MASS DISTRIBUTION OF PRODUCTS OF CLUSTER IMPACTS

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MASS DISTRIBUTION OF PRODUCTS OF CLUSTER IMPACTS

R. Beuhler, L. Friedman

To cite this version:

R. Beuhler, L. Friedman. MASS DISTRIBUTION OF PRODUCTS OF CLUSTER IMPACTS.

Journal de Physique Colloques, 1989, 50 (C2), pp.C2-127-C2-131. �10.1051/jphyscol:1989222�. �jpa-

00229420�

suppl6ment au 50, 1989

MASS DISTRIBUTION OF PRODUCTS OF CLUSTER IMPACTS

R.J. BEUHLER a n d L. FRIEDMAN

C h e m i s t r y D e p a r t m e n t , B r o o k h a v e n N a t i o n a l L a b o r a t o r y , U p t o n , L . I . , NY 1 1 9 7 3 , U.S.A.

Rdsumd

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L e s d i s t r i b u t i o n s d e m a s s e s d e s f r a g m e n t s m o l e c u l a i r e s e t a t o m i q u e : i o n i q u e s C m i s p a r t i r d e s u r f a c e s d e c a r b o n e p a r d e s i o n s a g r e g a t s m o n o - c h a r g e : e t c o n t e n a n t 8 0 m o l e c u l e s d ' e a u o n t 6 t e d e t e r m i n b e s . A v e c d e s c l u s t e r s d ' e n e r g i e c i n e t i q u e d e 2 4 0 k e V d e s r e n d e m e n t s s i g n i f i c a t i f s d e f r a g m e n t s m o l e c u l a i r e s c o n t e n a n t j u s q u ' h 2 1 a t o m e s d e c a r b o n e o n t k t 6 o b s e r v e s . L e s r e n d e m e n t s d ' i o n s f u r e n t u t i l i s e s p o u r e s t i m e r l e s r e n d e m e n t s r e l a t i f s d e s f r a g m e n t s n e u t r e s a v e c l ' h y p o t h k s e q u e l e s r e n d e m e n t s r e l a t i f s d e s p r o c e s s u s d ' 6 m i s : i o n d ' i o n s e t d e n e u t r e s e t a i e n t c o n d i t i o n n C s p a r de: f a c t e u r s c i n e t i q u e s q u i p o u v a i e n t @ t r e e s t i m e s d e f a s o n i n d e p e n d a n t e . L e s r e n d e m e n t s d e n e u t r e s d e d u i t s o n t s e r v i e n s u i t e e s t i m e r l a f r a c t i o n d e l ' e n e r g i e t o t a l e d u p r o j e c t i l e p o u r l ' b m i s s i o n . L e s r e s u l t a t s i n d i q u e n t q u ' u n e p a r t i e i m p o r t a n t e d e l ' e n e r g i e d i s p o n i b l e e s t i m p l i q u e e d a n s l e p r o c e s s u s d o e m i s s i o n s e c o n d a i r e p a r a g r e g a t .

A b s t r a c t - Mass d i s t r i b u t i o n s of i o n i c a t o m i c and m o l e c u l a r f r a g m e n t s s p u t t e r e d from c a r b o n s u r f a c e s by s i n g l y charged p o s i t i v e c l u s t e r i o n s c o n t a i n i n g 80 w a t e r molecules have been determined. With c l u s t e r k i n e t i c e n e r g y o f 240 keV s i g n i f i c a n t y i e l d s of m o l e c u l a r f r a g m e n t s c o n t a i n i n g u p t o 2 1 c a r b o n ' a t o m s were observed. I o n y i e l d s were u s e d t o e s t i m a t e r e l a t i v e y i e l d s of n e u t r a l f r a g m e n t s w i t h t h e assumption t h a t r e l a t i v e y i e l d s of t h e r e s p e c t i v e i o n i c and n e u t r a l s p u t t e r i n g p r o c e s s e s were determined by k i n e t i c f a c t o r s which c o u l d be e v a l u a t e d i n d e p e n d e n t l y . The d e r i v e d n e u t r a l y i e l d s were t h e n u s e d t o e s t i m a t e t h e f r a c t i o n of t o t a l p r o j e c t i l e energy u t i l i z e d i n e v a p o r a t i v e c o o l i n g , i . e . s p u t t e r i n g . The r e s u l t s i n d i c a t e a l a r g e f r a c t i o n of t h e energy a v a i l a b l e i s u s e d i n t h e c l u s t e r s p u t t e r i n g p r o c e s s .

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Energy t r a n s f e r p r o c e s s e s f o l l o w i n g h y p e r v e l o c i t y c l u s t e r i o n impact and p e n e t r a t i o n of s o l i d s u r f a c e s a r e e x p e c t e d t o d i f f e r from t h o s e i n v o l v e d i n t h e p e n e t r a t i o n of f a s t a t o m i c

p r o j e c t i l e s b e c a u s e of d i f f e r e n c e s i n t h e r e s p e c t i v e c o l l i s i o n s p i k e s g e n e r a t e d by d i f f e r e n t s i z e p r o j e c t i l e s . With much s m a l l e r s u r f a c e t o volume r a t i o s i n s p i k e s produced by c l u s t e r s , t h e r e l a t i v e i m p o r t a n c e of energy l o s s from t h e s p i k e by t h e r m a l c o n d u c t i o n i s reduced.

S p u t t e r i n g e f f i c i e n c i e s , b a s e d on t h e e n e r g y d e p o s i t e d by t h e p r o j e c t i l e , a r e h i g h e r . But t h e e x t e n t t o which energy i s d i s t r i b u t e d between s p u t t e r i n g and c o n d u c t i v e c o o l i n g can n o t be r e a d i l y determined from g r o s s s p u t t e r i n g y i e l d s . Mass and k i n e t i c energy d i s t r i b u t i o n s of s p u t t e r e d p r o d u c t s a r e r e q u i r e d a l o n g w i t h s p u t t e r i n g y i e l d s t o d e t e r m i n e t h e amount of e n e r g y d i s s i p a t e d i n t h e s p u t t e r i n g p r o c e s s .

We have s t u d i e d mass d i s t r i b u t i o n s of s p u t t e r e d i o n s produced by impact of w a t e r c l u s t e r i o n s on s o l i d c a r b o n t a r g e t s . The masses and e n e r g i e s of t h e p r o j e c t i l e s u s e d were s i m i l a r t o t h o s e u s e d i n e a r l i e r i n v e s t i g a t i o n s of c l u s t e r i m p a c t s o n t h i n carbon f i l m s / I / . I n t h i s work, t r a n s m i s s i o n e l e c t r o n microscopy was u s e d t o d e t e r m i n e morphology of h o l e s o r c r a t e r s made by c l u s t e r impact. E s t i m a t e s of h o l e o r c r a t e r s i z e and d e p t h were used t o o b t a i n c r u d e v a l u e s of s p u t t e r i n g y i e l d s . A model developed by Sigmund and C l a u s s e n / 2 / was u s e d t o c a l c u l a t e s p u t t e r i n g y i e l d s a s a f u n c t i o n of e n e r g y d e n s i t y i n c o l l i s i o n s p i k e s . The model c a l c u l a t i o n s were b a s e d on t h e a s s u m p t i o n of a s p u t t e r i n g p r o c e s s which e x c l u s i v e l y produced a t o m i c carbon. The v a l i d i t y of t h i s a s s u m p t i o n i s t e s t e d by d e t e r m i n a t i o n of mass

d i s t r i b u t i o n s of s p u t t e r e d p r o d u c t s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989222

JOURNAL DE PHYSIQUE

The major basis for the assumption of carbon atoms as the nearly exclusive product of cluster ion sputtering is found in consideration of the magnitude of the energy densities in cluster produced collision spikes and the assumption that particle evaporation takes place from very high energy density atomic assemblies. Under these circumstances, if sputtering generates large yields of molecular fragments, there will be a high probability of unimolecular decomposition of these fragments into atomic carbon in the time period of at least sec that elapses before these species can be identified and detected.

The problem of accurate determination of sputtered particle mass distributions requires either the efficient ionization of neutral products or the establishment of a linkage between directly sputtered ion yields and corresponding neutral products. Results from thin carbon films gave sputtering yields as high as 20,000 carbon atoms per impact of a 300 keV. 100 water molecule cluster ion. Ion yields are estimated at less than 1% of the neutral yield 131.

In this report, we estimate relative yields of neutral products by consideration of relative rates of evaporation of corresponding ionic and neutral carbon species. In the case of carbon, there is a fortuitous near identity in the heats of sublimation of C1, C2 and Cg neutral fragments with energies of sublimation of approximately 8 eV 1 4 1 . Similarly and possibly fortuitously, experimental values of the respective ionization potentials of these three carbon fragments fall within 0.1 eV 1 5 1 . If sputtering or evaporation is assumed to be governed by the kinetic equation,

where v is the frequency factor and E the activation energy then with the assumption of a constant value for the frequency factors1 in the respective rate processes the relative rates for the sputtering of the 1, 2, and 3 carbon neutral fragments will be the same. Similarly the ionic species will also be sputtered with the same relative rates. If sputtering takes place from systems with sufficiently high energy densities differences between activation energies for evaporation will be minimized and the respective rates for these processes will approach each other. It would then appear that a linkage between the relative abundances of ionic and neutral sputtering products could be established. In this way an estimate of the relative importance of conductive cooling in cluster sputtering might be established.

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Techniques for generation, mass analysis; acceleration, and detection of hypervelocity water cluster ions have been presented in previous publications / 6 , 7 / . Mass distributions of secondary product ions were determined using quadrupole mass analyzers which operated in the mass range of 2-1200 daltons. Earlier studies using time-of-flight mass analysis

demonstrated that most of the sputtered ion product mass distribution fell within this range (unpublished work). Secondary ions were collected from a target mounted at an angle of 45"

from the axis of the primary beam. The axis of the quadrupole mass analyzer was set at a 45' angle with respect to the target and 90' with respect to the axis of the primary beam. Ions rrere detected with an off-axis channel plate secondary electron multiplier.

The quadrupole mass analytical technique is relatively insensitive to components of ion velocity oriented along the axis of the quadrupole rods provided that these velocity components are not too large to permit a suitable ion residence time in the rod analyzer.

But velocity components at right angles to the axis of the secondary ion beam or not parallel to the beam axis reduce the probability of ion transit through the analyzer system.

Consequently an observational problem arises with ions sputtered with different collection efficiencies resulting from differences in initial kinetic energy distributions. The most reliable results are obtained with the lowest possible resolution of quadrupole mass analysis. The magnitude of errors arising from differences in collection efficiency can be estimated by determination of mass spectra with varying quadrupole resolution. The C1 ions were found to vary significantly with resolution. Much smaller variations were found with higher molecular weight species. C1 ion yields have been underestimated somewhat in our experiments because of collection efficiency problems but spectra presented in this paper were obtained under conditions which showed very minor changes of relative ion intensities with variations in resolution.

l ~ r e ~ u e n c ~ factors may indeed be inversely proportional to the square root of the masses of the gaseous product. The assumption of a constant value of ^V would then lead to

overestimation of relative yields of higher molecular weight products. Since these products requira less energy per carbon atom for sputtering, the assumption underestimates the fraction of energy dissipated in sputtering.

hydrogen which were attributed to surface contamination of the target by traces of diffusion pump oil. With primar beam intensities of 2.6.101° ions per second hitting an area covered by approximately l-lO1f carbon atoms, CI and C2 ions containing hydrogen in the secondary mass spectra were reduced with respect to pure C1 and C2 ions by more than a factor of ten.

Significant but smaller reductions were observed with higher molecular weight secondary carbon ions. If the assumption is made that the primary beam intensity at 2.6.10~~1~ is sufficient to clean the target surface and sputter ions before significant adsorption of oil can take place, a surface layer is removed in every second of bombardment and the sputtering yield is 4.10~ carbon atoms per projectile impact. Experimental data were taken for the most part with higher intensity primary beams with ion currents greater than 10lO/s.

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Our objective in the determination of mass distributions of secondary ions sputtered by cluster impacts from carbon surfaces is to obtain information on the energy transfer

processes involved in the cooling of collision spikes. Direct experimental determination of neutral product yields would be desirable but are not yet available. Such determinations involve observational problems associated with relative ionization and collection

efficiencies of the respective products. Alternatively a correlation must be found between ionic and neutral mass distributions with the latter clearly making up the bulk of sputtered product. With a correlation that establishes the mass distribution of the bulk of sputtered product and estimates of the heat of sublimation per carbon atom in the respective atomic and molecular carbon fragments, a calculation of a lower limit of the fraction of total

projectile energy utilized in sputtering is straightforward. Experimentally determined mass distributions of ionic products are presented in the Table. Neutral product yields estimated from these ionic yields are also presented. These estimated neutral distributions are then used along with available thermochemical data on heats of sublimation of neutral carbon fragments to estimate the fraction of the total projectile energy that is required for the sputtering processes. The assumptions that have been used in estimating the relations between neutral and ionic products and in estimating heats of sublimation of carbon fragments in those cases where experimental data are not available are presented in the following discussion. Our conclusion is that at least 20% of the projectile energy is used in sputtering. This is consistent with a shock wave model for sputtering /8/ which would leave a minimum amount of energy behind as heat in the target. More

detailed data on kinetic energy distributions of sputtered products is required to establish an energy balance that would illuminate this question.

Table 1. Normalized spectra of Ionized and Neutral Fragments From Impacts of 240 kV-80 Water Molecule Clusters on Carbon,

Number of Carbons Ion Mass Calculated Distributions % of Total Energy In Fragment Spectrum of Neutral Products to Form Products

JOURNAL

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The fact that ionic yields constitute a very small fraction of the total sputtering yield forces consideration of the probability of production of an ion as a function of the number of atoms in the ionic fragment. We assume a random distribution of charged species in the surface of the energetic atom assembly from which evaporation takes place. The larger the fragment that is evaporated the higher the probability that it will contain a charged atom.

If very large chunks of the assembly are ejected and undergo unimolecular decomposition in the gas phase, charge competition will determine the probability of the smaller or larger fragments becoming ion products. But there will still be a statistical probability favoring the retention of charge in the larger fragment. Consequently in the correlation of ionic and neutral yields, the secondary ion mass spectrum must first be normalized or corrected for the statistical probability of finding one of the rare positive ions in the system in the

particular product fragment. This normalization is carried out by dividing the relative intensity of the signal of ions containing n carbon atoms by n. This normalization drastically reduces estimates of correlated neutral yields of higher molecular weight fragments

.

Yields of neutral products may not exclusively be determined by simple statistical

considerations. Relative rates of evaporation of different size molecular fragments play a dominant role and these rates of evaporation are to a large extent dependent on energy densities in the parent energetic atom assemblies.

The three lowest molecular sputtered ionic products have been found to have very similar ionization potentials, 11.1 eV / 5 / and their neutral precursors have very similar heats of sublimation from the solid, approximately 8 eV 141. The relative rates of formation of these gaseous ions from solid carbon are very similar and the relative rates of sublimation of the corresponding three neutral carbon species are also very similar. It is reasonable to conclude that these relative ionic yields corrected for statistical factors reflect relative neutral yields in the sputtering process. This conclusion is based on the assumption that all sputtering products are generated directly from the energetic atom assembly in the solid surface. An alternative mechanism is the formation of sputtered products from the

unimolecular decomposition of excited larger fragments ejected from the solid. With this mechanism the relative probability of formation of a particular ion will depend on the energy density in the parent system and relative values of threshold energies for the competitive decomposition processes. Frequency factors in the competing rate processes must also be considered but with processes involving the elimination of one, two or three carbon atom fragments from a larger molecular assembly a reasonable first approximation is the assumption of very similar frequency factors for all three competitive decompositions. The arguments presented above for the similarities in rates with species having similar threshold energy requirements still apply. These arguments establish a good correlation between ionic and neutral fragments for roughly 60% of the neutral fragments observed or about 20% of the total amount of sputtered carbon.

The problem of the higher molecular weight fragments, particularly their linkage to ion yields of the same mass, remains. Ionization potentials of larger carbon fragments are expected to be somewhat lower than for atomic carbon or the C2 or C3 species, with values eventually approaching the work function of solid carbon, approximately 4.6 volts 191. A lower ionization potential would lead to a higher relative rate of ionic evaporation and thus reflect a somewhat lower relative yield for the corresponding neutral fragment of the same mass. The use of the assumption of equal ionization potentials for higher molecular weight ionic species leads to the calculation of a lower limit of the fraction of total energy dissipated in sputtering.

With inf0rmat.i.m on the mass distribution of a major portion of sputtered products it is possible to estimate the relative importance of thermal conduction and particle evaporation as cooling processes. Sputtering yields of roughly 20,000 atoms per cluster impact are estimated from the size of craters excavated from thin films by clusters containing 100 water molecules with kinetic energy of roughly 250 keV. A similar yield is indicated from data on the primary ion currents required to maintain surfaces free of pump oil at background pressures of the order of 10-7 torr. If the sublimation energy of C1, C2 and C3 neutral fragments is taken at approximately 8 eV, then about 20% of the total energy deposited is accounted for in the evaporation of these three neutral species. About 50% of the total projectile energy is used in the total sputtering process if one assumes an energy of sublimation of 2.6 eV per carbon atom for larger carbon fragments. We have assumed that the energy of sublimation per carbon atom for larger fragments is the same as that in the C3 fragments. We may have slightly overestimated the sublimation energy and in so doing

overestimated the energy required for the formation of these fragments. The magnitude of the error introduced in estimating heats of sublimation of larger carbon fragments is not large.

the stabilization energy fed back in the formation of multiply bonded unsaturated neutral carbon molecular fragments.

This estimate of energy used in sputtering does not include consideration of kinetic energy in the sputtered products. A more accurate estimate of the relative importance of conductive versus evaporative cooling requires data on product kinetic energies. Perhaps a more

important factor than the neglect of kinetic energy of sputtered product in the estimate of energy utilized in sputtering is the effect of the assumption that ionization potentials of molecular carbon fragments are constant and do not decrease with increasing fragment size for carbon fragments containing 4 or more carbon atoms. Realistic estimates of ionization potentials of the larger molecular fragments would lead to reduced values of estimated corresponding neutral fragment yields and indicate a larger fraction of carbon transported as atomic carbon in the sputtering process. This in turn would increase the estimate of the fraction of total energy used in evaporative cooling. It is clear that the magnitude of energy used in sputtering is crudely estimated. More precise values of sputtering yields are needed as well as information on kinetic energy distributions of sputtered products. The results so far clearly indicate that cluster impact sputtering deposits a larger fraction of projectile energy into evaporative rather than conductive cooling in sharp contrast to sputtering by fast atomic particles.

The conclusion that a major fraction of the projectile kinetic energy can be accounted for by evaporative cooling processes lends credence to sputtering models in which a jet of

supercritical gas is ejected from the collision spike /lo/ or a shock wave is assumed to carry out sputtered product. In both cases the time scale of the sputtering process is for the most part limited to the transit time of particles moving with the velocity of the shock or the velocity of atoms in the supercritical gas. These times are of the order of a few picoseconds and it is this very short time period that limits the extent of conductive cooling and energy equilibration in the solid target. To this extent the collision spike produced in solids by hypervelocity clusters may be considered a very high energy density assembly of atoms which is inertially confined.

This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.

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