3. High voltage MOSFET switch

positional X-Y deflectors mounted after the focusing lens and two apertures (A1, A2), one between the two deflector sets (A1) and the other after the deflectors (A2), to discriminate the secondary electrons from the internal scattering of the beam. The whole electron gun assembly is floated, which voltage is decides the electron beam energy. A pulsed switch with a variable ON time width is used to operate the electron gun. For pulsed extraction ToF, a high voltage in the push-pull mode with a 50 ns rise and fall time is used which is a home-built high voltage MOSFET switch. A cylindrical Faraday cup made from stainless steel (SS-304), of length 100 mm and radius 25 mm to collect electron beam. This Faraday cup is placed at a distance of 50 mm from the exit of the interaction region. The Faraday cup is biased to +36 V using batteries to collect projectile electrons as well as the secondary electrons produced by the primary beam due to collision with Faraday cup wall. The intended target molecule is introduced into the interaction zone that is well localized in space through a fine capillary of internal diameter 300 microns and length of 15 mm. To avoid any possible secondary electron emission due to the electron beam colliding with the capillary, the capillary exit is kept nearly 5 mm away from the center of the ToF interaction region. A channel electron multiplier (CEM) is used for the detection of molecular ions, which is biased to a voltage of 2600 V. The electron gun, the pusher and puller plates are pulsed as per our pulsing sequence for delayed time-of-flight mass spectrometry as shown in Figure 2. Molecular ions produced due to electron impact (in the well-focused interaction region) are accelerated by the electric field and compensated for the special spread in the second region before entering the field-free drift tube followed by the ion detector. A multi-hit time-to-digital converter [Agilent TDC (Model: U1051A)] is employed as part of data acquisition system. To filter out the noises picked up by the detector due to switching pulses, we use a desired gate pulse. Stainless steel (SS 304) high vacuum chamber with metal joints are used to place the whole experimental setup. For better alignment, the time-of-flight mass spectrometer is mounted along the axis of the chamber and electron gun in perpendicular to its axis which ensure the cross beam (perpendicular) interaction of projectile beam and the molecular jet. There are several auxiliary ports for pumping, electrical connections and vacuum gauges. The whole chamber is pumped by two

Mass Spectrometry - Future Perceptions and Applications

Figure 2.

28

Pulsing sequence used for delayed extraction time-of-flight mass spectrum.

Electron beams are very sensitive to electrode voltages and hence in a time of flight spectrometer used in electron impact studies, they need special arrangement to limit the effect of the extraction voltages on the pusher and puller. Usually, this can be achieved by pulsing the extraction voltages. Switching noise pickup on the detector channels are the major complications with this arrangement. The electron beam cannot be allowed to persist during the extraction process, failing to which the beam might cause a large number of secondary electrons produced due to the deflected beam hitting the electrodes and then causing additional undesired ionization events. While we extracting the recoil ions it becomes essential to blank the electron beam. For this purpose, the present experiment with electron impact ionization requires two sets of fast switches, one set for switching electron beam and the other for switching of pusher and puller simultaneously. By using commercial off the shelf power MOSFETs we built fast switches to achieve this. Very commonly used fast single output switches are detailed in few literature for various applications [21–24]. Dual output fast pulsers for time-of-flight mass spectrometry are also available commercially, but are quite expensive generally.

It is necessary to have a pair of switches which can operate in a synchronized manner to switch the pusher and puller plates in the spectrometer which enables us to control the extraction cycle accurately. We use a pair of fast power MOSFETs triggered in synchronism to achieve this. A brief schematic of the high voltage MOSFET switch circuit is as shown in Figure 3. In order to minimize the probability of damage to the external TTL pulse generator due to noise produced by the high

Figure 3. Schematic of high voltage MOSFET switch (push-pull).

displacement of the center of the interaction region along the ToF axis as well as in the direction perpendicular to the ToF axis (Figure 5b). From our simulation the collection efficiency is estimated by assuming a spherical distribution of the source of diameter 6 mm and assuming an rms velocity twice as large as the value of 300 K that is 480 μm/μs is as the worst case scenario and in all the cases, we could achieve 100% collection efficiency. The electron beam is pulsed with a width of 500 ns. By using deflector blank pulsing method the pulse rise and fall times of 50 ns was obtained, where we pulse the deflector just before the first aperture and hence the electron pulse was effectively on for about 400 ns. The molecular ion extraction field (i.e., pusher-puller plates) is pulsed to 125 V/cm to extract the ions immediately after electron beam pulse is deflected off. The pusher-puller pulse are delayed for delayed extraction of molecular ions, for various delay times with reference to the electron beam pulse. From our calculation 99.9% of the naphthalene intact ions are expected to have thermal velocities less than 480 μm/μs at room temperature and this implies that they can move only 2.5 mm in 5 μs, as our simulations gave us the freedom of shifting the source position of the naphthalene target over the range of 2.5 mm. Thus, we have considered 5 microseconds as the maximum value of our delayed extraction time. The effect of the rise time of the extraction voltage to the molecular velocity spread is numerically calculated, which is 20 μm/μs for naphthalene molecule. So by taking into consideration the spread in thermal velocity, as well as the field effect for naphthalene molecule, the total spread in the velocity is expected to be a maximum of 500 μm/μs. In this case also we could achieve collec-

Large Molecule Fragmentation Dynamics Using Delayed Extraction Time-of-Flight Mass…

Various projectile electron beam energy values with varying amounts of delay between the electron pulse-off and extraction pulse-on time are used for recording the Naphthalene mass spectra. The mass spectra obtained at different beam energies, as well as extraction delays, are systematically normalized to the single ioniza-

A typical mass spectra is dominated by singly ionized naphthalene molecule followed by prominent peaks originating due to acetylene evaporation losses, as well as intact di-cation, and di-cation with H2=2H2 loss as shown in Figure 6. There are several energetic fragmentation channels are also visible but are not as prominent. The intensity of single C loss and minimal intensity of 3C loss are hardly seen. The single ionization peak is preceded by one or two H atom loss is in general. The mass spectra reported here is not discernible due to our modest resolution. The possible loss of multiple H along with acetylene loss is negligible as shown in the acetylene loss region. On the other hand, the diacetylene loss region, shows two clear peaks due to loss of C4H2 and C4H4 and the latter could be due to the

sequential loss of two acetylene molecules. Significantly, the peak following mass at 64 mass units can be assigned clearly to the di-cation. The formation of exactly half mass fragment for naphthalene is highly improbable unless it is accompanied by several other peaks due to the concurrent loss of multiple H atoms and that would cause a large spread in the mass spectra. A dominant 2H=H2 loss channel is observed with about 50% of the di-cation peak intensity and a fast evaporation of neutral 2H or H2 and hence a large internal energy deposition due to low impact parameter collision is indicated distinct appearance of such a peak. We could ignore safely the 2H loss channel as it is observed to be less likely in previous investigations as

tion peak area. For comparison between different beam energies and delay combinations, the area of each individual peak after such normalization could

tion efficiency of 100% as per our simulation.

DOI: http://dx.doi.org/10.5772/intechopen.84407

5. Result and discussions

directly be considered.

31

#### Figure 4.

High voltage MOSFET switch output (a) push-pull mode, (b) rise time ð<50nSÞ in push mode, (c) fall time ð<50nSÞ in pull mode.

voltage section of the circuit, the TTL pulse is passed through several logic gates. This pulse is then used to trigger a MOSFET driver (IC31415P) and the output of the driver is fed to a toroidal transformer with single primary (two turns) and a pair of secondary (five turns each). Gate of the main power MOSFET is controlled by each of the two outputs, triggers a pair of small signal MOSFETs (BS170) which in turn control the. We have tested the switch with voltages as high as �300 V and could achieve rise/fall times of less than 50 ns with load (shown in Figure 4b, c). Figure 4a shows the fall/rise time at the end of the pulse was about 5 μs (note pushpull mode fall time) and was not necessary to improve as it played no role in the measurement.

A pair of deflectors and an aperture in between are used to achieve the electron beam pulsing. A single output MOSFET switch is used for one of the deflectors closer to the electron gun and this switch was derived from the same circuit described above but by using only one branch and using a cascade of four identical MOSFETs to achieve pulsing ability for voltages as high as �2000 V.
