, fixed incident ion speed) we founda threshold region in which the ion yields varied rapidly

PHYSICS TODAY MARCH 1992 3 3

with excitation density and which depended on the size ofthe ejected ion.14 Further, we found that absorbingmolecules on nitrocellulose, an extremely volatile materi-al, produced sharper mass spectra. This technique is oftenreferred to as "the Uppsala magic carpet."

Pressure pulse modelThe total ejection yield (that is, ionized and neutralmolecules) of intact leucine shown in figure 5b wasobtained in 1987 by amino-acid analysis of the sputteredmaterial.15 Those measurements and studies16 of molecu-lar ion ejection from Langmuir-Blodgett films (see thearticle by Vijendra K. Agarwal in PHYSICS TODAY, June1988, page 40) that had layers of fatty acids of differentmass made it obvious that a volume of material wasejected for each ion impact. Therefore Iosef S. Bitenskyand Edward S. Parilis suggested that ejection of intact ionsoccurred in response to a radial and axial shock waveemanating from the Coulomb explosion in the plasmaformed in the track of the fast heavy ion. However, thevery important observation that the dependence of thetotal intact leucine yield on (dE/dx)e differed from that ofthe intact leucine ion yield remained unexplained.15

A critical advance in understanding came withmolecular dynamics calculations17 based on molecularexpansion in the track. (See figure 7.) Those calculationsconfirmed the cubic dependence on (dE/dr)e of the totalyield. Measurements by Werner Ens and collaborators18

that showed the direction of ejection of the intact ions wascorrelated with the incident ion direction but was differentfrom the direction of ejection of fragment ions (see figure4b) provided another critical advance. Guided by theseresults and the shock model, we developed a simpleanalytic description of the ejection process.12

In this model the net impulse—the "pressure pulse"—produced by the transiently pressurized material in theion track determines the size of the ejected volume. Thecontinuum mechanics of this process, which occurs overtimes from 10~13 to 10~n sec and over distances on the or-der of nanometers, is far from being understood. Nonethe-less we use these ideas below to describe the response of

the solid to the electronic energy deposited by fast heavyions. The model roughly explains the dependence of thetotal intact leucine yield on (&E/dx)e seen in figure 5b, thedifference between that dependence and that of the intactleucine ions, and the direction of ejection of the intact ions.

The effects produced can be ordered according to theradial distance from the track, as indicated in figure 2.The outer-shell electrons of atoms in the core of the trackof a fast heavy ion are generally fully ionized, forming ahigh-temperature plasma. The ionization density de-creases out to the so-called Bohr adiabatic radius r~v/a>,beyond which dipole excitations are improbable. Theregion within this radius is labeled a in figure 2. At largerradial distances the secondary electrons produce ioniza-tion and excitation decreasing approximately as r~2 out tosome "maximum" secondary-electron transport distance.For a fission fragment from a 252Cf source, the energydeposited per unit path length is on the order oflOkeV/nm, forming the hot track core. This highlyexcited track produces axial and radial expansion of thematerial. Here we are particularly interested in the effectof this expansion on the surface and near-surface mole-cules.

In region a in figure 2 the material bonds arecompletely fragmented. For a very short time, while theelectrons lose their energy to vibrational excitations, thelocal electron density is lower than the ion density, leavingthe track positive. In this region structural and chemicalrearrangements can occur as a result of the fragmentationand the Coulomb energy. A well-known phenomenonmentioned earlier is the formation of etchable tracks ininsulators. For the organic solids of interest here, protonsare ejected from the surface with ejection energies thatvary in response to the local surface-ionization density.19

Based on the ejected proton speeds the field in the trackcore is estimated to persist for about 10~13 sec, a measureof the time during which the Coulomb repulsion is active.

Kicking out buckyballsBecause region a in figure 2 is rapidly and extensivelyfragmented, newly formed species are ejected in addition

Typical processes involving electron-hole pair formation caused by the

impact of a fast light ion (orange arrow).a: An electron produced near the

surface escapes, and the hole (red)migrates to the surface, where it is

trapped. Recombination with anelectron forms an excited species (star)

that tends to relax toward the groundstate by ejecting material and

luminescing, b: A secondary electronproduces another electron-hole pair,

and the holes migrate, are trapped,recombine and relax, producing defectsor chemical alterations, c: An electronand hole are quenched at the substrate

(green) in a thin sample. Figure 6

3 4 PHYSICS TODAY MARCH 1992

Classical molecular dynamicssimulation gives the illustrated result atabout 5X 10~12 sec after the passage ofa fast heavy ion. Internal vibrationalexcitations cause rapid expansion of themolecules in the track (blue spheres).Prompt ejection is seen from both thetrack and the surrounding material.(Adapted from ref. 1 7.) Figure 7

to protons and small ionized fragments. A quite remark-able result is that a single fast heavy ion impact causes onaverage about one ionized fullerene Cm to be ejected from(C2H2F2)n, with an enhanced peak in the mass spectra atm = 60. This peak represents the very stable "soccer-ball"species. (See the article by Donald R. Huffman in PHYSICSTODAY, November 1991, page 22.) The radial velocitydistribution of ejected fullerenes is not normal to thesurface and, as figure 2 indicates, roughly anticorrelateswith the incident ion direction. This suggests that theparticles are formed in region a and are ejected approxi-mately back along the track, indicating the occurrence ofan axial pulse. What is remarkable is that C-C bonds areso energetically favored that a rapid local collapse of thematerial occurs during the net outward expansion. Thatis, the volume occupied by 60 carbons in the originalmaterial is about eight times larger than that of vibration-ally relaxed Cm. Since the molecule produced is not fullyrelaxed, measuring the level of vibrational excitation isimportant in understanding the formation and ejectionprocess.

In contrast to the protons, light ionized fragments andbuckyballs, intact biomolecular ions exhibit ejectionvelocities with a significant radial component. When theincident ion enters at an angle, the ejection velocity of abiomolecule is on the opposite side of the normal from thatfor C + , as figure 2 indicates. From figure 2 and themolecular dynamics simulation shown in figure 7, one cansee that such a component is obtained if the species isejected from a region outside of the track core. Theimpulse received by a volume of material from thetransient pressure pulse12 can be roughly described as atrack of nearly simultaneous and additive small impulsesAp. (See figure 8.) The total momentum transferred to avolume of material is

(6)

where each excitation is located at z' along the track, r' isthe vector from the excitation to the volume receiving theimpulse, and we recall that A~1 is the average number ofexcitations per unit path length. If we now assume theimpulses Ap(r') decrease with distance r1 from their sourceas r"2, then the integral in equation 6 is easily done,

giving

(7)

where/? is the radial distance of the volume from the track,z is the volume's position along the track and

PUsing equation 7 we find that a surface molecule in

the plane formed by the ion track and the surface normalreceives an impulse with an angle of 45° ± 0/2 to thedirection of incidence, where 0 is the incident ion angle.The deflection-plate experiment of figure 4b gave ameasured average angle with respect to the track of about50° for 0 = 0° and 90° for 0 = 45°, as compared withpredictions of 45° and 67.5°, respectively, from equation 7.Adding surface binding to the model would increase thecalculated angle, giving close agreement with experiment.Wien confirmed the experimental results obtained withthe deflection plates in experiments that used a position-sensitive stop detector to measure the full radial velocitydistribution of intact dimer ions of valine.20

Since the ejection angles were roughly the same inexperiments using a multilayer of insulin as they werewhen insulin molecules were only adsorbed on a spin-coated nitrocellulose surface, the above results alsoindicate that the ejected large intact ions come predomi-nantly from species adsorbed on the surface. (OrderedLangmuir-Blodgett films may be exceptions.) Noting thatclose to the track core the ionization and fragmentationprobabilities increase, we deduce that the large intact ionsare the prompt surface ejecta from region b in figure 2, justoutside the track core. The radius of region b scales as(dE/dx)l/2 for a given ejected ion and for a fixed incidention velocity. This scaling gives the nearly linear depend-ence on {dE/dx)e observed for the ion yield at large(d£7cLt)e. It is also consistent with observations offragmentation in the time-of-flight measurements, indi-cating that the molecular ions typically have significantinternal energy. Therefore the internal and kineticenergies of the ions are a probe of the fast processesoccurring close to the track.

Equation 7 can also be used to estimate the total(ionized and neutral) ejecta volume.12 The volume inwhich the net impulse received normal to the surface is

PHYSICS TODAY MARCH 1992 3 5

Ap(r')

Pressure-pulse model computes the totalimpulse given to a volume of material bysumming the small impulses initiated at eachlocation along the track of the incidentparticle. (See equations 6 and 7 in thetext.) Figure 8

greater than some critical value determined by thematerial binding is region c of figure 2. For 6 = 0° this re-gion is roughly hemispherical, with a radius proportionalto (dE/dj:)e, giving the cubic yield dependence shown infigure 4b for leucine and found in Eu2O3 and in moleculardynamics simulations.20 The neutral molecules are ex-pected to have lower average internal energies becausethey are ejected at larger average radial distances fromthe track core. Ionizing these neutrals may thereforeenhance the sensitivity of plasma-desorption mass spec-trometry.

At larger distances, labeled region d in figure 2,transported thermal energy and the tail of the secondary-electron distribution act to alter the organic material, to alarge extent by cross-linking of molecules.21 Observingthe decrease in the yield as the number of ions per unitarea incident on the surface increases provides an easymeasurement of the maximum radius of this damagedregion.

Closing remarksElectronically induced sputtering or desorption is aubiquitous astrophysical phenomenon, playing an impor-tant role in planetary magnetospheres, cometary plasmasand the interstellar medium.3-22 Quite remarkably, thisprocess also turns out to be important for the biologicalsciences as a new mass spectrometric tool and is potential-ly an important future tool in macromolecule surface-adsorption studies.45 For the molecular insulators wehave discussed, the field spans a broad range of the physicsof the conversion of electronic energy into molecularmotion. Whereas ejection of molecules from low-tempera-ture condensed gases at low excitation densities can occurin response to individual energy-conversion events, theejection of protons and buckyballs indicates that the trackof a fast heavy ion acts like a nanometer-scale hot plasmafor short time periods, and the ejection of intact largemolecules is indicative of a nanometer-scale "shock."Given the long awareness of radiation damage to biologi-cal materials, it is amusing that proteins, compactbiological polymers, can now be used to probe the dynamicprocesses in the highly excited track of a heavy ion.

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