Time-resolved LMR measurement of the rate constants of the reactions SiH3+SiH3 and SiH3+Cl

Chemical Physics 181 (1994) 119-128 North-Holland

Time-resolved LMR measurement of the rate constants of the reactions SiH3 + SiH3 and SiH3 + Cl A.V. Baklanov and A.I. Chichinin Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation Received 26 November 1992; in final form 28 October 1993

The kinetics of the reactions SiHs + SiHs and SiHr + Cl were studied using time-resolved laser magnetic resonance (LMR ) . The silyl radical arising in KrF laser (248 run) photolysis of SiI-I,/COC12/Ar mixture was detected. The rate constants obtained from thedecaykineticsofS~,s~satT=295arekl(S~3+S~3)=(1.6~0.5)x10-11cm’/sandk~(S~,+C1)=(3.2f1.0)x10-” cm3/s. The values of k, and k2 were found to be unchanged under variation in argon pressure from 6 to 17.3 Torr. The other rate constants measured and estimated are k,(SiH,+Cl)=(2.5+0.5)x10-10 cm3/s, k4(SiH3+C0)<7x10-I6 I&/S, k~(SiHI+HCI)((1.8+0.5)~10-‘5cm3/s, k,r(SiH,+COCl,)~5x10-‘5cm3/s and kr(SiH3+NF3)<5x10-‘5cm’/s. Possible reasons for the lower rate constants of hydrogen atom abstraction by chlorine atom for the reaction involving SiH3 compared to that for the reaction with the molecule SiI-I, are discussed. It is assumed that in collision SiHr and Cl species are oriented so that the probability of their approach in the confguration favourable for the abstraction reaction decreases.

1.1nmucti0n

The first subject of this inquiry was the recombination reaction of silyl radicals: SiHJ + SiHs 3

(1)

This reaction plays an important part in forming sil-, icon films during chemical deposition from silane plasma [ 11. To estimate k,, Becerra and Walsh [ 2 ] used the available recombination rate constant of trimethylsilyl radicals in the gas phase and the measured ratio of the recombination rate constants of silyl and trimethylsilyl radicals in solution. Another estimation of k, is given by Kushner [ 3 1, who modeled the processes in glow-discharge plasma during the formation of silicon films. The first direct measurement of k1 was realized by Itabashi et al. in 1989 [ 41. They used a diode laser to investigate the decay kinetics of silyl radicals generated by impulsing discharge in the mixture SiH4/H2. For the generation of SiHS Loh et al. [ $61 used the reaction of a silane molecule with atomic chlorine, produced by photolysis of CCL. They detected the decay kinetics of SiHS using a diode laser. 0301-0104/94/$07.00

In 1990 they obtained an upper estimation of k, [ 5 1, and in 199 1 they measured it [ 61. In 199 1, Koshi et al. [ 71 measured k, using time-resolved mass spectrometry. They followed the decay kinetics of SiH3 arising in the flash-photolysis of the mixture CClJ SiH4/He. If SiH3 was generated by discharge [ 41 or by photolysis of CClJSiH., mixture [ 5-71, other radicals were produced simultaneously. In principle, reactions with these species could contribute in the decay kinetics of SiHs radicals. It was interesting for us to use another, free of such complications, procedure of SiH3 generation. In this work we also investigated the reaction of the silyl radical with a chlorine atom SiH3 + Cl 2

in order to compare kz with the rate constant of the reaction Cl+ SiH4 3

SiHs +HCl ,

(3)

which had been measured by several research groups [ 8- 111. To our knowledge, no data on the values of k2 have hitherto been published. In this work Cl atoms and SiHS radicals were gen-

0 1994 Elsevier Science B.V. All rights reserved.

SSDZ 0301-0104(93)E0392-9

(2)

120

A. V. Baklanov, A.I. Chichinin /Chemical Physics 181(1994) 119-128

erated by photolysis of SiH4/COC12/Ar mixture at 248 nm. Time-resolved laser magnetic resonance technique (LMR) was used for measurement of the silyl radical reaction kinetics.

2. Experimental The apparatus for time-resolved laser magnetic resonance has been described elsewhere [ 10,12- 15 1. We used a slow flow system consisting of a Pyrex cell 19 mm in diameter and inserted in the cavity of a CO2 laser between the poles of an electromagnet. The mixture SiH.,-COC12-Ar was pumped through the cell. The 12 cm long registration zone was affected by oscillating ( 150 kHz, amplitude 60 Gs) and constant magnetic fields. COz-laser radiation arrived at a GeHg photoresistor (53 K). The photoresistor signal synchronously detected at a frequency of 150 Hz was digitized and processed by a computer. The radiation of an excimer laser (ELI-94, NF,-Kr-He) entered the cell through a quartz window at small angle with the beam of the COZ laser. The diameter of the excimer laser beam was larger than that of the CO* laser beam. The radiation energy could be varied by means of special grids placed at the outlet window of the extimer laser. The maximum energy of our laser was z 30 mJ per pulse, the repetition rate being 3 pps. The frequency was chosen so as to allow the mixture in the cell to change completely between two laser flashes. Chlorine atoms were generated by photochemical decomposition of phosgene [ 16 ] : COC12+hv(248

nm)+2Cl$CO.

(4)

In the presence of silane, the fast reaction (3) took place. This scheme is convenient since, firstly, the photolysis of phosgene under 248 nm light irradiation yields only chlorine atoms and stable CO molecules [ 16 1. Secondly, under appropriate conditions one can derive the concentrations of Cl atoms resulting from the photodissociation of COC& from the risetime of SiHs accumulation kinetics. Thus, the problem of determining the absolute concentrations of SiH3 and Cl is resolved. We used the polarization El B at which the electric vector of COz radiation is perpendicular to the magnetic field. The 13C02 laser allowed detection of

both Cl atoms by llP(36) line (882.287 cm-‘) [ 17,18 ] and SiH, radicals by the laser lines 11 P ( 16) (900.369 cm-‘) and 1 lR(20) (928.657 cm-‘). The former of the two lines has already been used in previous investigations [ 12,13 1, the latter was discovered in this work. In both cases the LMR spectrum of SiH3 radicals was one broad line at weak magnetic fields ( < 0.5 kG). However, on our spectrometer the signal was 2-3 times more intense in the latter case. The signal kinetics in both cases were the same. It should be noted that the LMR spectra of SiH3 were studied only by Krasnoperov et al. [ 141. A list was published of CO,-laser lines showing the LMR spectra of SiH3. Unfortunately, no information on ‘3C02laser lines was given. We cannot argue that the lines of the SiH3 LMR spectra we used are the most intense ones. The silane we used indicated a purity of 99.99O/o. Phosgene was prepared using the reaction of oleum with tetrachloromethane, as described in ref. [ 19 1, then purified in a fractionating column to a purity of x 99%. The purity of CO, HCl, and NF, was approximately the same. The purity of argon was 99.99%. The procedure for data processing was the following. Typical kinetics of LMR signals of SiH3 radicals are presented in fig. 1. The experimental kinetics were treated by a least-squares fitting to the expression s(t)=S[exp(-f/rd)-exp(-t/r,)].

(5)

Here 7r corresponds to the appearance of SiH3 in reaction (3) and 7, corresponds to the decay of these radicals via chemical reactions and due to diffusion from the COZ laser beam. For reaction ( 1)) the signal decay kinetics were nonexponential since the reaction is second-order. However, the approximation of the kinetics by expression (5) for a certain time range is also good in this case.

3. Results In our experiments it was necessary to know the value of the absolute concentration of Cl atoms arising under the photolysing pulse. Our technique implies that the accuracy of this value and, hence, the errors of k, and k2 measurements depend on the accuracy of the k3 value.

A. K Baklanov,A.I. Chichinin/Chemical Physics181(1994) 1191128

Fig. 1. LMR signal kinetics of SiH3. Solid curves are least-squares fitted exponential curves. Upper curve: experimental conditions: Ph= 17.3 Torr; [SiH&= 1.1 x lOI’, [SiH&> Iah,; [COCl~]=6~10’~, [Cl],,=6~1O’~cm-~ (theconcentrationof chlorine atoms was found by formula (7) ). The exponent was constructed from the initial section of the decay kinetics. Lower curve: experimental conditions: [SiH&< [Cl],,; Pk= 17.3 Torq [SiH,]0=2.0X10L3, [COClI]=2.2X1016, [Cl]0=2X10’4cm-3. The exponent was constructed from the whole kinetics.

3.1. Measurement of the rate constant of the reaction CI+SiH,+SiH,+HCI Fig. 2 shows the reciprocal risetime (l/r,) of the SiHJ LMR-signal as a function of silane pressure at [ SiH410> [Cl],,. Table 1 presents the value of 5 derived from the slope of the straight line of fig. 2 and data published by other authors. Our value is seen to be in good agreement with those reported by Chesnokov and Panfilov [ 9 1, and Krasnoperov et al. [ 10 1. Niki et al. [ 111 have discussed a possible reason for the underestimated value of ka given by Shlyer et al. [ 8 1. We failed to account for the inconsistence of our value for ka with that measured by Niki et al. [ 111. Below we use the ka value obtained in this work. It is noteworthy, that no channels other than (3) have been revealed for the reaction Cl + SiH4 [ 111. As to the role of the spin-orbit-excited atom Cl(2P1,2) in all processes investigated in this work, we should mention the following. Firstly, the yield of excited chlorine atoms on the photolysis of COC12at 248 nm is below 5Oh [20]. Secondly, the rate constants of C1(2P1,2) quenching by COC12is 3x lo-” cm’/s [20]. Thus, at the phosgene concentrations used ( l-l 0 Torr) the decay of excited chlorine at-

121

Fig. 2. Reciprocal risetime of SiH3 signal versus silaue concentration. [COC12]=1.7x10’6, [C1]0=1.9X10’3 cmm3, P&=17.3 Torr. Table 1 Rate constants (k3) for the reaction Cl+ SiH,-t Measured value 5 (cm’/s)

References

(9.2f2.0)~10-” (1.7*0.5)X10-‘~ (2.3*0.5)x lo-r0 (4.3f0.5)X10-10 (2.5+0.5)x10-r0

(71

181 [91 [lOI this work

oms occurs much faster than do possible chemical reactions involving these atoms. This suggests that the role of the excited chlorine atom in the processes under investigation is negligible. 3.2. Determination of absolute concentrations of Cl atoms If the initial concentration of chlorine atoms [Cl] o is much higher than the initial concentration of silane [ SiH,] o, the risetime of the SiH3 signal can be expressed as follows: l/~r=k3[Cllo,

(6)

where [Cl], is the initial concentration of chlorine atoms. It can be calculated in the following way:

A.V; Baklanov, A.I. Chichinin /Chemical Physics 181(1994) 119-128

122

[Cllo=~,

(7)

where A=/3[COC12]

.

exp( -la[COC12])

(8)

Here /3 is the attenuation coefficient of the photolysing pulse energy (0 < /3< 1)) I= 37 cm is the distance from the inlet window to the middle of the detection zone, G= 9 x 1O-” [ 16 ] is the absorption cross-section of COC& at 248 nm, (Yis the chlorine atom yield; a[COCl,] = [Cl],atP= 1 and la[COC12] K 1. As is evident from formulae (6) and (7)) the unknown value (Ycan be determined from the dependence of 1/r, on kgl. This dependence is presented in fig. 3 from which one can find that cx= 0.11 O/6.Some experiments were carried out at a higher fluence of photolysing radiation which corresponded to CY = 1.O%.

constant. The initial concentration of chlorine atoms [ Cl ] o was varied by changing the pulse energy of the excimer laser. By this means the initial concentration of silyl radicals [ SiH,] o arising in the fast reaction (3) was varied. As illustrated in fig. 1, the decay kinetics of SiH3 under these conditions was not exponential. This may be accounted for assuming that process ( 1) makes the major contribution to these kinetics. On the assumption that all the processes resulting in reduced SiH3 concentration (including diffusion), except for ( 1)) are described by first-order reaction kinetics with the total rate constant 1/r, the decay kinetics of silyl radical is described by the formula

rw(---t/z) [SiH3]=[SiH3]o y= (2k,r[SiH,],)-’

(l+Y)[l-exp(-t/r)/(l+y)] .

’ (9)

3.3. Measurement of the rate constant of the reaction

SiH, + SiHp The recombination rate constant of silyl radicals was measured at [ SiH.,lo> [Cl],. In these experiments the silane and phosgene concentrations were

According to this formula, the decrease in SiH3 concentration should be nonexponential, which is what we observed in the experiment (fig. 1). In order to simplify the procedure for k, derivation from experimental data, we used the initial section of the decay kinetics of the SiH3 signal, where formula (9) is well approximated by the expression [SiH3] z [SiH310 exp( -t/r,,)

30-

l/r~=2k1[SiH3]o+l/r.

L

0

20 :

0A

Ooj I&A

(i-‘1

Fig. 3. Reciprocal risetime of SiHJ signal versus kd, where A is determinedfromtherelationship (8). [CoCl,]=2.0~ 10’7cm-3, P_,+ 17.3 Torr, the attenuation coefficient of radiation /3 varied from 1 to 30. The slope of the straight line corresponds to cr=l.lxlO-3.

,

(10) (11)

The addend in expression ( 11) includes contributions of all processes, except for radical recombination, which lead to a decrease in SiH3 concentration in the volume sounded by the COz laser (reactions with other components of the mixture and diffusion from the CO* laser beam). Expression ( 10) is a good approximation of formula (9 ) over a limited time range corresponding to 30-40% signal decay. The time domain was chosen in the following manner. Its initial point corresponded to the time t= 3-5 T=,,at which the addend in eq. (5) could be neglected. Since the ratio Q/Z, was 15-50, by this time the concentration [ SiH3] differed from [ SiH310 only by lo-20%. The final point of the time interval corresponded to a further 20% decrease in signal intensity. Thus, in the end of the time interval, the SiH3 concentration was less than

A. K Baklanov,A.I. Chichinin/ChemicalPhysics 181(1994) II%128

[ SiH3]c by no more than 30-40%. Within this time range experimental kinetics were fitted to eq. ( 10) and l/7* was derived. Since under our conditions each chlorine atom yields one silyl radical, we assumed that [ SiH310= [Cl],, and [Cl I0 was calculated by formulae (7) and (8). The value of a was determined as described above. The dependence of 1/TV on [ SiHS],-,is presented in fig. 4. From the slope of the straight line, which may be drawn through these points, using eq. ( 11) we obtained ki = ( 1.7 + 0.5) x lo-” cm’/s. This value corresponds to Pk= 17.3 Torr; at P,=6 Torr ki = (1.45 50.4) x 10-i’ cm’/s. The value of l/7 equal to the cut on the ordinate was also derived from the experimental dependence. The substitution of the ki and l/7 values into formula (9 ) allowed a quite accurate description of SiH3 signal decay kinetics over the whole time interval. This validates our procedure for the derivation of k, . In principle, the slope of the line in fig. 4 could be affected by interaction between SiH3 and CO appearing in the photolysis of phosgene, HCl arising simultaneously with SiH3, or silylene SiHz and silylsilylene SiH3SiH which are formed during the recombination of silyl radicals [ 21. Special measurements (see below) have shown that the reactions of SiH3 with CO

and HCl can be neglected in our conditions. Silylene and silylsilylene are also unable to affect substantially the decay kinetics of SiH3 since at [ SiH4] ,,> [ SiH310 one should expect that the silylenes would be embedded in silane with a rate constant close to the frequency of gas-kinetic collisions as it happens with SiHz [ 2 11. The final value of k, obtained by averaging its value argon pressures is measured at different k1=(1.6f0.5)x10-“cm3/s. 3.4. Measurement of the rate constant of the reaction SiH, + CL The rate constant of this reaction was measured at excess chlorine atoms [Cl],> [ SiH4],,. The initial concentration of chlorine atoms [Cl ] 0 was varied by changing the excimer laser pulse energy. The initial concentration [ SiH4] ,, and hence [ SiH3 ] o was kept constant. Fig. 1 shows typical kinetics obtained under these conditions. The signal risetime determined by reaction (3) is seen to be essentially shorter than the decay time determined by reaction (2). Fig. 5 presents the dependence of the reciprocal time of SiH3 signal decay ( l/7* ) on the concentration of chlorine atoms which was assumed constant and equal to the

25

20

SiHa + SiH3 -

!

k

k=(1.6f0.5)il~"cm3/~

Cl+SiHS-L 5

~O.~)XI~'~~/S

0

2

4

[c& Fig. 4. Reciprocal risetime of SiHs signal versus SiHs concentration. P,=17.3 Torq [SiH,],= 1.1 x 1015, [COClz] =6.0x 10” cm-3, (Y= 1.1 x lo-‘, j?= l-30. The k, value was obtained by averaging over all experimental series.

123

6

/id’ cm3

risetime of SiHs signal versus Fig. 5. Reciprocal [Cl].= [Cllo- [SiH& Pti= 17.3 Tom [SiH410=2.0x 1013, [COCII] = (l-2.6) x 10” cme3, (Y= 1 x lo-‘, fi= 1-14. The k2 value was obtained by averaging over all experimental scrics.

124

A. V. Baklanov,A.I. Chichinin/Chemical Physics181(1994) 119-128

difference [Cl],= [Cl],[SiH410, where [Cl], was calculated by formulae (7) and (8). By subtraction of [ SiH4] ,-,we took into account the fact that a number of chlorine atoms were consumed during SiHs formation in the fast reaction (3 ). The rate constant ofprocess(2)k2=(3.2+0.8)~10-11cm3/swasderived from the slope of the straight line obtained. This value of kz was obtained at PAr= 17.3 Torr. At P=6 Torr we obtained k,= (3.2+ 1.0) x 10-l’ cm3/s. As will be shown below, the influence of the reaction SiH3 + CO-+ on the slope of the line in fig. 5 may be neglected.

feet of reaction ( 5 ) may be disregarded. Using the values from figs. 4 and 5, one can give an upper estimate for the rate constant of the process SiH3 + COC&-+: k6<5x 10-15cm-3/s. In addition, in this work we attempted to measure the rate constant of the process SiH3 +NF3-+, assuming that NF, can be used as an acceptor of SiH3. The addition of NF, up to the concentration 2 X 1016 cmm3 caused no change in SiH3 signal decay time accurate to A( 1/rd) = 10’ s-‘. Thus, the upper estimate fork, is k7< 5x lo-l5 cm3/s.

3.5. Measurement of the rate constants of the reactions SiH, + CO(4), HCl(5’), COC1,(6), NF,(7)

4. Discussion 4.1. SiH,-?-SiH,-+

Since the photolysis of phosgene involves the formation of CO, the reaction between SiHs and CO could contribute to the slopes of the lines given in figs. 4 and 5. To take this contribution into account, we tried to measure the rate constant of this process. It turned out that the increase in CO concentration right up to [CO] = 1.4x 10” cme3 had no effect on the reciprocal decay time of the SiH3 signal within thelimitsofA(1/r~)=100s-‘.Hence,k,~7X10-’6 cm3/s. As the CO concentration in the experiments presented in figs. 4 and 5 was below 4 x 10’ 4 cm- 3, the contribution of this process could be ignored. Because reaction (3) yields not only SiH3, but also HCl, we tried to measure the rate constant (k,) of the reaction between SiH3 and HCl. The HCl concentration in the reactive mixture was increased up to 10” cme3. The reaction SiH,+HCl+ involves the formation of a hydrogen atom which, in principle, can react with COClz to yield HCl, CO, and Cl. To minimize the effect of the chlorine atoms on the SiH3 decay kinetics, we chose the initial conditions so that [ SiH3 ] ,,CK[Cl ] ,,. The SiH3 lifetime ( rd) reduced with increasing [HCl].Thek,value ((1.820.5)~10-‘~ cm3/s) was derived from the plot of 1/rd as a function of HCl concentration. Inasmuch as the purity of HCl we used was only 99%, the influence of more reactive admixtures, which can be present in HCl, on the SiH3 lifetime must not be ruled out. Thus, it would be more correct to consider the rate constant obtained to be an upper estimate of kS. Since the experimental data presented in figs. 4 and 5 correspond to the conditions when [HCl] < 3 x lOI cme3, the ef-

Table 2 lists k, values obtained in this work (1.6&0.5)x lo-” cm3/s and reported in the literature. Our value is smaller than the results of other measurements [ 4,6,7] by a factor of 5-10. Below we will consider the possible reasons of these descrepancies. The authors of ref. [ 41 studied the decay kinetics of silyl radicals generated by impulsing discharge in a SiH4/H2 mixture. The SiH3 decay kinetics observed were assumed to be determined by recombination of silyl radicals. The absolute concentration of SiH, was found using the Einstein coefficient calculated ab initio for the IR absorption band detected. The overestimated value of kl [ 41 can be accounted for by the fact that reactions with other reactive species arising in the discharge plasma contribute to the SiH3 decay kinetics. In addition, it is impossible to estimate the accuracy of the Einstein coefficient used. In refs. [ 6,7] reaction (3 ) was used for SiH3 generation, with atomic chlorine produced by photolysis of CC14. For absolute calibration of SiH, Lob and Table 2 Rate constants (k, ) for the reaction SiH, + SiH,-+ Measured value k, (cm3/s)

References

(1.5+O.6)x1O-‘o (7.9?2.9)xlO-” (1.2+0.4)x10-lo (1.6iO.S)xlO-”

141 [61 171 this work

A. K BakIanov, A.I. Chichinin /Chemical Physics 181(1994) 119-128

Jasinski [ 6 ] measured diode laser radiation absorption at the R ( 3 ) transition of HCl molecules, arising in reaction (3) simultaneously with SiHS radicals. The authors compared this absorption with the absorption of the known HCl quantity in a bulb experiment and so they estimated the initial concentration of silyl radicals. This procedure is correct if the populations of the probed levels in the kinetic experiment correspond to room temperature, as in the stationary one. This is very important, because it is known [ 221 that a substantial fraction (0.41+ 0.09) of HCl molecules, arising in reaction (3), are vibrationally excited. The authors assumed that in the kinetic experiment the vibrational relaxation was completed to the moment of HCl absorption measurement. We think that this assumption needs to be discussed. One of the HCl absorption kinetics, used for calibration, is shown in the inset of fig. 3 [ 6 1. It is seen that the rising part of this kinetics consists of two components: a fast and a slow one. The authors connect the fast component to absorption of non-equilibrium HCl, arising in reaction ( 3 ) , and the slow component to relaxation of initial HCl distribution to the equilibrium one. It is seen that the amplitude of the slow component is not greater than that of the fast one. It is possible to estimate what the ratio of these amplitudes might be in the case of complete HCl relaxation. On the time scale of the experiment being discussed, it is possible to assume the rotational distribution of HCl to be the equilibrium one. Taking into account the yield of vibrationally excited HCl arising in reaction ( 3 ) [ 22 1, we have estimated the ratio of amplitudes in the case of complete HCl relaxation. For absorption at R ( 3 ) transition, used in ref. [ 61, this ratio must be equal to 4.2Zf:q. For the kinetics being discussed (inset of fig. 3 [ 6 ] ) , the ratio of amplitudes is less than this value. If the slow component is related with relaxation, then this circumstance indicates that the relaxation of HCl is not completed at the maximum of the absorption kinetics in this experiment. If this is so, the authors of ref. [ 6 ] underestimated the quantity of HCl arising in reaction (3), and so underestimated the initial concentration of SiH3, and so overestimated the rate constant of silyl radicals recombination. For verification of their calibration technique Loh and Jasinski [ 6 ] estimated the yield of atomic chlorine arising from Ccl, photolysis under their conditions. The re-

125

sults of the estimations, based on literature data, and on their measurement of HCl concentration, coincide. This circumstance is important, but the question pointed above is still open. Koshi et al. [7] also used reaction (3) for SiHS generation. Atomic chlorine was also produced by photolysis of CCL, at 193 nm. For calibration of the initial SiH3 concentration the authors measured the concentration of HCl arising in reaction (3) simultaneously with that of SiH3 radical, mass-spectrometrically. On the basis of these measurements the authors concluded that the yield of Cl atoms under photolysis of CC& was 0.37Ohwhen the fluence of laser radiation was 12 mJ cm-‘. Under the conditions used by Koshi et al. the reaction of Cl atom with silane molecule had to be the main process of Cl atom consumption. So every Cl atom had to produce one HCl molecule. So we can estimate the cross-section of Cl atom formation by CC& photolysis at 193 nm. The result is 3.2 x lo-l9 cm’. At the same time it is possible to calculate this cross-section on the basis of recommended data [ 23 ] on CCL, absorption cross-section and quantum yield of Cl atom at 193 nm. This estimate gives 1.3~ lo- l8 cm’, which is higher by a factor of 4 than the estimate made on the basis of the results of ref. [ 7 1. So we can suppose that in ref. [ 7 ] the quantity of HCl and so the initial concentration of SiHs was underestimated and as a result the silyl recombination rate constant was overestimated. The use of Ccl4 photolysis for Cl atom generation could be also the reason of overestimation of k, in refs. [ 6,7]. It is known [ 231 that CC& and Ccl2 species arise in photolysis of Ccl, (2~ 193 nm) simultaneously with Cl atoms. So the reactions of SiH3 radical with these species could contribute to silyl radical decay kinetics, experimentally observed under the conditions of refs. [ 6,7]. Loh and Jasinski [ 61 excluded the contribution of such processes. They observed that approximately 90% of the silyl produced transformed to disilane, being the final product of silyl recombination reaction. It should be noted that this conclusion is based on the results of the calibration technique used by Loh and Jasinski for silyl concentration measurement [ 61. We argumented above that this technique could underestimate the concentration of silyl radicals. If this was so, the possibility of reactions with Ccl2 and CC& could not be excluded. Loh and Jasinski [ 61 and Koshi et al. [ 71

126

A.V. Baklanov, A.I. Chichinin /Chemical Physics 181(1994)

observed the formation of some quantities of substance, proposed to be SiH&l. Possibly, this substance arised in the above mentioned reactions. Photolysis of phosgene (A=248 nm) gives rise to only atomic chlorine and stable carbon oxide [ 16 1. So we believe that our scheme of Cl atom generation eliminates the interference from reactions of this type. The rate constant k, measured in this work is independent of the total pressure (in our experiments 6 and 17.3 Torr). This is consistent with the existing knowledge of the mechanism of reaction ( 1) [ 2 ] : SiH3 + SiH3 -+ Si, Hz + SiH3 + SiH3,

(12a)

-+ SiHz + SiH4,

t 12b)

-+ SiH3 SiH + Hz,

(12c)

only the k3 value was used (see above). Hence, the ratio k3/k2 remains unchanged with changed k3. Thus, the radical SiH3 is noticeably more “inert” towards the chlorine atom than the silane molecule. This is rather surprising. The question arises how this result can be accounted for. The mechanism of reaction (3) isknown [ll]: SiH4 + Cl *

It is believed that the recombination results in the formation of a vibrationally excited disilane molecule which can then decompose via several channels. The reactions yielding silylenes ( 12b ) , ( 12~) (not the reverse dissociation ( 12a) ) dominate. Hence, the buffer gas pressure will affect only the ratio of the products and has no effect on the recombination rate of silyl radicals. Kamisako et al. [ 241 have found experimentally that at argon pressures close to those used in this work, channel ( 12d) may be considered negligible compared to channels ( 12b) and ( 12~). Hence, under our condition, variations in buffer gas pressure should have no effect on the ratio of recombination products, as well. 4.2. SiH,+ Cl-+ As to the reactions of the silyl radical with atoms, in the literature, to our knowledge, there are available only the rate constants for the reactions with hydrogen atom k(SiH,+H)= (2+ 1) x lo-” cm3/s [6] and with bromide atom k(SiH3+Br)=4x lo-” cm3/s [25]. It is of interest to compare the rate constants of processes (2) and ( 3). The ratio of the measured values of k2 and k3 is k3/k2= 7.6 rf:2.4. The error incorporates only the error included in the kz value. The fact is that in the calculation of the absolute concentration of chlorine atoms required for determining k2,

[26,27]

.

For reaction ( 2 ) , two mechanisms are possible: the direct abstraction of hydrogen atom, SiH3 + Cl 5

(12d)

SiH3 + HCl,

Mg = - 12.1 kcal/mol

Mg=-31.1 fM SilHs .

119-128

SiH2 + HCl, kcal/mol

[26,27]

or the formation of vibrationally molecule SiH,Cl*,

,

@a)

excited silylchloride

SiH3 + Cl -!%L SiH3 Cl*, Mg = - 107.6 kcal/mol which further decomposes

[ 26,271 , via H2 abstraction

(2b) [ 28 ] :

SiH3 Cl*-+ SiHCl + Hz, AHE =47.2 kcal/mol

[ 26,271 .

Consider the factors which can in principle account for the unexpected value kz/k3= 7.6 -t 2.4. It is simpler to compare the rate constants of reactions (2a) and ( 3)) which proceed via the same mechanism. Since k,= k,,+ kzb, k3/kzaa 7.6. The first and the most natural explanation of the latter relation may be the fact that the activation barrier for reaction (2a) would be higher than that for reaction ( 3 ). This explanation however seems to be unlikely. Note that the k3 value is close to the gas kinetic limit for the rate constant of the bimolecular reaction and, hence, the energy barrier for Cl + SiH4-+SiH3 + HCl reaction is absent. We assume that the energy barrier is also absent in the reaction Cl + SiH3-SiH2 + HCl because these reactions proceed via the same mechanism (the abstraction of a hydrogen atom from the silicium atom by a chlorine atom, with the unpaired electron of SiH3 taking no part in the chemical reaction) and the reaction of abstraction from a radical is more exothermic. We pro-

A. K Baklanov,AI. Chichinin/ChemicalPhysics IS1 (1994) 119-128

teed from the idea that for reactions of the same type increased reaction exothermicity leads to a decreased activation barrier. This idea is validated by the fact that reactions of the same type show a correlation between the value of energy barrier and the thermal effect of the reaction. Numerous examples of such correlations have been discussed by Kondratyev et al. [29]. Reaction (2a) is more exothermic than reaction (3). Hence, if the reaction barrier is absent in reaction ( 3 ) , it should be absent in reaction (2a). In principle, the ratio k,/k,, could be affected by the “spin factor”. In reaction (3), one of the reactants (the chlorine atom) and one of the products (SiHs) show a nonzero electron spin equal to l/2, i.e. the spins of reactants and products are equal. The assumed restriction of electron spin change during the reaction will not impose restrictions on the rate constant of this reaction k3. In reaction (2) both reactants have electron spin l/2 and according to the moment addition rule can yield products with net spins 0 and 1, i.e. in the singlet and triplet states. Let the reaction yielding triplet-state products have a significant energy barrier. Then, in the adiabatic approximation, only one of four collisions of reactants, namely that corresponding to the singlet state, could result in product formation. In this case, a four-fold decrease in kza relative to k3 could be accounted for by the effect of the “spin factor”. However, for silylene the triplet state lies quite low AE( 3B1-‘A,) x 14 kcal/mol [ 301, hence the exothermicity of reaction (2a), yielding SiHz in the triplet state, is AH: = - 17 kcal/mol. Proceeding from the foregoing, we believe that such exothermicity suggests no energy barrier for this channel as well. Hence, one has no reason to believe that the “spin factor” is responsible for the decrease in kza relative to k3. The only undeniable evidence for the relation k3 z+kza is that the number of H atoms in the SiH4 molecule is larger than that in SiH3. However, the ratio is only 4/3. Thus, basing on the factors we considered above, even disregarding reaction (2b), it is difficult to account for the experimental ratio k3/k2= 7.6. In our mind, the fact that the probability of hydrogen atom abstraction from SiH3 is less than the probability of hydrogen atom abstraction from SiH4 may be associated with the following. The potential of in-

0.1

127

0

bl)

b2)

Fig. 6. Schematic of the approach trajectory of the chlorine atom and silyl radical. (a) The orientation effect resulting in a decreased probability of hydrogen atom abstraction. (bl, b2) Inversion of silyl radical in approaching the chlorine atom.

teraction of SiH3 with a Cl atom is characterized by a deep well responsible for the Si-Cl bonding in the SiH3Cl molecule. The maximum depth of the well corresponds to the orientation of the third-order symmetry axis of SiH3 towards the Cl atom. When the Cl atom deflects from this axis, the energy of atom-radical interaction decreases. As a result, in collision the symmetry axis of silyl radical will be oriented to the chlorine atom. The probability of the approach of the reacting species in the configuration favourable for hydrogen atom abstraction will thus decrease (see fig. 6a). The same effect will be observed in inverting the pyramidal radical SiHS when the chlorine atom approaches from the pyramide base, as shown in fig. 6b. The free radical SiH3 has already an inversion barrier as low as 1868 cm-’ [ 3 11, or %5.3 kcal/mol. The presence of an attractive interaction with the chlorine atom will decrease this barrier.

Acknowledgement The authors express their thanks to L.N. Krasnoperov for discussions which stimulated this work and to V.P. Strunin for the mass-spectrometric analysis of the substances used and for the supply with silane.

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