1. Introduction

Large molecules in general and in particular, polycyclic aromatic hydrocarbons (PAHs) are found in the terrestrial environment as well as in the interstellar medium abundantly [1]. They are primarily formed on earth by the incomplete combustion of organic molecules. The origin of Diffuse Interstellar Bands (DIBs), that are absorption features seen in the spectra of astronomical objects in optical and infrared wavelengths has been attributed to PAHs [2]. Considering the abundance of high energy radiation in the interstellar medium, it remains an interesting endeavor to understand the mechanism behind the survivability of PAHs in such environments. Because of such astronomical significance, the interaction of PAHs with photons and charged particles has seen renewed interest in the last couple of decades. Even in recent times, several results have been reported globally on the topic of ion-PAH collisions [3–6]. In addition, detailed experiments are being carried out using synchrotron radiation sources [7]. High energy electron interaction

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

diacetylene (C4H2) evaporation process to identify the influence of plasmon exci-

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

instrumentation presented here is very effective under such conditions.

The pulsed extraction of ions in a ToF setup can be used to analyze the evolution of a time-dependent population of various fragmentation channels. If the decay constants are in the range of 10<sup>5</sup> <sup>10</sup>6, then the thermal velocity dispersion does not affect the collection efficiency if it becomes possible to probe the system within sufficient time. Even though such a unique system has limited applicability, it has helped in our present investigation to study the evaporative loss from a PAH molecule due to electron impact and the decay rate for acetylene and diacetylene loss in naphthalene has been shown to have decay constant in the range of 10<sup>5</sup> <sup>10</sup><sup>6</sup> sec <sup>1</sup> at 8–9 eV internal energy [12, 18]. So, the decay time of acetylene and diacetylene is of the order of 1–10 μs. The ionization potential of naphthalene is 8.12 eV [19] and the plasmon resonance of naphthalene is observed at 17 eV [4]. The

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

tation at different electron beam energies up to 1 keV.

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

2. Experimental set-up

Figure 1.

27

The developed experimental set-up.

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 atoms in the molecule.

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 molecules [17].

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

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

diacetylene (C4H2) evaporation process to identify the influence of plasmon excitation at different electron beam energies up to 1 keV.

The pulsed extraction of ions in a ToF setup can be used to analyze the evolution of a time-dependent population of various fragmentation channels. If the decay constants are in the range of 10<sup>5</sup> <sup>10</sup>6, then the thermal velocity dispersion does not affect the collection efficiency if it becomes possible to probe the system within sufficient time. Even though such a unique system has limited applicability, it has helped in our present investigation to study the evaporative loss from a PAH molecule due to electron impact and the decay rate for acetylene and diacetylene loss in naphthalene has been shown to have decay constant in the range of 10<sup>5</sup> <sup>10</sup><sup>6</sup> sec <sup>1</sup> at 8–9 eV internal energy [12, 18]. So, the decay time of acetylene and diacetylene is of the order of 1–10 μs. The ionization potential of naphthalene is 8.12 eV [19] and the plasmon resonance of naphthalene is observed at 17 eV [4]. The instrumentation presented here is very effective under such conditions.
