2. Experimental set-up

with PAHs, on the other hand, has not been investigated in the context of radiation

High energy photon impact studies are often made using synchrotron radiation sources [8–10] with atoms or molecules as targets. Such mass spectrometric techniques along with the secondary electron selectivity methods like photoelectronphotoion coincidence (PEPICO) spectroscopy and threshold photoelectronphotoion coincidence (T-PEPICO) spectroscopy helps in determining the appearance energy and time scales of various dissociation channels of molecules very accurately. By modeling the line shape of the mass spectrum that arises due to slow decay, corresponding decay constant can be measured for microsecond range [11]. Longer decay times are probed using ion traps with variable extraction time [12]. It is essential to note that the excitation mechanism in conjunction with the appropriate electron spectrometer gives a very narrow range of internal energy left in the molecule. In particular, near the threshold, the internal energy is generally larger than the original thermal energy of the molecule, leading to resulting decay constants also range in a narrow band of values. Considering that the Arrhenius law decay rates are extremely sensitive to the internal energy, this factor very importantly implies that the decay rates with such secondary electron gated species will lie in a narrow range. Charged particle interaction with molecules will have a much broader range of internal energies and a precise value of decay constant cannot be obtained even with a suitable secondary electron energy gating. Hence modeling of exact decay curve for charged particle collision induced dissociation is deemed impractical. Moreover often minor fragmented peaks will interfere with the tail of mass spectrum due to isotopic effects or due to the presence of many hydrogen

Electron impact ionization is one of the oldest mass spectrometric tools but it mainly focuses on identifying the possible ionization and fragmentation channels particularly between 70 and 100 eV energy. Typically, the mass spectrometric data available in the database is taken in this range, because ionization cross-section normally peaks in this region [13]. In past, several electron impact ionization investigations have been done mainly on inert gasses, diatomic or triatomic molecular gasses. Several experiments and modeling attempts have been made for such studies with electron energy up to few keV [14–16]. But such studies are very rare for larger molecules; the main reason is the complexity of a large number of decay channels, difficulties in separating indirect from direct ionization processes. For large molecules there are few attempts have been made in some specific cases for target specific energy loss modeling within the charged particle interaction with

The stability of PAHs during the interaction of charged particles, cosmic rays and photon sources in the interstellar medium is of our interest [4, 7]. It has been shown conclusively that for charged particle interaction with naphthalene, the plasmon excitation is a major channel particularly at high velocities of projectile wherein the other processes have diminishing cross section [4]. It is also seen that acetylene (C2H2) loss comes as a by-product of such plasmon excitation with a very specific range of decay constants. We use naphthalene as a model since it exhibits many general spectroscopic and structural properties of larger PAHs [18]. An attempt is made to investigate the interaction of high energy electron beam with PAHs and assess the time dependence of C2H2 evaporation in comparison with the other channels using a Time-of-Flight (ToF) mass spectrometer. Recent studies on benzene using PEPICO highlight the importance of the time variation of the decay channel [11]. We explore the time evolution of various decay channels within the first 5 μs of naphthalene ionization. This is achieved by the correlated pulsing of the electron source and recoil ion extraction field. We specifically target C2H2 and

tolerance of PAHs.

Mass Spectrometry - Future Perceptions and Applications

atoms in the molecule.

molecules [17].

26

For detection of product ions we use a Wiley-McLaren type time-of-flight mass spectrometer [20] with the pulsed extraction technique. The experimental set-up with the data acquisition system is shown in Figure 1. This experimental set-up consists of pusher (labeled as P1 in Figure 1) as well as puller (labeled as P2 in Figure 1) plates of thickness 1 mm and outer diameter of 105 mm and the puller plate has an opening diameter of 26 mm and for field uniformity that is covered with a nickel mesh characterized by 40 lines per inch, which allows a transmission efficiency of 95% of the molecular ions. For the field-free drift of ions, we have a drift tube of length 200 mm with an opening of 25 mm covered by a nickel mesh for field uniformity. Between pusher (P1) and puller plates (P2) there is 16 mm gap while that between the puller and the drift tube is 5 mm. A low current – high energy (1–5 keV) electron gun is used for ionizing the target molecule, which is custom made using CRT tube. Electron gun produces electron from filament via thermionic emission with a current of typically about 180 mA. There are two sets of

Figure 1. The developed experimental set-up.

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.

turbo molecular vacuum pumps backed by a rotary pump. The pressure of the chamber goes down to 5:<sup>8</sup> <sup>10</sup><sup>8</sup> mbar, that rises up to a maximum of 5:<sup>0</sup> <sup>10</sup><sup>7</sup>

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

required due to the relatively large vapor pressure at 300 K. With the naphthalene target (has a purity greater than 98 %), the chamber pressure was maintained

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

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

mbar with the target gas. With naphthalene as the target, no heating was

are also available commercially, but are quite expensive generally.

at <sup>3</sup>:<sup>0</sup> <sup>10</sup><sup>7</sup> mbar.

Figure 3.

29

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

3. High voltage MOSFET switch

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

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

Figure 2. Pulsing sequence used for delayed extraction time-of-flight mass spectrum.

Large Molecule Fragmentation Dynamics Using Delayed Extraction Time-of-Flight Mass… DOI: http://dx.doi.org/10.5772/intechopen.84407

turbo molecular vacuum pumps backed by a rotary pump. The pressure of the chamber goes down to 5:<sup>8</sup> <sup>10</sup><sup>8</sup> mbar, that rises up to a maximum of 5:<sup>0</sup> <sup>10</sup><sup>7</sup> mbar with the target gas. With naphthalene as the target, no heating was required due to the relatively large vapor pressure at 300 K. With the naphthalene target (has a purity greater than 98 %), the chamber pressure was maintained at <sup>3</sup>:<sup>0</sup> <sup>10</sup><sup>7</sup> mbar.
