icetcs-cug-578-page-proceedings-ksew

A typical time of flight spectrum (singles or one dimensional time of flight spectrum)
of CO2 ions produced in such collision is shown in Figure 3.

Figure-3 here

The time of flight of different ions are obtained from their mass to charge (m/q) ratios,

where m is the mass of ion and q is its charge state. The different peaks of the ions arise due
to direct and/or dissociative ionization of the CO2 molecule. The CO2+ and CO2++ ions are
mainly produced by direct ionization process, while the fragment ions CO+, O+, C+, C2+ and
O2+ are generated from the dissociative ionization process of a multiply charged CO2

molecule. A more detailed study of the total- and partial ionization cross sections of six

ionic fragments of the CO2 molecule under the impact of 10-26 keV electrons can be found

in our recent paper [8]. The two sharp peaks beside the CO2+ main peak are attributed to

13C16O2 and 12C18O16O ions (see, Fig. 3). The momentum resolutions ∆ v x, y and ∆ v z for
p p

the CO2+ ion are found to be respectively 6.3±0.1 a.u. and 8.7±0.1 a.u. and the resolution of

position ∆x is found to be 0.07 cm. This uncertainty is caused mainly due to the finite width

of the collision volume (~2mm). The ion-ion two dimensional coincidence map of two

particle events observed in collision of 26 keV electrons with CO2 at extraction field of
350V/cm is shown in Figure 4. O+f and O+b refer to forward and backward motion of
emitted O+ ions during their birth time respectively.

Figure-4 here

It is seen from this figure that the ion-ion coincidence mapping is a powerful tool to
study all possible dissociation channels involved in a given collision reaction. The ion-ion
coincidence map is a plot of the time of flight (t1) of the first fragment ion versus the time of
flight (t2) of the second fragment ion. The shape of islands in the coincidence map gives the
information about fragmentation dynamics of the complete and the incomplete
fragmentation processes, the slope of island indicates about the charge and the momenta of
involved ions and its width gives information about the momentum of the undetected
particles (mostly neutrals). In the Figure, different islands are marked with their
corresponding fragmentation channels for both complete and incomplete Coulomb
explosion reactions. From the knowledge of momenta of different fragment ions, various

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dynamical parameters, for example, the KERD, number of charge state distributions, bond
angles, possible number of breakup channels and the involved potential energy surfaces of
the fragmenting precursor ion can be deduced from such experiments.

The KER distributions for dissociation channels of CO22+ and CO23+ are presented in
Fig. 5(a) [10]; Fig. 5(b) and 5(c) represent KER distributions for dissociation channels of
N2O2+ and N2O3+ [14], respectively. The vertical arrows represent the predictions of KER
under CE model considering point charges on atoms at equilibrium distances of the neutral
molecule; this model predicts higher values of KER as compared to our experimental data.

(a) (b) (c)
O++CO+
C++O++O+

Figure 5: KER distributions for dissociation channels of CO2q+ and N2Oq+ (q=2,3)
The bond angle and mechanism of dissociation for N2O3+ ion into N++N++O+ channel

can be obtained by considering the parameters defined in Fig. 6 (a). The average bond angle
of dissociating N2O3+ is observed to be 140º (see, Fig. 6(b)); the distribution of χ around 90º
shows the concerted nature of fragmentation (Fig. 6(c)) where both the bonds (N-N and N-
O) break simultaneously and the terminal N and O ions fly back-to-back with large
momentum leaving the central atom with almost negligible momentum [14]. By applying
similar methodology for CO23+, the bond angle is observed to be 120º and its dissociation
also follows concerted mechanism [9].

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(a) (b) (c)

Figure 6: The distribution of N-N-O bond angle θ (left) and the distribution of angle χ
(right) as observed in N++N++O+ fragmentation channel of N2O3+ for 10 keV
electron impact with N2O.

To illustrate further the dissociation dynamics, the Newton diagrams are obtained by
taking the momentum vector of the ion arriving first at MCP as reference along x-axis and
the momentum distribution of its concomitant fragments (second ion and third ion/neutral)
are plotted with respect to it above and below the x-axis (see, Fig. 7a). In case of the two-
body dissociation process, for example, O++CO+, momentum vectors of most of the CO+
ions are distributed around 167º±10º with respect to the momentum vectors of O+ ions
plotted along the x-axis [9]. The Newton diagram for C++O++O+ channel is shown in Fig.
7(b) which shows that both the O+ ions are ejected at about 102º with respect to the
momentum vector of the C+ ion plotted along x-axis. This diagram indicates the alteration of
molecular geometry for the triply ionized state of a CO2 molecule [9]. Similarly, Fig. 7(c)
shows that most of the terminal N+ and O+ ions are emitted respectively at 90º and 106º
relative to the central N+ ion [14]

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(a) (b) (c)

Figure 7: Newton diagrams for fragmentation channels (a) O++CO+, (b) C++O++O+, and(c)
N++N++O+ originated from the dissociation of CO22+, CO23+ and N2O3+,
respectively in keV electron impact.

CONCLUSIONS

We have used a linear TOF mass spectrometer equipped with a multi-hit position
sensitive detector to measure the momentum vectors of up to four concomitant fragment
ions generated from keV electron collisions with molecules (CO2 and N2O). The KER
distributions for different dissociation channels observed in these collision events are
obtained. The Coulomb explosion model is found to overestimate our experimental KER
values due to its oversimplified point charge assumption. The bent geometrical states are
observed during fragmentations of triply ionized CO2 and N2O with an average bond angle
of 120º and 140º, respectively; the dissociation of both of these ions follow a concerted
pathway. The Newton diagrams plotted for the dissociation channels of CO23+ and N2O3+
also verify the concerted mechanism of dissociation for these ions.

Acknowledgments

Authors are grateful to receive financial supports from Department of Science and
Technology (DST), New Delhi, Board of Research in Fusion Science and technology
(BRFST), Institute for Plasma Research (IPR), Gandhinagar and Council for Scientific and
Industrial Research (CSIR), New Delhi during the progress of this work.

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REFERENCES

[1] D. Mathur, Phys. Rep. 391, 1 (2004).
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(2000).
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Spielberger, L., Recoil-ion momentum spectroscopy, J. Phys. B: At. Mol. Opt. Phys, 1997,
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[8] Bhatt, P., Singh, R., Yadav, N. and Shanker, R., Partial ionization cross sections of a CO2
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[9] Bhatt, P., Singh, R., Yadav, N. and Shanker, R.,Dissociative ionization cross sections for 12
keV electron impact on CO2, Phys. Rev. A, 2011, 84, 042701-6
[10] Bhatt, P., Singh, R., Yadav, N. and Shanker, R., Relative partial ionization cross sections of
N2O under 10-25 keV electron impact, Phys. Rev. A, 2012, 85, 034702-5
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[13]. Jagutzki, O., Mergel, V., Ullmann-Pfleger, K., Spielberger, L., Meyer, U., Dörner, R. and
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xy

Electron Gun Collision-zone Drift Tube
(2-50 keV) 4.2 cm
Extraction
zone R10
2.1 cm

–

e R1

Channeltron for z
Electron detection

Hypodermic needle M1 M2 M3
assembly for Gas inlet
Dual MCP + DLD40
effusive gas-jet for ion detection

Faraday Cup

CI + Counter

Figure 1. Schematic of the experimental setup for obtaining the ion-ion and electron-ion
coincidences: CI- Current Integrator, MCP- Micro-channel plate, DLD- Delay line
detector, M1, M2 and M3-Transmission meshes, R1 to R10- Rings. The interaction
zone is enclosed between M1 (electron extraction electrode) and R1 (ion-extraction
ring). A uniform electric field in extraction zone is maintained through a step down
resister network.

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Fig. 2: Photograph of the existing experimental set up

Figure 3. TOF mass spectrum of ions arising from 12 keV electron impact on CO2 molecule at
extraction voltage 185 V/cm.; two small peaks besides the main peak of CO2+ refer to the
isotopic CO2+ ion peaks . Spectra of C2+ and O2+ ion peaks are shown in the inset.

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