Electron ionization of open/closed chain isocarbonic molecules relevant in plasma
processing: Theoretical cross sections
Umang R. Patel, K. N. Joshipura, Harshit N. Kothari, and Siddharth H. Pandya
Citation: The Journal of Chemical Physics 140, 044302 (2014); doi: 10.1063/1.4862056
View online: http://dx.doi.org/10.1063/1.4862056
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THE JOURNAL OF CHEMICAL PHYSICS 140, 044302 (2014)
Electron ionization of open/closed chain isocarbonic molecules relevant in
plasma processing: Theoretical cross sections
Umang R. Patel,1,2,a) K. N. Joshipura,2 Harshit N. Kothari,3 and Siddharth H. Pandya2
1
Gandhinagar Institute of Technology, Moti Bhoyan, Gandhinagar-382721, Gujarat, India
Sardar Patel University, Vallabh Vidyanagar-388120, Gujarat, India
3
Universal College of Engineering and Technology, Moti Bhoyan, Gandhinagar-382721, Gujarat, India
2
(Received 20 October 2013; accepted 31 December 2013; published online 23 January 2014)
In this paper, we report theoretical electron impact ionization cross sections from threshold to
2000 eV for isocarbonic open chain molecules C4 H6 , C4 H8 , C4 F6 including their isomers, and
closed chain molecules c-C4 H8 and c-C4 F8 . Theoretical formalism employed presently, viz., Complex Scattering Potential-ionization contribution method has been used successfully for a variety of
polyatomic molecules. The present ionization calculations are very important since results available
for the studied targets are either scarce or none. Our work affords comparison of C4 containing
hydrocarbon versus fluorocarbon molecules. Comparisons of the present ionization cross sections
are made wherever possible, and new ionization data are also presented. © 2014 AIP Publishing
LLC. [http://dx.doi.org/10.1063/1.4862056]
I. INTRODUCTION
Collisions of electrons with molecular targets, leading to
various processes, are found to occur in many natural and laboratory environments. Such collision processes lead to both
chemical and physical changes of matter in environments associated with radiation chemistry, plasma enhanced chemical
vapour deposition, lighting industry, and plasma processing
of materials for microelectronics. In most cases, collisional
interactions of low to intermediate energy electrons are the
precursors for the production of critical species in low temperature plasmas and plasma processing gas discharges.1, 2 About
one third of the processes needed to make a modern microchip
involved plasma based processes. Both hydrocarbons and
perfluorocarbons play an important role in plasma assisted
fabrication processes. Therefore, both classes of molecules
have received research attention from theorists as well as
experimentalists over the past few years. It appears that C4 H6 ,
C4 H8 , C4 F6 , c-C4 H8 , and c-C4 F8 are some of the molecules
for which electron scattering data, especially, ionization cross
sections are not available. A lot of work has been done on
the elastic and total cross sections but very less work has
been reported on electron ionization of the title molecules.3
Kwitnewski et al.4 have proposed a regression formula which
relates the grand total cross section and the total ionization
cross section for electron scattering and have also calculated
Binary encounter Bethe (BEB) cross sections for different
isomers of C4 H6 using the formula given in Ref. 2. Bart et al.5
have measured electron ionization cross section from threshold to 220 eV for a range of halogenated methanes and small
perfluorocarbons including C4 F6 . They5 have compared their
maximum ionization cross section with the calculated values
from Deutsch–Maerk (DM) model6 and BEB method.2 In the
BEB method, the total ionization cross section per molecular
a) Email: [email protected]
0021-9606/2014/140(4)/044302/8/$30.00
orbital (σ BEB ) is expressed in terms of basic molecular
properties.
Now, among the present targets, perfluorocyclobutane
(c-C4 F8 ) is a processing gas in plasma etching.7–9 Electron
impact on c-C4 F8 generates large quantities of CF radicals
and Cx Fy species which are very much useful in developing plasma devices and are more significant since the experimental study of electron interactions with Cx Fy is difficult.10
There have been two measurements of partial ionization cross
sections for c-C4 F8 obtained by Jiao et al.11 and Toyoda
et al.,12 and they11, 12 have measured cross sections for dissociative ionization of c-C4 F8 by electron impact leading to the
formation of Cx Fy species. An observation in general is that
parent ionization is dominant over the other outgoing species,
but in the case of c-C4 F8 none of the measurements11–13
have detected c-C4 F8 + parent ion, which indicates that the
ground state of c-C4 F8 + ion is not bound in Franck-Condon
region. In the last few years, experimental work has been done
by Szmytkowski and Kwitnewski14–16 on electron collisions
with isomers of small hydrocarbons and fluorocarbons, and
they14–16 have focused on the isomer effect. The main goal in
Refs. 14–16 was to find the difference in the total cross sections due to the differences in the structural geometry. Bettega
et al.17 and Lopes et al.18 noticed that cross sections for the
closed chain molecules are smaller than their open chain isomers. Ionization cross sections, especially, experimental data
of all the title targets except c-C4 F8 are scarce or nonexistent.
The scenario depicted above points to the need of the
study taken up in this paper. We report here the electron impact ionization cross sections for c-C4 F8 and for the isomers
of C4 H6 , C4 F6 , and C4 H8 . Very little work, if any, has been
reported on these species to the best of our knowledge. The
present study is also guided by small but important differences in the first ionization energies of the present targets,
as reported in different sources.5, 10, 19–21 We have employed
here the Complex Scattering Potential ionization contribution
140, 044302-1
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TABLE I. Properties of the targets studied presently.
Target
Chemical formula
Geometry
1,3 Butadiene
C4 H6
CH2 = CH − CH = CH2
2 Butyne
C4 H6
CH3 − C ≡ C − CH3
Perfluorobutadiene 1,3
C4 F6
CF2 = CF − CF = CF2
Perfulorobutyne 2
C4 F6
CF3 − C ≡ C − CF3
1 Butene
C4 H8
CH2 = CH − CH2 − CH3
2 Butene
C4 H8
CH3 − CH = CH − CH3
Cyclobutane
Perfluorocyclobutane
c-C4 H8
c-C4 F8
Ionization energy (eV) Bond length (Å)
H2C
CH2
H2C
CH2
F2C
CF2
F2C
CF2
9.082a
9.14b
9.580c
9.5d
12.3f
9.5d
12.3f
9.55c
9.86g
9.1c
9.38g
9.8c
12.1h
C–H 1.108a
C–H 1.116c
C≡C 1.214c
C–F 1.323e
C–F 1.333e
C≡C 1.198d
C–H 1.09c
C–H 1.09c
C–H 1.09c
C–F 1.33h
a
CRC references.19
Reference 2.
c
CCCBDB.20
d
NIST Chem web book.21
e
Reference 32.
f
Reference 5.
g
Reference 4.
h
Reference 10.
b
(CSP-ic) method developed and applied successfully by us,
over a wide range of molecular targets in the recent years.22–28
The present calculations have been performed in the group
additivity approach, as has been necessary for the large polyatomic molecules, each consisting of several functional chemical groups. Relevant properties of the target molecules, viz.,
geometry, first ionization threshold, and bond lengths are
given in Table I. Section II describes the theoretical methodology adopted here, followed by the results and discussions
(Sec. III) along with conclusions (Sec. IV).
density. The molecular spherical charge density is constructed
through a single-centre expansion of the atomic charge density at the molecular mass-centre.22 The static potential is
determined directly from the target charge density and the
exchange potential is calculated using the models of Ref. 29.
The total complex potential V(r, Ei ) introduced in the
Schrödinger equation, along with its solution obtained numerically, leads to the total (complete) cross section QT defined as
follows,
II. THEORETICAL METHODOLOGY
where, Qel is the total elastic cross section while its inelastic
counterpart is denoted by Qinel . The total (cumulative) inelastic cross section Qinel includes all energetically allowed electronic excitation as well as ionization channels of scattering,
so that
Qinel (Ei ) =
Qexc (Ei )+
Qion (Ei ).
(2)
At the incident energies (Ei ) from ionization threshold
to 2000 eV, it becomes meaningful to represent the electronmolecule system by a complex (spherical) potential, which
seeks to club together all admissible inelastic (including ionization) channels in the background of elastic scattering. For
an electron interacting with a molecule (or a functional chemical group), the total complex potential V(r, Ei ) = VR (r, Ei )
+ iVI (r, Ei ), consists of real potential VR and imaginary
potential VI . The real part comprises of static, exchange,
and polarization potentials while the imaginary part is the
inelastic “absorption” potential Vabs . Further, r is the radial
distance from the mass-centre of the target. The basic input
in constructing all these model potentials is the target charge
QT (Ei ) = Qel (Ei ) + Qinel (Ei ),
(1)
On the right-hand side of Eq. (2), the first term Qexc , to be
called electronic sum, includes all accessible electronic
excitation channels in the target, and the second term Qion
stands for the total cross sections of all allowed (parent, dissociative, single, double, etc.) ionization channels. Our focus is
on finding total ionization cross section, and the second term
will henceforth be denoted simply by Qion .
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J. Chem. Phys. 140, 044302 (2014)
The absorption potential mentioned above is an energy
dependant potential that accounts for all possible inelastic
scattering channels cumulatively, and has the generic form,
first developed by Staszewska et al.,30 in atomic units
Vabs (r, Ei ) = − 12 ρ(r)Vloc σee
1 = −ρ(r) T2loc 2 10k8π3 E
F i
2
× θ p − kF2 − 2 (A1 + A2 + A3 ). (3)
Here, Vloc is the local speed of the external electron, and σ ee
denotes the average cross section for binary collision of the
external electron with one of the target electrons. In Eq. (3),
p2 = 2Ei , kF is the Fermi wave-vector magnitude and is an
energy parameter. Blanco and Garcia31 suggested a variable
form of in order to account for screening effects of the target charge cloud on Vabs . The modification introduced by us
is to consider as a function of Ei , as discussed in Ref. 28.
Accordingly,
(Ei ) = 0.8I + β(Ei − I ).
(4)
Let us denote by “Ep ” the value of Ei at which our Qinel attains
its maximum. In Eq. (4) β is then obtained by requiring that
= I + 1 (eV) at Ei = Ep , beyond which is held constant.
The expression for (Ei ), in Eq. (4), is meaningful since fixed at I would not allow even excitation at incident energy
Ei ≤ I. On the other hand, if the parameter is much less
than the ionization threshold, then Vabs becomes unduly high
near the peak position. In short the present form of (Ei ), in
Eq. (4), balances all these aspects and allows us to obtain
the satisfactory values of Qion for a given target.28 With the
Vabs thus modified, we solve the Schrödinger equation numerically, to extract the complex partial wave phase-shifts δ l , for
different angular momenta l at desired energies.
The inelastic cross section Qinel is not a directly measurable quantity in a single experiment, but in view of Eq. (2),
we have in general,
Qinel (Ei ) ≥ Qion (Ei ).
(5)
At incident energies above I, the ionization processes begin to
play a dominant role due to the availability of infinitely many
open channels of scattering. There is no rigorous way of projecting out Qion from the theoretical quantity Qinel . Hence,
we have introduced an approximation by defining a ratio
function,
R(Ei )=
Qion (Ei )
.
Qinel (Ei )
(6)
Obviously, R = 0 when Ei ≤ I. For a number of stable
atomic-molecular targets like Ne, Ar, O2 , N2 , CH4 , H2 O, etc.,
for which several experimental ionization cross section datasets are known accurately,22–28 the ratio is seen to be rising steadily as the energy increases above the threshold, and
approaching unity at high energies. Thus,
R(Ei ) = 0, for Ei ≤ I,
(7a)
R(Ei ) = Rp , for Ei = Ep ,
(7b)
R(Ei ) ≤ 1, for Ei Ep .
(7c)
Here, Rp = 0.7 stands for the value of R at Ei = Ep . The choice
of this value22–28 is approximate but crucial. The peak position
Ep of the cross section Qinel occurs at incident energy where
the discrete excitation-sum is decreasing while Qion is rising
fast, suggesting the Rp value to be between 0.5 and 1. We follow the general observation22–28 that at energies close to peak
of Qinel the ionization contribution Qion is about 70%–80% in
the total inelastic cross section Qinel and it increases with energy. An indirect support in this regard is also obtained from
the correlation between the maximum ionization cross section
denoted by σ max , and the molecular properties, viz., polarizability and ionization threshold. It has been demonstrated in
our publications22–28 that total ionization cross sections can
be reasonably determined from this approximation and the resulting Qion are within the experimental uncertainties of about
10%–15%. Now, for the actual calculation of Qion from Qinel
we need R as a continuous function of energy Ei . Hence, we
represent26 the ratio R (Ei ) in the following manner,
ln Ui
C2
+
, (8)
R(Ei ) = 1 − f (Ui ) = 1 − C1
(Ui + a)
Ui
where Ui = Ei /I is a dimensionless and target-specific variable
corresponding to energy Ei . The reason for adopting a particular functional form of f (Ui ), i.e., second term of the righthand side of Eq. (8) is as follows. As Ei increases above I, the
ratio R increases from zero and approaches value 1, since the
ionization contribution rises and the discrete excitation-sum
in Eq. (2) decreases. The discrete excitation cross sections,
dominated by dipole transitions, fall off as ln(Ui )/Ui at high
energies. Accordingly, the decrease of the function f(Ui ) must
also be proportional to ln(Ui )/Ui in the high range of energy.
However, the two-term representation of f(Ui ) given in Eq. (8)
is more appropriate since the 1st term in the square bracket
ensures a better energy dependence at low and intermediate
Ei . Equation (8) involves dimensionless parameters C1 , C2 ,
and a, characteristic of the target in question. The three conditions stated in Eqs. (7a)–(7c) are used to determine these three
parameters, in an iterative manner.22 Thus, we first assume
a = 0 and consider a two-parameter expression in Eq. (8).
We employ therein the two conditions (7a) and (7b) to obtain C1 and C2 . The two-parameter equation is then used to
determine the value of R at a high energy Ei = 10 Ep , and
the same value is employed in Eq. (7c) to obtain the new set
of three parameters C1 , C2 , and a. Having thus obtained the
parameters we calculate Qion from Eq. (6), and therefore generate Rp value from these Qion . The resulting Rp value is used
next as an input to Eq. (7b) iteratively to finally calculate Qion .
Equations (5)–(8) describe our CSP-ic method.
The above calculation is basically carried out for a
functional group in the target molecule in question. As the
studied molecules are complex, a single centre approach is
not feasible for the entire molecule. Hence, the calculation is
made on a molecular functional group as a scattering centre,
and the group additivity method is employed, where the
geometrical structure of the molecule is taken into account.
The single centre charge density is obtained for a group in the
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FIG. 1. Total ionization cross section for e− – 1,3 butadiene. Black solid
line: the present Qion ; red solid line: Qion by Kim and Irikura;2 and blue solid
line with square: Qion by Kwitnewski et al.4
FIG. 2. Total ionization cross section for e− – 2 butyne. Blue solid line: the
present Qion ; and red solid line with square: Qion by Kwitnewski et al.4
molecule by combining appropriate atomic charge densities,
and is renormalized to account for the molecular bonding.
This also involves the bond lengths in the functional group.
Specifically, the group additivity is adopted in following
manner. Consider as an example, the molecule C4 H6 (1,3
butadiene) having four groups, viz., CH2 , CH, CH, and CH2
(Table I). Electron impact ionization cross sections for each
of these groups are obtained using the ionization threshold
of the C4 H6 itself, and the respective Qion is added to obtain
finally the total ionization cross section of C4 H6 .
investigation. The present result has a good general agreement with the regression formula, an approximation made by
Kwitnewski et al.,4 in the entire range of energy within the error bars, of course with a shift in the peak. Neither BEB cross
sections nor any other theoretical or experimental results are
available for this isomer of C4 H6 . Therefore, we thought it
worthwhile to report our theoretical values on this target.
III. RESULTS AND DISCUSSIONS
In this paper, the CSP-ic method followed by group additivity approach has been adopted to determine total ionization
cross sections for isocarbonic plasma molecules C4 H6 , C4 H8 ,
C4 F6 including their isomers, together with c-C4 H8 and cC4 F8 . We discuss below the results obtained for these targets
separately.
C. Perfluorobutadiene 1,3 CF2 = CF − CF
= CF2 (C4 F6 )
Next, we consider the electron impact ionization cross
section for C4 F6 (perfluorobutadiene 1,3), as shown by graphical plots in Fig. 3. This case makes an interesting study since
there is a significant difference between its first ionization
energy as quoted in Refs. 21 and 5. For this molecule, we
A. 1,3 Butadiene CH2 = CH − CH = CH2 (C4 H6 )
Electron impact ionization cross section for the open
chain C4 H6 (1,3 butadiene) molecule is plotted along with
compared data in Fig. 1. The present Qion matches well with
the BEB cross sections calculated by Kim and Irikura.2 A
small shift in the magnitude at lower energy, along with a
small difference in the peak position is found due to the
difference in the ionization threshold. The ionization potential used in present calculation is 9.082 eV19 while in the
BEB calculation2 it is 9.14 eV. A good general accord within
the error bars is found with the data calculated through a
regression formula by Kwitnewski et al.4
B. 2 Butyne CH3 − C ≡ C − CH3 (C4 H6 )
Figure 2 shows the electron impact ionization cross section for C4 H6 (2 butyne) for which there is only one previous
FIG. 3. Total ionization cross section for e− – perfluorobutadiene 1,3. Blue
solid line: the present Qion at I = 12.3 eV; black solid line: the present Qion
at I = 9.5 eV; and red triangles: Qion by Bart et al.5
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TABLE II. Comparison of maximum cross sections of C4 F6 .
Present peak values (Å2 )
Target C4 F6
(two isomers)
σ max (Å2 )
Bart et al.5
σ max (Å2 ) BEB
value in Ref. 5
σ max (Å2 ) DM
value in Ref. 5
I = 9.5 eV
I = 12.3 eV
CF2 CFCFCF2
CF3 CCCF3
10.46
10.27
10.70
10.43
13.02
13.51
11.32
12.31
9.57
10.41
have calculated Qion with I = 9.5 eV—the threshold given in
Ref. 22, and also with I = 12.3 eV as quoted by Bart et al.5
for their ionization measurements. The differences in the
results that we find in Fig. 3 are understood in terms of the
two threshold values, and one finds that our Qion with threshold value I = 12.3 eV has a more satisfactory agreement with
the experimental data of Ref. 5. Comparison of the present
maximum ionization cross section (σ max ) is shown in Table II.
D. Perfluorobutyne 2 CF3 − C ≡ C − CF3 (C4 F6 )
Figure 4 shows the electron ionization cross sections for
the C4 F6 isomeric target perfluorobutyne 2. The entire calculation has been done in the same way as in the previous case,
i.e., perfluorobutadiene 1,3. The ionization threshold of the
present isomer is not available in literature, and hence Qion
calculations are done at the values I = 9.5 eV and 12.3 eV, as
in the previous isomeric case. The threshold value quoted by
Bart et al.5 in their measurements is 12.3 eV. For the present
isomer, our Qion results with I = 12.3 eV agree well with measurement of Bart et al.,5 as can be seen from Fig. 4. With
I = 9.5 eV, the present Qion are higher, as expected. Comparison of maximum ionization cross section σ max is made again
in Table II, where our peak value is in accordance with both
experimental and BEB value given in Ref. 5.
FIG. 4. Total ionization cross section for e− – perfluorobutyne 2. Blue solid
line: the present Qion at I = 12.3 eV; black solid line: the present Qion with I
= 9.5 eV; and red squares: Qion by Bart et al.5
E. 1 Butene CH2 = CH − CH2 − CH3 (C4 H8 )
Consider now the hydrocarbon C4 H8 , i.e., 1 butene for
which the electron impact ionization cross sections are exhibited along with comparison in Fig. 5. In this case, we have
only one comparison that comes from the calculated BEB
cross sections of Kim and Irikura.2 The BEB results evaluated at ionization threshold 9.86 eV2 differ from our results
in the peak region, since in the present work we have adopted
the threshold value 9.55 eV (see Table I). The two theoretical results show a small difference in the peak position, but
tend to merge at higher energies (Fig. 5). Notably, there are no
measurements for Qion in this case. Differences in the two calculated results are attributed to the approximations involved in
the theories.
F. 2 Butene CH3 − CH = CH − CH3 (C4 H8 )
For the other C4 H8 isomer, viz., 2 butene, the electron
ionization results are plotted in Fig. 6. A trend similar to
the previous case of 1 butene is observed here also, and
the difference can be attributed to the two slightly different
threshold values. Also we have calculated Qion for the isomer
cyclobutane (c-C4 H8 ) but no comparison is found, so we have
not shown it in Fig. 6, but it has been considered below in the
discussion on isomer effects. No experimental measurements
of Qion of C4 H8 molecules are available for comparison.
FIG. 5. Total ionization cross section for e− – 1 butene. Red solid line: the
present Qion ; and blue solid line: Qion by Kim and Irikura.2
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Patel et al.
FIG. 6. Total ionization cross section for e− – 2 butene. Black solid line: the
present Qion ; and purple solid line: Qion by Kim and Irikura.2
G. Perfluorocyclobutane (c-C4 F8 )
Figure 7 shows our electron ionization cross sections for
c-C4 F8 or perfluorocyclobutane, together with comparisons.
Electron impact on c-C4 F8 generates a large number of Cx Fy
species and these are quite important in plasma processing.
Our present result matches very well with the Qion measurements of Toyoda et al.,12 at all incident energies from threshold to peak position, beyond which the data of Ref. 12 are not
available. The other set of data, measured by Jiao et al.,11 are
clearly on higher side. Beran and Kevan33 have measured Qion
for a large number of molecules, including c-C4 F8 , for 70 eV
electron impact. These one point data are also nearer to the
present result (Fig. 7). The recommended data on perfluorocyclobutane given by Christophorou and Olthoff10 were based
on the two measurements, viz., Refs. 11 and 12, so that their
FIG. 7. Total ionization cross section for e− – c-C4 F8 . Black solid line: the
present Qion ; red triangle: Qion by Toyoda et al.;12 green triangle: Qion by
Jiao et al.;11 brown star: Qion at 70 eV by Beran and Kevan;33 and blue solid
line: Qion recommended by Christophorou and Olthoff.10
J. Chem. Phys. 140, 044302 (2014)
FIG. 8. Total ionization cross section for isomers of C4 H6 . Black solid line:
the present 1,3-butadiene; and red solid line: the present 2 butyne.
values10 appear to lie between Refs. 11 and 12, especially in
the region of maximum. Now, one can see that our theory is
closer to Toyoda et al.,12 rather than Ref. 11.
In all such cases a new set of recommended data can now
be prepared by including the present results.
H. Isomer effect
So far in Figures 1–7 we have shown the electron
impact ionization cross sections for various isocarbonic hydrocarbons and fluorocarbons separately. In this sub-section,
let us examine how the cross section varies with the isomers
of the same molecule, and also how the changes in molecular
geometry are reflected in the scattering processes.
In the next three graphical plots, we have shown the isomer effect for the isomers of C4 H6 , C4 H8 , and C4 F6 . The electron impact ionization cross sections for two isomers of C4 H6 ,
i.e., 1,3 butadiene and 2 butyne, are compared in Fig. 8. Both
the isomers are open chain hydrocarbons. However, 1,3 butadiene is a compound with conjugated carbon–carbon double
bonds and CH2 group on either end of a chain. The 2 butyne
has one triple bond flanked with two C–C bonds and methyl
groups attached on the edges of the molecule. From Fig. 8,
it is seen that various arrangements of the same constituent
atoms in the isomers of the C4 H6 molecules influence slightly
the shape and magnitude in the low energy region, while at
high energy, the cross sections almost merge with each other.
Figure 9 depicts the electron impact ionization cross sections for the two C4 F6 isomers. We have calculated Qion for
both the isomers at ionization threshold following,21 as well
as at the value used by Bart et al.5 for their measurement. All
the data for the C4 F6 isomers are not directly available from
a single source, so we have found it from the different published works and standard data bank source.21 Perfluorobutadiene 1,3 is the simplest conjugated double bonded perfluorocarbon species with the two trifluoromethyl group lying in different planes. Its isomeric counterpart perfluorobutyne 2 is a
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molecule is smaller than that for open chain molecules. This
is similar to the observations of Bettega et al.,17 who have
noticed that the isomer effect is more evident for closed chain
molecules rather than the open chain molecules.
IV. CONCLUSIONS
FIG. 9. Total ionization cross section for isomers of C4 F6 . Black solid line:
the present perfluorobutyne 2; red solid line: the present perfluorobutadiene
1,3 at I = 9.5 eV; brown solid line: the present perfluorobutyne 2; and green
solid line: the present perfluorobutadiene 1,3 at I = 12.3 eV.
linear molecule with C–C triple bond and with trifluoromethyl
group at each end of the molecule. Due to its very low global
warming potential, perfluorobutadiene 1,3 is considered as a
substitutive component of plasma fabricating nano-electronic
devices and therefore there is a need for comprehensive data
for electron assisted processes in this compound. As shown in
Fig. 9, the Qion for the perfluorobutadiene 1,3 is found to be
somewhat lower than that of perfluorobutyne 2, at low energies. At high energies beyond the peak the cross sections tend
to merge with each other. From Figs. 8 and 9, we can say that
isomer effect is significant mainly at low impact energies.
In Fig. 10, the present cross sections for the isomers of
the C4 H8 have been shown. Here, we have compared Qion for
closed chain (c-C4 H8 ) with open chain C4 H8 (1 butene and
2 butene). It is observed that Qion for closed chain (compact)
FIG. 10. Total ionization cross section for isomers of C4 H8 . Black solid
line: the present cyclobutane, red solid line: 1 butene, and green solid line:
2 butene.
Thus in this work, the CSP-ic method has been successfully applied for calculating Qion of a few polyatomic
isocarbonic hydrocarbon and fluorocarbon molecules. We
have compared our results with the available measurements
or theory reported by other workers, and the accord with
available data is good, considering the limitation of our
method. Accordingly, the new results exhibited in this paper
are also reliable. It appears that not enough work has been
done previously on ionization of the targets studied here, and
hence the present work assumes importance. Also we have
shown the effect of the isomers of the same number of atomic
constituents on the magnitude of the respective cross sections.
The isomer effect is found to be more evident at low incident
energies, as well as in closed chain molecules. The present
results on previously studied targets along with the new
results reported here will be useful in plasma applications.
Finally, the present work now motivates us to study
the isomer effect on the electron ionization of various C3
hydrocarbons and fluorocarbons, and this investigation is
underway.
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