International Journal of Mass Spectrometry 376 (2015) 1–5
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry
journal homepage: www.elsevier.com/locate/ijms
Rate constants of electron attachment to alkyl iodides measured by
photoionization electron attachment ion mobility spectrometry
(PI-EA-IMS)
Hui Gao a,c , Wenqi Niu b , Chaoqun Huang a , Yan Hong a,c , Chengyin Shen a ,
Hongmei Wang c , Yan Lu a , Xiaojing Chen a , Lei Xia a,∗ , Haihe Jiang a , Yannan Chu a,∗
a
Laboratory Medical Optical and Mass Spectrometry, Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of
Sciences, No. 350, Scientific Road, Hefei 230031, China
b
School of Science, Anhui Agricultural University, Hefei 230036, China
c
Laboratory of Environmental Spectroscopy, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, No. 350, Scientific Road, Hefei
230031, China
a r t i c l e
i n f o
Article history:
Received 20 June 2014
Received in revised form 1 November 2014
Accepted 2 November 2014
Available online 8 November 2014
Keywords:
Photoionization
Ion mobility spectrometry
Electron attachment
Rate constant
Alkyl iodide
a b s t r a c t
The photoionization electron attachment ion mobility spectrometry (PI-EA-IMS), with photoelectrons
formed by photoionization of organic compound like acetone, has been developed to study electron
attachment reactions. With this apparatus, the rate constants for electron attachment to alkyl iodides
including ethyl iodide (C2 H5 I), 1-propyl iodide (1-C3 H7 I), 1-butane iodide (1-C4 H9 I) and 2-propyl iodide
(2-C3 H7 I) have been determined over the average electron energy from 0.29 to 0.96 eV. The rate constants
are in the order of magnitude of ∼10−9 cm3 molecule−1 s−1 . The experimental measurements show that
for straight-chain alkyl iodides, the values of the rate constants follow the order of: k(C2 H5 I) < k(1C3 H7 I) ≈ k(1-C4 H9 I) which can be explained by the energy threshold for the formation of iodine anion via
dissociative electron attachment. For the isomers, 2-C3 H7 I has a lower rate constant than 1-C3 H7 I which
may be caused by the effect of branched chain.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Electron-molecule reaction is the basic process in radiation
chemistry [1], plasma etching [2] and gas laser [3]. In general, electron attachment reaction can be studied by some conventional
techniques such as high-Rydberg collision ionization [4], rare-gas
photoionization [5], crossed beams [6], electron cyclotron resonance [7], the Cavalleri electron density sampling [8], electron
swarm [9], flowing-afterglow/Langmuir-probe [10] and microwave
conductivity/pulsed radiolysis [11]. Moreover, ion mobility spectrometry (IMS) also can be used to study electron attachment
reaction.
Recently, our research group has developed atmospheric pressure nitrogen corona discharge electron attachment ion mobility
spectrometry (APNCD-EA-IMS) [12] and photoemission electron
attachment ion mobility spectrometry (PE-EA-IMS) [13] to study
electron attachment to some chlorinated organic compounds.
∗ Corresponding authors. Fax: +86 551 65595311.
E-mail addresses: [email protected] (L. Xia), [email protected] (Y. Chu).
http://dx.doi.org/10.1016/j.ijms.2014.11.001
1387-3806/© 2014 Elsevier B.V. All rights reserved.
Particularly, in the PE-EA-IMS experiment, the electrons are generated by irradiating a metal grid surface with a 10.6 eV VUV Kr
lamp. Such an electron source has proved to be suitable for the rate
constant measurement for electron attachment to the compound
(CCl4 or CHCl3 ) with ionization energy (IE) below 10.6 eV. However,
we found that the PE-EA-IMS apparatus has some limitations especially for studying electron attachment to the organic compound
with low IE. In the PE-EA-IMS experiment, the photoemission electron current is weak, thus the curtain region, as originally employed
in the APNCD-EA-IMS [12], had to be removed in order to achieve a
determinable electron signal. Once there is not the curtain region,
the sample gas will fill the whole IMS tube. If the sample molecule
we studied has ionization energy less than 10.6 eV, it can suffer
from photoionization by the vacuum ultraviolet light. The photoionization and the electron attachment simultaneously happen in
the same area of IMS tube will influence the stability of electron
intensity and may complicate the ion mobility spectrum. To solve
this problem, in this work we propose the photoionization electron
attachment ion mobility spectrometry (PI-EA-IMS). This is based
on the fact that photoelectrons can be efficiently produced by photoionizing organic molecule like acetone [14]. With such a strong
2
H. Gao et al. / International Journal of Mass Spectrometry 376 (2015) 1–5
photoionization electron source, the curtain region in an IMS apparatus still can be adopted so as to segregate the electron reaction
and the electron production regions. In our newly developed PI-EAIMS instrument, the rate constants of electron attachment to low
IE organic compounds alkyl iodides such as C2 H5 I (IE = 9.35 eV), 1C3 H7 I (IE = 9.26 eV), 1-C4 H9 I (IE = 9.23 eV) and 2-C3 H7 I (IE = 9.19 eV)
[15] have been successfully determined over the average electron
energy 0.29–0.96 eV.
For electron attachment reactions of C2 H5 I, 1-C3 H7 I and 2C3 H7 I, in 1988 the pulse radiolysis-microwave cavity technique
was used to measure the rate constants over the average electron
energy 32.4–45.3 meV [16]. It was found that the rate constants
of C2 H5 I are less than that of 1-C3 H7 I and the isomer 2-C3 H7 I
has lower rate constants comparing to 1-C3 H7 I. In the range of
average electron energy 0.29–0.96 eV, only the rate constants of
C2 H5 I have been reported to be in the order of magnitude of
∼10−9 cm3 molecule−1 s−1 [17]. Up to now, the rate constants of
1-C4 H9 I have never been reported. Furthermore, electron attachment to these four alkyl iodides has not been studied using a same
experimental technique. Systematically studying these substances
is necessary for investigating their characteristics in the electron
attachment rate constants.
In the present work, electron attachment to four alkyl iodides
was studied by using the newly developed PI-EA-IMS instrument.
First, electron attachment to CCl4 was studied so as to check the
validity of the new apparatus. Subsequently the rate constants of
electron attachment to C2 H5 I, 1-C3 H7 I, C4 H9 I, 2-C3 H7 I were measured in the average electron energy range from 0.29 to 0.96 eV and
the differences in the rate constants were discussed.
2. Experimental
Fig. 1 is a schematic diagram of the homemade PI-EA-IMS apparatus which equipped with a new photoionization electron source.
The UV Kr lamp was located at the top of the ionization region and
was installed along the central axis of the drift tube. When organic
compound like acetone was introduced via the carrier gas 1 into the
ionization region, electrons were generated by the photoionization
reaction. Under the action of acceleration voltages U1 (∼400 V)
and U2 (∼600 V), the electrons could be dragged and guided into
the reaction region. There is a curtain region defined in space by
two perforated electrodes. As in the case of APNCD-EA-IMS [12],
a gas flow in the curtain region was used to prevent the sample
molecule diffusion into the photoionization region. The electric
field strength in the reaction or drift region could vary from 198
to 800 V cm−1 , corresponding to the average electron energy range
from 0.29 to 0.96 eV. In the experiment, the pulse width for the
Bradbury–Nielson ion shutter was 150 ␮s. The ion current accepted
by the Faraday plate was amplified with a gain of 109 V A−1 and fed
to a data acquisition and treatment computer system. All experiments were implemented at a temperature of 298 K and at ambient
pressure.
It is known that low energy electron reaction with halogenated
alkane molecules M proceeds usually via the dissociative electron
attachment process [18] as described by Eq. (1).
k
e + M −→I− + (M − I)
(1)
In the present experiments, the reactant gases were injected
either to the reaction region via the carrier gas 2 or to the drift
region through the drift gas. In both cases, the changes in the ion
mobility spectra might be used to investigate possible ion chemistry processes. In the case of measuring rate constant of electron
attachment, the regent gas was added to the drift region. When
the ion shutter on the IMS apparatus opened, the electron packet
would rapidly travel through the buffer gas in the drift region. If a
reagent gas was added to the drift region, due to electron attachment reactions, the negative ions could be generated on the path of
the electron movement, thus a special spatial distribution of ions
would be formed in the drift region. As a result, in the ion mobility spectrum, a negative ion intensity evolution with the drift time
could be observed. The intensity (i) of the formed anion I− can be
expressed as Eq. (2) [12,13].
k[M]L
i = k[M][e]0 tg exp −
w
t
1− d
tp
(2)
In Eq. (2), k is the electron attachment rate constant, tg is the
injection time of the electron packet, L is the length of drift region,
w is the drift velocities of the electron, td corresponds to the drift
time of the ions from a point located at x in the drift time region to
the Faraday plate, tp is the drift time of the ions from the shutter
to the Faraday plate, [M] and [e]0 are the concentrations of halogenated alkanes and electron, respectively. From Eq. (2), the natural
logarithm of the ion intensity versus the drift time obeys a linear
relationship [12,13].
ln i ∝
k[M]L
t
wtp d
(3)
In Eq. (3), if [M], L, w and tp are known, the electron attachment
rate constant k can be determined by the linear slope. In the present
work, the electron velocity w and its average electron energy were
taken from Ref [19].
The carrier, curtain and drift gases used in the experiment were
N2 with a purity of 99.9995%. The activated carbon and 13X molecular sieve were used to remove the impurities in N2 , such as water
vapor and other organic pollutants. The flow rates of carrier gas
Fig. 1. Schematic diagram of the photoionization ion mobility spectrometer (PI-EA-IMS).
H. Gao et al. / International Journal of Mass Spectrometry 376 (2015) 1–5
3
Fig. 2. The dependence of the electron intensity on acetone concentration.
1, carrier gas 2, curtain and drift gases were about 100, 100, 150,
668 mL min−1 , respectively. The chemical reagent CCl4 (Aladdin Co.,
Shanghai) has a purity of >99.7%. The purities of C2 H5 I, 1-C3 H7 I, 1C4 H9 I, and 2-C3 H7 I (Aladdin Co., Shanghai) are 99%. They were all
directly used without further purification. The halogenated alkanes
sample gases were prepared by using the syringe pump and N2 gas
flow dilution. The reagent concentrations were determined by the
saturation vapor pressure [20], the syringe pump speed and the
flow rate of carrier or drift N2 gas.
3. Results and discussion
3.1. Effect of acetone concentration
The electron intensity is related to the acetone concentration.
Fig. 2 gives the dependence of the electron intensity on the acetone concentration. The electron intensity increases rapidly with
the acetone concentration in the range from 7.5 to 42.5 ppm. As the
acetone concentration is higher than 57.5 ppm, the electron intensity gradually reaches a saturated state where the ultraviolet light
is almost completely consumed. In order to maintain enough electron intensity and to avoid possible effect from the ultraviolet light,
80 ppm acetone was used in the present PI-EA-IMS experiment.
Fig. 3. The ion mobility spectra for CCl4 sample introduction into (a) the reaction
region and (b) the drift region.
Fig. 4. (a) The ion mobility spectra for CCl4 introduction into the drift region at
different electric fields. (b) The plots for the natural logarithm of the ion intensity
versus the drift time in the incline part at different electric fields.
3.2. Tetrachloromethane
To check the reliability of the novel PI-EA-IMS apparatus, electron attachment reaction for CCl4 was firstly studied because its
rate constant was widely reported in the literature [11,21,22]. Fig. 3
displays the two ion mobility spectra measured with the CCl4 samples introduction to the reaction region and the drift region. In
each spectrum, there are two signal peaks closed to 0 and 6.08 ms
which are corresponding to the electrons and the negative ions
Fig. 5. The rate constants of electron attachment to CCl4 as a function of average
electron energy at 298 K. The experimental data are the average values from six
independent measurements and the error bars represent the standard deviations.
4
H. Gao et al. / International Journal of Mass Spectrometry 376 (2015) 1–5
Fig. 6. The ion mobility spectra for CCl4 , C2 H5 I, 1-C3 H7 I, 1-C4 H9 I and 2-C3 H7 I samples introduction into the reaction region.
Cl− (H2 O)n . In the case of CCl4 introduction into the drift region, the
spectrum exhibits a rising incline before Cl− (H2 O)n peak. Fig. 4(a)
shows that the similar phenomenon occurs when the drift electric field changes from 198 to 800 V cm−1 . The incline part reflects
the Cl− (H2 O)n formed along the path of the electron packet drift.
The natural logarithm of the signal intensity in the incline part
versus the drift time is shown in Fig. 4(b) which indeed exhibits
the straight line for every electric field. According to Eq. (3), the
electron attachment rate constants were calculated and the results
are presented in Fig. 5 together with the reported values in the literature [11,21,22]. It can be found that the present results are in good
Fig. 8. (a) The rate constants of electron attachment to C2 H5 I, 1-C3 H7 I and 1-C4 H9 I
as a function of average electron energy at 298 K. (b) The rate constant of electron
attachment to 1-C3 H7 I and 2-C3 H7 I as a function of average electron energy at 298 K.
The experimental data are the average values from ten independent measurements
and the error bars represent the standard deviations.
agreement with the literature data. This demonstrates the validity
of the PI-EA-IMS apparatus and experimental measurements.
3.3. Alkyl iodides
Fig. 7. The ion mobility spectra for C2 H5 I, 1-C3 H7 I, 1-C4 H9 I and 2-C3 H7 I introduction
into (a) the reaction region (b) the drift region.
Fig. 6 displays the ion mobility spectra when C2 H5 I, 1-C3 H7 I,
1-C4 H9 I and 2-C3 H7 I were introduced into the reaction region.
For comparison, the ion mobility spectrum arose from CCl4 was
included in Fig. 6. It is noted that the drift time (6.0 ms) of I− (H2 O)n
is shorter than that (6.08 ms) of Cl− (H2 O)n . Tabrizchi et al. [23] ever
recorded the ion mobility spectra for the CHCl3 and CH3 I systems
with a negative corona discharge electron source IMS at 433 K. Their
result shows that the drift time of I− (H2 O)n is longer than that of
Cl− (H2 O)n . The discrepancy can be explained by the degree of clustering for X− (H2 O)n depending on the temperature. Borsdorf et al.
[24] calculated the distribution of clusters for halide ions in a temperature range from 323 to 423 K. They found that the ion Cl− (H2 O)
is the main component at low temperature, while non-clustered
ion I− is the most abundant ion over the whole temperature range.
Cl− (H2 O) and I− may be the main ions in our low temperature
experimental condition, while Cl− and I− should be the main ion
species under Tabrizchi’s high temperature experiment.
As shown in Fig. 7, when 1-C4 H9 I, 1-C3 H7 I, C2 H5 I and 2-C3 H7 I
were introduced into the drift region, the ionic peak appeared at the
same drift time near 6.0 ms. This phenomenon shows that there was
no reaction between I− and neutral sample in the drift region. I− is
likely the unique ion species in both reaction and drift regions. In
addition, there is also an expected incline before I− peak which
is originated from the ions I− formed following the path of the
electron packet travel in the drift region.
H. Gao et al. / International Journal of Mass Spectrometry 376 (2015) 1–5
As in the case of CCl4 , the rate constants of electron attachment to C2 H5 I, 1-C3 H7 I, 1-C4 H9 I and 2-C3 H7 I were determined
in the average electron energy range from 0.29 to 0.96 eV. The
results are shown in Fig. 8. The rate constants are all in the
order of magnitude of ∼10−9 cm3 molecule−1 s−1 . For straightchain alkyl iodides, the rate constants of C2 H5 I is less than that
of 1-C3 H7 I and the rate constant difference is very small between
1-C3 H7 I and 1-C4 H9 I (see Fig. 8(a)). This implies that, for the
straight-chain alkyl iodides with more than two carbon atoms,
the carbon chain length has a minor effect on the electron capture process. This behavior may be explained by energy threshold
(Eth ) for I− formation in the electron attachment process. Eth
can be calculated using the bond dissociation energy D(C–I) and
electron affinity EA(I) of I: Eth = D(C–I) − EA(I). The experimental
D(C–I) values of C2 H5 I, 1-C3 H7 I and 1-C4 H9 I were reported to
be 2.255, 2.168 and 2.125 eV, respectively [25]. Based on these
values for D(C–I), because the EA(I) is definite (3.059 eV) [15],
the Eth values of these three alkyl iodides will follow the order:
Eth (C2 H5 I) < Eth (1-C3 H7 I) ≈ Eth (1-C4 H9 I). This may result in the rate
constant relationship: k(C2 H5 I) < k(1-C3 H7 I) ≈ k(1-C4 H9 I).
For the isomers, the rate constant of 2-C3 H7 I is lower than that of
1-C3 H7 I (see Fig. 8(b)). It may be caused by a different steric factor.
The volume of the alkyl group is larger than that of the hydrogen
atom which may hinder the possible electronic attack.
4. Conclusions
Using newly developed PI-EA-IMS, the electron attachment
reactions for a series of alkyl iodides have been studied. The
intensity of electrons formed by acetone photoionization is strong
enough to support the curtain region. Electron attachment to
CCl4 has been studied to demonstrate the validity of experimental method. Electron attachment rate constants for C2 H5 I,
1-C3 H7 I, 1-C4 H9 I and 2-C3 H7 I are all in the order of magnitude
of ∼10−9 cm3 molecule−1 s−1 at the average electron energy range
from 0.29 to 0.96 eV and follow the relationships of k(C2 H5 I) < k(1C3 H7 I) ≈ k(1-C4 H9 I) and k(2-C3 H7 I) < k(1-C3 H7 I). The differences
in the rate constants for these alkyl iodides can be reasonably
explained.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (21403245 and 21107112), Anhui Provincial Program
for Science and Technology Development (1301042095) and Direction Program of Hefei Center of Physical Science and Technology
(2012FXCX009).
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