4. Simulation

For optimized geometry as well as voltages for ion trajectory (as shown in Figure 5a, we performed the Simulation of our time-of-flight mass spectrometer with SIMION8.0 [25]. We simulated conditions with as much as 2.5 mm

#### Figure 5.

(a) Trajectory simulated for naphthalene (C10H8) with 480 μm/μs velocity from different position (colored as blue, green, black, white, brown) in the interaction region (b) Particle distribution in the interaction region. Red contours are the equipotential lines.

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

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 collection efficiency of 100% as per our simulation.

## 5. Result and discussions

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).

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

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

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

For optimized geometry as well as voltages for ion trajectory (as shown in Figure 5a, we performed the Simulation of our time-of-flight mass spectrometer

(a) Trajectory simulated for naphthalene (C10H8) with 480 μm/μs velocity from different position (colored as blue, green, black, white, brown) in the interaction region (b) Particle distribution in the interaction region.

with SIMION8.0 [25]. We simulated conditions with as much as 2.5 mm

MOSFETs to achieve pulsing ability for voltages as high as �2000 V.

measurement.

Figure 4.

ð<50nSÞ in pull mode.

Mass Spectrometry - Future Perceptions and Applications

4. Simulation

Figure 5.

30

Red contours are the equipotential lines.

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

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

## Mass Spectrometry - Future Perceptions and Applications

demonstrated by Jochims et al. [8] and thus unless stated explicitly otherwise, we will only refer to H2 loss possibility wherever the loss of 2 mass units is concerned. The effect is even stronger in the case of di-cation that a large peak is observed at 51 mass units coming from di-cation of naphthalene with the loss of one acetylenes and the acetylene loss competes with H loss in the case of single ionization. Similarly, this peak is again accompanied by H2 loss along with acetylene loss. As shown earlier in ion-PAH collisions studies [4, 6] we see the dominant H2 channel. It is C3H<sup>þ</sup> <sup>3</sup> and C3H<sup>þ</sup> <sup>4</sup> that dominate the spectra and the rest largely come as by-products of various decay channels in the mass region near 39 units.

For low-Z targets like hydrocarbons, the projectile energy dependence of the electron impact ionization process is very commonly known to peak at about 70– 100 eV and at higher energy, the cross section varies as ln E=E. Interestingly, the acetylene loss channel that shows a distinctly different behavior, as seen from Figure 7 except that all the channels considered here follow a similar trend. Thus, we have considered the following decay channels for our analysis.

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

<sup>8</sup> þ 2e�

<sup>6</sup> þ C2H2 þ 2e�

<sup>5</sup> þ C2H2 þ H þ 2e�

<sup>6</sup> þ C4H2 þ 2e�

<sup>5</sup> þ C2H2 þ C2H þ 2e�

<sup>4</sup> þ C4H4ð Þþ 2 � C2H2 2e�

<sup>3</sup> þ C2H2 þ C3H3 þ 2e�

<sup>4</sup> þ C4H2 þ C2H2 þ 2e�

<sup>3</sup> þ 2 � C2H2 þ C2H þ 2e�

<sup>2</sup> þ 3 � C2H2 þ 2e�

<sup>3</sup> þ C3H3 þ C4H2 þ 2e�

<sup>8</sup> þ 3e�

<sup>6</sup> þ 2H=H2 þ 3e�

<sup>6</sup> þ C2H2 þ 3e�

<sup>4</sup> þ C2H2 þ 2H=H2 þ 3e�

The loss of one or more H atom from the intact mono-cation is not shown explicitly in the list but is one of the most important channels. As observed by Gotkis et al., the primary decay channel for naphthalene is either H loss or acetylene

i. C10H8 <sup>þ</sup> <sup>e</sup>� ! C10H<sup>þ</sup>

ii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C8H<sup>þ</sup>

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

iii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C8H<sup>þ</sup>

iv. C10H8 <sup>þ</sup> <sup>e</sup>� ! C6H<sup>þ</sup>

v. C10H8 <sup>þ</sup> <sup>e</sup>� ! C6H<sup>þ</sup>

vi. C10H8 <sup>þ</sup> <sup>e</sup>� ! C6H<sup>þ</sup>

vii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C5H<sup>þ</sup>

viii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C4H<sup>þ</sup>

ix. C10H8 <sup>þ</sup> <sup>e</sup>� ! C4H<sup>þ</sup>

x. C10H8 <sup>þ</sup> <sup>e</sup>� ! C4H<sup>þ</sup>

xi. C10H8 <sup>þ</sup> <sup>e</sup>� ! C3H<sup>þ</sup>

xii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C10Hþþ

xiii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C10Hþþ

xiv. C10H8 <sup>þ</sup> <sup>e</sup>� ! C8Hþþ

xv. C10H8 <sup>þ</sup> <sup>e</sup>� ! C8Hþþ

33

Figure 6.

The mass spectrum of naphthalene(C10H8) at 1000 eV electron impact (Inset: naphthalene fragments).

Figure 7.

Projectile electron beam energy dependence of different decay channel with zero delay time extraction. Decay channels are labeled as listed above.

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

For low-Z targets like hydrocarbons, the projectile energy dependence of the electron impact ionization process is very commonly known to peak at about 70– 100 eV and at higher energy, the cross section varies as ln E=E. Interestingly, the acetylene loss channel that shows a distinctly different behavior, as seen from Figure 7 except that all the channels considered here follow a similar trend.

Thus, we have considered the following decay channels for our analysis.

<sup>8</sup> þ 2e�

ii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C8H<sup>þ</sup> <sup>6</sup> þ C2H2 þ 2e� iii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C8H<sup>þ</sup> <sup>5</sup> þ C2H2 þ H þ 2e� iv. C10H8 <sup>þ</sup> <sup>e</sup>� ! C6H<sup>þ</sup> <sup>6</sup> þ C4H2 þ 2e� v. C10H8 <sup>þ</sup> <sup>e</sup>� ! C6H<sup>þ</sup> <sup>5</sup> þ C2H2 þ C2H þ 2e� vi. C10H8 <sup>þ</sup> <sup>e</sup>� ! C6H<sup>þ</sup> <sup>4</sup> þ C4H4ð Þþ 2 � C2H2 2e� vii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C5H<sup>þ</sup> <sup>3</sup> þ C2H2 þ C3H3 þ 2e� viii. C10H8 <sup>þ</sup> <sup>e</sup>� ! C4H<sup>þ</sup> <sup>4</sup> þ C4H2 þ C2H2 þ 2e� ix. C10H8 <sup>þ</sup> <sup>e</sup>� ! C4H<sup>þ</sup> <sup>3</sup> þ 2 � C2H2 þ C2H þ 2e� x. C10H8 <sup>þ</sup> <sup>e</sup>� ! C4H<sup>þ</sup> <sup>2</sup> þ 3 � C2H2 þ 2e�

i. C10H8 <sup>þ</sup> <sup>e</sup>� ! C10H<sup>þ</sup>

xi. C10H8 <sup>þ</sup> <sup>e</sup>� ! C3H<sup>þ</sup> <sup>3</sup> þ C3H3 þ C4H2 þ 2e�

$$\text{xii.} \qquad \text{C}\_{10}\text{H}\_{8} + \text{e}^{-} \rightarrow \text{C}\_{10}\text{H}\_{8}^{++} + \text{3e}^{-}$$

$$\text{\textbf{x}\ddot{i}i.}\qquad\qquad\qquad\text{\textbf{C}\_{10}\textbf{H}\_{8} + \textbf{e}^{-} \rightarrow \textbf{C}\_{10}\textbf{H}\_{6}^{++} + 2\textbf{H}/\textbf{H}\_{2} + 3\textbf{e}^{-}$$

$$\text{xiv.}\qquad\qquad\text{C}\_{10}\text{H}\_{8} + \text{e}^{-} \to \text{C}\_{8}\text{H}\_{6}^{++} + \text{C}\_{2}\text{H}\_{2} + \text{3e}^{-}$$

$$\text{xv.}\qquad\text{C}\_{10}\text{H}\_{8} + \text{e}^{-} \rightarrow \text{C}\_{8}\text{H}\_{4}^{++} + \text{C}\_{2}\text{H}\_{2} + 2\text{H}/\text{H}\_{2} + 3\text{e}^{-}$$

The loss of one or more H atom from the intact mono-cation is not shown explicitly in the list but is one of the most important channels. As observed by Gotkis et al., the primary decay channel for naphthalene is either H loss or acetylene

demonstrated by Jochims et al. [8] and thus unless stated explicitly otherwise, we will only refer to H2 loss possibility wherever the loss of 2 mass units is concerned. The effect is even stronger in the case of di-cation that a large peak is observed at 51 mass units coming from di-cation of naphthalene with the loss of one acetylenes and the acetylene loss competes with H loss in the case of single ionization. Similarly, this peak is again accompanied by H2 loss along with acetylene loss. As shown earlier in ion-PAH collisions studies [4, 6] we see the dominant H2 channel. It is

The mass spectrum of naphthalene(C10H8) at 1000 eV electron impact (Inset: naphthalene fragments).

Projectile electron beam energy dependence of different decay channel with zero delay time extraction. Decay

of various decay channels in the mass region near 39 units.

Mass Spectrometry - Future Perceptions and Applications

<sup>4</sup> that dominate the spectra and the rest largely come as by-products

C3H<sup>þ</sup>

Figure 6.

Figure 7.

32

channels are labeled as listed above.

<sup>3</sup> and C3H<sup>þ</sup>

loss [12]. It should be noted that the loss of multiple H and acetylene fragments are the prominent statistical decay modes for PAHs and therefore they can be very useful in understanding the dynamics of the internal degrees of freedom in PAHs. Acetylene loss is clearly the most useful channel to study from the data we present here. Higher order ionizations like double and triple ionization are low impact parameter processes and thus a detailed direct coulomb interaction model is more relevant in such cases. In this study our main goal is to explore the plasmon excitation which is a large impact parameter process and it is known to produce singly charged cations for the case of PAHs [26]. We expect strong dynamical and statistical effects in the singly charged naphthalene ions and the associated evaporation products like single and double acetylene loss in our case. Various decay channels are referred in the manuscript according to the numbers given in the list above.

Jochims et al. are discussed the reaction sequence for acetylene loss and the formation of phenyl-acetylene C8H<sup>þ</sup> <sup>6</sup> from C10H8 in detail [26]. An intermediate is formed by the breaking of the transannular bond accompanied by the migration of H in the naphthalene cation and this is followed by the successive cleavage of C-C bond with a loss of C2H2 molecule and the formation of polyacetylene cation [26].

The first six decay channels are considered here with mono-cations and the rest may come from very energetic mono-cation or di-cation. Thus the processes governing the production of the latter decay channels can be treated as low impact parameter and high internal energy channels. And subsequently these decay channels are expected to have substantially large decay rates compared to the decay rates for mono-cations leading to decay channels ii and iii. This unique behavior is evident as shown in Figure 8a–c in which plot the relative intensity of all the decay channels at 250, 500 and 1000 eV electron impact energies. The very first clear observation is that the decay channels ii evolve very differently compared to the rest of the channels as a function of extraction delay and this can be seen in Figure 9a, b. The diacetylene loss channel show an increase in the yield fraction followed by a steady population or a slight decrease up to 5 μs extraction delay on the other hand, the last five decay channels show a clear decrease from the zero delay onwards. This decay behaviors indicates that the last five reactions have decay rates that are much larger than 10<sup>6</sup> and assuming these arise from the statistical dissociation.

population fraction. These channels from the zero delay time, indicating sufficiently fast decay accompanied by sufficient kinetic energy release to cause fast

(a) Decay channel ii (C2H2 Loss) and (b) decay channel iii (C4H2 or 2 C2H2 loss) at various electron

Various fragmentation channels of naphthalene at (a) 250 eV, (b) 500 eV, (c) 1000 eV electron impact

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

For PAHs in general and for naphthalene in particular a strong influence of the

The most dominant low energy channels commensurating with the excitation energy, in range of 7–8 eV leading to a total energy loss of 15–16 eV range is the decay channels leading to loss of acetylene and the loss of diacetylene. Acetylene loss shows much weaker energy dependence compared to the other channels which again is a well-known property of plasmon excitation. In the case of acetylene evaporation processes the time scales of few microseconds are seen and yields of

plasmon resonance excitation is known to be present. This effect has recently gained importance due to its possible role in the formation of molecular hydrogen and acetylene molecules in ISM and has been under investigation using far-UV photo-excitation as well as heavy ion-induced excitation. In this work, we have shown the effect of plasmon excitation under by high energy electron impact excitation. The time evolution of the acetylene evaporation, which is known to be a by-product of the plasmon excitation process, is measured for this study. A pulsed electron source along with the pulsed extraction of recoil ions using fast high voltage pulses is implemented and varying the extraction delay we observe the

parent and daughter ion yields of naphthalene molecule is observed.

dispersion of daughter ions from the interaction volume.

6. Conclusion

35

Figure 9.

Figure 8.

energies. Decay channels are labeled as listed above.

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

impact energies.

The interaction energy for the formation of larger sized fragments according to reactions iv - viii needs to be well above 22 eV as discussed by Ruhl et al. [18]. The plasmon excitation is expected to peak at about 17 eV [26]. In the data for each beam energy, we observe that compared to the acetylene loss the decay channel iii peak shows a faster rate of change and this can be understood from the fact that the decay rate of channel iii is marginally higher than that of the acetylene loss channel. Interestingly we see a gradual decrease in the relative intensity of all the daughter channels as a function of projectile energy. We can assign two major possibly for the decay channel iii: [1] neutral acetylene evaporation from mono-cation and [2] the dissociation of C4H<sup>þ</sup> <sup>2</sup> fragment. Moreover, this could also be the C4Hn fragment but it is not resolved well in our spectra. The population of evaporative products grows substantially in earlier times and after about 3 μs, it starts to drop again and behavior is seen even at 500 and 1000 eV beam energies. This suggesting that the first increase comes due to plasmon excitation and reduces its contribution as we go to higher beam energies and this observation is in complete agreement with the behavior seen in decay channel ii. On the other hand, the rest of the channels, show gradual and continuous decrease in yield with extraction delay time. In the case of decay channel ii, the scales are longer than that for channel ii is seen. The decay channel iv to viii represents more energetic processes and hence show a decrease in Large Molecule Fragmentation Dynamics Using Delayed Extraction Time-of-Flight Mass… DOI: http://dx.doi.org/10.5772/intechopen.84407

#### Figure 8.

loss [12]. It should be noted that the loss of multiple H and acetylene fragments are the prominent statistical decay modes for PAHs and therefore they can be very useful in understanding the dynamics of the internal degrees of freedom in PAHs. Acetylene loss is clearly the most useful channel to study from the data we present here. Higher order ionizations like double and triple ionization are low impact parameter processes and thus a detailed direct coulomb interaction model is more relevant in such cases. In this study our main goal is to explore the plasmon excitation which is a large impact parameter process and it is known to produce singly charged cations for the case of PAHs [26]. We expect strong dynamical and statistical effects in the singly charged naphthalene ions and the associated evaporation products like single and double acetylene loss in our case. Various decay channels are referred in the manuscript according to the numbers given in the list above. Jochims et al. are discussed the reaction sequence for acetylene loss and the

formed by the breaking of the transannular bond accompanied by the migration of H in the naphthalene cation and this is followed by the successive cleavage of C-C bond with a loss of C2H2 molecule and the formation of polyacetylene cation [26]. The first six decay channels are considered here with mono-cations and the rest

The interaction energy for the formation of larger sized fragments according to reactions iv - viii needs to be well above 22 eV as discussed by Ruhl et al. [18]. The plasmon excitation is expected to peak at about 17 eV [26]. In the data for each beam energy, we observe that compared to the acetylene loss the decay channel iii peak shows a faster rate of change and this can be understood from the fact that the decay rate of channel iii is marginally higher than that of the acetylene loss channel. Interestingly we see a gradual decrease in the relative intensity of all the daughter channels as a function of projectile energy. We can assign two major possibly for the decay channel iii: [1] neutral acetylene evaporation from mono-cation and [2] the

it is not resolved well in our spectra. The population of evaporative products grows

substantially in earlier times and after about 3 μs, it starts to drop again and behavior is seen even at 500 and 1000 eV beam energies. This suggesting that the first increase comes due to plasmon excitation and reduces its contribution as we go to higher beam energies and this observation is in complete agreement with the behavior seen in decay channel ii. On the other hand, the rest of the channels, show gradual and continuous decrease in yield with extraction delay time. In the case of decay channel ii, the scales are longer than that for channel ii is seen. The decay channel iv to viii represents more energetic processes and hence show a decrease in

<sup>2</sup> fragment. Moreover, this could also be the C4Hn fragment but

may come from very energetic mono-cation or di-cation. Thus the processes governing the production of the latter decay channels can be treated as low impact parameter and high internal energy channels. And subsequently these decay channels are expected to have substantially large decay rates compared to the decay rates for mono-cations leading to decay channels ii and iii. This unique behavior is evident as shown in Figure 8a–c in which plot the relative intensity of all the decay channels at 250, 500 and 1000 eV electron impact energies. The very first clear observation is that the decay channels ii evolve very differently compared to the rest of the channels as a function of extraction delay and this can be seen in Figure 9a, b. The diacetylene loss channel show an increase in the yield fraction followed by a steady population or a slight decrease up to 5 μs extraction delay on the other hand, the last five decay channels show a clear decrease from the zero delay onwards. This decay behaviors indicates that the last five reactions have decay rates that are much larger than 10<sup>6</sup> and assuming these arise from the statistical

<sup>6</sup> from C10H8 in detail [26]. An intermediate is

formation of phenyl-acetylene C8H<sup>þ</sup>

Mass Spectrometry - Future Perceptions and Applications

dissociation.

dissociation of C4H<sup>þ</sup>

34

Various fragmentation channels of naphthalene at (a) 250 eV, (b) 500 eV, (c) 1000 eV electron impact energies. Decay channels are labeled as listed above.

Figure 9. (a) Decay channel ii (C2H2 Loss) and (b) decay channel iii (C4H2 or 2 C2H2 loss) at various electron impact energies.

population fraction. These channels from the zero delay time, indicating sufficiently fast decay accompanied by sufficient kinetic energy release to cause fast dispersion of daughter ions from the interaction volume.

### 6. Conclusion

For PAHs in general and for naphthalene in particular a strong influence of the plasmon resonance excitation is known to be present. This effect has recently gained importance due to its possible role in the formation of molecular hydrogen and acetylene molecules in ISM and has been under investigation using far-UV photo-excitation as well as heavy ion-induced excitation. In this work, we have shown the effect of plasmon excitation under by high energy electron impact excitation. The time evolution of the acetylene evaporation, which is known to be a by-product of the plasmon excitation process, is measured for this study. A pulsed electron source along with the pulsed extraction of recoil ions using fast high voltage pulses is implemented and varying the extraction delay we observe the parent and daughter ion yields of naphthalene molecule is observed.

The most dominant low energy channels commensurating with the excitation energy, in range of 7–8 eV leading to a total energy loss of 15–16 eV range is the decay channels leading to loss of acetylene and the loss of diacetylene. Acetylene loss shows much weaker energy dependence compared to the other channels which again is a well-known property of plasmon excitation. In the case of acetylene evaporation processes the time scales of few microseconds are seen and yields of
