- Email: [email protected]
Radial velocity distributions of secondary ions ejected from PTFE by MeV atomic ions
__ __ f!!B
c2L!A - -
Nuclear Instruments
and Methodsin Physics ResearchB 107(1996) 308-3 12
k!lONil B
Beam Interactions with Materials 6 Atoms
ELSEVIER
Radial velocity distributions of secondary ions ejected from PTFE by MeV atomic ions R.M. Papal&o *, P. Demirev, J. Eriksson, P. Hkmsson,
B.U.R. Sundqvist
Division of Ion Physics, Dept. of Radiation Sciences, Uppsala University, Box 535, S 751-21, Uppsala. Sweden
Abstract
In this paper, systematic investigations of the initial radial velocity distributions of C,Fi secondary ions sputtered by 72.3 MeV 12’1+I3 ions from polytetrafluoroethylene (PTFE) foils are presented. The initial radial velocity distributions are obtained by monitoring the secondary ion yield as a function of the voltages applied to two sets of deflection plates installed parallel to the ion optical axis of a time-of-flight mass spectrometer. The mean radial velocity, ( ux), of C,FG ions (1 < R < 6 and 0 < m < 13) depends on the number of fluorine atoms, showing a periodic behaviour when plotted as a function of the ion mass. Ions with low fluorine content have ( ux) directed towards the primary ion’s line-of-incidence (positive mean velocities). Ions with a large number of fluorine atoms are ejected in an off-normal direction away from the incoming MeV ion trajectory (negative mean velocities). The ion emission can not be explained only by evaporative type processes. Mechanisms involving correlated momentum transfer e.g. pressure pulse or shock wave induced ejection, should also be taken into account. The data obtained for fluorocarbon ions from a purely fluorinated compound generalise a similar effect observed previously for C,HL ions, sputtered from different hydrogen containing polymer targets [R.M. PapalCo et al., Nucl. Instr. and Meth. B 91 (1994) 6671.
1. Introduction The interaction of swift heavy ions with dielectric materials causes, besides radiation damage, material ejection in a process called electronic sputtering [ 11.Studies of properties of ions sputtered electronically in a single ion impact, may provide direct in situ information on the complex physical and chemical processes occurring in the wake of the incident ions, as argued recently [Z-4]. For example, radial velocity distributions of sputtered intact biomolecular ions, small (molecular) fragment ions and fullerene ions ejected from polymers have given detailed insight into the time and spatial development of ion tracks [2,3]. Experimental results [5-71 and theoretical calculations indicate that high mass molecular ions (e.g. peptide ions) are ejected from the ultratrack preferentially in a direction normal to the ion track by a transient energy flow (pressure pulse [8] or shock wave [9]) propagating mdially from the track core. This is observed as an off normal maximum in the initial radial velocity distribution for such ions. On the other hand, the velocity distributions (more
* Corresponding
author.
precisely the mean velocity, ( ux), and the width, r) of low mass fragment (hydrocarbon) ions sputtered from polymers depend on the degree of hydrogenation of the secondary ions [3]. Ions with lower hydrogen content exhibit wider velocity distributions (i.e. higher (u,‘)) and ( uI> directed towards the primary ion trajectory [3]. Ions with higher hydrogen content have narrower velocity distributions (i.e. lower (u,‘)) and ( ux) directed away from the incident ion trajectory, in a similar fashion to massive bioorganic molecular ions. It was suggested that this behaviour reflects the radial profile of the deposited energy density in the heavy ion track, allowing to correlate the origin of the ionised ejecta relative to the point of impact of the fast heavy-ion [3]. Here an extension of these investigations for fluorocarbon ions ejected from polytetrafluoroethylene (PTFE) films is presented. A similar correlation between degree of fluorination and the initial radial velocity distributions of C,Fz secondary ions has been observed. This finding indicates that the effects observed in Ref. [3] are not coupled to a special property of hydrocarbon ions, and gives additional evidence for the connections between the properties of ejected ions, the deposited energy density profile and the fast physicochemical processes occurring in fast (u >> 0.22 cm/m, i.e. E x=- 25 keV/amu) ion tracks.
0168-583X/96/$15.00 0 19% Elsevier Science B.V. All rights reserved SSDlO168-583X(95)01147-1
R.M. Papal60 et al./Nucl.
Instr. and Meth. in Phys. Res. B 107 (1996) 308-312
309
2. The targets were 5 pm thick commercial polytetrafluoroethylene WIFE) foils ([-CF,CF,-I,, Goodfellow Ltd., England), mounted on an aluminium frame. A thin gold layer was deposited on the back of the foils to improve the electric contact with the sample holder. 72.3 MeV ‘*‘113+ ions from the Uppsala EN-tandem accelerator, incident on the target at an angle of 45”, and at a rate of - 2 X lo3 S -’ were used as projectiles. A reflection time-of-flight (TOF) spectrometer was used for secondary ion analyses [lo]. The positive secondary ions were accelerated by a potential V, of 14.00 kV, before traversing the field-free flight tube. The ions were then reflected in an electrostatic ion mirror at a potential V,,, = 15.66 kV correcting the dispersion in the initial axial velocity and improving the mass resolution [lo]. The ion flight time registration was performed by a time-to-digital converter (IPN, Orsay, France) with 0.5 ns time resolution. Typically 4 X lo6 primary ion impacts were accumulated for each mass spectrum. The initial radial (tangential) velocity distributions were obtained by monitoring the secondary ion yield as a function of the voltages applied to two sets of deflection plates (in the x- or y-direction perpendicular to the target surface normal). The plates were installed parallel to the ion optical axis of the TOF mass spectrometer. To determine u,, the yield of low mass ions was maximised by setting the appropriate potential on the y-deflection plates, and then acquiring data for different voltages V,, applied to the x-deflection plates. Conversion from deflection voltage units to velocity units was performed according to 111,121:
(1) where M is the ion mass, k, is a constant depending on V,, V,, the ion charge and on the geometrical parameters of the instrument [12]. V, in Eq. (1) is the voltage on the deflection plates required for an ion with a zero x-velocity component to reach the centre of the stop detector. The
8590
8600
8610
8620
8630
8640
t (ns)
0.8 s ‘2 9 ‘Z + a
0.6
0.4
Fig. 2. (a) Expanded view of the TOF mass spectrum of a F’TFE film in the region from 8590 to 8640 ns. The peaks for C2F+ and C,H: ions are indicated. (b) Radial velocity distributions for these two ions.
centroid of the velocity distribution of C,H: ions, shown previously to be ejected preferentially normal to the target surface (( ux> = 0) [ 111, was used for estimating V,,. The velocity distributions were obtained by fitting the experimental data to a Gaussian curve. The mean radial velocities ( vx), and the mean square velocities, (v:), were calculated using E$. (1) and averaging over the velocity distributions for each ion. Throughout the paper we have used the term “radial velocity” for brevity, although its x-component, u, - Fig. 1, was actually measured and reported.
3. Results and discussion
Stop detect01 Electrostatic
, /’
+x
72.3 M~V I I/‘Start detector ions , Fig. 1. Schematics of the TOF mass spectrometer (see text for details).
The first and basic procedure is to determine the chemical composition of each peak in the mass spectrum in the investigated mass range. It is essential to separate contributions from secondary ions close in mass (difference less then - 0.1 amu) but with distinct chemical compositions, as their velocity distributions may be very different. An example of this is given in Fig. 2 for C,F+ and C,H: ions (both with m/z = 43). The mean velocity and width of the velocity distributions for these two ions are quite different: e.g. ( vI) is 870 f 200 m/s for C2F+ and -660 f 200 m/s for C,HT. Clearly, a correlation between the velocity distribution and chemical composition
IV. CHEMICAL/BIOLOGICAL LET EFFECTS
R.M. PapaGo et al./Nucl.
310
o z x
0.76
.g
0.54
a
Instr. and Meth. in Phys. Res. B 107 (1996) 308-312
-mj ,
0.32
0.1
0
(
,
,
,
2
4
6
8
’ i1 10
12
Number of F atone XXI
0
-1090
-500 v,
Fig. 5. Average of the mean radial velocities of a group of C,Fz ions with a fixed m (n varying) as a function of the number of
09
Fig. 3. Radial velocity distributions (in voltage “units”, see note in the text) for C,Fz ions, m = 0.5, Il. The zero on the velocity scale, i.e. V,, is indicated by the vertical line.
of the secondary ions is only feasible when isobaric ions are mass separated. In the low mass region (up to - 100 u), the reflectron TOF mass spectrometer allows to resolve masses which differ by - 0.020 u. This is sufficient to unequivocally differentiate C,,Hi and C,Fz ions with the same integer mass, as in Fig. 2a. At masses higher than - 100 u the resolving power is smaller, but on the other hand, in this region, there are almost no contaminant C,H: ions detected. Thus the occurrence of “doublets” (ions with the same integer mass, but with different chemical composition), is greatly decreased in PTFE (as compared to e.g. PVDF [3]). It has allowed us to extend the analysed mass range to - 300 u, with a reliable assignment of the chemical composition of the ions. The time to mass calibration has been performed using intense peaks for which the chemical composition is undoubted and containing a different number of fluorine atoms (H+, CF+, CFT , C,Fl , C,F: , C3FT). This procedure gives more accurate masses as differences in the time-of-flight due to initial time spread during ion formation (coupled to the chemical composition of the secondary ions [13]) are
1500 loo0 500 B A >* ”
fluorine atoms, m (for details see text).
averaged out. With this calibration the mean deviation between the calculated and experimental masses for the - 90 ions analysed has been ( Am2)“* = 0.003 u. The positive ion spectrum of PTFE is, as expected, largely dominated by C,F,,, series with 1 5 R 5 9 and 0 zz m % 13 (no completely saturated ions, C,,Fn+z, have been observed). Several C+I; ions, 1 s n s 5 and 0 < p < 7 are present at low intensities. These ions originate most probably from hydrocarbon contaminant layers adsorbed on the PTFE film surface. Both types of ion series have been investigated. In Fig. 3 the radial velocity distributions for C,F,,, ions with m = 0, 5, 11 are shown. They are plotted for convenience as a function of the voltage at the deflection plates Vdr ‘. The ( ux) of the distributions changes from a positive value for Cl to a negative value for C,H:,. This trend is also observed for the other fluorocarbon ions. The ( uX) for some of these ions are plotted in Fig. 4 as a function of the ion mass, A smooth transition from positive to negative u,-velocities is observed as the number of fluorine atoms increases in each C,Fz series. For comparions is plotted in Fig. 4b ison, the (u,) for C,Hi (n = 2, 3, 4). The correlation between the ( uX> and the degree of hydrogenation of these hydrocarbon clusters is similar to the one observed for hydrogen-containing polymers like PVDF and polystyrene [3]. The average over n of the mean velocities, (Ud,, for fluorocarbon series, containing a fixed number of fluorine atoms m (i.e. series C,F, C,F,, up to C,F,,), has been estimated using:
mm=(%X _ nmin l +
O
%nal 1)
c ((%)n.m).
(2)
*nun
-500
where ((u,),,,) is the mean velocity of a cluster with n carbon atoms and m fluorine atoms and nmin and nmax are
-loo0 -1500
I
0
50
loo
20
150 200
I
30
I
I
40
50
I 4
&I
Mass (u)
Fig. 4. Mean radial velocities for (a) C,FG and(b) C,Hi ions as a function of mass. For clarity, in (a) each C,Fz series has a different symbol. The solid lines are only to guide the eye.
’ Since V,,a(M)‘/*u,, then. The width of the distributions on the voltage scale is directly proportional lo the mean “radial” energy, convenient for comparing ions with large differences in mass.
R.M. Papalio et al./Nucl.
Ins@. and Meth. in Phys. Res. B 107 (1996) 308-312
the smallest and the largest number of n in each series. The result is shown in Fig. 5 for m values up to 11. The
variation of the ( uX) with the number of carbon atoms n, in each C,Fz series, is related to the sizes of the error Although the values of the mean radial bars for am. velocities are mainly correlated to the number of fluorine atoms in the cluster, there is a tendency for the ( vX> to be closer to zero as the number of carbon atoms in the cluster increases (for a fixed m), i.e. for the heavier entities in the series. That is the origin of the bigger dispersion in the values of a,,, for e.g. m = 5 and 7.
If the C,,Fz ions were ejected predominantely by a thermal evaporation process, initial radial velocity distributions which are symmetric around the target surface normal (i.e. ( ur) = 0) would be expected. The fact that ( ur) varies from positive to negative values, indicates that a non-random momentum transfer contributes to the sputtering process. Positive mean initial velocities indicate ejection preferentially towards the primary ion’s line-of-incidence, suggesting ejection involving an axial expansion of the energised zone around the point of impact (a “gasflow” ejection [14,15]). Here the CT and C,,F+ are the eject, with the most pronounced positive “shift” of the velocity distributions. On the other hand, secondary ions with negative (II,) (ejection in an off-normal direction away from the MeV ion trajectory) are generally considered to be ejected by a correlated molecular motion close to the surface caused by a pressure pulse or shock wave propagating radially from the ion track [5,8]. This appears to be the case at least for the C,Fi species which have a large number of fluorine atoms (m > n). The transition from positive to negative values for ( uX> with increasing fluorine content in a C,FL series can be described by the simultaneous action of both mechanisms (to be discussed in detail in a separate paper). Fig. 6 shows the average over n (m constant, see Eq. (2)) for the full width at half maximum, r, of the velocity distributions of fluorocarbon series with fixed number of
c
5-It
I
I/T
IW
10
12
Numkr of F atoms
Fig. 6. Average of tbe r *( (* M( u: > - M( u,>*) of the velocity distributions of a C,F, ion series with a fixed m (varying n), as a function of the number of fluorine atoms tn. M is the secondary ion mass. For details of the averaging procedure see text.
311
fluorine atoms. The dependence of r on the degree of fluorination is not as clearly expressed as that for ( ur>. average values, (F2>,, for the Nevertheless, the r2 extreme cases m = 0, 1 and m = 11 are clearly different (- 5 eV, 4 eV and 2 eV, respectively). Since r’(eV> a M( uf) - M( u,)~, the values of r can be roughly interpreted as being a measure of the local “temperature” (energy density) at the zone of ejection. Assuming that, one may infer that the less fluorinated clusters (m = 0, 1) originate from the innermost zones of the MeV ion track where the deposited energy density is highest. In its turn, clusters with high fluorine content should come from the peripheral and “colder” regions of the track. One shall, however, consider the data from the width of the velocity distributions with care. First, the interpretation of the width in a maxwellian sense (i.e. as a measure of the local temperature, assuming thermal equilibrium) may not be necessarily valid. This is probably the case when the pressure pulse (shock wave) is the main ejection mechanism. Also, r may as well be enlarged by “non-thermal” sources, such as the angular spread of the secondary ions arising from ejection by two different processes (the pressure pulse and gas-flow), and/or the residual charge in the track at the time of ejection [ 16,171.The extra large r 2 of the velocity distributions of H+, HT, F’+, F+ and F: (12, 9, 10, 9 and 7 eV, respectively), may be explained in such a way.
4. Conclusions
A correlation between the velocity distributions and the degree of fluorination of C,F, positive ions sputtered by MeV heavy ions from PTFE targets has been demonstrated. Ions with low fluorine content have ( uX> directed towards the MeV incident ion trajectory (positive mean velocities). Ions with a large number of fluorine atoms are ejected in an off-normal direction away from the incoming MeV ion trajectory (negative mean velocities). This behaviour indicates that CL and C,F+ are most probably ejected by the axial expansion of the “hot” core of the track, while saturated fluorocarbon ions are sputtered most probably from the outermost part of the ion track by a pressure pulse (shock wave) induced ejection. The data obtained for fluorocarbon ions from a purely fluorinated compound generalise a similar correlation, observed previously, between the moments of the velocity distribution and the hydrogen content of C,,HL ejecta from hydrogencontaining polymers like PVDF and PS. This observation provides additional evidence for the connections between the properties of sputtered ionic species, the deposited energy density profile and the fast physico-chemical processes occurring in MeV ion induced tracks in organic solids.
IV. CHEMICAL/BIOLOGICAL
LET EFFECTS
312
R.M. Papal60 et al./Nucl.
Instr. and Meth. in Phys. Res. B 107 (1996) 308-312
Acknowledgements
This work has been supported by the Swedish Natural Sciences Research Council (NFR), the hgstrom and Cluster Consortia. R.M.P acknowledges the National Research Council of Brazil (CNPq) for a grant.
References
HI R.E. Johnson and B.U.R. Sundqvist, Phys. Today, March (1992) 28.
El B.U.R. Sundqvist, Int. J. Mass Spectrom. Ion Processes 126 (1993) 1. 131 R.M. PapalBo, G. Brinkmahn, D. FenyB, J. Eriksson, H.-F.
Kammer, P. Demirev, P. H&ansson and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 91 (1994) 667. [41 G. Betz and K. Wien, Int. J. Mass Spectrom. Ion Processes 140 (1994) 1. [51 W. Ens, B.U.R. Sundqvist, P. H&kansson, A. Hedin and G. Jonsson, Phys. Rev. B 39 (1989) 763. b1 R. Moshammer, R. Matthlus, K. Wien and G. Bolbach, in: Ion Formation from Organic Solids (IFOS V), eds. A. Hedin, B.U.R. Sundqvist and A. Benninghoven (Wiley. 1990) p. 17.
[7] J. Kopnizcky, C.T. Reimann, A. Hall& and B.U.R. Sundqvist, Phys. Rev. B 49 (1994) 625. [8] R.E. Johnson, B.U.R. Sundqvist, A. Hedin and D. Fenyb, Phys. Rev. B 40 (1989) 49; D. Fenyii and R.E. Johnson, Phys. Rev. B 46 (1992) 5090. [9] I. Bitensky and E.S. Parilis, NucI. Instr. and Meth. B 21 (1987) 26. [lo] G. Brinkmalm, P. H&ansson, J. Kjellberg, P. Demirev, B.U.R. Sundqvist, W. Ens, Int. J. Mass Spectrom. Ion Proc. 114 (1992) 183. [ 1l] D. Fenyii, A. Hedin, P. Hilkansson and B.U.R. Sundqvist, Int. J. Mass. Spectrom. Ion Proc. 100 (1990) 63. [ 121 G. Brinkmalm, P. Demirev, D. Feny6, P. H&kansson, J. Kopniczky and B.U.R. Sundqvist, Phys. Rev. B 47 (1993) 7560. [ 131 R.M. Papalto et al., to be published. [14] H.M. Urbassek and K.T. Waldeer, Phys. Rev. Lett. 67 (1991) 105. [ 151 I.S. Bitensky, P. Demirev and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 82 (1993) 356. [16] D. FenyB, P. Htlkansson and B.U.R. Sundqvist, Nucl. Instr. and Meth. B 84 (1994) 31. [ 171 R.D. Macfarlane and D.L. Jacobs, in: A. Benninghoven, ed., Proc. 4th Int. Conf. on Ion Formation from Organic Solids (IFOS IV), Mllnster, Germany, 1987 (Wiley, Chichester, 1989) p. 71.
c2L!A - -
Nuclear Instruments
and Methodsin Physics ResearchB 107(1996) 308-3 12
k!lONil B
Beam Interactions with Materials 6 Atoms
ELSEVIER
Radial velocity distributions of secondary ions ejected from PTFE by MeV atomic ions R.M. Papal&o *, P. Demirev, J. Eriksson, P. Hkmsson,
B.U.R. Sundqvist
Division of Ion Physics, Dept. of Radiation Sciences, Uppsala University, Box 535, S 751-21, Uppsala. Sweden
Abstract
In this paper, systematic investigations of the initial radial velocity distributions of C,Fi secondary ions sputtered by 72.3 MeV 12’1+I3 ions from polytetrafluoroethylene (PTFE) foils are presented. The initial radial velocity distributions are obtained by monitoring the secondary ion yield as a function of the voltages applied to two sets of deflection plates installed parallel to the ion optical axis of a time-of-flight mass spectrometer. The mean radial velocity, ( ux), of C,FG ions (1 < R < 6 and 0 < m < 13) depends on the number of fluorine atoms, showing a periodic behaviour when plotted as a function of the ion mass. Ions with low fluorine content have ( ux) directed towards the primary ion’s line-of-incidence (positive mean velocities). Ions with a large number of fluorine atoms are ejected in an off-normal direction away from the incoming MeV ion trajectory (negative mean velocities). The ion emission can not be explained only by evaporative type processes. Mechanisms involving correlated momentum transfer e.g. pressure pulse or shock wave induced ejection, should also be taken into account. The data obtained for fluorocarbon ions from a purely fluorinated compound generalise a similar effect observed previously for C,HL ions, sputtered from different hydrogen containing polymer targets [R.M. PapalCo et al., Nucl. Instr. and Meth. B 91 (1994) 6671.
1. Introduction The interaction of swift heavy ions with dielectric materials causes, besides radiation damage, material ejection in a process called electronic sputtering [ 11.Studies of properties of ions sputtered electronically in a single ion impact, may provide direct in situ information on the complex physical and chemical processes occurring in the wake of the incident ions, as argued recently [Z-4]. For example, radial velocity distributions of sputtered intact biomolecular ions, small (molecular) fragment ions and fullerene ions ejected from polymers have given detailed insight into the time and spatial development of ion tracks [2,3]. Experimental results [5-71 and theoretical calculations indicate that high mass molecular ions (e.g. peptide ions) are ejected from the ultratrack preferentially in a direction normal to the ion track by a transient energy flow (pressure pulse [8] or shock wave [9]) propagating mdially from the track core. This is observed as an off normal maximum in the initial radial velocity distribution for such ions. On the other hand, the velocity distributions (more
* Corresponding
author.
precisely the mean velocity, ( ux), and the width, r) of low mass fragment (hydrocarbon) ions sputtered from polymers depend on the degree of hydrogenation of the secondary ions [3]. Ions with lower hydrogen content exhibit wider velocity distributions (i.e. higher (u,‘)) and ( uI> directed towards the primary ion trajectory [3]. Ions with higher hydrogen content have narrower velocity distributions (i.e. lower (u,‘)) and ( ux) directed away from the incident ion trajectory, in a similar fashion to massive bioorganic molecular ions. It was suggested that this behaviour reflects the radial profile of the deposited energy density in the heavy ion track, allowing to correlate the origin of the ionised ejecta relative to the point of impact of the fast heavy-ion [3]. Here an extension of these investigations for fluorocarbon ions ejected from polytetrafluoroethylene (PTFE) films is presented. A similar correlation between degree of fluorination and the initial radial velocity distributions of C,Fz secondary ions has been observed. This finding indicates that the effects observed in Ref. [3] are not coupled to a special property of hydrocarbon ions, and gives additional evidence for the connections between the properties of ejected ions, the deposited energy density profile and the fast physicochemical processes occurring in fast (u >> 0.22 cm/m, i.e. E x=- 25 keV/amu) ion tracks.
0168-583X/96/$15.00 0 19% Elsevier Science B.V. All rights reserved SSDlO168-583X(95)01147-1
R.M. Papal60 et al./Nucl.
Instr. and Meth. in Phys. Res. B 107 (1996) 308-312
309
2. The targets were 5 pm thick commercial polytetrafluoroethylene WIFE) foils ([-CF,CF,-I,, Goodfellow Ltd., England), mounted on an aluminium frame. A thin gold layer was deposited on the back of the foils to improve the electric contact with the sample holder. 72.3 MeV ‘*‘113+ ions from the Uppsala EN-tandem accelerator, incident on the target at an angle of 45”, and at a rate of - 2 X lo3 S -’ were used as projectiles. A reflection time-of-flight (TOF) spectrometer was used for secondary ion analyses [lo]. The positive secondary ions were accelerated by a potential V, of 14.00 kV, before traversing the field-free flight tube. The ions were then reflected in an electrostatic ion mirror at a potential V,,, = 15.66 kV correcting the dispersion in the initial axial velocity and improving the mass resolution [lo]. The ion flight time registration was performed by a time-to-digital converter (IPN, Orsay, France) with 0.5 ns time resolution. Typically 4 X lo6 primary ion impacts were accumulated for each mass spectrum. The initial radial (tangential) velocity distributions were obtained by monitoring the secondary ion yield as a function of the voltages applied to two sets of deflection plates (in the x- or y-direction perpendicular to the target surface normal). The plates were installed parallel to the ion optical axis of the TOF mass spectrometer. To determine u,, the yield of low mass ions was maximised by setting the appropriate potential on the y-deflection plates, and then acquiring data for different voltages V,, applied to the x-deflection plates. Conversion from deflection voltage units to velocity units was performed according to 111,121:
(1) where M is the ion mass, k, is a constant depending on V,, V,, the ion charge and on the geometrical parameters of the instrument [12]. V, in Eq. (1) is the voltage on the deflection plates required for an ion with a zero x-velocity component to reach the centre of the stop detector. The
8590
8600
8610
8620
8630
8640
t (ns)
0.8 s ‘2 9 ‘Z + a
0.6
0.4
Fig. 2. (a) Expanded view of the TOF mass spectrum of a F’TFE film in the region from 8590 to 8640 ns. The peaks for C2F+ and C,H: ions are indicated. (b) Radial velocity distributions for these two ions.
centroid of the velocity distribution of C,H: ions, shown previously to be ejected preferentially normal to the target surface (( ux> = 0) [ 111, was used for estimating V,,. The velocity distributions were obtained by fitting the experimental data to a Gaussian curve. The mean radial velocities ( vx), and the mean square velocities, (v:), were calculated using E$. (1) and averaging over the velocity distributions for each ion. Throughout the paper we have used the term “radial velocity” for brevity, although its x-component, u, - Fig. 1, was actually measured and reported.
3. Results and discussion
Stop detect01 Electrostatic
, /’
+x
72.3 M~V I I/‘Start detector ions , Fig. 1. Schematics of the TOF mass spectrometer (see text for details).
The first and basic procedure is to determine the chemical composition of each peak in the mass spectrum in the investigated mass range. It is essential to separate contributions from secondary ions close in mass (difference less then - 0.1 amu) but with distinct chemical compositions, as their velocity distributions may be very different. An example of this is given in Fig. 2 for C,F+ and C,H: ions (both with m/z = 43). The mean velocity and width of the velocity distributions for these two ions are quite different: e.g. ( vI) is 870 f 200 m/s for C2F+ and -660 f 200 m/s for C,HT. Clearly, a correlation between the velocity distribution and chemical composition
IV. CHEMICAL/BIOLOGICAL LET EFFECTS
R.M. PapaGo et al./Nucl.
310
o z x
0.76
.g
0.54
a
Instr. and Meth. in Phys. Res. B 107 (1996) 308-312
-mj ,
0.32
0.1
0
(
,
,
,
2
4
6
8
’ i1 10
12
Number of F atone XXI
0
-1090
-500 v,
Fig. 5. Average of the mean radial velocities of a group of C,Fz ions with a fixed m (n varying) as a function of the number of
09
Fig. 3. Radial velocity distributions (in voltage “units”, see note in the text) for C,Fz ions, m = 0.5, Il. The zero on the velocity scale, i.e. V,, is indicated by the vertical line.
of the secondary ions is only feasible when isobaric ions are mass separated. In the low mass region (up to - 100 u), the reflectron TOF mass spectrometer allows to resolve masses which differ by - 0.020 u. This is sufficient to unequivocally differentiate C,,Hi and C,Fz ions with the same integer mass, as in Fig. 2a. At masses higher than - 100 u the resolving power is smaller, but on the other hand, in this region, there are almost no contaminant C,H: ions detected. Thus the occurrence of “doublets” (ions with the same integer mass, but with different chemical composition), is greatly decreased in PTFE (as compared to e.g. PVDF [3]). It has allowed us to extend the analysed mass range to - 300 u, with a reliable assignment of the chemical composition of the ions. The time to mass calibration has been performed using intense peaks for which the chemical composition is undoubted and containing a different number of fluorine atoms (H+, CF+, CFT , C,Fl , C,F: , C3FT). This procedure gives more accurate masses as differences in the time-of-flight due to initial time spread during ion formation (coupled to the chemical composition of the secondary ions [13]) are
1500 loo0 500 B A >* ”
fluorine atoms, m (for details see text).
averaged out. With this calibration the mean deviation between the calculated and experimental masses for the - 90 ions analysed has been ( Am2)“* = 0.003 u. The positive ion spectrum of PTFE is, as expected, largely dominated by C,F,,, series with 1 5 R 5 9 and 0 zz m % 13 (no completely saturated ions, C,,Fn+z, have been observed). Several C+I; ions, 1 s n s 5 and 0 < p < 7 are present at low intensities. These ions originate most probably from hydrocarbon contaminant layers adsorbed on the PTFE film surface. Both types of ion series have been investigated. In Fig. 3 the radial velocity distributions for C,F,,, ions with m = 0, 5, 11 are shown. They are plotted for convenience as a function of the voltage at the deflection plates Vdr ‘. The ( ux) of the distributions changes from a positive value for Cl to a negative value for C,H:,. This trend is also observed for the other fluorocarbon ions. The ( uX) for some of these ions are plotted in Fig. 4 as a function of the ion mass, A smooth transition from positive to negative u,-velocities is observed as the number of fluorine atoms increases in each C,Fz series. For comparions is plotted in Fig. 4b ison, the (u,) for C,Hi (n = 2, 3, 4). The correlation between the ( uX> and the degree of hydrogenation of these hydrocarbon clusters is similar to the one observed for hydrogen-containing polymers like PVDF and polystyrene [3]. The average over n of the mean velocities, (Ud,, for fluorocarbon series, containing a fixed number of fluorine atoms m (i.e. series C,F, C,F,, up to C,F,,), has been estimated using:
mm=(%X _ nmin l +
O
%nal 1)
c ((%)n.m).
(2)
*nun
-500
where ((u,),,,) is the mean velocity of a cluster with n carbon atoms and m fluorine atoms and nmin and nmax are
-loo0 -1500
I
0
50
loo
20
150 200
I
30
I
I
40
50
I 4
&I
Mass (u)
Fig. 4. Mean radial velocities for (a) C,FG and(b) C,Hi ions as a function of mass. For clarity, in (a) each C,Fz series has a different symbol. The solid lines are only to guide the eye.
’ Since V,,a(M)‘/*u,, then
R.M. Papalio et al./Nucl.
Ins@. and Meth. in Phys. Res. B 107 (1996) 308-312
the smallest and the largest number of n in each series. The result is shown in Fig. 5 for m values up to 11. The
variation of the ( uX) with the number of carbon atoms n, in each C,Fz series, is related to the sizes of the error Although the values of the mean radial bars for am. velocities are mainly correlated to the number of fluorine atoms in the cluster, there is a tendency for the ( vX> to be closer to zero as the number of carbon atoms in the cluster increases (for a fixed m), i.e. for the heavier entities in the series. That is the origin of the bigger dispersion in the values of a,,, for e.g. m = 5 and 7.
If the C,,Fz ions were ejected predominantely by a thermal evaporation process, initial radial velocity distributions which are symmetric around the target surface normal (i.e. ( ur) = 0) would be expected. The fact that ( ur) varies from positive to negative values, indicates that a non-random momentum transfer contributes to the sputtering process. Positive mean initial velocities indicate ejection preferentially towards the primary ion’s line-of-incidence, suggesting ejection involving an axial expansion of the energised zone around the point of impact (a “gasflow” ejection [14,15]). Here the CT and C,,F+ are the eject, with the most pronounced positive “shift” of the velocity distributions. On the other hand, secondary ions with negative (II,) (ejection in an off-normal direction away from the MeV ion trajectory) are generally considered to be ejected by a correlated molecular motion close to the surface caused by a pressure pulse or shock wave propagating radially from the ion track [5,8]. This appears to be the case at least for the C,Fi species which have a large number of fluorine atoms (m > n). The transition from positive to negative values for ( uX> with increasing fluorine content in a C,FL series can be described by the simultaneous action of both mechanisms (to be discussed in detail in a separate paper). Fig. 6 shows the average over n (m constant, see Eq. (2)) for the full width at half maximum, r, of the velocity distributions of fluorocarbon series with fixed number of
c
5-It
I
I/T
IW
10
12
Numkr of F atoms
Fig. 6. Average of tbe r *( (* M( u: > - M( u,>*) of the velocity distributions of a C,F, ion series with a fixed m (varying n), as a function of the number of fluorine atoms tn. M is the secondary ion mass. For details of the averaging procedure see text.
311
fluorine atoms. The dependence of r on the degree of fluorination is not as clearly expressed as that for ( ur>. average values, (F2>,, for the Nevertheless, the r2 extreme cases m = 0, 1 and m = 11 are clearly different (- 5 eV, 4 eV and 2 eV, respectively). Since r’(eV> a M( uf) - M( u,)~, the values of r can be roughly interpreted as being a measure of the local “temperature” (energy density) at the zone of ejection. Assuming that, one may infer that the less fluorinated clusters (m = 0, 1) originate from the innermost zones of the MeV ion track where the deposited energy density is highest. In its turn, clusters with high fluorine content should come from the peripheral and “colder” regions of the track. One shall, however, consider the data from the width of the velocity distributions with care. First, the interpretation of the width in a maxwellian sense (i.e. as a measure of the local temperature, assuming thermal equilibrium) may not be necessarily valid. This is probably the case when the pressure pulse (shock wave) is the main ejection mechanism. Also, r may as well be enlarged by “non-thermal” sources, such as the angular spread of the secondary ions arising from ejection by two different processes (the pressure pulse and gas-flow), and/or the residual charge in the track at the time of ejection [ 16,171.The extra large r 2 of the velocity distributions of H+, HT, F’+, F+ and F: (12, 9, 10, 9 and 7 eV, respectively), may be explained in such a way.
4. Conclusions
A correlation between the velocity distributions and the degree of fluorination of C,F, positive ions sputtered by MeV heavy ions from PTFE targets has been demonstrated. Ions with low fluorine content have ( uX> directed towards the MeV incident ion trajectory (positive mean velocities). Ions with a large number of fluorine atoms are ejected in an off-normal direction away from the incoming MeV ion trajectory (negative mean velocities). This behaviour indicates that CL and C,F+ are most probably ejected by the axial expansion of the “hot” core of the track, while saturated fluorocarbon ions are sputtered most probably from the outermost part of the ion track by a pressure pulse (shock wave) induced ejection. The data obtained for fluorocarbon ions from a purely fluorinated compound generalise a similar correlation, observed previously, between the moments of the velocity distribution and the hydrogen content of C,,HL ejecta from hydrogencontaining polymers like PVDF and PS. This observation provides additional evidence for the connections between the properties of sputtered ionic species, the deposited energy density profile and the fast physico-chemical processes occurring in MeV ion induced tracks in organic solids.
IV. CHEMICAL/BIOLOGICAL
LET EFFECTS
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Instr. and Meth. in Phys. Res. B 107 (1996) 308-312
Acknowledgements
This work has been supported by the Swedish Natural Sciences Research Council (NFR), the hgstrom and Cluster Consortia. R.M.P acknowledges the National Research Council of Brazil (CNPq) for a grant.
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