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An ion optical system for the detection of charged fragments with high acceptance in a time-of-flight mass spectrometer
Nuclear Instruments and Methods in Physics Research B71 (1992) 330-338 North-Holland
Beam Interactions with Materials & Atoms
An ion optical system for the detection of charged fragments with high acceptance in a time-of-flight mass spectrometer Rainer Lork, Mark Bends, Rolf Jürgen Berger, Dieter Gassen, Klaus Schäfer, Marius Tybislawski and Wolfgang Neuwirth 1. Plrysikalisches Institut, Universität zu Köln, Zh1picher Sir. 77, W-5000 Köln 41, Germany Received 23 December 1991 and in revised form 22 April 1992
The ion optical system, described in this paper, has been designed to obtain a high transmission (close to unity) through a time-of-fligbt mass spectrometer, which analyses the charged fragments resulting from ion-molecule collisions with high time resolution. An ion optical conduction without losses is an indispensable condition to measure correlations between several charged fragments produced in one ion-molecule collision directly via time-of-flight analysis. If the transmission is not affected by the initial kinetic energy of the fragments (several eV are released if the dissociation occurs from an excited dissociatioe state), and the initial velocity distribution is isotropic, line shape analysis can now be used to evaluate the initial energy distribution of fragments originating from a specific decay channel, which can be selected just by their correlation. Furthermore, the realized detection method, which is based on counting the secondary electrons released by the fragment striking an ion-converter dynode, can be extended to deliver further information, such as the fragment's charge state . Various measurements, using methyl chloride as the target gas and analysing initial energy as well as correlation phenomena, prove that the realized system meets all the requirements high-resolution time-of-flight analysis poses while representing an essential experimental progress, which was mainly due to the discovery of an Ton optical element consisting of concentric electrodes that we call "Doppelring" . 1. introduction The interaction of fast ions with molecules produces a wide range of excitation and reaction phenomena . In the energy range up to 150 keV, the projectile velocity is comparable with the average velocity of the valence electrons of the target atoms; therefore the influence of the molecular structure is pronounced. For the investigation of those interactions which lead to charged reaction products, we have developed an apparatus, which has been described in [1] . It consists of a 150 kV ion accelerator providing the projectiles (H + , H'-+, or H3 ions), a molecular beam system creating the gas target, and a time-of-flight mass spectrometer (TOFMS) to analyse the charged reaction products. The range of reactions which can be investigated with this apparatus extends from a simple ionization of the target molecule to its fission into several fragments (charged or not). Moreover, these fragments can start with considerable initial energy, released if the dissociation occurs from a repulsive excited state. All imaginable charged fragments are observed, even those which cannot be explained by the ground state bond structure of the original molecule . But these specific reaction channels appear with very different abundances, which
makes a thorough investigation very interesting. The measurable quantities are the cross section for the production of the fragments, their initial energy, the probability for a correlated production of several specific fragments in a single collision and the dependence of these quantities on the energy and type of projectiles. In order to make clear the experimental possibilities of the existing apparatus [1], a short description of the flight time measuring method is presented. The basic setup is shown in fig. 1 . The masses are identified by their flight time, which is measured in the following way : The time of production of the fragments is fixed by pulsing the projectile beam . Simultaneously with each projectile pulse (with a width adjustable from 0 .5 ns up and a repetition rate up to 10 kHz) a "clock" is started . The produced charged fragments are extracted by the electric field of an ion optical system. After drifting through a field-free tube, they are post-accelerated by an additional ion optical system and focused on a detector (previously a channeltron). The detector signal released by an impinging fragment stops the running clock. Hitherto, the flight time (Tt - m/q ) has been measured with a time-to-amplitude converter (TAC), and the corresponding spectrum has been generated, via analog-digital conversion (ADC), in a PC .
0168-583X/92/$05.00 © 1992 - Elsevier Science Publishers B .V. All rights reserved
R. Lark et al. / TOFspectrometer for charged fragments Projectiles
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time, the fragments move as if they would have started with the same kinetic energy directly towards the detector . The turn-around time is determined by the axial components of the initial velocities the fragments are ejected with and the local electric field strength in the interaction region . Radial velocity components influ-
ence the flight time only in second order, even if inhomogeneities in the extraction field exist. Therefore, the turn-around tim,, of just those fragments which start with maximum axial initial velocity corresponds to the width of the line in the flight time
spectrum . Other effects on the line width due to different spatial start parameters in the finite interaction volume (flight time differenceson different trajectories through the spectrometer and varying path lengths Fig. 1 . Basic scheme of the time-of-flight mass spectrometer and simplified circuit diagram.
With this setup the flight time measurement of only one fragment per projectile pulse has been possible until now. Frequently, a single ion-molecule collision does not lead to only one charged fragment, but to several
simultaneously. If the transmission through the spectrometer were complete, only the fastest, and therefore the lightest, would be detected . The slow (heavy) fragments are suppressed by the faster (lighter) ones. In order to reduce this suppressing effect, especially with correlated fragments, it was previously necessary to
lower the transmission considerably, so that the detected fragments were selected statistically. Therefore, the previous ion optical system deliberately conducted less than 10% of the produced charged fragments to the detector.
The trajectory of a charged particle depends in electrostatic potentials only on its initial kinetic energy, provided its charge state and spatial initial coordinates are given. Therefore, a reduction of the transmission from unity leads automatically to a dependence on
initial energy . This can falsify the measurement of the yield of fragments with different initial energies . Nevertheless, it is possible to investigate the relative yield of fragments as a function of the projectile energy (10-150 keV) [2] and sort (H +, H2+ , or H3 ions) [3]. Geometry and potential distribution of the previous spectrometer [1] have been designed particularly with respect to optimum time resolution . The flight time
spread of a specific fragment, due to different start parameters, is mainly affected by the initial kinetic
energy . Those fragments which start with a velocity component away from the detector, are first slowed down in the extraction field and then accelerated back towards the detector. After passing their original plane of production, delayed by the so-called turn-around
resulting from different places of origin, as described in [1]) are of second order and have actually been
minimized by an appropriate geometry and field distribution of the spectrometer. Thus in designing new ion optics, we profited from the experiences gained with the development of the previous spectrometer . Hence, to give some characteristic numbers, with the present arrangement described in section 2, all fright
time
differences between H+ fragments starting with the same axial component of initial velocity are, according to ion optical calculations, less than 2 ns for initial
energies up to 10 eV, compared with a flight time spread of 64 ns for protons with an initial energy of 10 eV . Further contributions to the line width arise from the electronics and the finite projectile pulse duration,
both affecting every fragment in the same way. The overall time resolution of the instrument can in principle be determined from the line width of those fragments which start with thermal energies only . Here, we obtained line widths ranging from 4-10 ns for masses from 1-100 amu with flight times between 2.5 and 25 ws.
A general formula for the transformation from measured flight time into initial energy distribution of a specific fragment was explicitly deduced in [4] under the condition of a complete transmission through the spectrometer, an isotropic initial velocity distribution
and a homogeneous extraction field. The latter condition is fairly well satisfied by our new ion optical system, inasmuch as the field gradients in the interaction region are less than t0.3 V/mmz, compared with a mean extraction field of about 14 V/mm, and will therefore contribute only in second order to the flight time distribution . If an anisotropy in the initial velocity distribution of a decay channel exists (only the axis of the projectile beam could serve as preferential direction), the corresponding time-of-flight line shape would result from the convolution of initial energy and angular distributions of the underlying dissociation process . Initial energies up to 15 eV have been observed [5] . But the line shapes measured with the previous system
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indicated that fragments with high initial momentum perpendicular to the symmetry axis of the spectrometer were detected with distinctly lower probability than those starting with smaller transversal momentum. If such an initial energy dependent loss of fragments can be avoided by using appropriate ion optics, the initial energy distribution of a specific fragmentation channel can be calculated via line shape analysis [4]. The knowledge of these initial energy distributions allows conclusions about the molecular states involved in the underlying dissociation process, thus giving additional information about how fragmentation occurs. A further essential contribution to the identification of reaction channels in ion-molecule collisions consists in finding out correlations between fragments, i .e . the simultaneous production of more than one charged fragment in one ion-molecule collision. The direct measurement of such correlations still requires a substantial extension of the apparatus, namely a transmission close to unity in combination with a so-called multiple stop system, which allows simultaneous measurement of the flight times o¬ several fragments. A transmission close to unity for every fragment is only possible if it is independent of tl, -- initial energy. The ion optical task is then to focus fragments with high initial energy, wherever they are created (within the interaction region) and in whichever direction they are ejected. A transmission as close to unity as possible is a basic condition for both precise and quantitative measurements of correlations between fragments and that of their initial energy distribution . New experimental evidence about the fragmentation are realized by com-
bining both aspects. Hence it becomes possible to analyse the initial energy distribution not only of a specific fragment, but even of the fragments originating from a specific reaction channel, which is fixed by a correlated pair of fragments. Fragmentation processes sometimes lead to multiply charged fragments. If a doubly charged fragment with odd mass number appears with half-integral masses in the time-of-flight spectrum, it can then be identified clearly. Two fragments with the same massto-charge ratio, however, are indistinguishable in the spectrum, as their flight times are identical . But the doubly charged fragment receives twice the kinetic energy in the electric fields of the spectrometer. If the chosen detector system allows the determination, of not only the flight time, but also the fragment's energy and thus its charge state, an unambiguous mass classification will be possible . The energy resolution of the hitherto employed channeltron, however, is not sufficient to get any information apart from the flight time. This was the motive to develop an alternative detection method with the option to extract additional information from the pulse height of the detector signal in future investigations . 2. Ion optics of the time-of-flight-mass spectrometer For the purposes mentioned in the introduction, a higher transmission through the spectrometer was needed. The ion optical design of the hitherto existing system thus had to be refined in order to obtain better focusing qualities, but without losing its excellent time
Projectiles
Fig. 2. Trajectories (full lines) through the extraction and acceleration region. The blown-up sketch of the interaction region (hatched area) illustrates their start parameters. The electric potentials of the elements are given in kV and equipotential lines (dotted lines) are drawn (with a potential difference of 200 V).
R Lork et at. / TOFspectronteter for charged frnvntnts behaviour. Therefore, a more elaborate ion optics was necessary. In view of the variety of occurring start parameters (fragments are produced at positions up to 1.5 mm off the axis and ejected in all directions with initial kinetic energies up to 15 eV), every analytical approximation method fails (as, e .g. Gaussian optics for paraxial trajectories). Consequently, the potential 'distribution within the spectrometer has to be computed numerically by solving the Laplace equation under consideration of boundary conditions [6], where the task is to find these boundary conditions empirically by choosing an appropriate geometry and voltage distribution of the ion optical system . Its focusing qualities are controlled by computing the motion of ions with extreme but realistic start parameters. The optimum arrangement can only be found step by step, however, with feedback of gained experience. It was not possible to reach the required focusing properties with the usual repertoire of ion optical elements, to which, for example, the well-known einzel lens belongs [7]. The breakthrough was achieved by introducing a new ion optical element, which consists of two concentric ring electrodes ("Doppelring"), where the inner one has a higher negative potential, causing a stronger force towards the axis than common lenses. The spectrometer is basically divided into three consecutive units : an extraction-acceleration region, drift tube, and detector region. The drift tube is field free, resulting in the advantage of being able to calculate the ion optics of the extraction-acceleration and the detector region separately. After many numerical calculations, the arrangement shown in fig. 2 turned out to be the optimum one for the extraction-acceleration region. Here, the geometry and potential distribution and some paths of ions with extreme start parameters are drawn from the interaction region up to the drift tube . The occurring spatial initial coordinates are located within the interaction region, i .e . the intersection of projectile and molecular beam. The axial symmetry of the spectrometer makes the physical situation the same in every plane contain-
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ins the spectrometer axis. This diagram presents the flight paths of ions starting 1.5 nun off the axis in different directions with an initial energy of 10 eV. Those fragments which start perpendicular to the axis are the most difficult to focus because they soon diverge from the axis . If fragments are produced at finite distances from the ion optical axis, further complications turn up as it then becomes significant whether or not they begin perpendicularly away or towards the axis (trajectories a and b, respectively, in fig. 2). In this case, a simultaneous focusing of these two extreme fragment paths is not possible, because if one of them is focused on the axis, the other one will inevitably diverge . The compromise in the realized system is to make both meet at one point in the detector plane, which is, however, situated somewhat outside the ion optical axis . Nevertheless, the beam profile at the end of the drift tube (with a length of 105 cm) could be reduced to 3 cm with a divergence of less than one degree, compared with a diameter of 6 cm with the previous system. These values refer to an initial energy of 10 eV, which is rarely exceeded in fragmentation processes. But the intended detection method still necessitates concentration of the ions into a narrow and only slightly divergent beam. This has to be done by an additional ion optical system, following the field free drift tube . The fragments leave the drift tube with a kinetic energy of 3 .5 keV, according to the potential of the tube . For their final focusing into the detector system a voltage of up to 50 kV is available, which can be also used, as will be discussed later, for the detection process. Various numerical attempts, using conventional einzel and cylindrical lense- have failed . The curvature of equipotential lines, indicating radial electric field strength, remained too weak to give a sufficient attraction towards the ion optical axis. In particular, with einzel lenses the focusing was always too diffuse, especially for fragments moving at greater distances from the axis. The "Doppelring" was the crucial element to solve this ion optical problem. Fig. 3 shows the arrangement
Fig. 3. Focusing of fragments through the "Doppelring" ion optics. The electric potentials of the elements are given in kV and equipotential lines (dotted lines) are drawn (with a potential difference of 2.5 kV).
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of five of such elements, which accomplished the demanded properties right away. The attractive negative potentials of the inner rings of the "Doppelring" system decrease more rapidly than those of the outer rings, exactly in such a way that the electric forces are adjusted to the kinetic energy which the accelerated fragments have actually gained. The resulting curvature of the equipotential lines, i.e. the radial component of the electric field, is now strong enough to attract the fragments gradually but sufficiently towards the ion optical axis. Thus, initially parallel trajectories are focused into one point, analogous to light optics. All focal points are located within the same plane (focal plane). The distance of the focal points from the axis depends on the angle of the incoming trajectories relative to the axis. Here, fragments with an angle of 1° meet 2 mm away from the ion optical axis . But the angles caused by the ion optical system of the extraction-acceleration region remain below 1° for all occurring start parameters. Consequently, 4 mm represents a conservative estimate of the diameter of the focal spot. Furthermore, the angular spread after the ion optical system of the detector region remains small as well . The trajectories of the fragments have angles of less than 3° with respect to the axis, depending on their radial distance from the axis when entering the "Doppelring" ion optics . The parameters of the ion optical arrangements displayed in figs . 2-4 are the optimum ones for the given geometry and dimensions of our setup. For other configurations different potential distributions will result, but nevertheless the "Doppelring" elements with their favourable focusing properties will be a suitable means of designing similar instruments.
3. Detector system Ions of medium energy, in the range of a few tens of keV, can be detected via production of secondary electrons at a target surface . As shown in section 2, our new ion optical system prepares a relatively narrow and only slightly divergent beam of fragments, whose waist lies, moreover, in the field free region . Therefore it makes sense to place the ion-converter dynode just there, and to extract and focus the produced secondary electrons by a separate, likewise cylindrically symmetric, electron optical system (maximum voltage adjustable up to 50 kV), whose axis is orientated perpendicular to the spectrometer axis . In the region of the dynode, a weak electric field suffices to extract the secondary electrons without noticeably affecting the fast incoming ions. A design developed by Dietz and Sheffield [81 served as a prototype for this electron optical system. The complete arrangement of the ion and electron optics in the detector chamber is illustrated in fig. 4, showing also some typical electron paths. The secondary electrons are accelerated to kinetic energies up to 50 keV and focused head-on on a silicon surface barrier detector (Ortec model BA-014-050-100). Each of the secondary electrons produced by a single impinging fragment deposits an energy of up to 50 keV in the detector, depending on the chosen acceleration voltage . Hence, the pulse height of the detector signal is directly proportional to the number of secondary electrons released by one specific fragment. The energy resolution of the surface barrier detector (better than 14 keV) is sufficient to distinguish between n and n + 1 electron events, even if the energy per electron is
5cn Fig . 4. Design of the ion and electron optics in the detector chamber. The electric potentials of the elements are given in kV and typical electron paths through the electron optical system are shown.
R Lork et a1. / TOF spectrometer for chargedfragments reduced below 30 keV. To each peak in the pulse height spectrum thus corresponds a fixed number of secondary electrons. Its distribution and average value, the so-called average secondary electron yield, is characteristic for the specific fragment. These characteristics cannot be expressed in a general formula, because the secondary electron yield depends too strongly on the individual structure of the fragments [9,10] . Up to velocities corresponding to those of 50-keV protons, the secondary electron yield of a particular fragment increases proportional to its velocity. Because of their great penetration depth, protons generally produce the fewest secondary electrons, viz. only one or two in case of normal incidence. With aluminium as the dynode material, the secondary electron yield of protons reaches its maximum at an energy of 50 keV before falling again afterwards [11,12]. In order to detect the produced H + fragments with maximum efficiency, a post-acceleration voltage of 50 kV is therefore sufficient . This detector system, counting secondary electrons, is a device that is not restricted to providing fast electronic signals for time processing only, as already the previous channeltron did, but can in principle be extended to gather further information from the structured pulse height spectrum of secondary electrons that it delivers. 4. Measurements The further development of the apparatus described in this paper has been primarily undertaken to enhance the overall acceptance of our spectrometer (i .e. the product of transmission and detection efficiency). The ion optical improvements carried out in this respect increased the transmission, whereas the detection efficiency was raised as close to unity as possible, independent of the fragment sort, by using a new detection method . How far these aims were achieved with the new ion optical and detector system, will be revealed through the measurements presented in the following. Fig. 5 shows the time-of-flight mass spectrum of the fragments of CH 3Cl molecules, produced in collisions with 65-keV protons. All measurements were performed under single collision condition with an average yield of one charged fragment per every tenth projectile pulse, which was realized with a pulsed proton beam (about 1000 protons per pulse) and a surface density of a few 10" molecules/cm2 in the gas target. As with the previous system the time spread of the peaks is again mainly determined by the initial kinetic energy which the fragments receive in the dissociation process and not by instrumental effects, such as mentioned in the introduction. The combined influence of
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H+ -- CH3Cl
r
10
go
30
40
so -
41â
Mass [u] Fig. 5 . Time-of-flight mass spectrum of methyl chloride produced by collisions with 65-keV protons. these effects can be seen in the line shapes of those fragments that start with thermal energies only (e.g. the parent ion CH 3Cl + which delivers a line of symmetric shape with a width of 9 ns). The line width of thermal H + fragments, for comparison, was 4 ns. For purely thermal peaks this means a mass resolution of m/Om > 500. However, most of the fragments are produced with finite initial energy. For example, the lines of CI + (mass 35 or 37) and HCI + (mass 36 or 38), with a width of 170 ns and nearly rectangular shape, were just separated, indicating a mass resolution of about 40 for initial energies of the order of 2 eV. It was not the intention to obtain merely a high mass resolution, which could be raised by increasing the extraction field, but to gain a high sensitivity for the influence of initial energies on the line shapes . The principal difference compared with the previous spectrometer is that the line shape of the single peaks is no more affected by a loss due to incomplete transmission of fragments starting with high transversal momentum . With isotropic initial angular distribution one would expect fragments starting with a well defined energy to have a rectangular time-of-flight peak [4]. As can be seen in fig. 6, that is in fact the case with the flight time distribution of H3 fragments, originating from the dissociation of CH 3CI molecules and starting with a uniform initial energy of approximately 5 eV. This exceptional fragment serves especially well for this purpose, as it originates from a single reaction channel, namely the break-up of CH 3C1 into the correlated pair H3 and CCl + from a well defined excited state . The narrow initial energy distribution of this complete reaction channel (the two involved fragments complement each other to the full parent molecule) manifests itself in the steep flanks of the resulting time-of-flight distribution (see fig. 6). The line shapes of most other fragments of CH 3 C1 indicate that more than one excited state is involved. (More details are
li" Lork et a!. / TOFsix ctmnwter for chargedfragments presented in a subsequent paper.) With the former setup all time-of-flight peaks of the fragments of higher initial energy showed a deep dip in the center (dashed line in fig. 6), just at flight times related to initial directions perpendicular to the spectrometer axis. Here, all fluctuations on the plateau are purely statistical, so it can be concluded that the transmission through the spectrometer is better than 95% . A modification of the flight time measuring system, namely its extension to a so-called duo stop system by adding a second clock (TAC). allowed us to measure the flight times of two fragments per projectile pulse simultaneously. (In the future we will use a time-to-digital converter for eight stop events per one start event, so as to also straightforwardly measure higher correlations between the fragments.) A logic gate between the two TAC's, each connected to its own ADC, enables the second TAC only after the first one has been stopped by the first incoming fragment . Thus we obtained two time-of-flight spectra, and by setting up to seven windows around the expected masses of interest while recording the spectra, it became possible to measure the correlations between the respective fragment pairs. Problems with pileup arise only in the rare cases when in one ion-molecule collision two correlated fragments with flight time differences of less than one microsecond are produced. The ion optical parameters .-/q ps of our instrument give a flight time of 2 .5 x V, for a fragment of mass m (in amu) and charge q (in e). Accordingly, the flight time difference for the aforementioned correlated pair (H ., CC]') amounts to about 13 Ws. Various correlated fragments have been observed. A detailed analysis of the results will be reported in a subsequent paper . In this paper we discuss only those results that are relevant to show the operating conditions of the new system. With two clocks, the suppression effect, mentioned in the introduction, exists only if a third charged frag-
a 400
L
Û 200
4.0
Time of Flight
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lNsed
Fig. 6. Time-of-flight distribution of H3 fragments produced in collisions of 65-keV protons with CH 3 Cl molecules. The dashed line represents the line shape measured with the former system .
Table 1 Percentages (related to the total yield of all positively charged fragments) of the different orders of correlation between positively charged fragments produced in collisions of 65-keV protons with CH,Cl molecues . Fragment correlation CH_ICI + molecularion positive fragment without partner two positively charged fragments three positively charged fragments
Abundance 26% 61% 12% 0.2%
ment is produced during a single pulse. If one CH 3Cl molecule decays into three charged fragments, a hydrogen ion will be the fastest of them. It is possible to exclude in a separate measurement these light fragments (H +, H ; , and H 3 ) from the flight time measurement by starting the clock (TAO with a delay of somewhat more than the H ; flight time . A corresponding measurement with protons of 65 keV showed that the yield of the correlated fragment pairs (C +, CI'), (CH', CI'), and (CH", CI') increased by 15%, 6%, and 2%, respectively. This increase in yield is obviously due to triple correlation, being suppressed by the fast hydrogen ions in the first measurement . This suggests that the underlying transition leads over a triply charged CH 3 C1 3+ intermediate state . Up to now a doubly charged intermediate state is known in the fragmentation of CH 3Cl [13], but, as far as we know, a triply charged intermediate state of a molecule has never been observed before. Table 1 shows the fraction of the different degrees of correlation in the fragmentation of methyl chloride . These values, even the rather small one for triple correlation, significantly exceed the random coincidences. The fraction of random coincidences can be determined by the yield of those pairs of fragments which cannot be produced from one molecule, as, for example, (CH3 , CH 3 Cl + ). The measured proportions indicate very small but finite probabilities for higher correlations (more than double) in collision-induced molecular dissociation processes, which calls for a more thorough research on this subject. The whole acceptance of the spectrometer, that means the probability that a produced charged fragment will be recorded, is the product of the transmission through the spectrometer and the detection efficiency. The transmission, which is the probability for a produced fragment to reach the detector, is governed by the initial kinetic energy of the fragment and its spatial start parameters. These are assumed to be equally distributed for each fragment sort, whereas the detection efficiency, according to the physics of secondary electron emission, depends on the structure
R Lork et al./ TOF spectrometer for charged fragments and composition of the fragment and its velocity when reaching the ion-converter dynode . A simple method to determine the acceptance of the spectrometer quantitatively would consist in measuring the correlation between two fragments where the cross section for their production is known. But, unfortunately, no absolute data about such correlations can be found in the literature . Nevertheless, our measurements with CH3CI as target gas revealed a strongly correlated pair of fragments, namely H3 and CCl +, where with each H3 ion a CCI + ion was detected with a probability of 80% . Both ions emerged from one and the same dissociation act, sharing the released dissociation energy of approximately 5 eV inversely proportional to their masses, according to momentum conservation. Such a mass dependent sharing of initial energy leads in turn to exactly the same flight time spread and line shape for both correlated fragments, which was well reproduced here. Now we can give a conservative estimate of the overall acceptance of our spectrometer, even for fragments of higher initial energy. The lower limit is 80%, if the correlation between H3 and CCl + were complete, which is not at all conclusive, as a correlated production of H3 with a neutral partner cannot be observed in our experiments. All in all, both the transmission and the detection efficiency are now very close to unity, which has not been achieved simply by expanding the sensitive detector surface (e.g., by using a multi-channel-plate, with the disadvantage of insensitive areas between the channels), but by the introduction of advanced ion optics. The excellent focusing properties of the "Doppelring" ion optics provide the essential condition to open new fields of investigation . An important aspect, which has to be considered carefully and which possibly limits the properties of the detectcr system, is that of noise electrons, released by field emission from the electrodes at high negative potential . With a voltage of 30 kV applied to the ion and electron optical systems in the detector chamber, the background counting rate amounts to 5 electrons/s, but rises steeply with increasing voltage . We could not assemble our device in a "clean room", had to do without complex in situ electrode conditioning procedures and moreover were not able to reach a better pressure than 10' mbar (ion source and gas target form two unavoidable leaks) in our non-bakeable vacuum chamber, which altogether severely impairs the high voltage performance of the system [14]. Noise electrons are particularly disturbing in the measurement of correlations between the fragments, in that they produce a larger background and therefore more random coincidences. For this reason the spectra presented here have all been measured with a voltage of 30 kV instead of 50 kV . As already seen, this did not discernibly affect the ion optical properties of the
337
cd a `o
d
50
100
150
Energy NO]
Fig. 7. Pulse height spectrum (background subtracted) of secondary electrons (dots) at a voltage of 30 kV applied to the detector system, in comparison with an empirical fit of separate Gaussians (full line).
"Doppelring" optics, at least not up to initial kinetic energies of the fragments of 5 eV. In order to prove the efficiency of the new detecting device, the gas target was charged with air and a high voltage of 30 kV was applied to the detector chamber. The background corrected pulse height spectrum (fig. 7) measured with the new detector system, scows the clear structure of the detector signal of secondary electrons after 30-keV ion bombardment of the target dynode. Every line in the energy spectrum can be attributed to the number of secondary electrons released by the fragment and reaching the detector each with a kinetic energy of 30 keV. The good accordance between measured energy spectrum (dots) and empirical fit with separate Gaussians (full line), proves the statistical significance of this detection method. The detection efficiency for an individual fragment increases with the number of secondary electrons. Since each secondary electron is detected with a reduced probability of 80%, due to the loss of backscattered electrons [8], with two secondary electrons the detection efficiency raises to 96% but already exceeds 99% if a convoy of three or more secondary electrons will be produced. This estimation delivers a conservative value for the detection efficiency, as backscattered electrons also deposit a certain fraction of their energy in the silicon detector [15]. Whether this fraction can raise the detection efficiency significantly above 80% in the case of one secondary electron only depends on the trigger levels to discriminate against the noise. As can be seen in fig. 7, the average yield of secondary electrons lies in the range of five, but in almost every case more than two electrons are released by the impinging fragment so that we can conclude that this detection method is highly efficient. The foregoing statement must be qualified for H + fragments since they mostly produce, as mentioned in section 3, only one or two secondary electrons, and
.Ms
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therefore the ability that none are released cannot he neglected. The proton is one of the most abundant fragments of hydrocarbon molecules, but unfortunately it also stems from the fragmentation of the residual gas, where, besides H,O, hydrogen is a substantial component, due to the outflow from the (hydrogen) ion source . The fraction of hydrogen in the residual gas depends sensitb,ely on the operating conditions of the ion source, which makes a background correction of the H ' (just as well as H,) fragments in the time-offlight spectra very difficult. Further experimental effort will be necessary to overcome the remaining uncertainty in the spectroscopy especially of those H + fragments that are produced without charged partner (only these cause serious problems because the others can be unambiguously accounted for by their correlation). For all other fragments such restrictions do not exist . Acknowledgement This work was supported Forschungsgemeinschaft.
by
the
Deutsche
R [1] W.Y. Baek, D. Gassen, iC. Förster, K. Schäfer and W. Neuwirth, Nucl. Instr. and Meth . B58 (1991) 266.
[2] D . Gassen and W. Neuwirth (to be published). [3] W.Y . Back, D. Gassen, K . Förster, K. Schäfer and W. Neuwirth, Nucl . Instr. and Meth. (submitted). [4] K. Schäfer, W.Y. Baek, K. Förster, D. Gassen and W. Neuwirth, Z. Phys. D21 (1991) 137. [5] A.K. Edwards, R.M. Wood and M .F. Steuer, Phys. Rev. A16 (1977) 1385. [6] See e.g . A. Septier, in: Focusing of Charged Particles, vol. 1 (Academic Press, New York, 1967) p. 45. [7] See e.g. P.W. Hawker, in : Principles of Electron Optics, vol. 2 (Academic Press, London, 1989). [8] L.A. Dietz and J.C. Sheffield, Rev. Sci. Instr. 44 (1973) 183 . [9] R.J . Beuhler and L . Friedman, J . Appl. Phys. 48 (1977) 39'28. [10] E.V. Alonso, R.A. Baragiola, J . Ferr6n, M.M. Jakas and A. Oliva Florio, Phys. Rev. B22 (1980) 80. [11] H .H. Andersen and J .F. Ziegler, Hydrogen Stopping Powers and Ranges in All Elements, vol. 3 (Pergamon, New York, 1977). [12] B. Svensson and G. Holmén, J. Appl . Phys. 52 (1981) 6928. [13] M .N . Monce, A.K. Edwards, R.M. Wood, M .F. Steuer, A.V . Shah and P. Tabor, J. Chem. Phys.74 (1981) 2860. [14] See e.g. R.V. Latham, in: High Voltage Vacuum Insulation: The Physical Basis (Academic Press, London-New York, 1981) . [15] R. Moshammer and R. Matthäus, J. Phys. Coll . (1989) C2-111 .
Beam Interactions with Materials & Atoms
An ion optical system for the detection of charged fragments with high acceptance in a time-of-flight mass spectrometer Rainer Lork, Mark Bends, Rolf Jürgen Berger, Dieter Gassen, Klaus Schäfer, Marius Tybislawski and Wolfgang Neuwirth 1. Plrysikalisches Institut, Universität zu Köln, Zh1picher Sir. 77, W-5000 Köln 41, Germany Received 23 December 1991 and in revised form 22 April 1992
The ion optical system, described in this paper, has been designed to obtain a high transmission (close to unity) through a time-of-fligbt mass spectrometer, which analyses the charged fragments resulting from ion-molecule collisions with high time resolution. An ion optical conduction without losses is an indispensable condition to measure correlations between several charged fragments produced in one ion-molecule collision directly via time-of-flight analysis. If the transmission is not affected by the initial kinetic energy of the fragments (several eV are released if the dissociation occurs from an excited dissociatioe state), and the initial velocity distribution is isotropic, line shape analysis can now be used to evaluate the initial energy distribution of fragments originating from a specific decay channel, which can be selected just by their correlation. Furthermore, the realized detection method, which is based on counting the secondary electrons released by the fragment striking an ion-converter dynode, can be extended to deliver further information, such as the fragment's charge state . Various measurements, using methyl chloride as the target gas and analysing initial energy as well as correlation phenomena, prove that the realized system meets all the requirements high-resolution time-of-flight analysis poses while representing an essential experimental progress, which was mainly due to the discovery of an Ton optical element consisting of concentric electrodes that we call "Doppelring" . 1. introduction The interaction of fast ions with molecules produces a wide range of excitation and reaction phenomena . In the energy range up to 150 keV, the projectile velocity is comparable with the average velocity of the valence electrons of the target atoms; therefore the influence of the molecular structure is pronounced. For the investigation of those interactions which lead to charged reaction products, we have developed an apparatus, which has been described in [1] . It consists of a 150 kV ion accelerator providing the projectiles (H + , H'-+, or H3 ions), a molecular beam system creating the gas target, and a time-of-flight mass spectrometer (TOFMS) to analyse the charged reaction products. The range of reactions which can be investigated with this apparatus extends from a simple ionization of the target molecule to its fission into several fragments (charged or not). Moreover, these fragments can start with considerable initial energy, released if the dissociation occurs from a repulsive excited state. All imaginable charged fragments are observed, even those which cannot be explained by the ground state bond structure of the original molecule . But these specific reaction channels appear with very different abundances, which
makes a thorough investigation very interesting. The measurable quantities are the cross section for the production of the fragments, their initial energy, the probability for a correlated production of several specific fragments in a single collision and the dependence of these quantities on the energy and type of projectiles. In order to make clear the experimental possibilities of the existing apparatus [1], a short description of the flight time measuring method is presented. The basic setup is shown in fig. 1 . The masses are identified by their flight time, which is measured in the following way : The time of production of the fragments is fixed by pulsing the projectile beam . Simultaneously with each projectile pulse (with a width adjustable from 0 .5 ns up and a repetition rate up to 10 kHz) a "clock" is started . The produced charged fragments are extracted by the electric field of an ion optical system. After drifting through a field-free tube, they are post-accelerated by an additional ion optical system and focused on a detector (previously a channeltron). The detector signal released by an impinging fragment stops the running clock. Hitherto, the flight time (Tt - m/q ) has been measured with a time-to-amplitude converter (TAC), and the corresponding spectrum has been generated, via analog-digital conversion (ADC), in a PC .
0168-583X/92/$05.00 © 1992 - Elsevier Science Publishers B .V. All rights reserved
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time, the fragments move as if they would have started with the same kinetic energy directly towards the detector . The turn-around time is determined by the axial components of the initial velocities the fragments are ejected with and the local electric field strength in the interaction region . Radial velocity components influ-
ence the flight time only in second order, even if inhomogeneities in the extraction field exist. Therefore, the turn-around tim,, of just those fragments which start with maximum axial initial velocity corresponds to the width of the line in the flight time
spectrum . Other effects on the line width due to different spatial start parameters in the finite interaction volume (flight time differenceson different trajectories through the spectrometer and varying path lengths Fig. 1 . Basic scheme of the time-of-flight mass spectrometer and simplified circuit diagram.
With this setup the flight time measurement of only one fragment per projectile pulse has been possible until now. Frequently, a single ion-molecule collision does not lead to only one charged fragment, but to several
simultaneously. If the transmission through the spectrometer were complete, only the fastest, and therefore the lightest, would be detected . The slow (heavy) fragments are suppressed by the faster (lighter) ones. In order to reduce this suppressing effect, especially with correlated fragments, it was previously necessary to
lower the transmission considerably, so that the detected fragments were selected statistically. Therefore, the previous ion optical system deliberately conducted less than 10% of the produced charged fragments to the detector.
The trajectory of a charged particle depends in electrostatic potentials only on its initial kinetic energy, provided its charge state and spatial initial coordinates are given. Therefore, a reduction of the transmission from unity leads automatically to a dependence on
initial energy . This can falsify the measurement of the yield of fragments with different initial energies . Nevertheless, it is possible to investigate the relative yield of fragments as a function of the projectile energy (10-150 keV) [2] and sort (H +, H2+ , or H3 ions) [3]. Geometry and potential distribution of the previous spectrometer [1] have been designed particularly with respect to optimum time resolution . The flight time
spread of a specific fragment, due to different start parameters, is mainly affected by the initial kinetic
energy . Those fragments which start with a velocity component away from the detector, are first slowed down in the extraction field and then accelerated back towards the detector. After passing their original plane of production, delayed by the so-called turn-around
resulting from different places of origin, as described in [1]) are of second order and have actually been
minimized by an appropriate geometry and field distribution of the spectrometer. Thus in designing new ion optics, we profited from the experiences gained with the development of the previous spectrometer . Hence, to give some characteristic numbers, with the present arrangement described in section 2, all fright
time
differences between H+ fragments starting with the same axial component of initial velocity are, according to ion optical calculations, less than 2 ns for initial
energies up to 10 eV, compared with a flight time spread of 64 ns for protons with an initial energy of 10 eV . Further contributions to the line width arise from the electronics and the finite projectile pulse duration,
both affecting every fragment in the same way. The overall time resolution of the instrument can in principle be determined from the line width of those fragments which start with thermal energies only . Here, we obtained line widths ranging from 4-10 ns for masses from 1-100 amu with flight times between 2.5 and 25 ws.
A general formula for the transformation from measured flight time into initial energy distribution of a specific fragment was explicitly deduced in [4] under the condition of a complete transmission through the spectrometer, an isotropic initial velocity distribution
and a homogeneous extraction field. The latter condition is fairly well satisfied by our new ion optical system, inasmuch as the field gradients in the interaction region are less than t0.3 V/mmz, compared with a mean extraction field of about 14 V/mm, and will therefore contribute only in second order to the flight time distribution . If an anisotropy in the initial velocity distribution of a decay channel exists (only the axis of the projectile beam could serve as preferential direction), the corresponding time-of-flight line shape would result from the convolution of initial energy and angular distributions of the underlying dissociation process . Initial energies up to 15 eV have been observed [5] . But the line shapes measured with the previous system
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indicated that fragments with high initial momentum perpendicular to the symmetry axis of the spectrometer were detected with distinctly lower probability than those starting with smaller transversal momentum. If such an initial energy dependent loss of fragments can be avoided by using appropriate ion optics, the initial energy distribution of a specific fragmentation channel can be calculated via line shape analysis [4]. The knowledge of these initial energy distributions allows conclusions about the molecular states involved in the underlying dissociation process, thus giving additional information about how fragmentation occurs. A further essential contribution to the identification of reaction channels in ion-molecule collisions consists in finding out correlations between fragments, i .e . the simultaneous production of more than one charged fragment in one ion-molecule collision. The direct measurement of such correlations still requires a substantial extension of the apparatus, namely a transmission close to unity in combination with a so-called multiple stop system, which allows simultaneous measurement of the flight times o¬ several fragments. A transmission close to unity for every fragment is only possible if it is independent of tl, -- initial energy. The ion optical task is then to focus fragments with high initial energy, wherever they are created (within the interaction region) and in whichever direction they are ejected. A transmission as close to unity as possible is a basic condition for both precise and quantitative measurements of correlations between fragments and that of their initial energy distribution . New experimental evidence about the fragmentation are realized by com-
bining both aspects. Hence it becomes possible to analyse the initial energy distribution not only of a specific fragment, but even of the fragments originating from a specific reaction channel, which is fixed by a correlated pair of fragments. Fragmentation processes sometimes lead to multiply charged fragments. If a doubly charged fragment with odd mass number appears with half-integral masses in the time-of-flight spectrum, it can then be identified clearly. Two fragments with the same massto-charge ratio, however, are indistinguishable in the spectrum, as their flight times are identical . But the doubly charged fragment receives twice the kinetic energy in the electric fields of the spectrometer. If the chosen detector system allows the determination, of not only the flight time, but also the fragment's energy and thus its charge state, an unambiguous mass classification will be possible . The energy resolution of the hitherto employed channeltron, however, is not sufficient to get any information apart from the flight time. This was the motive to develop an alternative detection method with the option to extract additional information from the pulse height of the detector signal in future investigations . 2. Ion optics of the time-of-flight-mass spectrometer For the purposes mentioned in the introduction, a higher transmission through the spectrometer was needed. The ion optical design of the hitherto existing system thus had to be refined in order to obtain better focusing qualities, but without losing its excellent time
Projectiles
Fig. 2. Trajectories (full lines) through the extraction and acceleration region. The blown-up sketch of the interaction region (hatched area) illustrates their start parameters. The electric potentials of the elements are given in kV and equipotential lines (dotted lines) are drawn (with a potential difference of 200 V).
R Lork et at. / TOFspectronteter for charged frnvntnts behaviour. Therefore, a more elaborate ion optics was necessary. In view of the variety of occurring start parameters (fragments are produced at positions up to 1.5 mm off the axis and ejected in all directions with initial kinetic energies up to 15 eV), every analytical approximation method fails (as, e .g. Gaussian optics for paraxial trajectories). Consequently, the potential 'distribution within the spectrometer has to be computed numerically by solving the Laplace equation under consideration of boundary conditions [6], where the task is to find these boundary conditions empirically by choosing an appropriate geometry and voltage distribution of the ion optical system . Its focusing qualities are controlled by computing the motion of ions with extreme but realistic start parameters. The optimum arrangement can only be found step by step, however, with feedback of gained experience. It was not possible to reach the required focusing properties with the usual repertoire of ion optical elements, to which, for example, the well-known einzel lens belongs [7]. The breakthrough was achieved by introducing a new ion optical element, which consists of two concentric ring electrodes ("Doppelring"), where the inner one has a higher negative potential, causing a stronger force towards the axis than common lenses. The spectrometer is basically divided into three consecutive units : an extraction-acceleration region, drift tube, and detector region. The drift tube is field free, resulting in the advantage of being able to calculate the ion optics of the extraction-acceleration and the detector region separately. After many numerical calculations, the arrangement shown in fig. 2 turned out to be the optimum one for the extraction-acceleration region. Here, the geometry and potential distribution and some paths of ions with extreme start parameters are drawn from the interaction region up to the drift tube . The occurring spatial initial coordinates are located within the interaction region, i .e . the intersection of projectile and molecular beam. The axial symmetry of the spectrometer makes the physical situation the same in every plane contain-
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ins the spectrometer axis. This diagram presents the flight paths of ions starting 1.5 nun off the axis in different directions with an initial energy of 10 eV. Those fragments which start perpendicular to the axis are the most difficult to focus because they soon diverge from the axis . If fragments are produced at finite distances from the ion optical axis, further complications turn up as it then becomes significant whether or not they begin perpendicularly away or towards the axis (trajectories a and b, respectively, in fig. 2). In this case, a simultaneous focusing of these two extreme fragment paths is not possible, because if one of them is focused on the axis, the other one will inevitably diverge . The compromise in the realized system is to make both meet at one point in the detector plane, which is, however, situated somewhat outside the ion optical axis . Nevertheless, the beam profile at the end of the drift tube (with a length of 105 cm) could be reduced to 3 cm with a divergence of less than one degree, compared with a diameter of 6 cm with the previous system. These values refer to an initial energy of 10 eV, which is rarely exceeded in fragmentation processes. But the intended detection method still necessitates concentration of the ions into a narrow and only slightly divergent beam. This has to be done by an additional ion optical system, following the field free drift tube . The fragments leave the drift tube with a kinetic energy of 3 .5 keV, according to the potential of the tube . For their final focusing into the detector system a voltage of up to 50 kV is available, which can be also used, as will be discussed later, for the detection process. Various numerical attempts, using conventional einzel and cylindrical lense- have failed . The curvature of equipotential lines, indicating radial electric field strength, remained too weak to give a sufficient attraction towards the ion optical axis. In particular, with einzel lenses the focusing was always too diffuse, especially for fragments moving at greater distances from the axis. The "Doppelring" was the crucial element to solve this ion optical problem. Fig. 3 shows the arrangement
Fig. 3. Focusing of fragments through the "Doppelring" ion optics. The electric potentials of the elements are given in kV and equipotential lines (dotted lines) are drawn (with a potential difference of 2.5 kV).
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of five of such elements, which accomplished the demanded properties right away. The attractive negative potentials of the inner rings of the "Doppelring" system decrease more rapidly than those of the outer rings, exactly in such a way that the electric forces are adjusted to the kinetic energy which the accelerated fragments have actually gained. The resulting curvature of the equipotential lines, i.e. the radial component of the electric field, is now strong enough to attract the fragments gradually but sufficiently towards the ion optical axis. Thus, initially parallel trajectories are focused into one point, analogous to light optics. All focal points are located within the same plane (focal plane). The distance of the focal points from the axis depends on the angle of the incoming trajectories relative to the axis. Here, fragments with an angle of 1° meet 2 mm away from the ion optical axis . But the angles caused by the ion optical system of the extraction-acceleration region remain below 1° for all occurring start parameters. Consequently, 4 mm represents a conservative estimate of the diameter of the focal spot. Furthermore, the angular spread after the ion optical system of the detector region remains small as well . The trajectories of the fragments have angles of less than 3° with respect to the axis, depending on their radial distance from the axis when entering the "Doppelring" ion optics . The parameters of the ion optical arrangements displayed in figs . 2-4 are the optimum ones for the given geometry and dimensions of our setup. For other configurations different potential distributions will result, but nevertheless the "Doppelring" elements with their favourable focusing properties will be a suitable means of designing similar instruments.
3. Detector system Ions of medium energy, in the range of a few tens of keV, can be detected via production of secondary electrons at a target surface . As shown in section 2, our new ion optical system prepares a relatively narrow and only slightly divergent beam of fragments, whose waist lies, moreover, in the field free region . Therefore it makes sense to place the ion-converter dynode just there, and to extract and focus the produced secondary electrons by a separate, likewise cylindrically symmetric, electron optical system (maximum voltage adjustable up to 50 kV), whose axis is orientated perpendicular to the spectrometer axis . In the region of the dynode, a weak electric field suffices to extract the secondary electrons without noticeably affecting the fast incoming ions. A design developed by Dietz and Sheffield [81 served as a prototype for this electron optical system. The complete arrangement of the ion and electron optics in the detector chamber is illustrated in fig. 4, showing also some typical electron paths. The secondary electrons are accelerated to kinetic energies up to 50 keV and focused head-on on a silicon surface barrier detector (Ortec model BA-014-050-100). Each of the secondary electrons produced by a single impinging fragment deposits an energy of up to 50 keV in the detector, depending on the chosen acceleration voltage . Hence, the pulse height of the detector signal is directly proportional to the number of secondary electrons released by one specific fragment. The energy resolution of the surface barrier detector (better than 14 keV) is sufficient to distinguish between n and n + 1 electron events, even if the energy per electron is
5cn Fig . 4. Design of the ion and electron optics in the detector chamber. The electric potentials of the elements are given in kV and typical electron paths through the electron optical system are shown.
R Lork et a1. / TOF spectrometer for chargedfragments reduced below 30 keV. To each peak in the pulse height spectrum thus corresponds a fixed number of secondary electrons. Its distribution and average value, the so-called average secondary electron yield, is characteristic for the specific fragment. These characteristics cannot be expressed in a general formula, because the secondary electron yield depends too strongly on the individual structure of the fragments [9,10] . Up to velocities corresponding to those of 50-keV protons, the secondary electron yield of a particular fragment increases proportional to its velocity. Because of their great penetration depth, protons generally produce the fewest secondary electrons, viz. only one or two in case of normal incidence. With aluminium as the dynode material, the secondary electron yield of protons reaches its maximum at an energy of 50 keV before falling again afterwards [11,12]. In order to detect the produced H + fragments with maximum efficiency, a post-acceleration voltage of 50 kV is therefore sufficient . This detector system, counting secondary electrons, is a device that is not restricted to providing fast electronic signals for time processing only, as already the previous channeltron did, but can in principle be extended to gather further information from the structured pulse height spectrum of secondary electrons that it delivers. 4. Measurements The further development of the apparatus described in this paper has been primarily undertaken to enhance the overall acceptance of our spectrometer (i .e. the product of transmission and detection efficiency). The ion optical improvements carried out in this respect increased the transmission, whereas the detection efficiency was raised as close to unity as possible, independent of the fragment sort, by using a new detection method . How far these aims were achieved with the new ion optical and detector system, will be revealed through the measurements presented in the following. Fig. 5 shows the time-of-flight mass spectrum of the fragments of CH 3Cl molecules, produced in collisions with 65-keV protons. All measurements were performed under single collision condition with an average yield of one charged fragment per every tenth projectile pulse, which was realized with a pulsed proton beam (about 1000 protons per pulse) and a surface density of a few 10" molecules/cm2 in the gas target. As with the previous system the time spread of the peaks is again mainly determined by the initial kinetic energy which the fragments receive in the dissociation process and not by instrumental effects, such as mentioned in the introduction. The combined influence of
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H+ -- CH3Cl
r
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go
30
40
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41â
Mass [u] Fig. 5 . Time-of-flight mass spectrum of methyl chloride produced by collisions with 65-keV protons. these effects can be seen in the line shapes of those fragments that start with thermal energies only (e.g. the parent ion CH 3Cl + which delivers a line of symmetric shape with a width of 9 ns). The line width of thermal H + fragments, for comparison, was 4 ns. For purely thermal peaks this means a mass resolution of m/Om > 500. However, most of the fragments are produced with finite initial energy. For example, the lines of CI + (mass 35 or 37) and HCI + (mass 36 or 38), with a width of 170 ns and nearly rectangular shape, were just separated, indicating a mass resolution of about 40 for initial energies of the order of 2 eV. It was not the intention to obtain merely a high mass resolution, which could be raised by increasing the extraction field, but to gain a high sensitivity for the influence of initial energies on the line shapes . The principal difference compared with the previous spectrometer is that the line shape of the single peaks is no more affected by a loss due to incomplete transmission of fragments starting with high transversal momentum . With isotropic initial angular distribution one would expect fragments starting with a well defined energy to have a rectangular time-of-flight peak [4]. As can be seen in fig. 6, that is in fact the case with the flight time distribution of H3 fragments, originating from the dissociation of CH 3CI molecules and starting with a uniform initial energy of approximately 5 eV. This exceptional fragment serves especially well for this purpose, as it originates from a single reaction channel, namely the break-up of CH 3C1 into the correlated pair H3 and CCl + from a well defined excited state . The narrow initial energy distribution of this complete reaction channel (the two involved fragments complement each other to the full parent molecule) manifests itself in the steep flanks of the resulting time-of-flight distribution (see fig. 6). The line shapes of most other fragments of CH 3 C1 indicate that more than one excited state is involved. (More details are
li" Lork et a!. / TOFsix ctmnwter for chargedfragments presented in a subsequent paper.) With the former setup all time-of-flight peaks of the fragments of higher initial energy showed a deep dip in the center (dashed line in fig. 6), just at flight times related to initial directions perpendicular to the spectrometer axis. Here, all fluctuations on the plateau are purely statistical, so it can be concluded that the transmission through the spectrometer is better than 95% . A modification of the flight time measuring system, namely its extension to a so-called duo stop system by adding a second clock (TAC). allowed us to measure the flight times of two fragments per projectile pulse simultaneously. (In the future we will use a time-to-digital converter for eight stop events per one start event, so as to also straightforwardly measure higher correlations between the fragments.) A logic gate between the two TAC's, each connected to its own ADC, enables the second TAC only after the first one has been stopped by the first incoming fragment . Thus we obtained two time-of-flight spectra, and by setting up to seven windows around the expected masses of interest while recording the spectra, it became possible to measure the correlations between the respective fragment pairs. Problems with pileup arise only in the rare cases when in one ion-molecule collision two correlated fragments with flight time differences of less than one microsecond are produced. The ion optical parameters .-/q ps of our instrument give a flight time of 2 .5 x V, for a fragment of mass m (in amu) and charge q (in e). Accordingly, the flight time difference for the aforementioned correlated pair (H ., CC]') amounts to about 13 Ws. Various correlated fragments have been observed. A detailed analysis of the results will be reported in a subsequent paper . In this paper we discuss only those results that are relevant to show the operating conditions of the new system. With two clocks, the suppression effect, mentioned in the introduction, exists only if a third charged frag-
a 400
L
Û 200
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Time of Flight
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lNsed
Fig. 6. Time-of-flight distribution of H3 fragments produced in collisions of 65-keV protons with CH 3 Cl molecules. The dashed line represents the line shape measured with the former system .
Table 1 Percentages (related to the total yield of all positively charged fragments) of the different orders of correlation between positively charged fragments produced in collisions of 65-keV protons with CH,Cl molecues . Fragment correlation CH_ICI + molecularion positive fragment without partner two positively charged fragments three positively charged fragments
Abundance 26% 61% 12% 0.2%
ment is produced during a single pulse. If one CH 3Cl molecule decays into three charged fragments, a hydrogen ion will be the fastest of them. It is possible to exclude in a separate measurement these light fragments (H +, H ; , and H 3 ) from the flight time measurement by starting the clock (TAO with a delay of somewhat more than the H ; flight time . A corresponding measurement with protons of 65 keV showed that the yield of the correlated fragment pairs (C +, CI'), (CH', CI'), and (CH", CI') increased by 15%, 6%, and 2%, respectively. This increase in yield is obviously due to triple correlation, being suppressed by the fast hydrogen ions in the first measurement . This suggests that the underlying transition leads over a triply charged CH 3 C1 3+ intermediate state . Up to now a doubly charged intermediate state is known in the fragmentation of CH 3Cl [13], but, as far as we know, a triply charged intermediate state of a molecule has never been observed before. Table 1 shows the fraction of the different degrees of correlation in the fragmentation of methyl chloride . These values, even the rather small one for triple correlation, significantly exceed the random coincidences. The fraction of random coincidences can be determined by the yield of those pairs of fragments which cannot be produced from one molecule, as, for example, (CH3 , CH 3 Cl + ). The measured proportions indicate very small but finite probabilities for higher correlations (more than double) in collision-induced molecular dissociation processes, which calls for a more thorough research on this subject. The whole acceptance of the spectrometer, that means the probability that a produced charged fragment will be recorded, is the product of the transmission through the spectrometer and the detection efficiency. The transmission, which is the probability for a produced fragment to reach the detector, is governed by the initial kinetic energy of the fragment and its spatial start parameters. These are assumed to be equally distributed for each fragment sort, whereas the detection efficiency, according to the physics of secondary electron emission, depends on the structure
R Lork et al./ TOF spectrometer for charged fragments and composition of the fragment and its velocity when reaching the ion-converter dynode . A simple method to determine the acceptance of the spectrometer quantitatively would consist in measuring the correlation between two fragments where the cross section for their production is known. But, unfortunately, no absolute data about such correlations can be found in the literature . Nevertheless, our measurements with CH3CI as target gas revealed a strongly correlated pair of fragments, namely H3 and CCl +, where with each H3 ion a CCI + ion was detected with a probability of 80% . Both ions emerged from one and the same dissociation act, sharing the released dissociation energy of approximately 5 eV inversely proportional to their masses, according to momentum conservation. Such a mass dependent sharing of initial energy leads in turn to exactly the same flight time spread and line shape for both correlated fragments, which was well reproduced here. Now we can give a conservative estimate of the overall acceptance of our spectrometer, even for fragments of higher initial energy. The lower limit is 80%, if the correlation between H3 and CCl + were complete, which is not at all conclusive, as a correlated production of H3 with a neutral partner cannot be observed in our experiments. All in all, both the transmission and the detection efficiency are now very close to unity, which has not been achieved simply by expanding the sensitive detector surface (e.g., by using a multi-channel-plate, with the disadvantage of insensitive areas between the channels), but by the introduction of advanced ion optics. The excellent focusing properties of the "Doppelring" ion optics provide the essential condition to open new fields of investigation . An important aspect, which has to be considered carefully and which possibly limits the properties of the detectcr system, is that of noise electrons, released by field emission from the electrodes at high negative potential . With a voltage of 30 kV applied to the ion and electron optical systems in the detector chamber, the background counting rate amounts to 5 electrons/s, but rises steeply with increasing voltage . We could not assemble our device in a "clean room", had to do without complex in situ electrode conditioning procedures and moreover were not able to reach a better pressure than 10' mbar (ion source and gas target form two unavoidable leaks) in our non-bakeable vacuum chamber, which altogether severely impairs the high voltage performance of the system [14]. Noise electrons are particularly disturbing in the measurement of correlations between the fragments, in that they produce a larger background and therefore more random coincidences. For this reason the spectra presented here have all been measured with a voltage of 30 kV instead of 50 kV . As already seen, this did not discernibly affect the ion optical properties of the
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cd a `o
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Energy NO]
Fig. 7. Pulse height spectrum (background subtracted) of secondary electrons (dots) at a voltage of 30 kV applied to the detector system, in comparison with an empirical fit of separate Gaussians (full line).
"Doppelring" optics, at least not up to initial kinetic energies of the fragments of 5 eV. In order to prove the efficiency of the new detecting device, the gas target was charged with air and a high voltage of 30 kV was applied to the detector chamber. The background corrected pulse height spectrum (fig. 7) measured with the new detector system, scows the clear structure of the detector signal of secondary electrons after 30-keV ion bombardment of the target dynode. Every line in the energy spectrum can be attributed to the number of secondary electrons released by the fragment and reaching the detector each with a kinetic energy of 30 keV. The good accordance between measured energy spectrum (dots) and empirical fit with separate Gaussians (full line), proves the statistical significance of this detection method. The detection efficiency for an individual fragment increases with the number of secondary electrons. Since each secondary electron is detected with a reduced probability of 80%, due to the loss of backscattered electrons [8], with two secondary electrons the detection efficiency raises to 96% but already exceeds 99% if a convoy of three or more secondary electrons will be produced. This estimation delivers a conservative value for the detection efficiency, as backscattered electrons also deposit a certain fraction of their energy in the silicon detector [15]. Whether this fraction can raise the detection efficiency significantly above 80% in the case of one secondary electron only depends on the trigger levels to discriminate against the noise. As can be seen in fig. 7, the average yield of secondary electrons lies in the range of five, but in almost every case more than two electrons are released by the impinging fragment so that we can conclude that this detection method is highly efficient. The foregoing statement must be qualified for H + fragments since they mostly produce, as mentioned in section 3, only one or two secondary electrons, and
.Ms
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therefore the ability that none are released cannot he neglected. The proton is one of the most abundant fragments of hydrocarbon molecules, but unfortunately it also stems from the fragmentation of the residual gas, where, besides H,O, hydrogen is a substantial component, due to the outflow from the (hydrogen) ion source . The fraction of hydrogen in the residual gas depends sensitb,ely on the operating conditions of the ion source, which makes a background correction of the H ' (just as well as H,) fragments in the time-offlight spectra very difficult. Further experimental effort will be necessary to overcome the remaining uncertainty in the spectroscopy especially of those H + fragments that are produced without charged partner (only these cause serious problems because the others can be unambiguously accounted for by their correlation). For all other fragments such restrictions do not exist . Acknowledgement This work was supported Forschungsgemeinschaft.
by
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R [1] W.Y. Baek, D. Gassen, iC. Förster, K. Schäfer and W. Neuwirth, Nucl. Instr. and Meth . B58 (1991) 266.
[2] D . Gassen and W. Neuwirth (to be published). [3] W.Y . Back, D. Gassen, K . Förster, K. Schäfer and W. Neuwirth, Nucl . Instr. and Meth. (submitted). [4] K. Schäfer, W.Y. Baek, K. Förster, D. Gassen and W. Neuwirth, Z. Phys. D21 (1991) 137. [5] A.K. Edwards, R.M. Wood and M .F. Steuer, Phys. Rev. A16 (1977) 1385. [6] See e.g . A. Septier, in: Focusing of Charged Particles, vol. 1 (Academic Press, New York, 1967) p. 45. [7] See e.g. P.W. Hawker, in : Principles of Electron Optics, vol. 2 (Academic Press, London, 1989). [8] L.A. Dietz and J.C. Sheffield, Rev. Sci. Instr. 44 (1973) 183 . [9] R.J . Beuhler and L . Friedman, J . Appl. Phys. 48 (1977) 39'28. [10] E.V. Alonso, R.A. Baragiola, J . Ferr6n, M.M. Jakas and A. Oliva Florio, Phys. Rev. B22 (1980) 80. [11] H .H. Andersen and J .F. Ziegler, Hydrogen Stopping Powers and Ranges in All Elements, vol. 3 (Pergamon, New York, 1977). [12] B. Svensson and G. Holmén, J. Appl . Phys. 52 (1981) 6928. [13] M .N . Monce, A.K. Edwards, R.M. Wood, M .F. Steuer, A.V . Shah and P. Tabor, J. Chem. Phys.74 (1981) 2860. [14] See e.g. R.V. Latham, in: High Voltage Vacuum Insulation: The Physical Basis (Academic Press, London-New York, 1981) . [15] R. Moshammer and R. Matthäus, J. Phys. Coll . (1989) C2-111 .