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The effect of dose rate on imaging aromatic organic crystals
Ultramicvoscopy 23 (1987) 321-328 North-Holland, Amsterdam
321
THE EFFECT OF DOSE RATE ON IMAGING AROMATIC ORGANIC CRYSTALS J.R. FRYER Chemisto, Department, UnioersiO,of Glasgow, Glasgow GI2 8QQ, Scotland, UK Presented at Workshop January 1987; received at editorial office 20 July 1987
Previous studies on electron pulse radiolysis have indicated that the concentration of radiolytic products can be radiation-dose dependent. Experiments have been performed using aromatic monolayer films to determine if this dose-rate effect was significant in the electron microscope. Langmuir-Blodgett films of acetyi pyrene and 9-butyi-10-anthr3'i propionic acid were examinea under conventional diffraction and it was found that the first-order reflections faded after a dose of 3-4 e/~, 2. However, using a high-intensity electron beam for exposure times between 10 and 100 ms. lattice images were obtained after sustained electron doses of 90-100 e / A ~.
1. Introduction Previous studies on radiation damage in the electron microscope had examined the phenomena associated with the specimen and ability to image the specimen. This present work is concerned with the effect of exposure time on the ability to obtain lattice images from monolayer films of aromatic hydrocarbons. In our previous work we had first examined the statistical basis for resolving lattices and points in a decaying specimen in terms of the Rose equation [1,2]. Our work explained why much finer lattices could be resolved than the resolution limit deri~,ed from the extinction dose [3]. The form and nature of the specimen was then examined in some detail [4-6] showing that the extinction dose, measured from the fading of the diffraction pattern, had a linear dependence upon the thickness of the specimen. The specimens examined in detail were mainly aromatic hydrocarbons or phthaiocyanines, but subsequently Ohno [7] also showed a linear increase in extinction dose for aliphatic carboxylic acid multiple monolayers. These results largely explained the variations between measurements that had characterised quantitative radiation damage hitherto. In consequence, the specimen preparation conditions had to be rigorous. and Dorset prepared monolayer paraffin speci-
mens that could be compared in various laboratories [8]. The extensive quantitative work of Dorset [9-12] has shown that crystal bending and buckling was a feature of these weak aliphatic materials when exposed to the electron beam. In consequence, high resolution imaging of the hkO orientation of paraffins gave optical diffraction patterns that were usually incomplete and of lower than the theoretically expected intensity [13]. To reduce this difference, Henderson, Glaeser and coworkers [14,15] minimised the effects of of bending and buckling in the specimen by using very small illumination under low-dose conditions. The work on phthalocyanines [4-6] had also examined the effect of overlayers on the specimen and the change in radiation susceptibility caused by the presence of different substituent atoms on the molecule. This led to an explanation of radiation damage in terms of severance of these peripheral atoms leaving the excited molecule to ,either recombine with a n o m+'c r atom or" ut.,,.-,t,,.:tntc. i ttc effect of an overlying film was to maintain the concentration of released atoms within the cr\stal -increasing the chance of recombinatio~ but ultimately decay of the molecules in the presence of radiation would result. The importance of the nature of the molecular bonding was shov,n b\ Howie et al. [16], who found that at ver\ to~ voltages, < 1 keV. aromatic molecules did not
0304-3991/87/$03.50 "~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Di~is:~on)
J.R. Fryer / Effect of dose rate on imaging aromatic organic crystals
322
suffer damage but aliphatic molecules did. Stabilisation of an aromatic organic specimen could also be achieved b ) a d d i n g a halogen (C1 or Br)-containing compound to the specimen so that halogen peripheral atoms were released by th~ radiation and helped stabilise the specimen by combination with free radicals [17].
2. Basis of the present work The events of movement, loss of material, degradation of excited species and recombination are all aspects of the chemical reactions taking place subsequent to the initial effect of the electron beam. The manifestation of damage in the microscope is loss of clystalline structure in the specimen. It is therefore necessary to differentiate those reactions which excite molecules and atoms without altering their lattice positions and those which cause loss of structure. The initial events of electronic excitation - the major process at 100 key - take place in 10-~s s. This radiolysis produces free radicals and ions as major products. For alkane organic molecules the G value (events per 100 eV absorbed) for radical production is 4--6 compared to 0.1-0.8 for free ion production [18]- i,e, free radicals are the major product. Considering just these free radicals, there is the following reaction sequence [19] for a molecule A: A-~L~2 R. + Products. k~ is the rate constant for this reaction and depends on the dose rate. There is the subsequent recombination reaction" 2 R . - - ~ R 2. At ~tcauy ...... -'- state for a radical concentration it,,. " ]: d[R.
]/dt
= 2l,, -
2k2[R-12 = 0.
Therefore,
[R.]=tk,/k:) ''2, indicating a square-root dependence on dose rate. Pulse radiolysis studies [19] have shown that the
steady-state lifetime of the free radicals may be several milliseconds under dose rates similar to the electron microscope although the electron microscope gives a much higher total dose. The extent of the reaction is also limited by the diffusion coefficient of the radicals in the radiation spur, but this reaction does occur, as is shown by the lattice contraction around sites of radiation damage in phthalocyanines [4]. In general this reaction and recombination will lead to only minor structural deformation, unless the product R 2 degrades very rapidly. A major reaction pathway will be the reaction of the free radical with the surrounding matrix or with a small radical (formed as one of the products of the radiolysis of the molecule A) diffusing rapidly through the lattice. The extent of these reactions will be dependent upon the particular molecules concerned, the reactivity and geometry of the lattice site of the radical, diffusion coefficients, the concentration of small radicals and the reaction temperature. Again, these reactions may not severely affect the structure. Subsequent to these reactions, there is the degradation of unstable species formed in these processes and degradation caused by the radiolysis of the first products, reducing them to smaller molecules which are not caged by the lattice and can diffuse away. These latter reactions are responsible for the loss of structure. It is probable that it is these events which also caused lattice movement, as the voids created with the radicals in the initial radiolysis - another source of movem e n t - are present 10 -s s after the initial event [19], and lattice relaxation does not appear to be this fast. The conclusions from this array of ot,servations are: (1) The observed radiation damage in the microscope is the end result c,f many chemical reactions. (2) The rate and sequence of evems is dependent upon the individual molecules concerned. (3) Diffusion is an important parameter at all stages in the sequence, but the stages at which it can oe significantly altered by reduction in temperature or by carbon encapsulation are unknown and probably vary from one compound to another. (4) During the initial radiolysis, the various con-
J. R, Fryer / Effect of dose rate on imaging aromatic urgamc crystals
centrations of products are dose-rate dependent. In the case of competitive reactions, the nature of the subsequent reactions is dependent upon these concentrations. Although the ultimate result of the radiolysis will be structural degradation, it is possible that within a few miliseconds the extent of this degradation is dose-rate dependent for some compounds, It is this possibility which is being examined in this work.
323
and beam-resistant hydrocarbon and was prepared by vacuum sublimation onto KCI(100) maintained at 300 o C. It was used to illustrate the resolution obtainable in an organic specimen without the effects of severe radiation damage. Molecular structures of these materials are shown in fig. 1. The microscope used was a JEM 1200EX with a LaB 6 filament. The images were recorded on Kodak Industrex X-ray film or through an image intensifier. This latter system employed a Y A G single crystal phosphor (Agar Scientific} with a Mullard Badger intensifier. The image was displayed on a monitor and also held in frame store that permitted integration and stretch facilities for brightness and contrast (Microconsultants Crystal system). The image could then be passed to a computer (Olivetti M24) with the image processing system SEMPER (Synoptics Ltd.), so that the power spectrum and subsequent filtration could be performed. The fast beam switching system was optically coupled to the gun shift circuit, and exposure times ranging from 6,6 ms to 60 s could be obtained, When used with the image pick-up system for short exposure times, the frame store was set for integration over 4 frames - approximately 160 ms - and the exposure performed within that time interval. The persistence of the YAG crystal was sufficient to avoid raster effects. The Faraday cage was an insulated cup built into a standard form of a JEOL side-entry specimen holder. The cup was screened so that only electrons entering the cup were measured. The microscope displayed the beam current incident
3. E x p e r i m e n t
The specimens used were acetyl pyrene and 9-butyl-10-anthryl propionic acid in the form of Langmuir-Blodgett monolayer films [20,21]. Multilayer films of known thickness were made by repeated dipping of the specimen support film. P a r a f f i n - C 3 6 H 7 4 - was prepared in the hkO orientation by crystallisation on a carbon support film of a dilute (2%) solution in hexane and removal of excess liquid with a filter paper. This method gave monolayer crystals. Calcium pigment red is a typical azo pigment with the unfortunate qualities that crystals are rarely in excess of 50nm in extent, they cannot be prepared epitaxially from the vapour or melt because of chemical degradation and preparation from a drop or spray usually results in an agglomerated mass. It presents extreme difficulties for electron crystallography but can be regarded as typical of many organic and organometallic materials. Hexabenzocorone (MP 825 ° C) is a very stable
,......
C4H9
?0 i
.0
c00Ccl
-.
--7,(
pert2
cH2 COCH3
a
COOH
b
C
d
Fig. 1. Molecular structures ot': (a) acetyi pyrene: (b) C a anthracene (i.e O-butvl-10-anthrvl propionic ~:cid): tel pigment red 5-. ,,'.' hcxabenzocor. ,,-,,,..
J.R. Fryer / Effect of dose rate on imaging aromatic organic crystals
324
on the final screen, but it was not known whether any backscatter correction had been applied. At 120 keV the current shown on the display was 66% of the current measured by the Faraday cage at the specimen level. This is in reasonable agreement with the backscattering values measured by Nicholson [22]. This value was for a beam spread to fill the final screen. If the beam was concentrated into a small spot the value fell to 64%, and 60% was measured in the diffraction mode. The displayed current was used for the experimental work and appropriate corrections applied.
4. Results
Experiments with the Langmuir-Blodgett films initially concerned theil" radiation susceptibility. Fig. 2 shows the dependence of film thickness e/Ao2
10
;f 0
I
2
3
I,.
5
6
Number of layers Fig. 2. The dependence of extinction dose (in e/~Az) of first order reflections with specimen thickness in terms of molecular layers for acct.! py:ene.
Fig. 3. Image of C4 anthracene crystal embedded in a Langmuir-Blodgett monolayer film.
expressed in terms of molecular l a y e r s - against extinction dose of the first-order reflections as measured visually with a selected area aperture subtending 0.2 /~m2 on the specimen. The extinction dose for a monolayer was 5.3 e/A2; the linear dependence is shown in the graph. Correspondingly the extinction dose for the anthracene derivative was 3 e / A 2. Several efforts by different personnel were made to obtain lattice images of the anthracene derivative and the acetyl pyrene using conventional low-dose techniques. They were not successful, but using the rapid exposure technique wi~h a total dose of 95 e / A 2 in a duration of 22 ms, the image shown in fig. 3 was obtained from the anthracene derivative. It is probable that the sharply defined islands may be second layers, but the lattice structure of the surrounding monolayer can be seen. The lattice spacing is 0.5 nm and the random domain structure in the monolayer is evident in fig. 4. A similar exposure of the acetyl pyrene showed a general imaging of the major lattice structure. Hexabenzocoronene at a dose of 93 e / A 2 gave clear lattice images as shown in fig. 5. The normal extinction dose for this material is approximately 40 e/A'-. However, this result primarily showed that there was no large loss of micro-
J.R. Fryer / Effect of dose rate on imaging aromatic organic co'stals
Fig. 4. Crystalline domains in a film of C4 anthracene.
Fig. 5. Lattice image of hexabenzocoronene in a vacuum-sublimed epitaxiai film.
325
326
J.R. Fryer / Effect of dose rate on imaging aromatic organic crystals
scope resolution occurring through the large beam convergence angle necessary for the high beam intensity. The aromatic azo salt also g~ ve lattice images using short exposures and the image intensifier system. In this case the short exposure was made whilst integrating the collected image over several frames. Lattice structure could not be seen on the monitor, but second-order reflections were present in the computed transform. EpitaxiaUy grown paraffins did not show clear lattice images using the short-exposure method although there appeared some improvement in the diffraction patterns taken under these conditions. A possible source of error in these dose figures was in the time taken for the beam to traverse onto and from the specimen. However, this error was not thought to be significant because the exposure of the film was comparable with the density developed in films exposed to the same nominal dose under conventional conditions.
initial radiolysis as expressed by the rate k~ previously but the formation of a radical which can combine with activated molecule at the lattice site and delay its further radiolysis or degradation. This phenomenon may be confined to aromatic compounds, but as yet there is insufficient data to establish the limits of its generality.
Acknowledgements I wish to thank Miss C. McConnell for the use of some of her results on Langmuir-Blodgett films, Mr. J. Kirwen and Mr. J.S. Bhumbra for the design and construction of the beam-switching apparatus, and Professor R.M. Glaeser and Dr. D.L. Dorset with whom discussion modified my interpretation of some radiation damage phenomena.
References 5. Discussion and conclusions The results from the acetyl pyrene multilayers support those previously obtained by Ohno and ourselves. The significant point about these results is that there is some concentration dependence as a limiting parameter. If the molecular decay rate was short compared to other variables then the extinction dose was independent of thickness. The concentration dependence may arise from diffusion rates (i.e. diffusion in the film as distinct from diffusing away from it) or from the number of small free radicals available to recombine with larger radicals in lattice positions. There does not appear to be any simple relationship between thickness as expressed in terms of the number of molecules - as in the case of the LB film multilayers - and those occurring in crystalline materials such as the phthalocyanines, The ability of the aromatic compounds studied to sustain higher doses when given in a shorter time indicates that the rate-determining step in the decay process is not zero order with respect to dose. In the time scale involved it may not be an
[11 A. Rose, Advan. Electron. Electron Phys. 1 (1948) 131. [21 R.M. Glaeser, in: Physical Aspects of Electron Microscopy and Microbeam Analysis, Eds. B.M. Siegel and D.R. Beaman (Wiley, New York, 1975) p. 205. [31 J.R. Fryer and D.J. Smith. Proc. Roy. Soc. (London) A381 (1982) 225. [41 J.R. Fryer and F.M. Holland, Ultramicrescopy 11 (1983) 67. [51 J.R. Fryer and F.M. Holland, Proc. Roy. Soc. (London) A393 (1984) 352. [61 J.R. Fryer, Ultramicroscopy 14 (1984) 227. [71 T. Ohno, Ultramieroscopy 15 (1984) 319. [81 International Experimental Study Group, J. Microscopy 141 (1986) 385. 191 D,L. Dorset, J. Appl, Phys. 47 (1976) 780. IlO] D.L. Dorset, Z. Naturforsch. 33a (1978) 964. Ill1 D.L. Dorset, Ultramicroscopy 19 (1986) 311. [121 D.L. Dorset, Ultramicroscopy 21 (1987) 263. [131 R. Henderson and R.M. Glaeser, Ultramicroscopy 16 (1985) 139. 1141 K.H. Downing and R.M. Glaeser, Ultramicroscopy 20 (1986) 269. tZSl P. Bullough and R, Hcndcr.'~cn, Ultramicroscopy 21 (1947) 223. [16] A. Howie, F.J. Rocca and U. Valdr& Phil. Mag. B52 (1986) 71. [17] J.R. Fryer, C. McNee and F.M. Holland, Ultramicroscopy 14 (1984) 357, [181 G. Foldiak, Radiation Phys. Chem. 16 (1980) 451.
J.R. F~er / Effect of dose rate on imaging aromatic organic crystals
[19] G.R. Freeman, in: The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis, Eds. J.H. Baxendale and F. Busi (Reidel, Dordrecht. 1982) p. 19. [20] I.R. Paterson, G.J. Russell, D.B. Neal, M.C. Petty, G.C. Roberts, T. Ginnai and R.A. Hann, Phil. Mag. B54 (1986) 71.
327
[21] J.R. Fryer, R.A. Hann and B. Eyres, Nature 313 (1985) 382. [22] W.A.P. Nicholson, J. Microscopy 121 (1981) 141.
321
THE EFFECT OF DOSE RATE ON IMAGING AROMATIC ORGANIC CRYSTALS J.R. FRYER Chemisto, Department, UnioersiO,of Glasgow, Glasgow GI2 8QQ, Scotland, UK Presented at Workshop January 1987; received at editorial office 20 July 1987
Previous studies on electron pulse radiolysis have indicated that the concentration of radiolytic products can be radiation-dose dependent. Experiments have been performed using aromatic monolayer films to determine if this dose-rate effect was significant in the electron microscope. Langmuir-Blodgett films of acetyi pyrene and 9-butyi-10-anthr3'i propionic acid were examinea under conventional diffraction and it was found that the first-order reflections faded after a dose of 3-4 e/~, 2. However, using a high-intensity electron beam for exposure times between 10 and 100 ms. lattice images were obtained after sustained electron doses of 90-100 e / A ~.
1. Introduction Previous studies on radiation damage in the electron microscope had examined the phenomena associated with the specimen and ability to image the specimen. This present work is concerned with the effect of exposure time on the ability to obtain lattice images from monolayer films of aromatic hydrocarbons. In our previous work we had first examined the statistical basis for resolving lattices and points in a decaying specimen in terms of the Rose equation [1,2]. Our work explained why much finer lattices could be resolved than the resolution limit deri~,ed from the extinction dose [3]. The form and nature of the specimen was then examined in some detail [4-6] showing that the extinction dose, measured from the fading of the diffraction pattern, had a linear dependence upon the thickness of the specimen. The specimens examined in detail were mainly aromatic hydrocarbons or phthaiocyanines, but subsequently Ohno [7] also showed a linear increase in extinction dose for aliphatic carboxylic acid multiple monolayers. These results largely explained the variations between measurements that had characterised quantitative radiation damage hitherto. In consequence, the specimen preparation conditions had to be rigorous. and Dorset prepared monolayer paraffin speci-
mens that could be compared in various laboratories [8]. The extensive quantitative work of Dorset [9-12] has shown that crystal bending and buckling was a feature of these weak aliphatic materials when exposed to the electron beam. In consequence, high resolution imaging of the hkO orientation of paraffins gave optical diffraction patterns that were usually incomplete and of lower than the theoretically expected intensity [13]. To reduce this difference, Henderson, Glaeser and coworkers [14,15] minimised the effects of of bending and buckling in the specimen by using very small illumination under low-dose conditions. The work on phthalocyanines [4-6] had also examined the effect of overlayers on the specimen and the change in radiation susceptibility caused by the presence of different substituent atoms on the molecule. This led to an explanation of radiation damage in terms of severance of these peripheral atoms leaving the excited molecule to ,either recombine with a n o m+'c r atom or" ut.,,.-,t,,.:tntc. i ttc effect of an overlying film was to maintain the concentration of released atoms within the cr\stal -increasing the chance of recombinatio~ but ultimately decay of the molecules in the presence of radiation would result. The importance of the nature of the molecular bonding was shov,n b\ Howie et al. [16], who found that at ver\ to~ voltages, < 1 keV. aromatic molecules did not
0304-3991/87/$03.50 "~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Di~is:~on)
J.R. Fryer / Effect of dose rate on imaging aromatic organic crystals
322
suffer damage but aliphatic molecules did. Stabilisation of an aromatic organic specimen could also be achieved b ) a d d i n g a halogen (C1 or Br)-containing compound to the specimen so that halogen peripheral atoms were released by th~ radiation and helped stabilise the specimen by combination with free radicals [17].
2. Basis of the present work The events of movement, loss of material, degradation of excited species and recombination are all aspects of the chemical reactions taking place subsequent to the initial effect of the electron beam. The manifestation of damage in the microscope is loss of clystalline structure in the specimen. It is therefore necessary to differentiate those reactions which excite molecules and atoms without altering their lattice positions and those which cause loss of structure. The initial events of electronic excitation - the major process at 100 key - take place in 10-~s s. This radiolysis produces free radicals and ions as major products. For alkane organic molecules the G value (events per 100 eV absorbed) for radical production is 4--6 compared to 0.1-0.8 for free ion production [18]- i,e, free radicals are the major product. Considering just these free radicals, there is the following reaction sequence [19] for a molecule A: A-~L~2 R. + Products. k~ is the rate constant for this reaction and depends on the dose rate. There is the subsequent recombination reaction" 2 R . - - ~ R 2. At ~tcauy ...... -'- state for a radical concentration it,,. " ]: d[R.
]/dt
= 2l,, -
2k2[R-12 = 0.
Therefore,
[R.]=tk,/k:) ''2, indicating a square-root dependence on dose rate. Pulse radiolysis studies [19] have shown that the
steady-state lifetime of the free radicals may be several milliseconds under dose rates similar to the electron microscope although the electron microscope gives a much higher total dose. The extent of the reaction is also limited by the diffusion coefficient of the radicals in the radiation spur, but this reaction does occur, as is shown by the lattice contraction around sites of radiation damage in phthalocyanines [4]. In general this reaction and recombination will lead to only minor structural deformation, unless the product R 2 degrades very rapidly. A major reaction pathway will be the reaction of the free radical with the surrounding matrix or with a small radical (formed as one of the products of the radiolysis of the molecule A) diffusing rapidly through the lattice. The extent of these reactions will be dependent upon the particular molecules concerned, the reactivity and geometry of the lattice site of the radical, diffusion coefficients, the concentration of small radicals and the reaction temperature. Again, these reactions may not severely affect the structure. Subsequent to these reactions, there is the degradation of unstable species formed in these processes and degradation caused by the radiolysis of the first products, reducing them to smaller molecules which are not caged by the lattice and can diffuse away. These latter reactions are responsible for the loss of structure. It is probable that it is these events which also caused lattice movement, as the voids created with the radicals in the initial radiolysis - another source of movem e n t - are present 10 -s s after the initial event [19], and lattice relaxation does not appear to be this fast. The conclusions from this array of ot,servations are: (1) The observed radiation damage in the microscope is the end result c,f many chemical reactions. (2) The rate and sequence of evems is dependent upon the individual molecules concerned. (3) Diffusion is an important parameter at all stages in the sequence, but the stages at which it can oe significantly altered by reduction in temperature or by carbon encapsulation are unknown and probably vary from one compound to another. (4) During the initial radiolysis, the various con-
J. R, Fryer / Effect of dose rate on imaging aromatic urgamc crystals
centrations of products are dose-rate dependent. In the case of competitive reactions, the nature of the subsequent reactions is dependent upon these concentrations. Although the ultimate result of the radiolysis will be structural degradation, it is possible that within a few miliseconds the extent of this degradation is dose-rate dependent for some compounds, It is this possibility which is being examined in this work.
323
and beam-resistant hydrocarbon and was prepared by vacuum sublimation onto KCI(100) maintained at 300 o C. It was used to illustrate the resolution obtainable in an organic specimen without the effects of severe radiation damage. Molecular structures of these materials are shown in fig. 1. The microscope used was a JEM 1200EX with a LaB 6 filament. The images were recorded on Kodak Industrex X-ray film or through an image intensifier. This latter system employed a Y A G single crystal phosphor (Agar Scientific} with a Mullard Badger intensifier. The image was displayed on a monitor and also held in frame store that permitted integration and stretch facilities for brightness and contrast (Microconsultants Crystal system). The image could then be passed to a computer (Olivetti M24) with the image processing system SEMPER (Synoptics Ltd.), so that the power spectrum and subsequent filtration could be performed. The fast beam switching system was optically coupled to the gun shift circuit, and exposure times ranging from 6,6 ms to 60 s could be obtained, When used with the image pick-up system for short exposure times, the frame store was set for integration over 4 frames - approximately 160 ms - and the exposure performed within that time interval. The persistence of the YAG crystal was sufficient to avoid raster effects. The Faraday cage was an insulated cup built into a standard form of a JEOL side-entry specimen holder. The cup was screened so that only electrons entering the cup were measured. The microscope displayed the beam current incident
3. E x p e r i m e n t
The specimens used were acetyl pyrene and 9-butyl-10-anthryl propionic acid in the form of Langmuir-Blodgett monolayer films [20,21]. Multilayer films of known thickness were made by repeated dipping of the specimen support film. P a r a f f i n - C 3 6 H 7 4 - was prepared in the hkO orientation by crystallisation on a carbon support film of a dilute (2%) solution in hexane and removal of excess liquid with a filter paper. This method gave monolayer crystals. Calcium pigment red is a typical azo pigment with the unfortunate qualities that crystals are rarely in excess of 50nm in extent, they cannot be prepared epitaxially from the vapour or melt because of chemical degradation and preparation from a drop or spray usually results in an agglomerated mass. It presents extreme difficulties for electron crystallography but can be regarded as typical of many organic and organometallic materials. Hexabenzocorone (MP 825 ° C) is a very stable
,......
C4H9
?0 i
.0
c00Ccl
-.
--7,(
pert2
cH2 COCH3
a
COOH
b
C
d
Fig. 1. Molecular structures ot': (a) acetyi pyrene: (b) C a anthracene (i.e O-butvl-10-anthrvl propionic ~:cid): tel pigment red 5-. ,,'.' hcxabenzocor. ,,-,,,..
J.R. Fryer / Effect of dose rate on imaging aromatic organic crystals
324
on the final screen, but it was not known whether any backscatter correction had been applied. At 120 keV the current shown on the display was 66% of the current measured by the Faraday cage at the specimen level. This is in reasonable agreement with the backscattering values measured by Nicholson [22]. This value was for a beam spread to fill the final screen. If the beam was concentrated into a small spot the value fell to 64%, and 60% was measured in the diffraction mode. The displayed current was used for the experimental work and appropriate corrections applied.
4. Results
Experiments with the Langmuir-Blodgett films initially concerned theil" radiation susceptibility. Fig. 2 shows the dependence of film thickness e/Ao2
10
;f 0
I
2
3
I,.
5
6
Number of layers Fig. 2. The dependence of extinction dose (in e/~Az) of first order reflections with specimen thickness in terms of molecular layers for acct.! py:ene.
Fig. 3. Image of C4 anthracene crystal embedded in a Langmuir-Blodgett monolayer film.
expressed in terms of molecular l a y e r s - against extinction dose of the first-order reflections as measured visually with a selected area aperture subtending 0.2 /~m2 on the specimen. The extinction dose for a monolayer was 5.3 e/A2; the linear dependence is shown in the graph. Correspondingly the extinction dose for the anthracene derivative was 3 e / A 2. Several efforts by different personnel were made to obtain lattice images of the anthracene derivative and the acetyl pyrene using conventional low-dose techniques. They were not successful, but using the rapid exposure technique wi~h a total dose of 95 e / A 2 in a duration of 22 ms, the image shown in fig. 3 was obtained from the anthracene derivative. It is probable that the sharply defined islands may be second layers, but the lattice structure of the surrounding monolayer can be seen. The lattice spacing is 0.5 nm and the random domain structure in the monolayer is evident in fig. 4. A similar exposure of the acetyl pyrene showed a general imaging of the major lattice structure. Hexabenzocoronene at a dose of 93 e / A 2 gave clear lattice images as shown in fig. 5. The normal extinction dose for this material is approximately 40 e/A'-. However, this result primarily showed that there was no large loss of micro-
J.R. Fryer / Effect of dose rate on imaging aromatic organic co'stals
Fig. 4. Crystalline domains in a film of C4 anthracene.
Fig. 5. Lattice image of hexabenzocoronene in a vacuum-sublimed epitaxiai film.
325
326
J.R. Fryer / Effect of dose rate on imaging aromatic organic crystals
scope resolution occurring through the large beam convergence angle necessary for the high beam intensity. The aromatic azo salt also g~ ve lattice images using short exposures and the image intensifier system. In this case the short exposure was made whilst integrating the collected image over several frames. Lattice structure could not be seen on the monitor, but second-order reflections were present in the computed transform. EpitaxiaUy grown paraffins did not show clear lattice images using the short-exposure method although there appeared some improvement in the diffraction patterns taken under these conditions. A possible source of error in these dose figures was in the time taken for the beam to traverse onto and from the specimen. However, this error was not thought to be significant because the exposure of the film was comparable with the density developed in films exposed to the same nominal dose under conventional conditions.
initial radiolysis as expressed by the rate k~ previously but the formation of a radical which can combine with activated molecule at the lattice site and delay its further radiolysis or degradation. This phenomenon may be confined to aromatic compounds, but as yet there is insufficient data to establish the limits of its generality.
Acknowledgements I wish to thank Miss C. McConnell for the use of some of her results on Langmuir-Blodgett films, Mr. J. Kirwen and Mr. J.S. Bhumbra for the design and construction of the beam-switching apparatus, and Professor R.M. Glaeser and Dr. D.L. Dorset with whom discussion modified my interpretation of some radiation damage phenomena.
References 5. Discussion and conclusions The results from the acetyl pyrene multilayers support those previously obtained by Ohno and ourselves. The significant point about these results is that there is some concentration dependence as a limiting parameter. If the molecular decay rate was short compared to other variables then the extinction dose was independent of thickness. The concentration dependence may arise from diffusion rates (i.e. diffusion in the film as distinct from diffusing away from it) or from the number of small free radicals available to recombine with larger radicals in lattice positions. There does not appear to be any simple relationship between thickness as expressed in terms of the number of molecules - as in the case of the LB film multilayers - and those occurring in crystalline materials such as the phthalocyanines, The ability of the aromatic compounds studied to sustain higher doses when given in a shorter time indicates that the rate-determining step in the decay process is not zero order with respect to dose. In the time scale involved it may not be an
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J.R. F~er / Effect of dose rate on imaging aromatic organic crystals
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