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Gridless electron trap for a high-duty cycle magnetic bottle time-of-flight spectrometer
Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146900
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Gridless electron trap for a high-duty cycle magnetic bottle time-of-flight spectrometer
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Christoph Strobela, Gerd Gantefoera,*, Andras Bodib, Patrick Hembergerb a b
Department of Physics, University of Konstanz, Konstanz, Germany Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
A R T I C LE I N FO
A B S T R A C T
Keywords: Time-of-flight electron spectrometer Synchrotron radiation Magnetic-bottle spectrometer
Magnetic bottle time-of-flight electron spectrometers traditionally combine high detection efficiency with high energy resolution in low duty cycle, pulsed experiments. We proposed a new electron-trap design in which photoelectrons oscillate between a magnetic and an electrostatic mirror. Ideally, they accumulate in the trap for several microseconds before the electrostatic mirror is switched off. This opens up new applications at continuous or quasi-continuous light sources, such as synchrotron radiation facilities. In the original setup, we achieved high energy resolution using planar metal grids for the electrostatic mirror. However, the finite transmission of the meshes and the high oscillation frequency of electrons limit the duty cycle. In the first gridless design proposed herein, the energy resolution is lower, but the storage time exceeds 40 μs. Thus, at an operating frequency of 100 kHz (≙ 10 μs), 40% of all electrons emitted in 4π sr may be analyzed.
1. Introduction Electron spectroscopy relies on electrostatic hemispherical analyzers [1,2], velocity map imaging [3,4], as well as time-of-flight (ToF) approaches [5–9]. Among these, electrostatic hemispherical analyzers and ToF electron spectrometers may suffer from a low collection efficiency. In the case of the latter, this issue can be overcome by applying a magnetic field to collect virtually all photoelectrons emitted in 4π sr. The velocity vectors are aligned parallel to the axis of the strongly divergent field lines of the weak magnetic guiding field of the ionization region [10], and the divergent field acts as a mirror for the electrons emitted away from the electron detector. Such electrons turn back and travel down the field gradient along a helical path. Depending on the length of the flight tube, an energy resolution of ΔE/E ≈ 1–10% can be achieved at typical flight times on the order of a few μs in magnetic bottle time-of-flight (MB-ToF) spectrometers, which have become standard tools in gas-phase photoelectron spectroscopy [10–24]. Because of the need for a start signal in the ToF-analysis, MB-ToF spectrometers are usually combined with pulsed lasers [11–13,22,23], pulsed lamps [24] or synchrotron radiation facilities in single bunch operation [8,14,15,18,21] or using a chopper [16,17,19,20]. The maximum feasible repetition rate of these pulsed light sources is limited to ∼100 kHz by the time-of-flight of the photoelectrons (ca. 10 μs). On the one hand, even such repetition rates are out of reach using labbased light sources to cover the valence band energy range, including ⁎
experimentally challenging perturbative high-order harmonic generation schemes using lasers [25]. On the other, storage rings are user facilities, providing easily tunable synchrotron radiation in a broad energy range at a high flux. However, the time between two synchrotron light pulses (on the order of a few nanoseconds - a few microseconds, depending on the ring filling pattern) is too short to allow for electron ToF analysis. Consequently, velocity map imaging (VMI) is normally employed at synchrotron light sources when a high collection efficiency is of the essence, notably in gaseous coincidence experiments [26]. Because VMI only records momentum components perpendicular to the flight axis, the energy spectrum has to be reconstructed on the basis of this projection. Obtaining energy correlation matrices for multiple, simultaneously produced charged particles by reconstructing correlated VMI images of coincident species remains challenging [27]. Therefore, MB-ToF approaches are routinely invoked at synchrotron sources to detect and analyze simultaneously ejected photo- and Auger electrons in multiple ionization events in deep valence and core ionization experiments. Because they rely on chopped synchrotron radiation, most of the full photon flux is forgone and the signal levels are reduced by a factor of more than 1000 [16,17,19,20]. While low light intensities may be beneficial for coincidence experiments, they are insufficient for ion beam experiments. We also employed VMI and VUV synchrotron radiation to study the photoelectron spectrum of size-selected cluster ions prepared using a magnetron cluster source at a current of ca. 3 nA [28]. The most
Corresponding author. E-mail address: [email protected] (G. Gantefoer).
https://doi.org/10.1016/j.elspec.2019.146900 Received 26 June 2019; Received in revised form 15 October 2019; Accepted 21 October 2019 Available online 01 November 2019 0368-2048/ © 2019 Elsevier B.V. All rights reserved.
Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146900
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experiments, overall detection efficiency was estimated to be > 80%, limited only by the detection efficiency of the electron detector, be it a microchannel plate or a Channeltron, and the low transmission of the instrument for kinetic energies lower than 150 meV. The latter limitation is caused by surface inhomogeneities and stray magnetic fields, which affect the slowest electrons most. It could be overcome by applying a small field in the source region to guide even originally zero kinetic energy electrons into the flight tube. In our MB-ToF, a cylindrical cobalt samarium magnet with a diameter of 10 mm and a height of 10 mm provides the divergent magnetic field for the magnetic mirror. The maximum field strength is 0.7 T. The strength of the weak magnetic field in the drift region is 10 G. The cobalt samarium magnet is located 10 mm below the ionization region, which is rather small with a total volume of about 1 mm3. All surfaces near the photoelectron path are made of copper and coated with graphite to provide a homogeneous conductive surface and to ensure that no electric fields interfere with the trajectory of the photoelectrons. The electrons leave the divergent part of the strong magnetic field ca. 100 mm above the ionization region and enter the weak guiding field of the drift region. A weak electric field focuses them into the cone of a channeltron at the top of the flight tube 1.4 m above the ionization region. The external stray and Earth magnetic fields are compensated by large Helmholtz coils around the flight tube, which is not otherwise shielded. A total detection efficiency of > 80% and a typical energy resolution of 3% has been achieved using conventional excimer and Nd:YAG lasers with this instrument and without an electrostatic mirror. The best absolute energy resolution was 5 meV for electrons with a kinetic energy of ca. 1 eV [13]. This instrument was modified by the incorporation of an electrostatic mirror. A switchable repulsive electric field reflects the electrons back in the direction of the ionization volume, towards the magnetic mirror. The original electrostatic mirror (Fig. 1a) consists of two molybdenum grids 2 mm apart and 12 mm above the ionization volume with ca. 85% transmission each (45 wires per inch) [29]. The grid closer to the ionization area is always kept at ground potential and ensures that the volume of the photoelectron trap is free of electric fields. Thus, the electron kinetic energy is not altered in the trap. The second, top grid is connected to an external fast high voltage switch. When a negative voltage is applied to this grid, the two grids act as an electrostatic mirror. As long as the voltage applied is higher than the maximum kinetic energy of the electrons to be trapped, all electrons may theoretically be reflected back into the trap. After an accumulation period of a few μs, the electrostatic mirror is turned off by setting the second grid at ground potential. When the gate is opened, the accrued photoelectron cloud leaves the trap within approx. 0.5 μs and reaches the electron detector. Few electrons are inside the mirror at any given time, but they are inevitably perturbed and partly lost by the change in the potential when the gate is opened. In order for the remaining electrons inside the trap not to be affected, it is vitally important that the switching is instantaneous and that voltage fluctuations are negligible. They can be caused by stray capacitances and residual inductances. Therefore, the fast switch is connected to the repeller grid in the electrostatic mirror by low-inductance feedthroughs and RF-suitable cables to achieve rise and fall time of 40–50 ns. This allows us to operate the switch at a frequency of up to several hundred kHz. Slower rise and fall times result in distortions of the photoelectron spectrum. The grid design is limited by transmission issues. Etched grids typically have ca. 85% transmission for electrons. In each cycle in the trap, electrons pass the first grid twice, i.e., almost 30% of them are lost. If we consider 1–10 eV electrons with a velocity of ca. 1 m μs–1, alternating in the ca. 2 cm trap and passing the grounded grid twice every 4 cm of their path, their half-life in the trap is expected to be on the order of 0.1 μs, compromising the storage effect significantly. Therefore, a gridless design was constructed (Fig. 1 b), in which two electrodes generate the reflective electric field. The first electrode is always kept at ground potential and the second is switched similarly to
challenging aspect in the data analysis was the suppression of the background, which was due to photoemission due to scattered light, to cluster ions impinging on metal surfaces inside the spectrometer, and to photoionization of the background gas. The magnetic field inside the ionization volume in MB-ToF instruments effectively suppresses the detection of electrons formed outside of the ionization volume and eliminates the first two sources of background signal. However, at the low sample density of a size-selected cluster beam, it is imperative not to reduce the light intensity and make use of every photon the storage ring delivers. We set out to develop a high duty-cycle MB-ToF spectrometer to record photoelectron spectra of extremely dilute samples and to be operated at (quasi-)continuous light sources, such as synchrotron radiation facilities. The key new element is an electrostatic mirror, which is located between the drift region of the flight tube and the ionization volume. It reflects electrons, which are, thus, trapped between this electrostatic mirror and the magnetic mirror of the bottle [29]. We described three operation modes for the electron trap MB-ToF. First, without a storage effect, the gate voltage only determines whether prompt electrons pass through the gate. In the integral mode, the integrated photoelectron ToF spectrum appears as the rising and its mirror image as the falling edge in the detected signal. When there is a storage effect, electrons may accumulate in the trap for a few μs. When the electrostatic mirror is switched off, the cloud of stored electrons begins its journey to the electron detector together with the prompt electrons. If the gate is opened for a long time, the storage peak is superimposed on the integral signal. The recorded photoelectron ToF distribution corresponds to the ToF spectrum only in the third, time-offlight mode when the gate is opened for the shortest possible time to allow the trap to empty and to suppress the integral signal due to prompt electrons. Furthermore, it must of course be ensured that the trap does not alter the kinetic energy of the stored photoelectrons. In this mode, the time of flight is measured between switching off the electrostatic mirror and the detection of the electrons and the kinetic energy distribution of the photoelectrons is determined based on their ToF spectrum. Assuming no losses in the trap, i.e., a longer than the 10 μs electron lifetime at an extraction frequency of 100 kHz, such a setup can, in principle, be operated at 100% duty cycle. In a recent paper, Y. Hikosaka describes a remarkably similar design of a MB-ToF with an electrostatic gate [30]. Similar to our first version of the electrostatic mirror, it consists of two grids between the ionization area and the drift region. However, this arrangement serves only as a gate. It replaces a mechanical chopper that extends the duration between two light pulses. As a result, electron flight times of up to 10 μs can be measured, although the time between two light pulses is only about one μs. The paper deals with photoelectrons with kinetic energies of 20–60 eV. With a storage time of 10 μs these electrons pass the first grid several hundred times. Although it has a very high transmission (94%), virtually all stored electrons are absorbed by the grid. At these conditions, the storage effect is negligible. This paper is dedicated to the conundrum of the electrostatic mirror. In the original setup [29], we achieved good energy resolution (≈ 50 meV) at the expense of the duty cycle, due to the limited transmittance of the grids making up the electrostatic mirror. Here, we present a new version of this magnetic bottle time-of-flight electron spectrometer to overcome electron loss limitations in the grid design and demonstrate that high duty cycle (> 40%) is within reach when using a gridless mirror. Finally, we discuss the benefits of the two designs and argue that they can be combined to offer a high duty cycle and a high energy resolution at continuous ionization sources. 2. Experimental setup The modified magnetic bottle time-of-flight electron spectrometer is based on the standard magnetic bottle design as used in numerous previous experiments with pulsed lasers [12,13,22,23]. In these 2
Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146900
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Fig. 1. Two alternative designs of the electrostatic mirror for trapping photoelectrons between time-of-flight measurements. The magnetic mirror consists of a divergent magnetic field generated by a permanent magnet below the ionization volume. The electrostatic mirror above the ionization volume forms the top barrier of the trap. The original version of the electrostatic mirror consists of two metal grids (a). The gridless arrangement of two electrodes (b) is explored herein to avoid the transmittance-limited low electron lifetime in the original setup.
Fig. 2. Simulation of electron trajectories in the gridless electron trap design. No trapped electrons are lost and, in the absence of space charge effects, the electron lifetime in the trap is unlimited.
3. Results Argon was photoionized, and electron time-of-flight distributions were recorded using the two electrostatic mirror designs and continuous 22 eV synchrotron radiation from the VUV beamline of the Swiss Light Source [28,31,32]. Since the ionization potential of argon is ca. 15.7 eV [33], photoelectrons had a kinetic energy of ca. 6.7 eV. The gas pressure was 5·10–7 mbar and the photon flux about 1011 photons/s. Fig. 3a shows the spectrum obtained with the grid design. The electrostatic mirror was switched off for 5 μs at a repetition frequency of 80 kHz. The spectrum consists of an almost rectangular, 5 μs long feature, the integral peak, which corresponds to the constant production rate of photoelectrons in the ionization volume. The storage peak is the narrow feature at the beginning of the rectangular integral peak, which corresponds to photoelectrons accumulated while the electrostatic mirror was "on" and suddenly released towards the drift tube when the repeller grid in the electrostatic mirror was grounded [29]. The first version of the new instrument achieved a 5% energy resolution sufficient to resolve the spin–orbit splitting of the Ar+ cation and the H2+ vibrational progression. In principle, this energy resolution can be further improved by increasing the length of the drift zone. The present work focuses on increasing the amount of electrons accumulated in the trap, which depends crucially on increasing the
the second grid in the grid design. On the one hand, there are no transmission losses in the gridless electron trap (Fig. 2) unless the reflecting field is defocusing the electrons. On the other, the reflecting mirror volume is larger, which means that more electrons are in this volume when the gate is opened, and the electric field penetrates the volume of the trap, meaning that the kinetic energy of the photoelectrons may be perturbed depending on their position in the trap when the gate is opened. The electrodes are basically two apertures spaced by 12 mm. The first electrode is located 8 mm above the ionization region and has an opening of 5 mm. The second electrode has an opening of 8 mm. In our set-up, the electrodes need to have a conical shape (Figs. 1 and 2) to make room for the ion optics. The beam of size-selected cluster ions enters the ionization region through an Einzel lens from behind (not shown in the Figure). The second electrode is set to a negative voltage (gate closed) and for a short time set to ground potential (gate opened). It has been found that the magnitude of the repeller voltage in the range of 10–70 eV has no effect, provided, of course, that it is greater than the maximum kinetic energy of the photoelectrons to be analyzed.
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The spectrum recorded with the gridless electrode design over 30 s at a 50 kHz repetition rate has a similar shape (Fig. 3b). The Ar pressure was 2·10–7 mbar (compared to 5·10–7 mbar in Fig. 3a). Hence, the count rates are lower compared to Fig. 3a. There is a rectangular feature with a length equal to the turn-off time of the electrostatic mirror (10 μs). At the beginning of the rectangular feature, there is again the storage peak. There are two obvious differences to the spectrum achieved with the grid design: the storage peak is much more intense and much wider. From the number of counts in the storage peak and the count rate of the integral peak, the storage time is calculated to be 4.3 μs. This indicates an approximately 40-fold lifetime of the electrons in the trap and corresponds to a more than 40% duty cycle at 100 kHz, although the spectrum shown in Fig. 3b was recorded at a repetition rate of only 50 kHz. Such a dramatic increase in the duty cycle is not surprising, because the main cause of the losses, the limited transmission of the grids, has been eliminated. However, the duty cycle is only an upper limit to the collection efficiency of the spectrometer, i.e., to the chance that a photoelectron will contribute to the recorded spectrum. This is because the transmission of the instrument, the detection probability at the channeltron, and the limited reflectivity of the magnetic mirror affect both the storage and the integral peaks. The spectrum shows that the basic idea of combining a time-offlight electron spectrometer with an electron trap is principally sound. A high duty cycle of ca. 40% can be achieved in a gas-phase experiment using a magnetic bottle ToF spectrometer by including an electron trap regardless of the time structure of the light source. However, with the gridless design, the energy resolution needs to be optimized for general applicability at quasi cw-light sources. Fig. 4 shows a comparison of the time-of-flight photoelectron spectra of Ar taken at three different photon energies. Spectra on the left (Fig. 4a) were taken with the grid design and spectra on the right (Fig. 4b) with the gridless electrode design. The storage peak shifts to longer flight times with decreasing photon energy, as the electron kinetic energy decreases. At the employed gate voltage of 25 V and 45 V in the grid and gridless electrode designs, respectively, the relative energy resolution appears to be independent of the kinetic energy and is
Fig. 3. Comparison of time-of-flight photoelectron spectra obtained with the grid (a) and the gridless electrode design (b) shown in Fig. 1. Electrons are generated by photoionization of Ar gas with 22 eV synchrotron radiation. "off" and "on" refers to the voltage applied to the electrostatic mirror, i.e., opening and closing the gate towards the flight tube.
electron lifetime, i.e., the storage time, in the trap. Experimentally, the storage time can be determined by dividing the number of electrons in the storage peak by the production rate of photoelectrons, which is the count rate in the rectangular integral peak. In Fig. 3 a, the spectrum is a sum of 3 million pulses (1 min at 50 kHz). The total number of counts in the integral peak is 443,000 in the 2–6 μs interval. This corresponds to a count rate of 0.037 cts/μs. The total photoionization rate is somewhat higher, because prompt electrons also have to pass two grids and the transmission of the drift tube as well as the detection efficiency of electrons are both somewhat below 100%. In addition, about 1% of the photoelectrons are emitted almost perfectly parallel to the magnetic field lines and are not reflected by the magnetic mirror. Thus, the measured count rate in the integral peak is estimated to be approximately 80% of the total ionization rate in the ionization volume. However, these factors decrease the storage peak in equal measure. The total number of photoelectrons in the storage peak is 12,150 cts. This corresponds to a count rate of 0.0041 cts/pulse. Therefore, the number of electrons collected in the trap corresponds to the number of photoelectrons generated within 0.11 μs. This corresponds to 0.08 μs electron half-life in the trap, which agrees with theoretical considerations (see above) and is rather short. Thus, the storage effect is there, but it is weak. At the maximum repetition rate of 100 kHz and a storage time of 0.11 μs, only 1.1% of the theoretically detectable photoelectrons are in fact detected in the storage peak. Thus, the theoretically predicted and measured ca. 1% duty cycle is much below what is required.
Fig. 4. Comparison of time-of-flight photoelectron spectra taken with the grid (a) and the gridless electrode design (b) at three photon energies. The storage peak shifts to longer flight times with lower photon energies, as the electron kinetic energy decreases. The arrows in the spectra on the right indicate the theoretical electron time-of-flight corresponding to the kinetic energy calculated as the difference of the Ar ionization potential and the photon energy. In the series of spectra on the left, this corresponds to the position of the sharp storage peak. 4
Journal of Electron Spectroscopy and Related Phenomena 239 (2020) 146900
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ca. 5% for the grid design and ca. 30% for the electrode design. In the grid design, the storage peak at the lowest photon energy splits into two components corresponding to the two spin–orbit states of the Ar+ ion ground state, separated by 178 meV [29]. In the grid design, the storage time may be expected to increase with lower photon energies because slower electrons are formed, which pass through the grid less frequently. However, the effect is counteracted by the slow electrons being more susceptible to stray fields and is rather weak anyway because the velocity of the electrons varies only with the square root of the energy. In fact, within the energy range studied (2.2–10.2 eV), the storage time has also been found to be practically independent of the kinetic energy in both electron traps. For the spectra recorded with the gridless electrode design, the calculated flight times corresponding to the kinetic energies of the photoelectrons are also indicated. Most detected electrons have a longer ToF, corresponding to a lower energy, but there is also signal at higher kinetic energies. At first glance, it is surprising that the energy distribution is so broad. However, this can be explained by two effects. First, the electric field gradient is much weaker, making the reflection quite "soft". The electrons spend more time in this soft electric field as they eventually slow down and turn around. Second, the electric field penetrates more deeply into the volume of the trap. At different positions in the trap or in the electrostatic mirror, the electrons’ kinetic energy is perturbed to a varying degree. Because they spend more of their time in the electrostatic mirror during reflection, a larger fraction of electrons is located in the mirror when it is turned off and their kinetic energy is lowered. The electric field of the mirror also extends into the ionization volume and accelerates electrons towards the magnetic mirror. When the mirror field is switched off, these electrons have kinetic energies higher than the original energy. Since these electrons are faster, they spend less time in the magnetic mirror and their intensity is low. The measured time-of-flight spectrum reflects the distribution of the kinetic energies that the electrons had in the trap and the electrostatic mirror when the electric field was turned off instantaneously. Fig. 5 illustrates the shape of the storage peak qualitatively according to the above considerations. For the grid design (Fig. 5a), the kinetic energy of the electrons in the trap is constant and corresponds to the energy they have from the photoionization process (E0). At any given moment, only a few electrons are found in the electrostatic mirror being reflected because the field gradient is steep and the reflection is fast. These electrons have lower kinetic energies, resulting in a shoulder on the lower kinetic energy side. In the time-of-flight spectra, this explains the asymmetry of the storage peak towards longer flight times (see, e.g., Fig. 3a). For the gridless electrode design, the electric field is more extended and the field gradient is less steep (Fig. 5b). Most of the time the electrons spend during reflection in the electrostatic mirror. Accordingly, the peak in the photoelectron spectrum is broadened and shifted to lower kinetic energies. This picture qualitatively explains the differences in the time-of-flight spectra between the two designs of the electrostatic mirror. While the time-of-flight spectra measured in the gridless setup could be deconvoluted into kinetic energy spectra by recording calibrated ToF distributions belonging to monoenergetic electrons, we doubt that this approach would provide meaningful information at the current, broad response functions. This is also why we only discussed the experimental differences between the two designs based solely on time-of-flight distributions. Based on the comparison of the two designs, the following insights will help further development towards an optimal design:
Fig. 5. Storage peak shapes as influenced by the potential and the electron distribution in the trap (see text). Parts (a) and (b) refer to the two different gate designs displayed in Fig. 1. Arrows mark the ionization volume in the trap. Note that the schematic spectra on the right are energy spectra and not time-of-flight distributions.
• The time the electrons spend in the electrostatic field should be as
short as possible. This requires a steep field gradient, which is difficult to achieve without a grid. For low-energy electrons high, transmission grids could be used. Slow electrons pass through the grid only a few times during the storage time, so that the intensity loss is low. This is especially true when the trap is longer.
Therefore, increasing the length of the trap and using extremely high transmission grids should result in a significant increase in energy resolution (ΔE/E < 0.1). For low energy electrons (Ekin ≈ 1 eV), the intensity loss at the grid should still be low (< 50%).
4. Conclusions An electron spectrometer ideal for the study of extremely dilute samples will detect all electrons produced in the ionization volume and suppress all other electrons from outside this volume. Magnetic bottle time-of-flight spectrometers come close to this but normally have the disadvantage of requiring a pulsed light source. We developed this instrument further, equipping it with an electron trap based on an electrostatic mirror gating the entrance to the electron flight tube. The trap is formed by the magnetic mirror below and the electrostatic mirror above the ionization volume. While the electrostatic mirror is on, the electrons are trapped between the two mirrors and fly back and forth through the ionization volume. From time to time, the electrostatic mirror is turned off so that the accumulated electrons travel the regular path through the drift region of the spectrometer to the electron detector. First, the electron lifetime should be infinite in an effective trap. Thus, such a setup should allow us to detect virtually all photoelectrons
• The energy resolution is limited mainly by the effect of the trap on the energy distribution of the electrons. • The energy distribution broadening can be reduced by increasing the distance between the electrostatic mirror and the ionization region. The longer the zone with a negligible electric field, the larger the fraction of electrons with the unperturbed, original kinetic energy at the time when the gate is opened.
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regardless of the time structure of the light source. Second, an effective trap must not alter the electron kinetic energies and, thus, the energy resolution should be easy to increase by increasing the length of the drift tube. However, our data show that building an effective trap in these two regards simultaneously is not trivial. The crucial component is the electrostatic mirror. In order to achieve a high energy resolution, a strong and well-separated electric field is necessary, so that the electrons spend as little time in the mirror as possible. This condition can be met with metal meshes, which have the disadvantage of limited transmission. The trapped electrons pass through the first grid of the mirror twice in each cycle, resulting in extensive losses. In our case, a storage time of ca. 0.1 μs could be achieved based on grids with ca. 85% transmittance. Meshes with 99.5–99.9 % transmittance would be needed to approach > 4 μs storage time, an appropriate value assuming 100 kHz extraction frequency. We presented and tested another, gridless electrode configuration herein. This setup delivered the desired 40% duty cycle. However, electric field penetration and the resulting soft reflection process interfere with the energy resolution. Thus, the grid design achieves a high energy resolution while the gridless electrode design yields the required duty cycle. We have thus shown that magnetic bottle time-of-flight spectrometers can be used to study extremely dilute samples by suppressing electron collection from outside of the ionization volume at a high duty cycle or high energy resolution. Unfortunately, the two cannot yet be achieved simultaneously. Still, we believe that these insights and the knowledge gained in these proof-of-principle studies will help us achieve both design goals at the same time by combining the distinguishing features of both electron trap designs.
[2] [3] [4] [5] [6] [7] [8]
[9]
[10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22]
Acknowledgements
[23] [24]
Support by the German Research Foundation is highly appreciated (project GA 389/27-1). The measurements were performed at the VUV beamline (Swiss Light Source, Paul Scherrer Institute). A.B. and P.H. acknowledge funding by the Swiss Federal Office of Energy under contract SI/501269-01. The authors are grateful to the infrastructure groups at the SLS for the support and would like to thank Patrick Ascher for his assistance.
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Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Gridless electron trap for a high-duty cycle magnetic bottle time-of-flight spectrometer
T
Christoph Strobela, Gerd Gantefoera,*, Andras Bodib, Patrick Hembergerb a b
Department of Physics, University of Konstanz, Konstanz, Germany Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
A R T I C LE I N FO
A B S T R A C T
Keywords: Time-of-flight electron spectrometer Synchrotron radiation Magnetic-bottle spectrometer
Magnetic bottle time-of-flight electron spectrometers traditionally combine high detection efficiency with high energy resolution in low duty cycle, pulsed experiments. We proposed a new electron-trap design in which photoelectrons oscillate between a magnetic and an electrostatic mirror. Ideally, they accumulate in the trap for several microseconds before the electrostatic mirror is switched off. This opens up new applications at continuous or quasi-continuous light sources, such as synchrotron radiation facilities. In the original setup, we achieved high energy resolution using planar metal grids for the electrostatic mirror. However, the finite transmission of the meshes and the high oscillation frequency of electrons limit the duty cycle. In the first gridless design proposed herein, the energy resolution is lower, but the storage time exceeds 40 μs. Thus, at an operating frequency of 100 kHz (≙ 10 μs), 40% of all electrons emitted in 4π sr may be analyzed.
1. Introduction Electron spectroscopy relies on electrostatic hemispherical analyzers [1,2], velocity map imaging [3,4], as well as time-of-flight (ToF) approaches [5–9]. Among these, electrostatic hemispherical analyzers and ToF electron spectrometers may suffer from a low collection efficiency. In the case of the latter, this issue can be overcome by applying a magnetic field to collect virtually all photoelectrons emitted in 4π sr. The velocity vectors are aligned parallel to the axis of the strongly divergent field lines of the weak magnetic guiding field of the ionization region [10], and the divergent field acts as a mirror for the electrons emitted away from the electron detector. Such electrons turn back and travel down the field gradient along a helical path. Depending on the length of the flight tube, an energy resolution of ΔE/E ≈ 1–10% can be achieved at typical flight times on the order of a few μs in magnetic bottle time-of-flight (MB-ToF) spectrometers, which have become standard tools in gas-phase photoelectron spectroscopy [10–24]. Because of the need for a start signal in the ToF-analysis, MB-ToF spectrometers are usually combined with pulsed lasers [11–13,22,23], pulsed lamps [24] or synchrotron radiation facilities in single bunch operation [8,14,15,18,21] or using a chopper [16,17,19,20]. The maximum feasible repetition rate of these pulsed light sources is limited to ∼100 kHz by the time-of-flight of the photoelectrons (ca. 10 μs). On the one hand, even such repetition rates are out of reach using labbased light sources to cover the valence band energy range, including ⁎
experimentally challenging perturbative high-order harmonic generation schemes using lasers [25]. On the other, storage rings are user facilities, providing easily tunable synchrotron radiation in a broad energy range at a high flux. However, the time between two synchrotron light pulses (on the order of a few nanoseconds - a few microseconds, depending on the ring filling pattern) is too short to allow for electron ToF analysis. Consequently, velocity map imaging (VMI) is normally employed at synchrotron light sources when a high collection efficiency is of the essence, notably in gaseous coincidence experiments [26]. Because VMI only records momentum components perpendicular to the flight axis, the energy spectrum has to be reconstructed on the basis of this projection. Obtaining energy correlation matrices for multiple, simultaneously produced charged particles by reconstructing correlated VMI images of coincident species remains challenging [27]. Therefore, MB-ToF approaches are routinely invoked at synchrotron sources to detect and analyze simultaneously ejected photo- and Auger electrons in multiple ionization events in deep valence and core ionization experiments. Because they rely on chopped synchrotron radiation, most of the full photon flux is forgone and the signal levels are reduced by a factor of more than 1000 [16,17,19,20]. While low light intensities may be beneficial for coincidence experiments, they are insufficient for ion beam experiments. We also employed VMI and VUV synchrotron radiation to study the photoelectron spectrum of size-selected cluster ions prepared using a magnetron cluster source at a current of ca. 3 nA [28]. The most
Corresponding author. E-mail address: [email protected] (G. Gantefoer).
https://doi.org/10.1016/j.elspec.2019.146900 Received 26 June 2019; Received in revised form 15 October 2019; Accepted 21 October 2019 Available online 01 November 2019 0368-2048/ © 2019 Elsevier B.V. All rights reserved.
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experiments, overall detection efficiency was estimated to be > 80%, limited only by the detection efficiency of the electron detector, be it a microchannel plate or a Channeltron, and the low transmission of the instrument for kinetic energies lower than 150 meV. The latter limitation is caused by surface inhomogeneities and stray magnetic fields, which affect the slowest electrons most. It could be overcome by applying a small field in the source region to guide even originally zero kinetic energy electrons into the flight tube. In our MB-ToF, a cylindrical cobalt samarium magnet with a diameter of 10 mm and a height of 10 mm provides the divergent magnetic field for the magnetic mirror. The maximum field strength is 0.7 T. The strength of the weak magnetic field in the drift region is 10 G. The cobalt samarium magnet is located 10 mm below the ionization region, which is rather small with a total volume of about 1 mm3. All surfaces near the photoelectron path are made of copper and coated with graphite to provide a homogeneous conductive surface and to ensure that no electric fields interfere with the trajectory of the photoelectrons. The electrons leave the divergent part of the strong magnetic field ca. 100 mm above the ionization region and enter the weak guiding field of the drift region. A weak electric field focuses them into the cone of a channeltron at the top of the flight tube 1.4 m above the ionization region. The external stray and Earth magnetic fields are compensated by large Helmholtz coils around the flight tube, which is not otherwise shielded. A total detection efficiency of > 80% and a typical energy resolution of 3% has been achieved using conventional excimer and Nd:YAG lasers with this instrument and without an electrostatic mirror. The best absolute energy resolution was 5 meV for electrons with a kinetic energy of ca. 1 eV [13]. This instrument was modified by the incorporation of an electrostatic mirror. A switchable repulsive electric field reflects the electrons back in the direction of the ionization volume, towards the magnetic mirror. The original electrostatic mirror (Fig. 1a) consists of two molybdenum grids 2 mm apart and 12 mm above the ionization volume with ca. 85% transmission each (45 wires per inch) [29]. The grid closer to the ionization area is always kept at ground potential and ensures that the volume of the photoelectron trap is free of electric fields. Thus, the electron kinetic energy is not altered in the trap. The second, top grid is connected to an external fast high voltage switch. When a negative voltage is applied to this grid, the two grids act as an electrostatic mirror. As long as the voltage applied is higher than the maximum kinetic energy of the electrons to be trapped, all electrons may theoretically be reflected back into the trap. After an accumulation period of a few μs, the electrostatic mirror is turned off by setting the second grid at ground potential. When the gate is opened, the accrued photoelectron cloud leaves the trap within approx. 0.5 μs and reaches the electron detector. Few electrons are inside the mirror at any given time, but they are inevitably perturbed and partly lost by the change in the potential when the gate is opened. In order for the remaining electrons inside the trap not to be affected, it is vitally important that the switching is instantaneous and that voltage fluctuations are negligible. They can be caused by stray capacitances and residual inductances. Therefore, the fast switch is connected to the repeller grid in the electrostatic mirror by low-inductance feedthroughs and RF-suitable cables to achieve rise and fall time of 40–50 ns. This allows us to operate the switch at a frequency of up to several hundred kHz. Slower rise and fall times result in distortions of the photoelectron spectrum. The grid design is limited by transmission issues. Etched grids typically have ca. 85% transmission for electrons. In each cycle in the trap, electrons pass the first grid twice, i.e., almost 30% of them are lost. If we consider 1–10 eV electrons with a velocity of ca. 1 m μs–1, alternating in the ca. 2 cm trap and passing the grounded grid twice every 4 cm of their path, their half-life in the trap is expected to be on the order of 0.1 μs, compromising the storage effect significantly. Therefore, a gridless design was constructed (Fig. 1 b), in which two electrodes generate the reflective electric field. The first electrode is always kept at ground potential and the second is switched similarly to
challenging aspect in the data analysis was the suppression of the background, which was due to photoemission due to scattered light, to cluster ions impinging on metal surfaces inside the spectrometer, and to photoionization of the background gas. The magnetic field inside the ionization volume in MB-ToF instruments effectively suppresses the detection of electrons formed outside of the ionization volume and eliminates the first two sources of background signal. However, at the low sample density of a size-selected cluster beam, it is imperative not to reduce the light intensity and make use of every photon the storage ring delivers. We set out to develop a high duty-cycle MB-ToF spectrometer to record photoelectron spectra of extremely dilute samples and to be operated at (quasi-)continuous light sources, such as synchrotron radiation facilities. The key new element is an electrostatic mirror, which is located between the drift region of the flight tube and the ionization volume. It reflects electrons, which are, thus, trapped between this electrostatic mirror and the magnetic mirror of the bottle [29]. We described three operation modes for the electron trap MB-ToF. First, without a storage effect, the gate voltage only determines whether prompt electrons pass through the gate. In the integral mode, the integrated photoelectron ToF spectrum appears as the rising and its mirror image as the falling edge in the detected signal. When there is a storage effect, electrons may accumulate in the trap for a few μs. When the electrostatic mirror is switched off, the cloud of stored electrons begins its journey to the electron detector together with the prompt electrons. If the gate is opened for a long time, the storage peak is superimposed on the integral signal. The recorded photoelectron ToF distribution corresponds to the ToF spectrum only in the third, time-offlight mode when the gate is opened for the shortest possible time to allow the trap to empty and to suppress the integral signal due to prompt electrons. Furthermore, it must of course be ensured that the trap does not alter the kinetic energy of the stored photoelectrons. In this mode, the time of flight is measured between switching off the electrostatic mirror and the detection of the electrons and the kinetic energy distribution of the photoelectrons is determined based on their ToF spectrum. Assuming no losses in the trap, i.e., a longer than the 10 μs electron lifetime at an extraction frequency of 100 kHz, such a setup can, in principle, be operated at 100% duty cycle. In a recent paper, Y. Hikosaka describes a remarkably similar design of a MB-ToF with an electrostatic gate [30]. Similar to our first version of the electrostatic mirror, it consists of two grids between the ionization area and the drift region. However, this arrangement serves only as a gate. It replaces a mechanical chopper that extends the duration between two light pulses. As a result, electron flight times of up to 10 μs can be measured, although the time between two light pulses is only about one μs. The paper deals with photoelectrons with kinetic energies of 20–60 eV. With a storage time of 10 μs these electrons pass the first grid several hundred times. Although it has a very high transmission (94%), virtually all stored electrons are absorbed by the grid. At these conditions, the storage effect is negligible. This paper is dedicated to the conundrum of the electrostatic mirror. In the original setup [29], we achieved good energy resolution (≈ 50 meV) at the expense of the duty cycle, due to the limited transmittance of the grids making up the electrostatic mirror. Here, we present a new version of this magnetic bottle time-of-flight electron spectrometer to overcome electron loss limitations in the grid design and demonstrate that high duty cycle (> 40%) is within reach when using a gridless mirror. Finally, we discuss the benefits of the two designs and argue that they can be combined to offer a high duty cycle and a high energy resolution at continuous ionization sources. 2. Experimental setup The modified magnetic bottle time-of-flight electron spectrometer is based on the standard magnetic bottle design as used in numerous previous experiments with pulsed lasers [12,13,22,23]. In these 2
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Fig. 1. Two alternative designs of the electrostatic mirror for trapping photoelectrons between time-of-flight measurements. The magnetic mirror consists of a divergent magnetic field generated by a permanent magnet below the ionization volume. The electrostatic mirror above the ionization volume forms the top barrier of the trap. The original version of the electrostatic mirror consists of two metal grids (a). The gridless arrangement of two electrodes (b) is explored herein to avoid the transmittance-limited low electron lifetime in the original setup.
Fig. 2. Simulation of electron trajectories in the gridless electron trap design. No trapped electrons are lost and, in the absence of space charge effects, the electron lifetime in the trap is unlimited.
3. Results Argon was photoionized, and electron time-of-flight distributions were recorded using the two electrostatic mirror designs and continuous 22 eV synchrotron radiation from the VUV beamline of the Swiss Light Source [28,31,32]. Since the ionization potential of argon is ca. 15.7 eV [33], photoelectrons had a kinetic energy of ca. 6.7 eV. The gas pressure was 5·10–7 mbar and the photon flux about 1011 photons/s. Fig. 3a shows the spectrum obtained with the grid design. The electrostatic mirror was switched off for 5 μs at a repetition frequency of 80 kHz. The spectrum consists of an almost rectangular, 5 μs long feature, the integral peak, which corresponds to the constant production rate of photoelectrons in the ionization volume. The storage peak is the narrow feature at the beginning of the rectangular integral peak, which corresponds to photoelectrons accumulated while the electrostatic mirror was "on" and suddenly released towards the drift tube when the repeller grid in the electrostatic mirror was grounded [29]. The first version of the new instrument achieved a 5% energy resolution sufficient to resolve the spin–orbit splitting of the Ar+ cation and the H2+ vibrational progression. In principle, this energy resolution can be further improved by increasing the length of the drift zone. The present work focuses on increasing the amount of electrons accumulated in the trap, which depends crucially on increasing the
the second grid in the grid design. On the one hand, there are no transmission losses in the gridless electron trap (Fig. 2) unless the reflecting field is defocusing the electrons. On the other, the reflecting mirror volume is larger, which means that more electrons are in this volume when the gate is opened, and the electric field penetrates the volume of the trap, meaning that the kinetic energy of the photoelectrons may be perturbed depending on their position in the trap when the gate is opened. The electrodes are basically two apertures spaced by 12 mm. The first electrode is located 8 mm above the ionization region and has an opening of 5 mm. The second electrode has an opening of 8 mm. In our set-up, the electrodes need to have a conical shape (Figs. 1 and 2) to make room for the ion optics. The beam of size-selected cluster ions enters the ionization region through an Einzel lens from behind (not shown in the Figure). The second electrode is set to a negative voltage (gate closed) and for a short time set to ground potential (gate opened). It has been found that the magnitude of the repeller voltage in the range of 10–70 eV has no effect, provided, of course, that it is greater than the maximum kinetic energy of the photoelectrons to be analyzed.
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The spectrum recorded with the gridless electrode design over 30 s at a 50 kHz repetition rate has a similar shape (Fig. 3b). The Ar pressure was 2·10–7 mbar (compared to 5·10–7 mbar in Fig. 3a). Hence, the count rates are lower compared to Fig. 3a. There is a rectangular feature with a length equal to the turn-off time of the electrostatic mirror (10 μs). At the beginning of the rectangular feature, there is again the storage peak. There are two obvious differences to the spectrum achieved with the grid design: the storage peak is much more intense and much wider. From the number of counts in the storage peak and the count rate of the integral peak, the storage time is calculated to be 4.3 μs. This indicates an approximately 40-fold lifetime of the electrons in the trap and corresponds to a more than 40% duty cycle at 100 kHz, although the spectrum shown in Fig. 3b was recorded at a repetition rate of only 50 kHz. Such a dramatic increase in the duty cycle is not surprising, because the main cause of the losses, the limited transmission of the grids, has been eliminated. However, the duty cycle is only an upper limit to the collection efficiency of the spectrometer, i.e., to the chance that a photoelectron will contribute to the recorded spectrum. This is because the transmission of the instrument, the detection probability at the channeltron, and the limited reflectivity of the magnetic mirror affect both the storage and the integral peaks. The spectrum shows that the basic idea of combining a time-offlight electron spectrometer with an electron trap is principally sound. A high duty cycle of ca. 40% can be achieved in a gas-phase experiment using a magnetic bottle ToF spectrometer by including an electron trap regardless of the time structure of the light source. However, with the gridless design, the energy resolution needs to be optimized for general applicability at quasi cw-light sources. Fig. 4 shows a comparison of the time-of-flight photoelectron spectra of Ar taken at three different photon energies. Spectra on the left (Fig. 4a) were taken with the grid design and spectra on the right (Fig. 4b) with the gridless electrode design. The storage peak shifts to longer flight times with decreasing photon energy, as the electron kinetic energy decreases. At the employed gate voltage of 25 V and 45 V in the grid and gridless electrode designs, respectively, the relative energy resolution appears to be independent of the kinetic energy and is
Fig. 3. Comparison of time-of-flight photoelectron spectra obtained with the grid (a) and the gridless electrode design (b) shown in Fig. 1. Electrons are generated by photoionization of Ar gas with 22 eV synchrotron radiation. "off" and "on" refers to the voltage applied to the electrostatic mirror, i.e., opening and closing the gate towards the flight tube.
electron lifetime, i.e., the storage time, in the trap. Experimentally, the storage time can be determined by dividing the number of electrons in the storage peak by the production rate of photoelectrons, which is the count rate in the rectangular integral peak. In Fig. 3 a, the spectrum is a sum of 3 million pulses (1 min at 50 kHz). The total number of counts in the integral peak is 443,000 in the 2–6 μs interval. This corresponds to a count rate of 0.037 cts/μs. The total photoionization rate is somewhat higher, because prompt electrons also have to pass two grids and the transmission of the drift tube as well as the detection efficiency of electrons are both somewhat below 100%. In addition, about 1% of the photoelectrons are emitted almost perfectly parallel to the magnetic field lines and are not reflected by the magnetic mirror. Thus, the measured count rate in the integral peak is estimated to be approximately 80% of the total ionization rate in the ionization volume. However, these factors decrease the storage peak in equal measure. The total number of photoelectrons in the storage peak is 12,150 cts. This corresponds to a count rate of 0.0041 cts/pulse. Therefore, the number of electrons collected in the trap corresponds to the number of photoelectrons generated within 0.11 μs. This corresponds to 0.08 μs electron half-life in the trap, which agrees with theoretical considerations (see above) and is rather short. Thus, the storage effect is there, but it is weak. At the maximum repetition rate of 100 kHz and a storage time of 0.11 μs, only 1.1% of the theoretically detectable photoelectrons are in fact detected in the storage peak. Thus, the theoretically predicted and measured ca. 1% duty cycle is much below what is required.
Fig. 4. Comparison of time-of-flight photoelectron spectra taken with the grid (a) and the gridless electrode design (b) at three photon energies. The storage peak shifts to longer flight times with lower photon energies, as the electron kinetic energy decreases. The arrows in the spectra on the right indicate the theoretical electron time-of-flight corresponding to the kinetic energy calculated as the difference of the Ar ionization potential and the photon energy. In the series of spectra on the left, this corresponds to the position of the sharp storage peak. 4
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ca. 5% for the grid design and ca. 30% for the electrode design. In the grid design, the storage peak at the lowest photon energy splits into two components corresponding to the two spin–orbit states of the Ar+ ion ground state, separated by 178 meV [29]. In the grid design, the storage time may be expected to increase with lower photon energies because slower electrons are formed, which pass through the grid less frequently. However, the effect is counteracted by the slow electrons being more susceptible to stray fields and is rather weak anyway because the velocity of the electrons varies only with the square root of the energy. In fact, within the energy range studied (2.2–10.2 eV), the storage time has also been found to be practically independent of the kinetic energy in both electron traps. For the spectra recorded with the gridless electrode design, the calculated flight times corresponding to the kinetic energies of the photoelectrons are also indicated. Most detected electrons have a longer ToF, corresponding to a lower energy, but there is also signal at higher kinetic energies. At first glance, it is surprising that the energy distribution is so broad. However, this can be explained by two effects. First, the electric field gradient is much weaker, making the reflection quite "soft". The electrons spend more time in this soft electric field as they eventually slow down and turn around. Second, the electric field penetrates more deeply into the volume of the trap. At different positions in the trap or in the electrostatic mirror, the electrons’ kinetic energy is perturbed to a varying degree. Because they spend more of their time in the electrostatic mirror during reflection, a larger fraction of electrons is located in the mirror when it is turned off and their kinetic energy is lowered. The electric field of the mirror also extends into the ionization volume and accelerates electrons towards the magnetic mirror. When the mirror field is switched off, these electrons have kinetic energies higher than the original energy. Since these electrons are faster, they spend less time in the magnetic mirror and their intensity is low. The measured time-of-flight spectrum reflects the distribution of the kinetic energies that the electrons had in the trap and the electrostatic mirror when the electric field was turned off instantaneously. Fig. 5 illustrates the shape of the storage peak qualitatively according to the above considerations. For the grid design (Fig. 5a), the kinetic energy of the electrons in the trap is constant and corresponds to the energy they have from the photoionization process (E0). At any given moment, only a few electrons are found in the electrostatic mirror being reflected because the field gradient is steep and the reflection is fast. These electrons have lower kinetic energies, resulting in a shoulder on the lower kinetic energy side. In the time-of-flight spectra, this explains the asymmetry of the storage peak towards longer flight times (see, e.g., Fig. 3a). For the gridless electrode design, the electric field is more extended and the field gradient is less steep (Fig. 5b). Most of the time the electrons spend during reflection in the electrostatic mirror. Accordingly, the peak in the photoelectron spectrum is broadened and shifted to lower kinetic energies. This picture qualitatively explains the differences in the time-of-flight spectra between the two designs of the electrostatic mirror. While the time-of-flight spectra measured in the gridless setup could be deconvoluted into kinetic energy spectra by recording calibrated ToF distributions belonging to monoenergetic electrons, we doubt that this approach would provide meaningful information at the current, broad response functions. This is also why we only discussed the experimental differences between the two designs based solely on time-of-flight distributions. Based on the comparison of the two designs, the following insights will help further development towards an optimal design:
Fig. 5. Storage peak shapes as influenced by the potential and the electron distribution in the trap (see text). Parts (a) and (b) refer to the two different gate designs displayed in Fig. 1. Arrows mark the ionization volume in the trap. Note that the schematic spectra on the right are energy spectra and not time-of-flight distributions.
• The time the electrons spend in the electrostatic field should be as
short as possible. This requires a steep field gradient, which is difficult to achieve without a grid. For low-energy electrons high, transmission grids could be used. Slow electrons pass through the grid only a few times during the storage time, so that the intensity loss is low. This is especially true when the trap is longer.
Therefore, increasing the length of the trap and using extremely high transmission grids should result in a significant increase in energy resolution (ΔE/E < 0.1). For low energy electrons (Ekin ≈ 1 eV), the intensity loss at the grid should still be low (< 50%).
4. Conclusions An electron spectrometer ideal for the study of extremely dilute samples will detect all electrons produced in the ionization volume and suppress all other electrons from outside this volume. Magnetic bottle time-of-flight spectrometers come close to this but normally have the disadvantage of requiring a pulsed light source. We developed this instrument further, equipping it with an electron trap based on an electrostatic mirror gating the entrance to the electron flight tube. The trap is formed by the magnetic mirror below and the electrostatic mirror above the ionization volume. While the electrostatic mirror is on, the electrons are trapped between the two mirrors and fly back and forth through the ionization volume. From time to time, the electrostatic mirror is turned off so that the accumulated electrons travel the regular path through the drift region of the spectrometer to the electron detector. First, the electron lifetime should be infinite in an effective trap. Thus, such a setup should allow us to detect virtually all photoelectrons
• The energy resolution is limited mainly by the effect of the trap on the energy distribution of the electrons. • The energy distribution broadening can be reduced by increasing the distance between the electrostatic mirror and the ionization region. The longer the zone with a negligible electric field, the larger the fraction of electrons with the unperturbed, original kinetic energy at the time when the gate is opened.
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regardless of the time structure of the light source. Second, an effective trap must not alter the electron kinetic energies and, thus, the energy resolution should be easy to increase by increasing the length of the drift tube. However, our data show that building an effective trap in these two regards simultaneously is not trivial. The crucial component is the electrostatic mirror. In order to achieve a high energy resolution, a strong and well-separated electric field is necessary, so that the electrons spend as little time in the mirror as possible. This condition can be met with metal meshes, which have the disadvantage of limited transmission. The trapped electrons pass through the first grid of the mirror twice in each cycle, resulting in extensive losses. In our case, a storage time of ca. 0.1 μs could be achieved based on grids with ca. 85% transmittance. Meshes with 99.5–99.9 % transmittance would be needed to approach > 4 μs storage time, an appropriate value assuming 100 kHz extraction frequency. We presented and tested another, gridless electrode configuration herein. This setup delivered the desired 40% duty cycle. However, electric field penetration and the resulting soft reflection process interfere with the energy resolution. Thus, the grid design achieves a high energy resolution while the gridless electrode design yields the required duty cycle. We have thus shown that magnetic bottle time-of-flight spectrometers can be used to study extremely dilute samples by suppressing electron collection from outside of the ionization volume at a high duty cycle or high energy resolution. Unfortunately, the two cannot yet be achieved simultaneously. Still, we believe that these insights and the knowledge gained in these proof-of-principle studies will help us achieve both design goals at the same time by combining the distinguishing features of both electron trap designs.
[2] [3] [4] [5] [6] [7] [8]
[9]
[10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22]
Acknowledgements
[23] [24]
Support by the German Research Foundation is highly appreciated (project GA 389/27-1). The measurements were performed at the VUV beamline (Swiss Light Source, Paul Scherrer Institute). A.B. and P.H. acknowledge funding by the Swiss Federal Office of Energy under contract SI/501269-01. The authors are grateful to the infrastructure groups at the SLS for the support and would like to thank Patrick Ascher for his assistance.
[25] [26] [27] [28] [29] [30] [31] [32]
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