The Mass Gate

Vespel-Rod Wire 1 (-)

Wire 2 (+)

Steel-Rod Ion Beam

a) b)

Figure 4.20 Schematic views of the interleaved comb mass gate constructed for size selection of cluster ions. Two coplanar sets of stainless-steel wires tightly strung around rods comparable to a weaving loom. Due to the use of vespel rods and holders the two wires are insulated from each other and can be kept at different potentials. a) Schematic side view of the mass gate configuration. b) Schematic top view of the mass gate configuration.

One key feature of the experimental setup is the improved design of a pulsed ion gate (9 in figure 3.1), located in the first focal plane of the Re-TOFMS. The design of the mass gate is based on the original ideas of Cravath, Bradbury and Nielsen [275; 276] and enables the size selection of cluster ions prior to their surface interaction. It consists of two parallel coplanar sets of stainless-steel wires in a UHV-compatible frame, offering low capacity and allowing fast switching times (see figure 4.20)¹. The wire spacing of 500µm (wire diameter∅= 50µm) results in an optical transmission of 90%, the mass selection performance exceeds 190 at comparatively small deflection voltages of 150−500 V depending on the Re-TOFMS acceleration voltage (see subsection 4.2.4). The main advantage of the interleaved comb mass gate is its steep potential gradient (see figure 4.21 b). Due to the potential compensation of the alternating wire configuration, the potential declines exponentially to zero (see figure 4.21 a and b). Thus, unwanted field perturbations of the field free drift region of the mass spectrometer are minimized simultaneously improving resolution. In operation a bipolar potential is applied to the wires of the mass gate (as in figure 4.21 a). Every ion package which enters the mass gate is deflected by the wire potentials and leaves the normal beam trajectory. These ions collide with the exit slit of the mass gate or with the slit located in front of the reflectron and are “filtered” from the mass spectra. For

“gating” the desired ion mass (or cluster size) the potentials applied to the mass gate are switched off by fast push-pull switches (see section 3.4). These ions can pass the mass gate and fly on the normal ion trajectories to the detector. By switching the potentials to the wires on again, following ion packages are also

“filtered” from the beam. Thus the size selection performance of the mass gate depends on the resolution at the wire position plane and the switching speed of

1Designed and constructed by Ulf Bergmann

4. Chapter 4.1 TOFMS Optimization

0 200 400 600 800 1000 1200 1400 0

Figure 4.21 Depicted are SIMION simulations of the interleaved comb mass gate.

a) SIMION equipotential view of the alternating wire configuration. An alternating potential of ±100 V is applied to the wires. b)Graph showing the potential values from a) in dependence from the wire distance (in beam direction). The decline of the potential is well fitted by an exponential decay function.

the transistor switches. Ideally the mass gate is positioned in the space focus plane of the TOFMS accelerator for the highest size selection performance. Due to the principle of TOFMS mass resolution depends on many factors e. g. mass, beam diameter, ion package length and so on (see section 2.2.4). To estimate the performance of the mass gate before the implementation in the experimental setup we performed SIMION simulations. The simulation of the accelerator and the deflector were described before (in 4.1.2 and 4.1.3). In the simulations before the aim was to optimize each component of the TOFMS for optimal resolution.

In contrast for the mass gate a different approach is made. The question that raised was if it is possible to mass select big cluster sizes e. g. N ≈ (CO₂)⁺₁₀₀ with the present setup. Therefore the optimized three stage accelerator (see figure 4.9) and the optimized deflector (see figure 4.19) were used to simulate ion TOF distributions of (CO₂)⁺_n-cluster ions with N around 100 in the space focus plane (in contrast to the Ar⁺₂₅cluster ions used in the simulations before). Due to the use of much higher masses the kinetic energy in beam direction increases to about 9 eV per cluster ion (for (CO₂)⁺₁₀₀clusters assuming a beam velocity of^Duk

E= 630 m/s as in the case of Ar). In the following simulations the cluster-beam is represented by three ion groups. One ion group consists of 101 ions with the mass of a (CO₂)⁺_n clusters of the same size (N = 99,N = 100 orN = 101). The ions are positioned equidistantly in three lines in the first stage of the accelerator, forming one group.

The length of the ion group lines defines the beam diameter (experimentally defined by the skimmer diameter). To simulate the beam pulse length (given by the valve opening time), the first and the third ion groups are positioned symmetrically around the second ion package, which is located in the center of the acceleration stage (as in the case of the accelerator simulations see 4.1.2). Most of the simulations were performed with only the two acceleration stages of the three

4. Chapter 4.1 TOFMS Optimization

a) b)

TOF [µs]

Ion Counts

Figure 4.22 Histogram graphs of the TOF distribution for three different cluster sizes ((CO₂)⁺₉₉, (CO₂)⁺₁₀₀ and (CO₂)⁺₁₀₁) recorded at the space focus plane of the TOFMS accelerator (wire plane of the mass gate). a) TOF distribution for the two stage accelerator (L1 = 12 mm, L2 = 12 mm and LD = 326 mm) at 6 kV acceleration (U₀ = 6 kV, U₁ = 4873.65 V, ±95 V for deflection the optimum value adjusted by repeated simulations, 3 mm beam width and 15 mm beam pulse length).

The resolution calculated from the TOF spread of each cluster size is R = 650.1.

b)TOF distribution for the three stage accelerator (L₁ = 12 mm,L₂ = 12 mm,L₂= 26.5 mm and L_D = 299.5 mm) at 6 kV acceleration (U₀ = 6 kV, U₁ = 5218.82 V, U2 = 4466.55 V, ±95 V for deflection the optimum value adjusted by repeated simulations, 3 mm beam width and 15 mm beam pulse length). The resolution calculated from the TOF spread of each cluster size is R= 939.

stage accelerator to simulate a “worst case” scenario (L₁ = 12 mm, L₂ = 12 mm and the length of L₃ = 26.5 mm is added toL_d = 299.5 mm + L₃ = 326 mm for two stage operation and for three stage operation Ld = 299.5 mm). The time-of-flight distributions of the ions were recorded at the space focus plane located at the distance L_d. The resulting resolution was calculated with equation (2.36) by assuming a equipartition distribution for the starting ions (p(xs, i) = 1/n).

By displaying the TOF distribution of the three different cluster sizes ((CO₂)⁺₉₉, (CO₂)⁺₁₀₀ and (CO₂)⁺₁₀₁) in a histogram graph the TOF difference between the three sizes can be deduced. For successful mass gating the difference in arrival time for each cluster size must be greater than the minimum switching time of the transistor push-pull switches. Two representative histograms for 6 kV acceleration for two stage operation and three stage operation of the TOFMS accelerator are shown in figure (4.22). The resolution obtained by the TOF spread for the two stage configuration shown in figure (4.22 a) is R = 650.1 which is about 2/3 of the resolution obtained for the three stage configuration (figure 4.22 b) with R = 939. The arrival time difference between the last bin of the (CO₂)⁺₉₉ cluster ion package and the first bin of the (CO₂)⁺₁₀₁ cluster ion package is 180 ns for the two stage configuration (4.22 a) and 220 ns respectively for the three

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4. Chapter 4.1 TOFMS Optimization

Acceleration Voltage [kV]

R (t/∆t)

Figure 4.23 Influence of the acceleration voltage on the resolution at the space focus plane (wire plane of the mass gate) obtained from the TOF distribution for three different cluster sizes ((CO₂)⁺₉₉, (CO₂)⁺₁₀₀ and (CO₂)⁺₁₀₁). SIMION TOFMS resolution for the two stage accelerator configuration (L1 = 12 mm, L2 = 12 mm and L_D = 326 mm) at 1–6 kV acceleration (±50–95 V for deflection, optimum value adjusted by repeated simulations, 3 mm beam width and 15 mm beam pulse length).

stage configuration (4.22 b). The minimum output pulse width of the transistor switches is 200 ns. Thus it would be barely possible to mass select cluster sizes around (CO₂)⁺₁₀₀ with both configurations (two stage and three stage at 6 kV acceleration). For lower acceleration voltages the arrival time difference in the focus plane increases with decreasing acceleration voltage (e. g. 210 ns at 3 kV and 230 ns at 2 kV two stage acceleration). In contrast to this the calculated resolution decreases with decreasing acceleration voltage as might be expected. In this case the ion package peaks gain on width and overlap which each other for acceleration voltages below 2 kV (two stage acceleration). Therefore a clean mass selection for acceleration voltages below 2 kV and cluster sizes around N = 100 is not possible due to overlapping peaks even the high resolution (R = 250 at 1 kV two stage acceleration). Contrary to the mass resolution definition (see subsection 2.2.4) for

“clean” mass selection a high enough mass resolution (narrow peak width) and additionally sufficient arrival time delay between the ion packages of different mass is required (no overlap of mass peaks). For acceleration voltages above 1 kV the arrival time difference between the three cluster sizes is higher than 200 ns and the resolution even for the two stage system is larger than R= 250. Thus it can be assumed that for these configurations a mass selection performance above N = 100 can be expected. Note here that two factors decide the mass selection performance for a given configuration. The first factor is the time difference between the different cluster size peaks which is given by the acceleration voltage

4. Chapter 4.1 TOFMS Optimization

a) b)

Beam Width [mm] Beam Pulse Length [mm]

R (t/∆t)

Figure 4.24 Influence of the beam properties (beam width and beam pulse length) on the resolution at the space focus plane (wire plane of the mass gate) obtained from the TOF distribution for three different cluster sizes ((CO₂)⁺₉₉, (CO₂)⁺₁₀₀ and (CO₂)⁺₁₀₁). a) SIMION TOFMS resolution for the two stage accelerator config-uration (L1 = 12 mm, L2 = 12 mm and LD = 326 mm) at 6 kV acceleration (±95 V for deflection, beam pulse length fixed at 15 mm) and variation of the beam width. b)SIMION TOFMS resolution for the three stage accelerator configuration (L1 = 12 mm, L2 = 12 mm, L3 = 26.5 mm and LD = 299.5 mm) at 6 kV accelera-tion (±95 V for deflecaccelera-tion, optimum value adjusted by repeated simulaaccelera-tions beam width fixed at 3 mm) and variation of the beam pulse length.

(for larger acceleration voltages this time difference decreases). The other factor is the available resolution thus the peak width for a given configuration which is narrower for larger acceleration voltages or three stage operation. In the figure (4.23) the dependence of the resolution from the acceleration voltage is depicted.

As mentioned above with increasing acceleration voltage the resolution of the system at the space focus plane increases similar to a root function. Besides the acceleration voltage the two main beam properties (beam width and beam pulse length) also affect the resulting resolution. The influence of these two properties on the resulting resolution is depicted in figure (4.24). For comparison in figure (4.24 a) the behavior of the two stage accelerator for increasing beam width and in figure (4.24 b) the behavior of the three stage accelerator for increasing beam pulse length are depicted. In both cases with increasing values for the beam properties the resolution of the system decreases nearly exponentially. These results show that the resolution of the system can be improved by reducing the beam width or beam pulse length. The beam width is given by the skimmer diameter (∅ = 3 mm) and the orthogonal beam velocity (v⊥). By focusing the beam with an einzel lens into the TOFMS accelerator the beam width can be reduced. However, strong focusing with an einzel lens generates a focal point after which the beam is widened (similar behavior like a focusing optical lens).

Therefore a slightly focusing of the beam would be preferred. In both cases a

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4. Chapter 4.1 TOFMS Optimization uniform beam width along the whole beam pulse length cannot be achieved. It must be also noted that beam focusing is limited by the coulomb repulsion of the ions limiting the final beam width. Regarding the beam pulse length, this value can be reduced by pulsed ionization of the cluster beam. This can lead to reduced beam intensity which is unwanted for cluster experiments where high beam intensities are required (e. g. scattering experiments). The results of these

“worst” case simulations obtained in this subsection show that it must be possible to mass select cluster sizes aroundN = 100 with the present accelerator, deflector and mass gate configuration. This mass selection performance was also approved by experimental results which are summarized in subsection (4.2.4).