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High energy carbon cluster beams
International Journal of Mass Spectrometry and Ion Processes 130 (1994) 73-82 0168-I 176/94/$07.00 0 1994 - Elsevier Science Publishers B.V. All rights reserved
73
High energy carbon cluster beams K.Boussofiane-Baudin,
A. Brunelle, P. Chaurand, J. Depauw, S. Della-Negra, P. Hilcansson+, Y. Le Beyec*
Institut de Physique Nuclkaire, F-91406 Orsay, France
(Received 20 May 1993; accepted 30 July 1993) Abstract Carbon cluster beams with mega-electronvolt energies have been accelerated in the Orsay MP tandem accelerator. In a conventional cesium sputter ion source, negative beams of C,, Cc and C, cluster ions were produced and injected into the accelerator with a terminal voltage of 5.5 and 4 MV, respectively. Different charge exchange and decay processes in the terminal as well as in the beam line are described. Energy and time-of-flight measurements show that different carbon cluster ions can be formed, accelerated and transported far away from the accelerator and arrive intact at the experimental
Key work
Carbon clusters; Time of flight; Sputter ion source
1. Introduction
For many years atomic bombardment of solid materials has been used to eject ions from surfaces. The energy of impact has been in the range of a few kilo-electronvolts to tens of kiloelectronvolts where the dominant energy loss mode is attributed to nuclear cascade collisions. More recently, fast heavy ion projectiles with energies of several tens of mega electronvolts have also been used to bombard solid films of insulating materials (mainly organic solids) in the electronic stopping power regime. One of the surprising consequences of the large amount of electronic energy deposition is the emission of large intact molecules from the surface (or near the surface) of the solid [l-4]. The well-known method
* Corresponding author. t Permanent address: Division of Ion Physics, Department of Radiation Sciences, Uppsala University, Box 535, S-751 21 Uppsala, Sweden. SSDI 0 168-1176(93)03902-X
plasma desorption mass spectrometry using tission fragments from a 252Cf source with energies of 0.5-l MeV u-’ is today an efficient technique used in mass spectrometry to identify large biomolecules [5,6]. The studies of collisions of particles with solid surfaces have mainly been investigated with atomic ions, although there have been some pioneering experiments with polyatomic ions [7-241. A systematic study with various types of cluster projectiles and energies has focused on non-linear effects in secondary ion emission [25]. In those experiments the maximum energy of impact was below 100 keV and the projectile mass-to-charge (m/q) values were ranging between m/q 200 and 1000. To reach the electronic regime for energy deposition with cluster ions it is necessary to increase the total energy to several mega electronvolts. A particle accelerator such as a tandem accelerator has to be used for this purpose. In the interaction of cluster ions with solids a
74
K. Boussofiane-Baudin
large amount of energy can be deposited in a small volume of a solid. Several interesting phenomena can result from this transient, nonequilibrium energy deposition and very little is known about these extreme conditions created by cluster ion impact. The emission of secondary species is a channel of energy dissipation, and mass spectrometry will be used in the future to analyse the sputtered ions. In this work we describe a method to accelerate carbon cluster ions Cz’ to high energy. Time-of-flight (TOF) mass spectrometry is used to identify the cluster ion beam constituents with the most probable m/q value. With the help of energy measurements the final identification of the beam constituents is done and it is shown that intact carbon cluster ion beams can be obtained at the experimental site.
et al./Inr. J. Mass Spectrom. Ion Processes 130 (1994)
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2. Experimental procedure 2.1. The tandem accelerator Figure 1 shows a schematic view of the accelerator and the experimental set-up. In a standard sputter ion source, Csf ions of 20 keV were used to bombard a carbon target. Negative carbon cluster ions were extracted from the source and mass analysed in a 35” injector magnet before being preaccelerated to around 180 keV. No special target preparation was needed in order to obtain carbon clusters. Figure 2 shows typical currents measured for different carbon cluster ion beams in a Faraday cup at the low energy side of the accelerator, prior to acceleration. After pre-acceleration the mass selected beam passes a buncher and deflector plate arrangement before entering the tandem
I I ~Expenmedl
L-IJLL ToF
!_llLL Energy
Fig. 1. Schematic view of the accelerator with the experimental set-up. In a sputter ion source negative carbon cluster ions are produced. They are pre-accelerated to about 180 keV and then injected into the tandem accelerator. The negatively charged cluster ions are accelerated towards the terminal on positive high voltage, about 5 MV in this study. In charge-exchange collisions with nitrogen gas in a stripper channel the cluster ions become positively charged and are accelerated a second time. The beam is pulsed and the TOF is measured from the deflection-electronics to a stop detector in the experimental chamber.
K. Boussofiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994)
4
_I
d,
2- lo*, lo’: 100 1 0
I 1
12c-
1 5
7
n
b
8
9
,
I i , , ,
l0 11 12 13 14 15 16
n (numberof atoms)
Ftg. 2. Measured currents in the low energy Faraday cup for different carbon cluster ions produced by sputtering of carbon by a 20 keV Csc beam. If the clusters have a mass larger than the mass of a Cs atom, great care must be taken to avoid interferences from mixed carbon-cesium clusters [26].
accelerator where it is accelerated towards the terminal at a positive potential of several million volts. In the terminal, at the centre of the accelerator, the negatively charged cluster ions pass a strip-
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per channel containing N2 gas. Owing to collisions between the gas molecules and the cluster ions they lose one or more electrons and become intact neutral cluster molecules or positively charged cluster ions. Some of the clusters break into fragments in the terminal and are accelerated in the second step as smaller cluster ions, or they continue with the constant speed obtained in the first acceleration step as neutral molecules. The gas pressure in the stripper channel is of great importance for the charge-exchange process and must be very low to avoid extensive fragmentation. 2.2. The experimental set-up After being accelerated the high energy beam is transported about 15 m to the 90” analysing magnet. As the magnetic field is not high enough to deflect singly charged high-mass cluster ions, an experimental chamber has been mounted after the analysing magnet in the 0” direction (see Fig. 1). The chamber has two microchannel plate detec-
-
10000 1t c1
1
1t 2
8000 C,2t
injected
5.13 MV
6000 z? 5 s 4000
/I'+ 1
L
d
52010
16000
16800
17600
18400
19200
20000
20800
21600
Time of flight (nsec) Fig. 3. TOF spectrum measured between a start pulse from the beam pulsing electronics and a stop detector in the expertmental chamber. q ions were injected into the accelerator with a terminal voltage of 5 MV. The ions CT and C!,‘, 1 < n C 8, are produced at the terminal by break-up of the Cc ions in the charge-exchange processes. The aluminium ions come from the target holder and are accelerated to the terminal as Al; ions.
K. Boussofiane-Baudin et aLlInt. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
76
tors, MCPl and MCP2, and a solid state silicon detector. All detectors can be used to give stop signals for time-of-flight (TOF) measurements but only the silicon detector can give an energy signal. One of the detectors, MCP2, is mounted at a fixed position at the back of the chamber while the other two can be moved in and out of the beam. The distance between the detectors MCPl and MCP2 is 80 cm. A pulsed beam was run through the accelerator and the TOF was measured with a multistop timeto-digital converter (model CTN-M2, Institut de Physique Nucleaire, Orsay, France) between a start pulse from the electronics for the deflection plates and a stop pulse from one of the stop detectors. Energy spectra were also recorded with the silicon detector in coincidence with peaks in the corresponding time-of-flight spectrum. Three experiments are discussed in this paper and all give very similar results. In the first run, Table 1 Calculated and measured time-of-flight the Orsay tandem accelerator
C, ions with an energy of 180 keV were injected into the tandem accelerator with a terminal voltage of 4.95 MV. In the second run, 180 keV C; ions were accelerated with 5.13 MV. In the third run, 163 keV C, ions were accelerated with 4.0MV. In the following text, these experiments are referred to as “the C8 case”, “the C9 case” and “the Cl2 case” respectively. Because of the similarity between the different cases only the C9 case is described in detail in the following text. 3. Results 3.1. TOF measurements
An example of a TOF spectrum, when C, ions were injected into the accelerator, is shown in Fig. 3. In this case the silicon detector was used as stop detector. The peak assignments have been done by comparing the measured TOF values with the
values for different cluster ions and neutrals
produced
when 180 keV Cc ions were injected into
T-P
E
T,I - Tap
5.72 6.31 6.91 7.50 8.09 8.68 9.27 9.86 10.45
5.72 3.16 2.30 1.87 1.62 1.45 1.32 1.23 1.16
1000 220 254 62 48 56 8 3 21
16 326.7 17 152.0 17 674.2 18 047.2 18 332.3 18 559.4 18 743.8 18 916.2 19032.6
16 340.6 17 164.3 17 682.6 18 053.6 18 336.8 18 562. I 18 746.5 18900.7 19031.8
13.9 12.3 8.4 6.4 4.5 2.7 2.7 -15.5 - 0.8
5.31
0.59
78
20 748.6
20731.6
-17
10.86 12.03 13.21 14.39
10.86 4.01 2.64 2.06
130 4 1 1
15 628.7 16795.5 17429.1 17 886.4
15 643.7 16 809.7 17447.8 17881.7
15 14.2 18.7 - 4.7
15.99
15.99
c’+ I
3
15283.7
15 302.3
18.6
7.79 6.46
3.90 3.22
Al;+ Al;+
6 38
18 193.4 17 295.0
18 203.9 17313.2
10.2 18.2
The acceleration voltage was 5.13 MV. The time T is the time-of-flight between a start signal from the beam pulsing electromcs and a stop signal from a silicon stop detector in the experimental chamber. All energies are in mega-electronvolts and the flight times in nanoseconds.
K. Boussofiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994)
corresponding ones calculated from a computer program which takes into account all dimensions and different voltages of the accelerator. As can be seen from Table 1 the agreement between the measured and calculated TOF values is very good, with a typical deviation of less than 0.5%. One should keep in mind that this type of TOF spectrum is not of the same type as the ones obtained in an ordinary TOF mass spectrometer. As a consequence these spectra can not be calibrated with the simple formula TOF = A[(m/q)]“* + B (A and B are constants) used in TOF mass spectrometry. Another procedure often used for the TOF measurements is the following. Two TOF spectra using the detectors MCPl and MCP2 as stop detectors respectively are recorded. By determining the time difference between corresponding peaks in these two spectra, the TOF for the ions needed to travel the distance between the stop detectors is obtained. This distance is 80cm and it is straightforward to calculate TOF values and compare with the measured ones. The agreement is often within 0.576 1% between these values although the flight path between the detectors is short. The spectrum in Fig. 3 shows some very typical features observed in the C8 and C9 cases, as well as in the Cl2 case. 3.1.1. The presence of a neutral C.90peak. These molecules originate from C, ions that have been accelerated to the terminal and then neutralised in the charge-exchange process; they are not accelerated in the second step of the accelerator and thus have only half the energy of the intact CF ions, 5.3 1 MeV in this case, provided they still are intact after the terminal. By applying a magnetic field in the analysing magnet this peak will not be influenced, nor will the peaks on the right hand side (not seen in the figure) with longer flight times, as they all are neutrals. However, all the peaks with shorter flight times than the neutral C8 peak will disappear almost completely and they must therefore be ions. The neutral components of the different ion peaks are thus very small. It is not clear
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II
where the neutrals with longer flight times than the Ct molecules are formed. 3.1.2. The presence of a Cp ion peak. These intact cluster ions correspond to accelerated Cc ions that have changed their charge from -1 to +l in the stripper channel, without fragmentation, and thus obtained the energy 10.45MeV. Multiply charged intact cluster ions have not been observed either in the C8 case or in the Cl2 case. The region between the CF ion peak and the corresponding neutral Ct peak normally contains no peaks because no clusters with TOF values in this time window can be formed when C, ions are injected into the accelerator. 3.1.3. The presence of cluster ions of the type Cz+ where n< 9. These ions correspond to accelerated Cc ions which break into smaller cluster ions in the charge exchange process in the stripper channel before they are accelerated further in the second step. The energies of these ions are (5.13 +O.l8)n/9 +q 5.13MeV. The only intense multiply charged ion observed so far is the C: ion followed by a weak Ci’ ion. For even shorter flight times than for this ion, no peaks are observed. Multiply charged cluster ions have only been observed for the rather weak C:+, Cp and C*+ I ions . However, for cluster ions CL being accelerated to the terminal, all ions Cz’, n G k, with the same n/q value will have the same TOF through the accelerator and will leave the accelerator with the same velocity. Therefore, with only TOF measurements it is impossible to distinguish ions like C:’ and Ci’ from each other. The possibility that multiply charged ions are hidden in the spectra can not therefore be excluded. The C;+ peak can, for example, contain contributions from Cy ions as well as from C3+ ions. In F$. 3 two peaks appear at the same TOF as for the Cg’ and CF ions. The corresponding peaks have not however been observed in the C8 or in the Cl2 case, and in fact these ions are Alif and Ali+ coming from injected Ali ions which break up at
78
K. BoussoJiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
the terminal. The mass of Al4 is the same as for Cs, namely m/z 108, and both ions can thus pass the injector magnet. The aluminium cluster ions are produced in the sputter ion source when a part of the Csf ion beam hits the aluminium target holder. This has been carefully checked in a separate experiment. The effect illustrates a problem when accelerating cluster ion beams. Different cluster ion beams with the same m/z value (or very close) can be injected into the accelerator and owing to breakups at the terminal, many different combinations can occur in a TOF spectrum and interfere with the correct peaks. With TOF measurements one can only conclude that a certain cluster has been accelerated to a certain energy when it leaves the accelerator. If the cluster then decays (owing to collisions or too-high internal energy) in flight in the beam line, the fragments will continue with the same velocity as the cluster had before the decay, and the time measurement will be the same as for an intact cluster. This problem will be further discussed in section 3.2. The TOF value for a cluster passing the accelerator is determined by time contributions at the low energy side where the cluster ions move with a low velocity before they have been accelerated. If the clusters continue from the terminal as neutrals, instead of singly charged cluster ions, there will only be a 9% increase in the TOF value in the C9 case, as can be seen in Table 1.
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490
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ap
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IO 0
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4
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3 c
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42
28
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24
3.2. Energy measurements To investigate if the clusters are intact, not only when they leave the accelerator but also when they have travelled 15 m in the beam line and arrive in the experimental chamber, energy measurements have been performed with a solid state silicon detector. The detector, manufactured by Ortec, has no entrance gold window but instead an ionimplanted contact layer at the surface with a thickness of x 500 A. The active area of the detector has a diameter of 8mm. The energy spectra were recorded in coincidence with the corresponding TOF spectrum which means that a window was
400
600
800
lcO0
1;
Channels u. energy Fik. 4. Energy spectra recorded in coincidence with windows around the (a) Cl+, (b) C:‘, (c) Ci’ and (d) C$ peaks, respectively, in the corresponding time-of-flight spectrum. The different fragment ions correspond to clusters that have decayed after leaving the accelerator.
K. Boussofiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994)
set around a peak in the TOF spectrum and the energy signals were only registered for events in that time window. The data acquisition system used in the experiment allows six windows to be set simultaneously in one TOF spectrum. Figure 4(a)-(d) shows such energy spectra with a time window set around the peaks C:+, Ci+,Cp and Ct respectively. From the energy calibration, described below, one can conclude that almost all the cluster ions are intact when hitting the detector. This is also the case for the C:+, C:+, Cy, Cp and Ci+ ions not shown here. However, some of the cluster ions decay in flight after leaving the accelerator. The small peaks in e.g. Fig. 4(a) correspond to ions from decayed C:+ cluster ions and it will be shown below that, in this case, the peaks correspond to the ions Cc, (1 < n < 4). In Fig. 4(d) intact Ci neutral molecules can be observed but most of the clusters have decayed and the peaks Cs and Ci are dominating the spectrum. The fragment Cy is not detected because the energy is below the electronic threshold used in this experiment. It is impossible to tell if these neutral fragments are created already in the charge-exchange process in the stripper channel or if they are fragments from C8 clusters that leave the terminal intact but then decay somewhere between the terminal and the experimental chamber. In all energy spectra a peak is observed at channel 661 corresponding to the energy for the Ci’ ion formed at the terminal. The presence of this extra peak in all spectra is due to an artifact in the data acquisition system but it will not influence the energy measurements. The energy spectra have been analysed in the following way. It is assumed that a cluster breaks immediately into atoms when it hits the detector. These atoms penetrate the entrance window or an inactive “dead layer” in the detector and loose some energy which is not registered by the detector. The atoms are then slowed down and stopped in the detector. Only the electronic part of the energy loss will create electron-hole pairs that will give rise to an output signal. This signal is assumed
19
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to be the linear sum of the contributions from the individual atoms. The important parameter for a cluster is thus the amount of energy per atom rather than the total energy of the cluster. The first step in the analysis is to determine the centroid of all the peaks shown in Fig. 4 and calculate the corresponding energy these clusters and fragments are assumed to have when they are stopped in the detector. The energy loss in the inactive layer at the entrance of the detector should be included in the calculations. In calculating this energy loss correction one should keep in mind the following. Consider e.g. the cluster ion C:+ with its corresponding fragment ions shown in Fig. 4(a). The first calculated energy value for the Ci’ ion should be subtracted with an energy loss dE but the last value for the C!,+ cluster ion should be reduced by 5dE. (Remember the distinction between fragment ions CA’ created in decays after the accelerator and cluster ions CA+created at the terminal of the accelerator!) As the nuclear stopping is very small compared with the electronic stopping in this case no correction is needed for the fact that the detector only registers electronic
8
1 J
64The enqes
2-
o;,,,,,,,,,,,,,,,,,,,,,,,,,,, __. mo 400 0
are corrected
for o “deod layer” of 500 i 600
Km
PmJ
1200
-1
1400
Channel number Fig. 5. Plot of calculated energies of clusters and fragments versus the corresponding channel numbers observed m energy spectra when C< ions were injected into the tandem accelerator: (W ) data points for clusters or atomic ions created at the terminal; (IJ) data points for fragments coming from decays after the acceleration. The line is a least squares fit to all data points for carbon atomic ions. The energy value have been corrected for an energy loss in a dead layer in the detector with an assumed thickness of 500 A.
K. Boussojiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
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energy loss. Two general features have been observed when analysing the energy measurements. (1) In a plot of the calculated energies versus the corresponding centroid values for a particular cluster with its fragments, the energy values follow a straight line. In such a plot the different fragments have the same energy/atom value. It is however observed that the lines systematically become slightly steeper for heavier clusters than for lighter ones. It seems as if the energy lost by a heavy cluster in the detector is not recorded properly. (2) In a plot of the calculated energies versus the corresponding centroid values for all ions with the same number of atoms, the energy values also follow a straight line. The slope of these lines also increases slightly with the number of atoms per cluster. Figure 5 shows a summary of the calculated energies for all fragments and clusters in the C9 case as a function of the corresponding channel numbers. The data points for ions created at the terminal are marked with filled squares to distinguish them from ions coming from decays after the acceleration which are marked with open squares. Tandem accelerator 6 Y Exit Terminal Entrance A
k.0
Beam line
-ck” ’
CnO”
n
-Ck”
-
-C,o
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CnO n
-d -
-ck’-
B
ck’+ -ck’+
t’
C
Cn4+ “
,c:;::
nck
ncr-_d
C,,‘l+ C C+I+ -d n
-
p
Fig. 6. A summary of observed charge-exchange and decay processes and where they take place when Ci ions are injected into the tandem accelerator. A refers to the information of neutral intact or fragment molecules at the terminal. It is not possible to tell if an observed neutral fragment comes from an intact neutral molecule that decays in flight or if it is formed directly in the terminal. B refers to charge-exchange without fragmentation in the terminal. Only singly charged cluster ions have been observed so far. C refers to charge-exchange at the terminal with fragmentation. The Cy ion is the only intense multiply charged ion observed so far. d refers to decay in flight of the clusters after acceleration.
As can be seen in the figure the data do not fall on one calibration line for the detector. The deviation from the line for the C1 data is larger for larger cluster ions. This effect will be further discussed in section 4. Figure 6 gives a summary of the different processes described above leading to the different cluster beams studied. The figure also shows schematically where these processes take place in the accelerator and beam line. 4. Discussion When looking at TOF spectra like the ones in Fig. 3 one should keep in mind that the intensity distribution for the different peaks in the spectra depends critically on the tuning of the accelerator and beam line parameters. This is of course nothing strange but a user of cluster ion beams has to pay a lot of attention to this fact. Different cluster beams, produced in the break-up of clusters in the terminal, requires also quite different values of the steerings in order to get them into the experimental chamber. However, the peaks Ci’, Ci’, and C8 and the corresponding peaks in the C8 and Cl2 case are always present and constitute a characteristic fingerprint for cluster beams also observed for beams of gold [27] and C& clusters [28]. Another characteristic signature is the lack of peaks between the intact singly charged cluster ion and the corresponding neutral one in the TOF spectra. In the energy calibration of the silicon detector it is not clear why the data do not follow on a single calibration line, see Fig. 5. Consider e.g. the Ct point and the Ci+ point. The calculated energies are 10.8 and 9.9 MeV respectively including a correction for a “dead layer” of 500 A. The measured detector response for the Cp cluster ion is however too low compared with a calibration line defined by the C1 atomic ion data points. There are however several important differences between the two cases. (1) The correction for the energy loss in the “dead layer” is much more critical for the cluster ion than the atomic ion. In this example the energy-
K. Boussofiane-Baudin
et aLlint. J. Mass Spectrom.
Ion Processes
loss/atom value is about the same in the two cases but the Cp energy should be subtracted by nine times this value and the Cp energy only by once this value. (2) The range of the C:’ ion in silicon is about 10pm which is very long compared with the assumed thickness of the “dead layer”, 0.05 pm. For the carbon atoms from the CF cluster ion, the range is only about 1.5 pm. In the case of the cluster, atoms will therefore be slowed down and stopped close to the “dead layer” at the surface of the detector. This may introduce a decrease of the amplitude in the output signal. (3) The assumption that the contributions from the different atoms in a cluster can be added to give a correct output signal corresponding to the impact of a cluster on the detector may be totally wrong. Many atoms hitting the detector at the same time may induce plasma like conditions with again a possible loss in amplitude in the output signal as a consequence. (4) Very little is known about energy measurements with silicon detectors of high-energy carbon clusters and there might be new effects involved. In any event the two ways of plotting the energy data as mentioned in the text constitute a self consistent way of analysing the data. If a peak is misinterpreted and assigned with the wrong energy it will not fall on the corresponding curves. The energy measurements can therefore be used to investigate if the clusters reaching the experimental chamber are intact or not. One can argue however that it is not possible to claim that the clusters are intact when they hit the detector because they can decay just before entering the detector. How does one know that the measured energy signal from a Ci+ ion is not n Ci atoms hitting the detector at the same time? It is reasonable to assume that the clusters decay with a certain rate that is not changing with time or location in the beam line. If many decays happen in the vicinity of the detector it should also often happen that at least one of the cluster constituents should miss the detector. In the latter case the detected energy should be less than for the CA+
130 (1994)
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peak but intense peaks of this type are not observed in the energy spectra for the carbon cluster ions. The fact that only one large peak is observed in a particular spectrum is thus strong evidence that the corresponding cluster was intact when it hit the detector. 5. Conclusions With the help of TOF and energy measurements it has been demonstrated that high energy stable carbon cluster ion beams CA’ (2s.n~ 12) can be produced and transported to the experimental chamber to investigate the interaction of fast clusters with solid surfaces. In particular the beams C:+, C:+ and Ci’ have been discussed. It is however known that other carbon clusters can be produced in a cesium sputter ion source. As long as the cluster ions remain stable it should be possible to accelerate them to high energy and identify them with the described technique in this paper. Carbon cluster ions C; (n < 19) have for example been accelerated to 324 keV with a Van de Graaf accelerator [29]. The shape of the carbon cluster ions depends on the number of carbon constituents [30]. Linear structures and closed structures can therefore be accelerated to mega electronvolt energies. The amount of energy loss in a solid and the secondary emission yield when bombarding a surface will be measured as a function of size and shape of these new projectiles in future experiments. This will be an intermediate step in the use of heavier exotic projectiles with large energy in similar experiments. Acknowledgements
The enthusiastic support from the staff of the accelerator are gratefully acknowledged. In particular, many thanks to J.M. Curaudeau, J.P. Mouffron and B. Waast. One of us (P.H.) would like to acknowledge the support from the Swedish Natural Science Research Council.
K. Boussoflane-Baudin et a/./M. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
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4
R.E. Johnson and B.U.R. Sundqvist, Physics Today, March (1992) 28. K. Wien, Nucl. Instrum. Methods B, 65 (1992) 149. Y. Le Beyec, in A.C.A. Souza, E.F. da Silveira, J.C. Nogueira, M.A.C. Nascimento and D.P. Almeida (Eds.), Collision Processes of Ion, Positron, Electron and Photon Beams with Matter, World Scientific, Singapore, 1992, p. 15. B.U.R. Sundqvist, in A.C.A. Souxa, E.F. da Silveira, J.C. Nogueira, M.A.C. Nascimento and D.P. Almeida (Eds.), Collision Processes of Ion, Positron, Electron and Photon Beams with Matter, World Scientific, Singapore, 1992, p. 59. D.F. Torgerson, R.P. Skowronski and R.D. Macfarlane, Biochim. Biophys. Res. Commun., 60 (1974) 616. P. Roepstorff, in D.M. Desiderio (Ed.), Mass Spectrometry of Peptides, CRC Press, Boca Raton, FL, 1990, p. 65. H. Dietzel, Cl. Neukum and P. Rauser, J. Geophys. Res., 77 (1972) 1375. D. Smith and N.G. Adams, J. Appl. Phys., 6 (1973) 700. H.H. Andersen and H.L. Bay, J. Appl. Phys., 45 (1974) 953, 46 (1975) 2416. D.A. Thompson and S.S. Johar, Appl. Phys. Lett., 34 (1979) 342. S.S. Johar and D.A. Thompson, Surf. Sci., 90 (1979) 319. R.J. Beuhler and L. Friedman, Nucl. Instrum. Methods, 170 (1980) 309. W. Knabe and F.R. Krueger, Z. Naturforsch Teil A, 37 (1982) 1335. J.P. Thomas, P.E. Filpus-Luyckx, M. Fallavier and E.A. Schweikert, Phys. Rev. L&t., 55 (1985) 103. M.W. Matthew, R.J. Beuhler, M. Ledbetter and L. Friedman, Nucl. Instrum. Methods B, 14 (1986) 448.
15 R. Beuhler and L. Friedman, Chem. Rev., 86 (1986) 521. 16 W. Reuter, Anal. Chem., 59 (1987) 2081. 17 J.P. Thomas, P.E. Oladipo and M. Fallavier, Nucl. Instrum. Methods B, 32 (1988) 354. 18 M. Salehpour, D.L. Fishel and J.E. Hunt, Int. J. Mass Spectrom. Ion Processes, 84 (1988) R7. 19 M. Salehpour, D.L. Fishel and J.E. Hunt, Phys. Rev. B, 38 (1988) 12320. 20 A. Hedin, P. HBkansson, B.U.R. Sundqvist and R.E. Johnson, J. Phys., 50 C2 (1989) 15. 21 J.P. Thomas, A. Oladipo and M. Fallavier, J. Phys., 50 C2 (1989) 195. 22 R.J. Beuhler, G. Friedlander and L. Friedman, Phys. Rev. Lett., 63(12) (1989) 1292. 23 M.G. Blain, S. Della-Negra, H. Joret, Y. Le Beyec and E.A. Schweikert, Phys. Rev. Lett., 63 (15) (1989) 1625. 24 M. Fallavier, J. Kemmler, R. Kirsch, J.C. Poizet, J. Remillieux and J.P. Thomas, Phys. Rev. Lett., 65 (1990) 621. 25 M. Benguerba, A. Brunelle, S. Della-Negra, J. Depauw, H. Joret, Y. Le Beyec, M.G. Blain, E.A. Schweikert, G. Ben Assayag and P. Sudraud, Nucl. Instrum. Methods B, 62 (1991) 8. 26 R. Middleton, Nucl. Instrum. Methods, 144 (1977) 373. 27 Ch. Schoppmann, P. Wohlfart, D. Brandl, M. Sauer, Ch. Tomaschko, H. Voit, K. Boussofiane, A. Brunelle, P. Chaurand, J. Depauw, S. Della-Negra, P. Hiikansson and Y. Le Beyec, Nucl. Instrum. Methods B, 82 (1993) 156. 28 S. Della-Negra, A Brunelle, Y. Le Beyec, J.M. Curaudeau, J.P. Mouffron, B. Waast, P. Hiikansson, B.U.R. Sundqvist and E. Parilis, Nucl. Instrum. Methods B, 74 (1993) 453. 29 R. Vandenbosch, D. Ye J. Neubauer, D.I. Will and T. Trainor, Phys. Rev. A, 46 (1992) 5741. 30 P. Joyes, Les AgrCgats Inorganiques Ekmentaires, Les Editions De Physique, Paris, 1990.
73
High energy carbon cluster beams K.Boussofiane-Baudin,
A. Brunelle, P. Chaurand, J. Depauw, S. Della-Negra, P. Hilcansson+, Y. Le Beyec*
Institut de Physique Nuclkaire, F-91406 Orsay, France
(Received 20 May 1993; accepted 30 July 1993) Abstract Carbon cluster beams with mega-electronvolt energies have been accelerated in the Orsay MP tandem accelerator. In a conventional cesium sputter ion source, negative beams of C,, Cc and C, cluster ions were produced and injected into the accelerator with a terminal voltage of 5.5 and 4 MV, respectively. Different charge exchange and decay processes in the terminal as well as in the beam line are described. Energy and time-of-flight measurements show that different carbon cluster ions can be formed, accelerated and transported far away from the accelerator and arrive intact at the experimental
Key work
Carbon clusters; Time of flight; Sputter ion source
1. Introduction
For many years atomic bombardment of solid materials has been used to eject ions from surfaces. The energy of impact has been in the range of a few kilo-electronvolts to tens of kiloelectronvolts where the dominant energy loss mode is attributed to nuclear cascade collisions. More recently, fast heavy ion projectiles with energies of several tens of mega electronvolts have also been used to bombard solid films of insulating materials (mainly organic solids) in the electronic stopping power regime. One of the surprising consequences of the large amount of electronic energy deposition is the emission of large intact molecules from the surface (or near the surface) of the solid [l-4]. The well-known method
* Corresponding author. t Permanent address: Division of Ion Physics, Department of Radiation Sciences, Uppsala University, Box 535, S-751 21 Uppsala, Sweden. SSDI 0 168-1176(93)03902-X
plasma desorption mass spectrometry using tission fragments from a 252Cf source with energies of 0.5-l MeV u-’ is today an efficient technique used in mass spectrometry to identify large biomolecules [5,6]. The studies of collisions of particles with solid surfaces have mainly been investigated with atomic ions, although there have been some pioneering experiments with polyatomic ions [7-241. A systematic study with various types of cluster projectiles and energies has focused on non-linear effects in secondary ion emission [25]. In those experiments the maximum energy of impact was below 100 keV and the projectile mass-to-charge (m/q) values were ranging between m/q 200 and 1000. To reach the electronic regime for energy deposition with cluster ions it is necessary to increase the total energy to several mega electronvolts. A particle accelerator such as a tandem accelerator has to be used for this purpose. In the interaction of cluster ions with solids a
74
K. Boussofiane-Baudin
large amount of energy can be deposited in a small volume of a solid. Several interesting phenomena can result from this transient, nonequilibrium energy deposition and very little is known about these extreme conditions created by cluster ion impact. The emission of secondary species is a channel of energy dissipation, and mass spectrometry will be used in the future to analyse the sputtered ions. In this work we describe a method to accelerate carbon cluster ions Cz’ to high energy. Time-of-flight (TOF) mass spectrometry is used to identify the cluster ion beam constituents with the most probable m/q value. With the help of energy measurements the final identification of the beam constituents is done and it is shown that intact carbon cluster ion beams can be obtained at the experimental site.
et al./Inr. J. Mass Spectrom. Ion Processes 130 (1994)
73-82
2. Experimental procedure 2.1. The tandem accelerator Figure 1 shows a schematic view of the accelerator and the experimental set-up. In a standard sputter ion source, Csf ions of 20 keV were used to bombard a carbon target. Negative carbon cluster ions were extracted from the source and mass analysed in a 35” injector magnet before being preaccelerated to around 180 keV. No special target preparation was needed in order to obtain carbon clusters. Figure 2 shows typical currents measured for different carbon cluster ion beams in a Faraday cup at the low energy side of the accelerator, prior to acceleration. After pre-acceleration the mass selected beam passes a buncher and deflector plate arrangement before entering the tandem
I I ~Expenmedl
L-IJLL ToF
!_llLL Energy
Fig. 1. Schematic view of the accelerator with the experimental set-up. In a sputter ion source negative carbon cluster ions are produced. They are pre-accelerated to about 180 keV and then injected into the tandem accelerator. The negatively charged cluster ions are accelerated towards the terminal on positive high voltage, about 5 MV in this study. In charge-exchange collisions with nitrogen gas in a stripper channel the cluster ions become positively charged and are accelerated a second time. The beam is pulsed and the TOF is measured from the deflection-electronics to a stop detector in the experimental chamber.
K. Boussofiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994)
4
_I
d,
2- lo*, lo’: 100 1 0
I 1
12c-
1 5
7
n
b
8
9
,
I i , , ,
l0 11 12 13 14 15 16
n (numberof atoms)
Ftg. 2. Measured currents in the low energy Faraday cup for different carbon cluster ions produced by sputtering of carbon by a 20 keV Csc beam. If the clusters have a mass larger than the mass of a Cs atom, great care must be taken to avoid interferences from mixed carbon-cesium clusters [26].
accelerator where it is accelerated towards the terminal at a positive potential of several million volts. In the terminal, at the centre of the accelerator, the negatively charged cluster ions pass a strip-
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15
per channel containing N2 gas. Owing to collisions between the gas molecules and the cluster ions they lose one or more electrons and become intact neutral cluster molecules or positively charged cluster ions. Some of the clusters break into fragments in the terminal and are accelerated in the second step as smaller cluster ions, or they continue with the constant speed obtained in the first acceleration step as neutral molecules. The gas pressure in the stripper channel is of great importance for the charge-exchange process and must be very low to avoid extensive fragmentation. 2.2. The experimental set-up After being accelerated the high energy beam is transported about 15 m to the 90” analysing magnet. As the magnetic field is not high enough to deflect singly charged high-mass cluster ions, an experimental chamber has been mounted after the analysing magnet in the 0” direction (see Fig. 1). The chamber has two microchannel plate detec-
-
10000 1t c1
1
1t 2
8000 C,2t
injected
5.13 MV
6000 z? 5 s 4000
/I'+ 1
L
d
52010
16000
16800
17600
18400
19200
20000
20800
21600
Time of flight (nsec) Fig. 3. TOF spectrum measured between a start pulse from the beam pulsing electronics and a stop detector in the expertmental chamber. q ions were injected into the accelerator with a terminal voltage of 5 MV. The ions CT and C!,‘, 1 < n C 8, are produced at the terminal by break-up of the Cc ions in the charge-exchange processes. The aluminium ions come from the target holder and are accelerated to the terminal as Al; ions.
K. Boussofiane-Baudin et aLlInt. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
76
tors, MCPl and MCP2, and a solid state silicon detector. All detectors can be used to give stop signals for time-of-flight (TOF) measurements but only the silicon detector can give an energy signal. One of the detectors, MCP2, is mounted at a fixed position at the back of the chamber while the other two can be moved in and out of the beam. The distance between the detectors MCPl and MCP2 is 80 cm. A pulsed beam was run through the accelerator and the TOF was measured with a multistop timeto-digital converter (model CTN-M2, Institut de Physique Nucleaire, Orsay, France) between a start pulse from the electronics for the deflection plates and a stop pulse from one of the stop detectors. Energy spectra were also recorded with the silicon detector in coincidence with peaks in the corresponding time-of-flight spectrum. Three experiments are discussed in this paper and all give very similar results. In the first run, Table 1 Calculated and measured time-of-flight the Orsay tandem accelerator
C, ions with an energy of 180 keV were injected into the tandem accelerator with a terminal voltage of 4.95 MV. In the second run, 180 keV C; ions were accelerated with 5.13 MV. In the third run, 163 keV C, ions were accelerated with 4.0MV. In the following text, these experiments are referred to as “the C8 case”, “the C9 case” and “the Cl2 case” respectively. Because of the similarity between the different cases only the C9 case is described in detail in the following text. 3. Results 3.1. TOF measurements
An example of a TOF spectrum, when C, ions were injected into the accelerator, is shown in Fig. 3. In this case the silicon detector was used as stop detector. The peak assignments have been done by comparing the measured TOF values with the
values for different cluster ions and neutrals
produced
when 180 keV Cc ions were injected into
T-P
E
T,I - Tap
5.72 6.31 6.91 7.50 8.09 8.68 9.27 9.86 10.45
5.72 3.16 2.30 1.87 1.62 1.45 1.32 1.23 1.16
1000 220 254 62 48 56 8 3 21
16 326.7 17 152.0 17 674.2 18 047.2 18 332.3 18 559.4 18 743.8 18 916.2 19032.6
16 340.6 17 164.3 17 682.6 18 053.6 18 336.8 18 562. I 18 746.5 18900.7 19031.8
13.9 12.3 8.4 6.4 4.5 2.7 2.7 -15.5 - 0.8
5.31
0.59
78
20 748.6
20731.6
-17
10.86 12.03 13.21 14.39
10.86 4.01 2.64 2.06
130 4 1 1
15 628.7 16795.5 17429.1 17 886.4
15 643.7 16 809.7 17447.8 17881.7
15 14.2 18.7 - 4.7
15.99
15.99
c’+ I
3
15283.7
15 302.3
18.6
7.79 6.46
3.90 3.22
Al;+ Al;+
6 38
18 193.4 17 295.0
18 203.9 17313.2
10.2 18.2
The acceleration voltage was 5.13 MV. The time T is the time-of-flight between a start signal from the beam pulsing electromcs and a stop signal from a silicon stop detector in the experimental chamber. All energies are in mega-electronvolts and the flight times in nanoseconds.
K. Boussofiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994)
corresponding ones calculated from a computer program which takes into account all dimensions and different voltages of the accelerator. As can be seen from Table 1 the agreement between the measured and calculated TOF values is very good, with a typical deviation of less than 0.5%. One should keep in mind that this type of TOF spectrum is not of the same type as the ones obtained in an ordinary TOF mass spectrometer. As a consequence these spectra can not be calibrated with the simple formula TOF = A[(m/q)]“* + B (A and B are constants) used in TOF mass spectrometry. Another procedure often used for the TOF measurements is the following. Two TOF spectra using the detectors MCPl and MCP2 as stop detectors respectively are recorded. By determining the time difference between corresponding peaks in these two spectra, the TOF for the ions needed to travel the distance between the stop detectors is obtained. This distance is 80cm and it is straightforward to calculate TOF values and compare with the measured ones. The agreement is often within 0.576 1% between these values although the flight path between the detectors is short. The spectrum in Fig. 3 shows some very typical features observed in the C8 and C9 cases, as well as in the Cl2 case. 3.1.1. The presence of a neutral C.90peak. These molecules originate from C, ions that have been accelerated to the terminal and then neutralised in the charge-exchange process; they are not accelerated in the second step of the accelerator and thus have only half the energy of the intact CF ions, 5.3 1 MeV in this case, provided they still are intact after the terminal. By applying a magnetic field in the analysing magnet this peak will not be influenced, nor will the peaks on the right hand side (not seen in the figure) with longer flight times, as they all are neutrals. However, all the peaks with shorter flight times than the neutral C8 peak will disappear almost completely and they must therefore be ions. The neutral components of the different ion peaks are thus very small. It is not clear
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II
where the neutrals with longer flight times than the Ct molecules are formed. 3.1.2. The presence of a Cp ion peak. These intact cluster ions correspond to accelerated Cc ions that have changed their charge from -1 to +l in the stripper channel, without fragmentation, and thus obtained the energy 10.45MeV. Multiply charged intact cluster ions have not been observed either in the C8 case or in the Cl2 case. The region between the CF ion peak and the corresponding neutral Ct peak normally contains no peaks because no clusters with TOF values in this time window can be formed when C, ions are injected into the accelerator. 3.1.3. The presence of cluster ions of the type Cz+ where n< 9. These ions correspond to accelerated Cc ions which break into smaller cluster ions in the charge exchange process in the stripper channel before they are accelerated further in the second step. The energies of these ions are (5.13 +O.l8)n/9 +q 5.13MeV. The only intense multiply charged ion observed so far is the C: ion followed by a weak Ci’ ion. For even shorter flight times than for this ion, no peaks are observed. Multiply charged cluster ions have only been observed for the rather weak C:+, Cp and C*+ I ions . However, for cluster ions CL being accelerated to the terminal, all ions Cz’, n G k, with the same n/q value will have the same TOF through the accelerator and will leave the accelerator with the same velocity. Therefore, with only TOF measurements it is impossible to distinguish ions like C:’ and Ci’ from each other. The possibility that multiply charged ions are hidden in the spectra can not therefore be excluded. The C;+ peak can, for example, contain contributions from Cy ions as well as from C3+ ions. In F$. 3 two peaks appear at the same TOF as for the Cg’ and CF ions. The corresponding peaks have not however been observed in the C8 or in the Cl2 case, and in fact these ions are Alif and Ali+ coming from injected Ali ions which break up at
78
K. BoussoJiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
the terminal. The mass of Al4 is the same as for Cs, namely m/z 108, and both ions can thus pass the injector magnet. The aluminium cluster ions are produced in the sputter ion source when a part of the Csf ion beam hits the aluminium target holder. This has been carefully checked in a separate experiment. The effect illustrates a problem when accelerating cluster ion beams. Different cluster ion beams with the same m/z value (or very close) can be injected into the accelerator and owing to breakups at the terminal, many different combinations can occur in a TOF spectrum and interfere with the correct peaks. With TOF measurements one can only conclude that a certain cluster has been accelerated to a certain energy when it leaves the accelerator. If the cluster then decays (owing to collisions or too-high internal energy) in flight in the beam line, the fragments will continue with the same velocity as the cluster had before the decay, and the time measurement will be the same as for an intact cluster. This problem will be further discussed in section 3.2. The TOF value for a cluster passing the accelerator is determined by time contributions at the low energy side where the cluster ions move with a low velocity before they have been accelerated. If the clusters continue from the terminal as neutrals, instead of singly charged cluster ions, there will only be a 9% increase in the TOF value in the C9 case, as can be seen in Table 1.
5ocI--
0
290
490
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ap
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, 4OCI-
IO 0
C51+
Si Energy spectra
I-
200
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s
0
c ‘+
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.iz 1.
$1'
4
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3 c
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42
28
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0
96
72
4a
24
3.2. Energy measurements To investigate if the clusters are intact, not only when they leave the accelerator but also when they have travelled 15 m in the beam line and arrive in the experimental chamber, energy measurements have been performed with a solid state silicon detector. The detector, manufactured by Ortec, has no entrance gold window but instead an ionimplanted contact layer at the surface with a thickness of x 500 A. The active area of the detector has a diameter of 8mm. The energy spectra were recorded in coincidence with the corresponding TOF spectrum which means that a window was
400
600
800
lcO0
1;
Channels u. energy Fik. 4. Energy spectra recorded in coincidence with windows around the (a) Cl+, (b) C:‘, (c) Ci’ and (d) C$ peaks, respectively, in the corresponding time-of-flight spectrum. The different fragment ions correspond to clusters that have decayed after leaving the accelerator.
K. Boussofiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994)
set around a peak in the TOF spectrum and the energy signals were only registered for events in that time window. The data acquisition system used in the experiment allows six windows to be set simultaneously in one TOF spectrum. Figure 4(a)-(d) shows such energy spectra with a time window set around the peaks C:+, Ci+,Cp and Ct respectively. From the energy calibration, described below, one can conclude that almost all the cluster ions are intact when hitting the detector. This is also the case for the C:+, C:+, Cy, Cp and Ci+ ions not shown here. However, some of the cluster ions decay in flight after leaving the accelerator. The small peaks in e.g. Fig. 4(a) correspond to ions from decayed C:+ cluster ions and it will be shown below that, in this case, the peaks correspond to the ions Cc, (1 < n < 4). In Fig. 4(d) intact Ci neutral molecules can be observed but most of the clusters have decayed and the peaks Cs and Ci are dominating the spectrum. The fragment Cy is not detected because the energy is below the electronic threshold used in this experiment. It is impossible to tell if these neutral fragments are created already in the charge-exchange process in the stripper channel or if they are fragments from C8 clusters that leave the terminal intact but then decay somewhere between the terminal and the experimental chamber. In all energy spectra a peak is observed at channel 661 corresponding to the energy for the Ci’ ion formed at the terminal. The presence of this extra peak in all spectra is due to an artifact in the data acquisition system but it will not influence the energy measurements. The energy spectra have been analysed in the following way. It is assumed that a cluster breaks immediately into atoms when it hits the detector. These atoms penetrate the entrance window or an inactive “dead layer” in the detector and loose some energy which is not registered by the detector. The atoms are then slowed down and stopped in the detector. Only the electronic part of the energy loss will create electron-hole pairs that will give rise to an output signal. This signal is assumed
19
73-82
to be the linear sum of the contributions from the individual atoms. The important parameter for a cluster is thus the amount of energy per atom rather than the total energy of the cluster. The first step in the analysis is to determine the centroid of all the peaks shown in Fig. 4 and calculate the corresponding energy these clusters and fragments are assumed to have when they are stopped in the detector. The energy loss in the inactive layer at the entrance of the detector should be included in the calculations. In calculating this energy loss correction one should keep in mind the following. Consider e.g. the cluster ion C:+ with its corresponding fragment ions shown in Fig. 4(a). The first calculated energy value for the Ci’ ion should be subtracted with an energy loss dE but the last value for the C!,+ cluster ion should be reduced by 5dE. (Remember the distinction between fragment ions CA’ created in decays after the accelerator and cluster ions CA+created at the terminal of the accelerator!) As the nuclear stopping is very small compared with the electronic stopping in this case no correction is needed for the fact that the detector only registers electronic
8
1 J
64The enqes
2-
o;,,,,,,,,,,,,,,,,,,,,,,,,,,, __. mo 400 0
are corrected
for o “deod layer” of 500 i 600
Km
PmJ
1200
-1
1400
Channel number Fig. 5. Plot of calculated energies of clusters and fragments versus the corresponding channel numbers observed m energy spectra when C< ions were injected into the tandem accelerator: (W ) data points for clusters or atomic ions created at the terminal; (IJ) data points for fragments coming from decays after the acceleration. The line is a least squares fit to all data points for carbon atomic ions. The energy value have been corrected for an energy loss in a dead layer in the detector with an assumed thickness of 500 A.
K. Boussojiane-Baudin et al./Int. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
80
energy loss. Two general features have been observed when analysing the energy measurements. (1) In a plot of the calculated energies versus the corresponding centroid values for a particular cluster with its fragments, the energy values follow a straight line. In such a plot the different fragments have the same energy/atom value. It is however observed that the lines systematically become slightly steeper for heavier clusters than for lighter ones. It seems as if the energy lost by a heavy cluster in the detector is not recorded properly. (2) In a plot of the calculated energies versus the corresponding centroid values for all ions with the same number of atoms, the energy values also follow a straight line. The slope of these lines also increases slightly with the number of atoms per cluster. Figure 5 shows a summary of the calculated energies for all fragments and clusters in the C9 case as a function of the corresponding channel numbers. The data points for ions created at the terminal are marked with filled squares to distinguish them from ions coming from decays after the acceleration which are marked with open squares. Tandem accelerator 6 Y Exit Terminal Entrance A
k.0
Beam line
-ck” ’
CnO”
n
-Ck”
-
-C,o
n
CnO n
-d -
-ck’-
B
ck’+ -ck’+
t’
C
Cn4+ “
,c:;::
nck
ncr-_d
C,,‘l+ C C+I+ -d n
-
p
Fig. 6. A summary of observed charge-exchange and decay processes and where they take place when Ci ions are injected into the tandem accelerator. A refers to the information of neutral intact or fragment molecules at the terminal. It is not possible to tell if an observed neutral fragment comes from an intact neutral molecule that decays in flight or if it is formed directly in the terminal. B refers to charge-exchange without fragmentation in the terminal. Only singly charged cluster ions have been observed so far. C refers to charge-exchange at the terminal with fragmentation. The Cy ion is the only intense multiply charged ion observed so far. d refers to decay in flight of the clusters after acceleration.
As can be seen in the figure the data do not fall on one calibration line for the detector. The deviation from the line for the C1 data is larger for larger cluster ions. This effect will be further discussed in section 4. Figure 6 gives a summary of the different processes described above leading to the different cluster beams studied. The figure also shows schematically where these processes take place in the accelerator and beam line. 4. Discussion When looking at TOF spectra like the ones in Fig. 3 one should keep in mind that the intensity distribution for the different peaks in the spectra depends critically on the tuning of the accelerator and beam line parameters. This is of course nothing strange but a user of cluster ion beams has to pay a lot of attention to this fact. Different cluster beams, produced in the break-up of clusters in the terminal, requires also quite different values of the steerings in order to get them into the experimental chamber. However, the peaks Ci’, Ci’, and C8 and the corresponding peaks in the C8 and Cl2 case are always present and constitute a characteristic fingerprint for cluster beams also observed for beams of gold [27] and C& clusters [28]. Another characteristic signature is the lack of peaks between the intact singly charged cluster ion and the corresponding neutral one in the TOF spectra. In the energy calibration of the silicon detector it is not clear why the data do not follow on a single calibration line, see Fig. 5. Consider e.g. the Ct point and the Ci+ point. The calculated energies are 10.8 and 9.9 MeV respectively including a correction for a “dead layer” of 500 A. The measured detector response for the Cp cluster ion is however too low compared with a calibration line defined by the C1 atomic ion data points. There are however several important differences between the two cases. (1) The correction for the energy loss in the “dead layer” is much more critical for the cluster ion than the atomic ion. In this example the energy-
K. Boussofiane-Baudin
et aLlint. J. Mass Spectrom.
Ion Processes
loss/atom value is about the same in the two cases but the Cp energy should be subtracted by nine times this value and the Cp energy only by once this value. (2) The range of the C:’ ion in silicon is about 10pm which is very long compared with the assumed thickness of the “dead layer”, 0.05 pm. For the carbon atoms from the CF cluster ion, the range is only about 1.5 pm. In the case of the cluster, atoms will therefore be slowed down and stopped close to the “dead layer” at the surface of the detector. This may introduce a decrease of the amplitude in the output signal. (3) The assumption that the contributions from the different atoms in a cluster can be added to give a correct output signal corresponding to the impact of a cluster on the detector may be totally wrong. Many atoms hitting the detector at the same time may induce plasma like conditions with again a possible loss in amplitude in the output signal as a consequence. (4) Very little is known about energy measurements with silicon detectors of high-energy carbon clusters and there might be new effects involved. In any event the two ways of plotting the energy data as mentioned in the text constitute a self consistent way of analysing the data. If a peak is misinterpreted and assigned with the wrong energy it will not fall on the corresponding curves. The energy measurements can therefore be used to investigate if the clusters reaching the experimental chamber are intact or not. One can argue however that it is not possible to claim that the clusters are intact when they hit the detector because they can decay just before entering the detector. How does one know that the measured energy signal from a Ci+ ion is not n Ci atoms hitting the detector at the same time? It is reasonable to assume that the clusters decay with a certain rate that is not changing with time or location in the beam line. If many decays happen in the vicinity of the detector it should also often happen that at least one of the cluster constituents should miss the detector. In the latter case the detected energy should be less than for the CA+
130 (1994)
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peak but intense peaks of this type are not observed in the energy spectra for the carbon cluster ions. The fact that only one large peak is observed in a particular spectrum is thus strong evidence that the corresponding cluster was intact when it hit the detector. 5. Conclusions With the help of TOF and energy measurements it has been demonstrated that high energy stable carbon cluster ion beams CA’ (2s.n~ 12) can be produced and transported to the experimental chamber to investigate the interaction of fast clusters with solid surfaces. In particular the beams C:+, C:+ and Ci’ have been discussed. It is however known that other carbon clusters can be produced in a cesium sputter ion source. As long as the cluster ions remain stable it should be possible to accelerate them to high energy and identify them with the described technique in this paper. Carbon cluster ions C; (n < 19) have for example been accelerated to 324 keV with a Van de Graaf accelerator [29]. The shape of the carbon cluster ions depends on the number of carbon constituents [30]. Linear structures and closed structures can therefore be accelerated to mega electronvolt energies. The amount of energy loss in a solid and the secondary emission yield when bombarding a surface will be measured as a function of size and shape of these new projectiles in future experiments. This will be an intermediate step in the use of heavier exotic projectiles with large energy in similar experiments. Acknowledgements
The enthusiastic support from the staff of the accelerator are gratefully acknowledged. In particular, many thanks to J.M. Curaudeau, J.P. Mouffron and B. Waast. One of us (P.H.) would like to acknowledge the support from the Swedish Natural Science Research Council.
K. Boussoflane-Baudin et a/./M. J. Mass Spectrom. Ion Processes 130 (1994) 73-82
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