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The pre-knock-on concept
Ultramicroscopy North-Holland
THE PRE-KNOCK-ON Martyn
CR.
Department Received
41
10 (1982) 41-44 Publishing Company
SYMONS
of Chemistry,
5 March
CONCEPT
The University,
Leicesier,
UK
1982
Mass-loss is commonly observed in electron-microscopy. Possible causes are discussed, and a novel mechanism, described as “the pre-knock-on process”, is postulated as a possibly significant process for systems at low temperature containing small molecules. A brief outline of the familiar knock-on process for molecular displacements in solids induced by high energy radiation is presented. The idea is that when there is insufficient energy available in elastic collisions to achieve such displacements, a localised, directed, shock-wave might be generated. It is suggested that if such a wave were to reach a surface, one or more surface molecules might be displaced. Some of these might pass into the gas phase, giving mass-loss, whilst some might build up surface defects without actually being lost.
1. Introduction The most important event in radiation chemistry is molecular ionization. Ways in which such events can modify the results of electron-microscopic studies have been discussed elsewhere [ 11. It seems most probable that, in addition to the chemical changes induced in a given molecule, the resulting relaxation or displacements of surrounding molecules is of prime importance in causing the blurring or fading effect associated especially with the spots of higher-order reflexes that is characteristic of radiation damage. At high doses these effects will lead to a more or less amorphous structure ending up with overall fading associated with a diffuse distribution of electrons scattered out of the primary beam.
2. Mass-loss However, another problem facing electron-microscopists involves overall fading, which is attributed to mass-loss. In this case diffuse scattering is of minor importance because electrons not scattered into discrete orders are left behind in the primary beam. As the amount of material falls, the 0304-3991/82/0000-0000/$02.75
0 1982 North-Holland
total available information is reduced, and the degree of certainty is reduced. This phenomenon is also frequently explained in terms of radiation damage in the chemical sense. Thus one could argue that radiation damage leads to fragmentation into small molecular or atomic units which can diffuse to the surface and pass into the gasphase. However, in my view [ 11, this is unlikely to be the major source of mass loss for most materials, especially in low temperature studies. Thus, at ca. 4 K, even hydrogen atoms are effectively trapped and can be studied by ESR spectroscopy. The most probable species to be lost in this way is molecular hydrogen, but loss of hydrogen by most molecules represents only a minor change, and the remaining radicals or unsaturated molecules will occupy almost the same sized site as that of the original molecule. This damaged unit is unlikely to move any more readily than the parent molecules, so the only effect should be a small local change in positions. At ca. 4 K, and even at 77 K in many cases, larger molecules, such as CH,, NH,, H,O, N,, O,, etc., are generally not formed, and even if they are, they will not normally migrate through the lattice. Indiscriminate fragmentation does not, in fact, occur in radiation chemistry, and at low temperatures, primary ionization events usually
42
M.C.R.
Symons / Pre-knock-on
dominate, fragmentation as such being relatively unimportant. I conclude that it is necessary to turn elsewhere for an explanation of mass-loss.
3. Local heating The most obvious explanation is a heating effect [2,3]. Local heating could, in principle, lead to evaporation, but this would be markedly temperature dependent, and should be eliminated at 4 K. In fact, this is not what is observed, the temperature effect being relatively minor in most systems. Furthermore, if a heating effect were involved, mass-loss should increase rapidly with dose-rate, since the higher the rate, the higher the effective temperature. In fact, there is no dose-rate dependence for mass-loss, so that heating as such can be dismissed. The purpose of the present article is to suggest a novel mechanism for mass-loss, called the “preknock-on” effect. I present only a qualitative outline of the proposed phenomenon.
4. Knock-on phenomena
It is well established that high energy particles can induce atomic or molecular displacements, an effect usually labelled “knock-on”. This is usually a very minor effect for most systems, with chemical damage dominating. However, in materials such as semiconductors or metals, where chemical damage is unimportant, knock-on events can dominate. These events are mostly associated with heavy particle beams, and with neutron beams. However, although fast electrons dissipate most of their energy in materials by causing excitation and ionization, they can also experience elastic collisions, and cause knock-on events [4]. Elastic collisions occur when fast electrons interact electrically with nuclei. If energy is lost by the electrons thereby it appears exclusively as kinetic energy of the recoil atom. The degree of energy transfer depends on many factors, but the maximum transferable energy (E,) is given by:
E =2(E+2Muc2)E m MC2
'
concept
where E = the kinetic energy of the electron, M,, = the rest mass of the electron, c = the velocity of light, and M = the mass of the nucleus. The threshold for a knock-on event is in the 6680 eV range, being normally in the region of 25 eV. These collisions deflect the electrons (Rutherford scattering). The displaced particle in turn replaces one of its near neighbours, the choice depending on the initial direction of the knock-on. Computer simulations suggest that a directed knock-on shock-wave then moves at high velocity, the velocity decreasing as the amplitude decreases until, ultimately, an interstitial or shared interstitial is formed. For further details of knock-on effects, see, for example, refs. [5-lo].
5. The pre-knock-on
effect
In most electron-microscopes, the energy per electron is insufficient to induce knock-on events, and therefore this phenomenon is ignored. Nevertheless, elastic collisions, deflecting the electrons. do occur, and the basis of the pre-knock-on concept is that this causes an atom or molecule to move quite violently within its normal site even though it does not acquire the threshold energy needed for actual displacement. This, I suggest, sets up a unique, local, shock-wave within the substrate. If this has not been dissipated by the time it reaches a surface, there could still be sufficient energy to cause molecular loss from the surface. A qualitative picture of the process envisaged is given in fig. 1. The energy required for such vaporization is always far less than that required for a knock-on event, so, provided the
.
.
.
.
.
e----_-.:111:.*_ e-
/
.
.
.
.
.
.
.
,
.
.
.
.
.
.
.
.
.
.
.
.
.
Fig. I. Possible mechanism for a pre-knock-on event. showing how a directed and localised shock-wave might result in surface loss with no net displacement within the lattice.
M. C. R. Symons
shock-wave can reach a suitable surface, molecular loss is quite a reasonable event. Such events could be responsible for at least some of the mass-loss. The phenomenon should be almost independent of temperature and dose-rate. These events may be more important in electronmicroscope work than in normal radiation studies because of the extremely thin samples involved. Events involving large angles of deflection for the electrons could result in waves directed along or close to the shortest axis so that dissipation of the wave is less likely to occur than in larger samples normally used in ESR or other spectroscopic studies.
/ Pre-knock-on
concept
43
efficient, but for the soft-balls, damping occurs and the disturbance is rapidly lost. Indeed, for higher molecular weight materials, mass-loss by pre-knock-on evaporation must be drastically reduced. In accord with this is the fact that compounds such as sucrose lose mass readily at room temperature but hardly at all at low temperature [ 11,121. In this case, chemical damage is almost certainly responsible. At room temperature, formation of small molecules such as H, and H,O will play an important role, and these will be readily lost. At low temperatures, secondary processes are suppressed, and mass-loss is reduced accordingly. Thus the pre-knock-on mechanism is probably only important for atomic and smallmolecule systems such as rare-gas crystals or ice.
6. Use of embedding techniques Surface coverings may well help to prevent such losses. What is required is that the molecule at the surface should collide with one of the molecules of the embedding material and recoil back into its original site. How the embedding material then gets rid of the excess energy is not really significant. Clearly, this requirement will not always be fulfilled. The escaping molecule may move too far away before being stopped, or it may be deflected from its path at some angle such that it does not return to the original site. Both these events could have an effect comparable with real mass-loss. Thus, careful design of the surface covering material may prove to be helpful.
7. Effect of molecular size The pre-knock-on effect should be especially important for small molecules or atoms having well-defined lattices. In amorphous materials such shock-waves would be rapidly dissipated and give rise to local heating. For solids comprising large molecules, especially bio-molecules, it is difficult to see how such a specific wave could be propagated, and molecular loss at a surface will clearly be far less important. One can liken the process for small and large molecules to events involving hard and soft spheres in contact. For the hardsphere system, propagation of a collision is highly
8. Bubbling This phenomenon is discussed in depth by Dubochet [13]. It probably has many causes, but the possibility that a pre-knock-on effect can contribute should be considered. I confine attention to the bubbling effect observed for pure water. As discussed in elsewhere [14], I think it is unlikely that radiation products are responsible for massloss at low temperatures, and hence it is also unlikely that they are responsible for bubbling, though they may well contribute. I suggest that the following mechanism may be of significance. Imagine that a pre-knock-on event displaces one or more water molecules from a crystal surface. Depending on the direction of the phonon shockwave, these may be displaced along the surface or, alternatively, may be re-scavenged by the surface at a site one or two molecular distances from the original site. These molecules will be held by only one or possibly two hydrogen bonds, and hence will act as good points for attachment of further water molecules. Thus a three-dimensional structure possibly resembling an incipient clathrate cage could accumulate. This might be “fixed” by a molecule such as H, from the ice, or possibly N, or 0, from the gas-phase. Ultimately, a clathrate cage might be completed, having, say, ca. 30 water molecules in its structure.
44
M. C.R. Symons / Pm-knock-on
9. Conclusions I conclude that, for small molecule systems, the pre-knock-on mechanism could contribute to bubble formation.
Acknowledgements I thank Professor E. Zeitler for encouraging me to write this article, and Dr. R. Doll for helpful comments.
References [l] M.C.R. Symons, Ultramicroscopy 10 (1982) 15. [2] G. Siegel, Der Einfluss tiefer Temperaturen auf die Strahlenschadigung van Bakterien und organischen Kristallen
concept
durch 100 keV-Electronen, Thesis, Technische Universitat Berlin (1970)D83. A.V. Crewe and M.S. Isaacson, Ultrami]31 K. Ramamurti, croscopy 1 (1975) 156. ]41 J.W. Corbett. Electron Radiation Damage in Semiconductors (Academic Press. New York, 1966). [A G.J. Dienes and G.H. Vineyard, Radiation Effects in Solids (Interscience, New York, 1957) pp. 6-28. [61 R.S. Nelson, The Observation of Atomic Collisions in Crystalline Solids (North-Holland, Amsterdam, 1968). [71 R.H. Silsbee, J. Appl. Phys. 28 (1957) 1246. PI O.S. Oen and M.T. Robinson, Appl. Phys. Letters 2 (1963) 83. I91 J.A. Brinkman, J. Appl. Phys. 25 (1954) 961. [lOI C. Erginsoy, G.H. Vineyard and A. Englert, Phys. Rev. 133 (1964) A595. Res. 52 (1975) 276. [Ill J. Dubochet, J. Ultrastruct. [I21 R. Freeman, K.R. Leonard and J. Dubochet. in: Proc. 7th European Congr. on Electron Microscopy. The Hague, 1980, p. 640. J. Lepault, R. Freeman, A. Berriman and [I31 J. Dubochet, J.-C. Homo, J. Microscopy, in press. 10 (1982) 97. [I41 M.C.R. Symons, Ultramicroscopy
THE PRE-KNOCK-ON Martyn
CR.
Department Received
41
10 (1982) 41-44 Publishing Company
SYMONS
of Chemistry,
5 March
CONCEPT
The University,
Leicesier,
UK
1982
Mass-loss is commonly observed in electron-microscopy. Possible causes are discussed, and a novel mechanism, described as “the pre-knock-on process”, is postulated as a possibly significant process for systems at low temperature containing small molecules. A brief outline of the familiar knock-on process for molecular displacements in solids induced by high energy radiation is presented. The idea is that when there is insufficient energy available in elastic collisions to achieve such displacements, a localised, directed, shock-wave might be generated. It is suggested that if such a wave were to reach a surface, one or more surface molecules might be displaced. Some of these might pass into the gas phase, giving mass-loss, whilst some might build up surface defects without actually being lost.
1. Introduction The most important event in radiation chemistry is molecular ionization. Ways in which such events can modify the results of electron-microscopic studies have been discussed elsewhere [ 11. It seems most probable that, in addition to the chemical changes induced in a given molecule, the resulting relaxation or displacements of surrounding molecules is of prime importance in causing the blurring or fading effect associated especially with the spots of higher-order reflexes that is characteristic of radiation damage. At high doses these effects will lead to a more or less amorphous structure ending up with overall fading associated with a diffuse distribution of electrons scattered out of the primary beam.
2. Mass-loss However, another problem facing electron-microscopists involves overall fading, which is attributed to mass-loss. In this case diffuse scattering is of minor importance because electrons not scattered into discrete orders are left behind in the primary beam. As the amount of material falls, the 0304-3991/82/0000-0000/$02.75
0 1982 North-Holland
total available information is reduced, and the degree of certainty is reduced. This phenomenon is also frequently explained in terms of radiation damage in the chemical sense. Thus one could argue that radiation damage leads to fragmentation into small molecular or atomic units which can diffuse to the surface and pass into the gasphase. However, in my view [ 11, this is unlikely to be the major source of mass loss for most materials, especially in low temperature studies. Thus, at ca. 4 K, even hydrogen atoms are effectively trapped and can be studied by ESR spectroscopy. The most probable species to be lost in this way is molecular hydrogen, but loss of hydrogen by most molecules represents only a minor change, and the remaining radicals or unsaturated molecules will occupy almost the same sized site as that of the original molecule. This damaged unit is unlikely to move any more readily than the parent molecules, so the only effect should be a small local change in positions. At ca. 4 K, and even at 77 K in many cases, larger molecules, such as CH,, NH,, H,O, N,, O,, etc., are generally not formed, and even if they are, they will not normally migrate through the lattice. Indiscriminate fragmentation does not, in fact, occur in radiation chemistry, and at low temperatures, primary ionization events usually
42
M.C.R.
Symons / Pre-knock-on
dominate, fragmentation as such being relatively unimportant. I conclude that it is necessary to turn elsewhere for an explanation of mass-loss.
3. Local heating The most obvious explanation is a heating effect [2,3]. Local heating could, in principle, lead to evaporation, but this would be markedly temperature dependent, and should be eliminated at 4 K. In fact, this is not what is observed, the temperature effect being relatively minor in most systems. Furthermore, if a heating effect were involved, mass-loss should increase rapidly with dose-rate, since the higher the rate, the higher the effective temperature. In fact, there is no dose-rate dependence for mass-loss, so that heating as such can be dismissed. The purpose of the present article is to suggest a novel mechanism for mass-loss, called the “preknock-on” effect. I present only a qualitative outline of the proposed phenomenon.
4. Knock-on phenomena
It is well established that high energy particles can induce atomic or molecular displacements, an effect usually labelled “knock-on”. This is usually a very minor effect for most systems, with chemical damage dominating. However, in materials such as semiconductors or metals, where chemical damage is unimportant, knock-on events can dominate. These events are mostly associated with heavy particle beams, and with neutron beams. However, although fast electrons dissipate most of their energy in materials by causing excitation and ionization, they can also experience elastic collisions, and cause knock-on events [4]. Elastic collisions occur when fast electrons interact electrically with nuclei. If energy is lost by the electrons thereby it appears exclusively as kinetic energy of the recoil atom. The degree of energy transfer depends on many factors, but the maximum transferable energy (E,) is given by:
E =2(E+2Muc2)E m MC2
'
concept
where E = the kinetic energy of the electron, M,, = the rest mass of the electron, c = the velocity of light, and M = the mass of the nucleus. The threshold for a knock-on event is in the 6680 eV range, being normally in the region of 25 eV. These collisions deflect the electrons (Rutherford scattering). The displaced particle in turn replaces one of its near neighbours, the choice depending on the initial direction of the knock-on. Computer simulations suggest that a directed knock-on shock-wave then moves at high velocity, the velocity decreasing as the amplitude decreases until, ultimately, an interstitial or shared interstitial is formed. For further details of knock-on effects, see, for example, refs. [5-lo].
5. The pre-knock-on
effect
In most electron-microscopes, the energy per electron is insufficient to induce knock-on events, and therefore this phenomenon is ignored. Nevertheless, elastic collisions, deflecting the electrons. do occur, and the basis of the pre-knock-on concept is that this causes an atom or molecule to move quite violently within its normal site even though it does not acquire the threshold energy needed for actual displacement. This, I suggest, sets up a unique, local, shock-wave within the substrate. If this has not been dissipated by the time it reaches a surface, there could still be sufficient energy to cause molecular loss from the surface. A qualitative picture of the process envisaged is given in fig. 1. The energy required for such vaporization is always far less than that required for a knock-on event, so, provided the
.
.
.
.
.
e----_-.:111:.*_ e-
/
.
.
.
.
.
.
.
,
.
.
.
.
.
.
.
.
.
.
.
.
.
Fig. I. Possible mechanism for a pre-knock-on event. showing how a directed and localised shock-wave might result in surface loss with no net displacement within the lattice.
M. C. R. Symons
shock-wave can reach a suitable surface, molecular loss is quite a reasonable event. Such events could be responsible for at least some of the mass-loss. The phenomenon should be almost independent of temperature and dose-rate. These events may be more important in electronmicroscope work than in normal radiation studies because of the extremely thin samples involved. Events involving large angles of deflection for the electrons could result in waves directed along or close to the shortest axis so that dissipation of the wave is less likely to occur than in larger samples normally used in ESR or other spectroscopic studies.
/ Pre-knock-on
concept
43
efficient, but for the soft-balls, damping occurs and the disturbance is rapidly lost. Indeed, for higher molecular weight materials, mass-loss by pre-knock-on evaporation must be drastically reduced. In accord with this is the fact that compounds such as sucrose lose mass readily at room temperature but hardly at all at low temperature [ 11,121. In this case, chemical damage is almost certainly responsible. At room temperature, formation of small molecules such as H, and H,O will play an important role, and these will be readily lost. At low temperatures, secondary processes are suppressed, and mass-loss is reduced accordingly. Thus the pre-knock-on mechanism is probably only important for atomic and smallmolecule systems such as rare-gas crystals or ice.
6. Use of embedding techniques Surface coverings may well help to prevent such losses. What is required is that the molecule at the surface should collide with one of the molecules of the embedding material and recoil back into its original site. How the embedding material then gets rid of the excess energy is not really significant. Clearly, this requirement will not always be fulfilled. The escaping molecule may move too far away before being stopped, or it may be deflected from its path at some angle such that it does not return to the original site. Both these events could have an effect comparable with real mass-loss. Thus, careful design of the surface covering material may prove to be helpful.
7. Effect of molecular size The pre-knock-on effect should be especially important for small molecules or atoms having well-defined lattices. In amorphous materials such shock-waves would be rapidly dissipated and give rise to local heating. For solids comprising large molecules, especially bio-molecules, it is difficult to see how such a specific wave could be propagated, and molecular loss at a surface will clearly be far less important. One can liken the process for small and large molecules to events involving hard and soft spheres in contact. For the hardsphere system, propagation of a collision is highly
8. Bubbling This phenomenon is discussed in depth by Dubochet [13]. It probably has many causes, but the possibility that a pre-knock-on effect can contribute should be considered. I confine attention to the bubbling effect observed for pure water. As discussed in elsewhere [14], I think it is unlikely that radiation products are responsible for massloss at low temperatures, and hence it is also unlikely that they are responsible for bubbling, though they may well contribute. I suggest that the following mechanism may be of significance. Imagine that a pre-knock-on event displaces one or more water molecules from a crystal surface. Depending on the direction of the phonon shockwave, these may be displaced along the surface or, alternatively, may be re-scavenged by the surface at a site one or two molecular distances from the original site. These molecules will be held by only one or possibly two hydrogen bonds, and hence will act as good points for attachment of further water molecules. Thus a three-dimensional structure possibly resembling an incipient clathrate cage could accumulate. This might be “fixed” by a molecule such as H, from the ice, or possibly N, or 0, from the gas-phase. Ultimately, a clathrate cage might be completed, having, say, ca. 30 water molecules in its structure.
44
M. C.R. Symons / Pm-knock-on
9. Conclusions I conclude that, for small molecule systems, the pre-knock-on mechanism could contribute to bubble formation.
Acknowledgements I thank Professor E. Zeitler for encouraging me to write this article, and Dr. R. Doll for helpful comments.
References [l] M.C.R. Symons, Ultramicroscopy 10 (1982) 15. [2] G. Siegel, Der Einfluss tiefer Temperaturen auf die Strahlenschadigung van Bakterien und organischen Kristallen
concept
durch 100 keV-Electronen, Thesis, Technische Universitat Berlin (1970)D83. A.V. Crewe and M.S. Isaacson, Ultrami]31 K. Ramamurti, croscopy 1 (1975) 156. ]41 J.W. Corbett. Electron Radiation Damage in Semiconductors (Academic Press. New York, 1966). [A G.J. Dienes and G.H. Vineyard, Radiation Effects in Solids (Interscience, New York, 1957) pp. 6-28. [61 R.S. Nelson, The Observation of Atomic Collisions in Crystalline Solids (North-Holland, Amsterdam, 1968). [71 R.H. Silsbee, J. Appl. Phys. 28 (1957) 1246. PI O.S. Oen and M.T. Robinson, Appl. Phys. Letters 2 (1963) 83. I91 J.A. Brinkman, J. Appl. Phys. 25 (1954) 961. [lOI C. Erginsoy, G.H. Vineyard and A. Englert, Phys. Rev. 133 (1964) A595. Res. 52 (1975) 276. [Ill J. Dubochet, J. Ultrastruct. [I21 R. Freeman, K.R. Leonard and J. Dubochet. in: Proc. 7th European Congr. on Electron Microscopy. The Hague, 1980, p. 640. J. Lepault, R. Freeman, A. Berriman and [I31 J. Dubochet, J.-C. Homo, J. Microscopy, in press. 10 (1982) 97. [I41 M.C.R. Symons, Ultramicroscopy