Kinetics of low energy electron attachment to some chlorosilanes in the gas phase

Journal Pre-proofs Research paper Kinetics of low energy electron attachment to some chlorosilanes in the gas phase B. Michalczuk, W. Barszczewska PII: DOI: Reference:

S0009-2614(19)31037-1 https://doi.org/10.1016/j.cplett.2019.137056 CPLETT 137056

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Chemical Physics Letters

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18 November 2019 17 December 2019 21 December 2019

Please cite this article as: B. Michalczuk, W. Barszczewska, Kinetics of low energy electron attachment to some chlorosilanes in the gas phase, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.137056

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Kinetics of low energy electron attachment to some chlorosilanes in the gas phase B. Michalczuk*, W. Barszczewska Siedlce University, Faculty of Sciences, 3 Maja 54, 08-110 Siedlce, Poland Abstract The rate coefficients (k’s) and the activation energies (Ea’s) of thermal electron attachment for SiCH3Cl3, SiH(CH3)2Cl, SiHCH3Cl2 and Si(C2H5)3Cl have been measured. The electron attachment processes in the chlorosilane-carbon dioxide mixtures have been investigated using a Pulsed Townsend technique in the temperature range 298K-378K. The increase of the rate coefficients with temperature follows the Arrhenius law and the Ea’s have been obtained from the slope of the ln(k) vs. T-1. The rate coefficients at 298K are equal to 8.72×10-11cm3s−1, 6.72×10-11cm3s−1, 16.8×10-11cm3s−1 and 3.27×10-11cm3s−1 and activation energies are: 0.32eV, 0.24eV, 0.25eV and 0.21eV, respectively for SiCH3Cl3, SiH(CH3)2Cl, SiHCH3Cl2 and Si(C2H5)3Cl.

Keywords: chlorosilanes, electron attachment rate coefficient, activation energy, Pulsed Townsend technique.

*corresponding author: Bartosz Michalczuk, Siedlce University, Faculty of Sciences, 3 Maja 54, 08-110 Siedlce, Poland; Phone: +48256431005; E-mail: [email protected]

1

Introduction Electron interactions with molecules and the behaviour of slow electrons in gases under an applied electric field have been studied experimentally since the early 20th century (e.g., see Refs. [1, 2, 3, 4, 5]). This basic knowledge has underpinned many technologies and is still underpinning many of today's technological advancements. In contrast to the last past decades, when mainly noble gases discharges were studied, the current interests are spread over a large variety of complex chemical systems. In particular, electronegative gases and plasmas attract much attention, in applications related to surface processing, environmental studies for waste gas disposal, atmospheric science. These play an important role in the plasma processing of microelectronic devices and many others [6, 7]. Therefore, there are many situations in modern plasma physics in which the role of the decomposition products of halogen-containing electronegative gases is significant. Such studies constitute an important part of atomic and molecular physics and they have resulted in a valuable database on electron interaction processes in plasma processing gases, including electron collision cross sections, electron transport, and rate coefficients. This knowledge is extensively used in studies of the fundamental aspects of electronegative plasmas and in numerous applications of gaseous media containing negative ions [8]. As the world becomes more and more driven by electronic devices and computers, the semiconductor industry is growing at a fast pace, placing more and more emphasis on the most efficient and cost-effective tools and processes. To do this, it is crucial to understand in detail the interactions between species which occur within the plasma. Because of this, it is important to know precisely how electrons can interact with neutral species, both in terms of collision energetics and reaction rate data. This is a key requirement for the progression of industrial plasma technology [9]. However there still relatively little is known on such processes, which is predominantly caused by the lack of experimental data for the wealth of interactions that can exist in various industrial plasmas. Thus, knowledge of these primary electron interactions can be used to control the important species in the plasmas of many modern technologies, especially in the microelectronics industry. Silanes and its halogenated derivatives are versatile materials used in a wide range of applications including adhesion promoters, coupling agents, crosslinking agents, dispersing agents, and surface modifiers [10, 11]. They are of interest because the decomposition of silane in low-pressure plasmas, as well as the reduction of chlorosilanes by hydrogen in plasmas, are important industrial processes for the manufacture of silicon and the deposition 2

of silicon films. In the presence of silicon wafers, any plasma processing will naturally generate silane derivates as products or intermediates. Electron attachment to silane and its halogenated analogues is an important primary step in the chemical processes which ensue when these gases are introduced into a plasma. The present work is a continuation of our previous studies where we have presented results on fundamental quantities such as rate coefficients (k’s) and activation energies (Ea’s) for thermal electron capture by some halocarbons [12, 13,14], perfluoroethers [15], halogenated alcohols [16, 17] using Pulsed Townsend technique also known as swarm method. Here we report for the first time the swarm results of electron attachment to SiCH3Cl3, Si(CH3)2ClH, SiH(CH3)Cl2 and Si(C2H5)3Cl in carbon dioxide as a buffer gas. Experiment The experiments were carried out with the Pulsed Townsend (PT) technique apparatus. This technique allows us to study electron attachment processes not only at the room temperature but also at higher temperatures. The experimental apparatus and measurement procedure has been already described in detail previously [12], therefore we will give only a brief description. The experimental setup consists of the stainless steel chamber of 700 cm3 volume with two parallel electrodes (Figure 1). The chamber can be heated using heating jackets produced by Watlow Company. Electronic control enables us to stabilize the temperature within 1 ◦C. The electron acceptor was introduced into the chamber with the excess of carbon dioxide as a buffer gas. An electron swarm is produced at the cathode using a 5 ns Nd:YAG laser operating on fourth harmonic (266 nm, 10 Hz). The electron swarm traverses to the collecting electrode (anode) under applied uniform electric field, through the gas mixture containing buffer gas (carbon dioxide) and an electron acceptor of a certain concentration. The drifting electrons create a pulse change in the potential of the collecting electrode. The pulse signal is amplified, registered on the oscilloscope and saved in the computer memory. The experiment was performed in such a way that each chlorosilane–carbon dioxide mixture was first introduced into the chamber at the applied total pressure (ca. 350–400 Torr). A total number of 50 pulses were registered for a given E/N and averaged. The procedure was repeated usually for five E/N values in the rather wide range (1.5×10-17 – 3.0×10-17 V cm2 molec.-1), where electrons in carbon dioxide are in thermal equilibrium with gas molecules. Next, the mixture was heated to a higher temperature and the measurement

3

procedure was repeated in the temperature range 298 K to 378 K. The whole experiment was carried out for a few different initial concentrations of chlorosilanes in carbon dioxide. In experiments, where the influence of carbon dioxide concentration on the rate coefficient was investigated (Figure 4), the procedure was as follows: Each of chlorosilane carbon dioxide mixture was introduced into a chamber at the highest applied total pressure (ca. 750 Torr). Next, the mixture was pumped out to a lower pressure and the procedure was repeated for 9 consecutive pressures in the range 300-750 Torr. Figure 2 shows the example pulse which is the result of averaging of the 50 consecutive pulses for the mixture of chlorodimethylsilane – carbon dioxide. The procedure of the averaging of the 50 consecutive pulses was described previously in Reference [12]. The electron attachment rate coefficients were determined from the shape of the output signal of the electron pulse. The experimental curves as shown in Figure 2 were fitted using Equation (I) [12]:

V()  B  exp(k  N a  )  exp( / t1 )

(I)

where: B – apparatus arbitrary constant, k – electron capture rate coefficient, Na – electron acceptor concentration, t1 – time constant of the preamplifier (t1=RC=400 μs). As all the values except B and k are known, the computer task is to simulate them. Since B and k influence the potential very differently, the simulation gives unequivocal results on condition the electron acceptor concentration and temperature are such that the rate of electron disappearance is in the range of (0.2–1.0) ×105 s-1 . The examined compounds and appropriate purities are as follows: SiCH3Cl3 (Aldrich, 99%), Si(CH3)2ClH (Aldrich, 98%), SiH(CH3)Cl2 (Aldrich, >97%) and Si(C2H5)3Cl (Aldrich, 99%). All compounds were purified by the vacuum freeze-pump-thaw technique. The carbon dioxide with a quoted purity 99.998% was obtained from Fluka and used as delivered.

4

Results and Discussion The present data were obtained using the swarm method, known as the Pulsed Townsend technique, that allows us to study electron attachment processes at elevated temperature. We have measured rate coefficients and the activation energies for following molecules:

trichloromethylsilane

(SiCH3Cl3),

chlorodimethylsilane

(Si(CH3)2ClH),

dichloromethylsilane (SiHCH3Cl2) and chlorotriethylsilane (Si(C2H5)3Cl) (for the molecular structures see Figure 3) over the temperature range T=298–378 K. T refers to both, the electron temperature and the gas temperature (T = Te = TG). All of the presently obtained results are collected in Table 1 together with available literature data that are necessary for further discussion. To the best of our knowledge there are no available kinetic data on electron attachment to these molecules. However, there are papers about electron interactions with some chlorosilanes, but provided data in those articles are different from those we present in this work. For instance, Vought [18] has determined ionisation energies for SiCl4 and SiCl2 while Wilkerson et al. [19] in his paper has analysed resonant electron capture in Si(NCO)4 and SiCl4. The chlorides and bromides of silicon were studied by Pabst et al. [20] for low energy electrons dissociative resonance capture processes. The ions found by Pabst were: MX3-, MX2-, X2and X- (M is silicone atom and X corresponds to chlorine or bromine). Hashino et al. [21] compared negative ion formation as a result of dissociative electron attachment to GeH4, CH4 and SiH4 using crossed-beam method with electron energy region from 6 to 11 eV. Another interesting work done by Wan et al. [22] deals with electron attachment to chlorosilanes (SiH2Cl2, SiHCl3 and SiCl4) and chloromethanes (CCl4, CHCl3, and CH2Cl2) where the cross sections for dissociative attachment (DEA) for electrons in the energy range 0.2 to 5.0 eV have been measured. A further crossed-beam measurements carried out by Bӧhler et al. [23] on Me3SiCl and PhMe2SiCl for possible enhancing effect of the phenyl group on the Si-Cl bond dissociation. Bjarnason et al. [24] also using crossed-beam technique investigated dissociative electron attachment and dissociative ionisation of chlorinated cyclic silane derivatives (11-dichloro-1-silacyclohexane and silacyclohexane). One of the latest paper which deals with silanes and their interactions with electrons is work by Kumar et. al [25] where results for electron attachment to the SiCl4 are presented. Measurements performed with crossed beam apparatus reveals that for interactions with electrons with energy range 0 to 10 eV result with ions Cl-, SiCl3-, SiCl2-, Cl2- and SiCl4-. Additionally, experimental results are supported by theoretical calculations (Density Functional Theory – DFT). 5

In our swarm experiment, the disappearance of the electrons from the swarm is monitored. The rate coefficients obtained in the swarm experiment correspond to total attachment processes including all the reaction channels. All investigated chlorosilanes were diluted in carbon dioxide which is good for electron thermalization. The example results (at 298 K) are shown in Figure 3 in terms of the rate coefficients vs. carbon dioxide concentrations. As it is seen, for the case of SiH(CH3)2Cl shown in Figure 3, as well as for the other ones we have investigated (Table 1), rate coefficient does not depend on the carbon dioxide concentration. We have also found that there is no influence of scavenger concentration on the rate coefficient. This means that only two-body process, electron attachment by a single molecule, takes place. SiCnHmClx +e-  products There are no data for thermal electron attachment for chlorosilanes, therefore, the examination of the obtained results will base mainly on their comparison with the kinetic data for other molecules of similar structure. In general chlorinated silanes are not very effective electron scavengers. It can be also stated that comparing with chlorinated hydrocarbons, chlorinated silanes are worse electron scavengers. But it is valid only when chlorinated hydrocarbons have in structure three or more chlorine atoms. Comparing SiH(CH3)2Cl with CH3CHClCH3 which have one Cl atom at the identical position but for silane chlorine is bonded to the Si atom and for 2chloropropane, chlorine is bonded to the carbon. Rate coefficients (k’s) for chlorosilane is one order of magnitude higher than k for chloropropane (6.72×10-11 cm3 s−1 for chlorosilane vs. 3.8×10-12 cm3 s−1 [26] for chloropropane). When we compare chlorinated silane and chlorinated hydrocarbon with two chlorine atoms, the rate coefficient for chlorosilane is one order of magnitude higher. For instance, SiHCH3Cl2 and corresponding chloro- hydrocarbon CHCl2CH3 have rate coefficients as follows: 1.68×10-10 cm3 s−1 for chlorosilane and 2.1×10-11 cm3 s−1 [27] for the chlorinated derivative of ethane. When three chlorine atoms are present rate coefficient for chlorinated hydrocarbon dramatically increases. SiCH3Cl3 with k = 8.72×10-11 cm3 s−1 vs. CH3CCl3 k = 1.5×10-8 cm3 s−1 [28]. Also Si(C2H5)3Cl has rate coefficient (k) similar to other measured chlorosilanes which is 3.27×10-11 cm3 s−1 (we have not

found

data

for

corresponding

chlorinated

hydrocarbon

in

the

literature).

It is well known fact that for halogenated hydrocarbons rate coefficients increase when number of halogen atoms also increases. For example, this tendency is observable for chlorinated derivatives of ethane. CH3CH2Cl, CH2ClCH2Cl, CHCl2CH2Cl, CHCl2CHCl2 6

have rate coefficients 3.4×10-14 cm3 s−1 [29], 3.2×10-11 cm3 s−1 [27], 3.7×10-10 cm3 s−1 [30], 3.2×10-8 cm3 s−1 [26] respectively, so it is clearly seen that by increasing the number of chlorine atoms from one to four, the rate coefficient increases 6 orders of magnitude. But not only the number of halogen atoms affects the value of the rate coefficient but also their localization in the carbon chain. For instance CHCl2CHCl2 with k = 3.2×10-8 cm3 s−1 [26] and CH2ClCHCl3 with k = 3.1×10-7 cm3 s−1 [28] or CH3CH2CHCl2 and CH3CCl2CH3 with rate coefficients equal to k = 5.7×10-11 cm3 s−1 [26] and k = 6.3×10-12 cm3 s−1 [26], but here difference is just one order of magnitude. In the case of chlorinated silanes, we do not observe that relationship. The rate coefficients for the set of chlorosilanes are within the same order of magnitude (10-11 cm3 s-1, Table 1 ). This is a remarkable observation and strongly contrasts with the general behavior of the hydrocarbons where substitution by chlorine usually leads to a strong increase of the electron scavenging properties resulting in a strong Cl− signal right at threshold (0 eV) [4, 5, 9]. The beam studies have shown [23, 24, 31] that low energy electron attachment to chlorinated silanes generates a variety of negative fragment ions via different resonances. These fragment ions are the result of dissociative electron attachment (DEA) processes ranging from simple bond cleavages to surprisingly complex unimolecular decompositions involving multiple bond cleavages, atom transfer in the transient negative ion (TNI) and formation of new bonds. For all investigated molecules, the main product of electron capture is the chloride ion. Comparing electron attachment by halogenated derivatives of silane with analogous hydrocarbons it can be stated that processes for these two groups are different due to the structure of molecules and the strength of the C – Cl and Si – Cl bonds. While for the chlorinated derivatives of hydrocarbons electron attachment leads to the break of C – Cl bond with Cl- as a main observed ion for the lowest energy of electrons, for chlorinated silanes greater variety of generated ions is observed and for much higher electron’s energies [22, 31]. We have also measured rate coefficients in the temperature range 298 K – 378 K. The results for the investigated systems in terms k vs. T and ln(k) vs. 1/T are shown in Figure 5. These data represent the average of a few measurements (three to six series) carried out at different initial pressures of chlorosilanes in the range 0.042–0.045 Torr, 0.018–0.022 Torr, 0.064–0.077 Torr and 0.097–0.125 Torr for SiCH3Cl3, SiHCH3Cl2, SiH(CH3)2Cl, Si(C2H5)3Cl, respectively. In all cases, we observed the increase of the rate coefficients with the temperature according to the Arrhenius function (ln(k) = ln(A) - Ea/kBT) what means that 7

the investigated processes require some activation energies. The validity of the Arrhenius equation has been theoretically investigated by Fabrikant and Hotop [32] by means of Rmatrix calculations. They have demonstrated that in both exo- and endothermic reactions the equation holds over a finite intermediate temperature range. Since the present experiments have been conducted in the temperature range where, according to above-mentioned calculations, the Arrhenius law holds and because we observe clear linear dependence of ln(k) vs. 1/T the activation energies can indeed be obtained from the slope of the curves in Figure 5 (right panel). The activation energies obtained for all investigated molecules are collected in Table 1 - column (Ea). To our knowledge, these values were obtained for the first time too. Usually, the activation energy is viewed as the crossing point between neutral and anionic potential curves where resonances are low in energy. However, due to the tunneling effect that can take place from the vibrational level below the barrier, it is likely that the activation energy of the process is slightly lower than the energy of the crossing point. The activation energy thus defines effective barrier energy for the electron attachment process. There are three main experimental techniques to investigate electron capture processes by molecules in the gas phase. The thermal electron capture rate constants and activation energies can be measured using various swarm experiments. The dependence of the cross section for dissociative electron attachment (DEA) on electron energy can be obtained in the beam experiment. The vertical attachment energy (VAE) can be obtained from electron transmission spectroscopy (ETS). These data describe the same process and should be interconnected. This follows from the fact that the rate constant for the thermal electron capture strongly depends on the extent of overlap between the shape and position of DEA cross section peak and the Maxwell–Boltzman distribution of both electron and molecules energies [26, 33, 34]. The DEA peak energy position and cross section, in turn, depend on VAE and partly on an autodetachment rate. Up to date, there are several approaches that aim to correlate the values of rate coefficients with the molecular and structural parameters and its ability to capture an electron. First serious attempt to connect these data was made by Christophorou et al. [27]. He has plotted the logarithm of the rate constant for thermal electron attachment as a function of VAE and obtained crudely linear dependence for a large set of halocarbons available at that time. Aflatooni et al. [35, 36, 37] have demonstrated that there exists a good correlation between the peak cross section for DEA and VAE for chloro- and chlorofluoroalkanes. They have also calculated the thermal attachment rate constants from 8

their beam data and have shown that they correlate well with the swarm ones and that the log (kth) is a linear function of VAE. Hotop et al. [38] have shown the relation of the rate coefficients values with the polarizability of the molecule. Another relation has been established by Szamrej et al. [26, 29, 39]. They have demonstrated that the rate coefficients depend on the polarizability of the attaching center of the molecule, a part of a molecule that is responsible for the capture of the electron. Dashevskaya et al. [40] based on Vogt-Wannier model [41] and its extension by Fabrikant et al. [42] have made analytical approximations for cross sections and thermal rate coefficients for electron capture by polarizable and dipolar target molecules. They found that both the polarizability and dipole moment of the molecule has a little effect on the rate coefficient and can be omitted in the considerations. The capture is the first step towards electron attachment and kcap can be understood only as an upper limit for the attachment process. Thus the experimental attachment rate coefficients (k’s) are (except SF6 and CCl4, for which activation energy is 0 eV) even a few orders of magnitude lower than the capture rate constants kcap (see Table 1 in Reference 40). In our case for the chlorosilanes (which are a weak electron scavengers) the values of the rate coefficients k’s (10−11cm3s−1, Table 1) are smaller than kcap (3×10−7cm3 s−1). In our previous work, we proposed [16, 43] the exponential relationship between the dissociative electron attachment (DEA) rate coefficients determined at room temperature k (298 K) and the activation energy. This dependence can be represented by the equation: k=Aexp(-Ea/kBT) where A is preexponential factor, kB is Boltzmann constant and Ea is the activation energy. In Figure 6 are presented thermal electron capture rate coefficients as a function of the activation energies for chlorinated silanes (present data) together with our previous experimental data for chlorinated alkanes and chlorinated alcohols (Table 1).

For

comparison we chose only these data for which the rate coefficient and the activation energy were determined in the same laboratory. The solid line shown in Figure 6 is resulting from simple Arrhenius equation where the preexponential factor corresponds to the maximum value of the rate coefficient equivalent to the zero energy of activation. As the A parameter we set up the thermal rate coefficient for CCl4 molecule (k = 3.79 × 10−7cm3s−1[44]), which is known as a very efficient electron scavenger. The factor A can be also interpreted based on the structure of the target molecule assuming that we treat an electron as the de Broglie wave. Since maximum s-wave cross section of electron scattering can be described by the expression (/2)2 where  is the de Broglie wavelength for the electron of energy 3/2kBT, the factor A can be treated as the theoretical limit for the attachment rate coefficient 5.0×10−7 9

cm3 s−1 at 298 K. This type of assumption could be successfully used in the case of molecules for which the change of A is so small. As it is shown in Figure 6 the most of the experimental data is practically nearby the theoretical line. This shows that indeed the activation energy is the main factor determining the rate coefficient for thermal electron capture. In the case, when the data points are far away from the theoretical line the other reasons should be considered. One can notice that in the case of large activation energies, the rate coefficients at room temperature are so small, that even small impurities in the samples have a significant impact on the efficiency of electron attachment processes. This has been described in detail previously [12]. This can offer a good criterion for judging the quality of the experimental results if both the rate coefficient and activation energy are known. Additionally, the experimental kinetic results are much more reliable if both parameters i.e. the reaction rate coefficients and the activation energies are determined in the same experiment. Conclusions In this paper, results from the study of thermal electron capture by SiCH3Cl3, SiH(CH3)2Cl, SiHCH3Cl2, and Si(C2H5)3Cl in the gas phase are presented. The measurements have been performed in the temperature range T = 298–378K (T = Te = TG), the determined rate coefficients for thermal electron capture processes show an Arrheniustype rise with temperature increasing. Only the second order processes were responsible for the electron capture. The rate coefficients at 298K are equal to 8.72×10-11cm3s−1, 6.72×1011cm3s−1,

16.8×10-11cm3s−1 and 3.27×10-11cm3s−1 and activation energies are: 0.32eV,

0.24eV, 0.25eV and 0.21eV, respectively for SiCH3Cl3, SiH(CH3)2Cl, SiHCH3Cl2 and Si(C2H5)3Cl. The rate coefficients (k’s) and the activation energies (Ea’s ) were determined for the first time. The dependence of the thermal electron attachment rate coefficients (k’s) on the activation energies (Ea’s) has been demonstrated. To get more details about low energy interaction with chlorinated derivatives of silane, additional measurements including broader temperature range or crossed – beam studies are needed.

10

Table 1. Thermal electron attachment rate coefficients (k298’s) at temperature 298 K and activation energies (Ea’s) for all investigated chlorosilanes and literature data for selected chloroalkanes and chlorinated alcohols. Trange - temperature range in which Ea’s were obtained.

11

Table 1.

CH3SiHClCH3

k298 (cm3 s-1) 6.72±0.08×10-11

Ea (eV) 0.24±0.013

Trange (K) 298-368

SiHCl2CH3

16.8±0.06×10-11

0.25±0.097

298-378

CH3SiCl3

8.72±0.07×10-11

0.32±0.020

298-378

Si(C2H5)3Cl

3.27±0.02×10-11 1.1×10-17(300K) [45] 1.8×10-13(300K) [45] 3.9×10-7(300K) [46] 3.4× 10-14 [29] 3.2× 10-11 [27]

0.21±0.010 0.67[45]

298-378 700-1100

0.39[45]

495-973

0.00 [46] 0.24 [47]

205-590 298 298

2.1 × 10-11 [27] 1.5×10-8 [28] 3.8×10-8 [30] 3.7×10-10 [30] 3.2×10-10 [28] 3.2×10-8 [26]

0.25 [47] 0.109 [28] 0.09 [30] 0.16 [30] 0.128 [28] -

298 298-470 298-373 298-343 298-385 298

Molecules Chlorinated silanes (present data)

CH3Cl CH2Cl2 CCl4 CH3CH2Cl CH2ClCH2Cl CHCl2CH3 CH3CCl3 Chlorinated alkanes

Chlorinated alcohols

CHCl2CH2Cl CHCl2CHCl2 CH2ClCCl3 CH3CHClCH3

3.1×10-7[28]

-

298

3.8×10-12 [26]

-

298

CH3CH2CH2Cl

3.6×10-13 [27]

-

298

CH3CH2CHCl2

5.7×10-11 [26]

-

298

CH3CCl2CH3 CH3CHClCH2CH3

6.3×10-12 [26] 2.0×10-15 [30]

0.55 [30]

298 343-373

CH2ClCHClCH2Cl

1.7×10-10 [13]

0.16 [13]

298-358

CH2ClCH2OH CH2ClCH2CH2OH CH2ClCH(OH)CH2Cl CH2ClCHClCH2OH CH2ClCH(OH)CH2OH

4,9±0.7×10-12 [17] 2.4±0.6×10-12 [17] 1.9±0.5×10-11 [17] 9.0±1.7×10-11 [17] 1.3±0.1×10-11 [17]

0.26±0.01 [17] 0.23±0.02 [17] 0.34±0.01 [17] 0.37±0.02 [17] 0.39±0.03 [17]

298-338 298-368 298-378 298-358 298-358

12

Figure 1. Scheme of the reaction chamber of SWARM experiment. (1- laser beam, 2cathode, 3- anode, 4- connection with vacuum line, 5- high voltage, 6- preamplifier, 7baratron). Figure 2. Averaged voltage signal for the mixture of SiH(CH3)2Cl – carbon dioxide obtained at 298 K and E/N=1.8×10-17 V cm2 molec.-1. Figure 3. Structures of investigated molecules. Figure 4. Rate coefficient of two-body process as a function of carbon dioxide concentration in the mixture of SiH(CH3)2Cl – carbon dioxide at 298 K. The concentration of SiH(CH3)2Cl was in the range 1.9×1015 molec.cm-3 - 2.4×1015 molec.cm-3. Figure 5. Dependence of k vs. T (left panel) and ln(k) vs. 1/T (right panel) for investigated molecules. Figure 6. Thermal electron capture rate coefficients as a function of the activation energies: () - present data, our previous data for: () - chlorinated alkanes, () – chlorinated alcohols, labelled by numbers as follows (for details see Table 1): (1) - CH3CCl3, (2) - CHCl2CH2Cl, (3) - CH2ClCHClCH2Cl, (4) - CH3CHClCH2CH3, (5) - CH2ClCH2CH2OH, (6) CH2ClCH2OH,

(7)

-

CH2ClCH(OH)CH2Cl,

(8)

-

CH2ClCHClCH2OH,

(9)

-

CH2ClCH(OH)CH2OH, (solid line - theoretically obtained values of the rate coefficients for T = 298K considering electron as the de Broglie wave).

13

Figure 1.

14

2,8

Voltage (V)

2,1

1,4

0,7

0,0 0

2

4

Time (10-6s)

Figure 2.

15

5

7

Figure 3.

16

7

k (10

-11

3 -1

cm s )

8

6

5 0.8

1.0

1.2

1.4

1.6 19

1.8

2.0 -3

[CO2] (10 molec. cm )

Figure 4.

17

2.2

2.4

2.0

Si(CH3)2Cl

SiCH3Cl3

Si(C2H5)3Cl

1.0

-22

0.5

-23

0.0

-24

300

310

320

Si(C2H5)3Cl

-21

lnk

k (10-9dm3molec.-1s-1)

SiHCH3Cl2

SiCH3Cl3

1.5

290

Si(CH3)2Cl

-20

SiHCH3Cl2

330

340

350

360

370

380

0.0026

0.0028

0.0030

T-1(K-1)

T (K)

Figure 5.

18

0.0032

0.0034

-3

10

-5

10

(1)

-7

10

(2)

3 -1

k (cm s )

-9

10

(8)

-11

(3)

10

(5)

-13

10

(6)

(7)

(9)

(4)

-15

10

-17

10

-19

10

0.0

0.1

0.2

0.3

0.4

Ea (eV)

Figure 6.

19

0.5

0.6

0.7

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The thermal electron attachment rate coefficients (k’s) for chlorinated silanes have been measured The activation energies (Ea’s) for chlorinated silanes were determined The dependence of the thermal electron attachment rate coefficients (k’s) on the activation energies (Ea’s) has been demonstrated

Bartosz Michalczuk: Software, Conceptualization, Data curation, Visualization, Investigation, Writing- Reviewing and Editing, Resources Wiesława Barszczewska: , Methodology, Writing- Original draft preparation, Supervision, Validation

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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