15890
J. Phys. Chem. 1995,99, 15890-15898
Selected Ion Flow Drift Tube Studies of the Reactions of Sf(4S) with C h , CZHZ,CZ&, and C3H8’ P. ZakouAl,”$ J. Glosik,*J$*V. Skalskf? and W. Lindinger’ Department of Electronics and Vacuum Physics, Mathematics and Physics Faculty, Charles University, V HoleioviEkhch 2, Prague 8, Czech Republic, and Institut &r Ionenphysik, Leopold Franzens Universitat, Technikerstrasse 25, A-6020 Innsbruck, Austria Received: March 6, 1995; In Final Form: June 27, 1995@ The reaction rate coefficients, k, and the product distributions for the reactions of ground-state S+(4S)with small hydrocarbon molecules CH4, C2H2, C2H4, and C3H8 have been measured as a function of reactant ion/ reactant neutral average center-of-mass kinetic energy, KEcM, in a selected ion flow drift tube (SIFDT) apparatus. The measurements have been performed at a temperature 298 f 2 K with helium as a buffer gas. The values k of the studied reactions have pronounced negative energy dependencies. For some reactions k decreases by about one order of magnitude when K E h is increasing from near thermal values to a few electronvolts. This is interpreted in terms of a simple model assuming the reactions to proceed via the formation of long-lived complexes. The lifetimes of these intermediate complexes against decomposition back to reactants and forward to products (or corresponding unimolecular rate coefficients k-I and k2) govern the overall rate of the reactions. It is found from the kinetics of the studied reactions that a power law of the form k-llk2 = const(KECM)m,where m is a constant parameter, can be used to describe the energy dependencies of the overall reaction rate coefficients of studied reactions.
I. Introduction Studies of ion-molecule reactions of the sulfur ion S+ are mostly motivated by the importance of these reactions in interstellar cloud^.^-^ However, not only the reactions of the S+ ion are important; reactions of sulfur-bearing ions, especially SH+?are very interesting as well. Research on reactions of the S+ ion has been carried out in ICR flowing afterglow^,^^'^ flow drift tube experiments,” and selected ion flow tube (SET) experiment^.^.'^.'^ A guided ion beam apparatus has recently been used by Armentrout and co-workers to study the kinetic energy dependencies of the reactions of atomic sulfur ions with molecular hydrogen and its isotopic variant^.'^,'^ In a flow drift tube study carried out by Dotan et a1.,l1the mobility of S+ in He has also been measured. Recently the structure, thermochemistry, and reactivity of some sulfurbearing ions with neutral molecules have been the subject of theoretical studies (see, e.g., refs 16 and 17). The reaction of S+ with acetylene has been discussed in a theoretical study of Barrientos and Largo.I6 In recent selected ion flow drift tube (SIFDT) studies in our laboratory we have investigated a number of fast ion-molecule reactions, the reaction rate coefficients of which decrease strongly when &M (the reactant iodreactant molecule average center-of-mass collision energy) increases from thermal values up to a few electron volt^.'^-^^ The presently reported reactions of S + also have similar dependencies on KEcM. We show that such reactions appear to proceed via the formation of longlived complexes, the rate coefficients for the decomposition of which (back to reactants and forward to products) depend on K&M in a power-law fashion. The formation of the ion-molecule collision complex is limited by the capture rate coefficient, k.In all the following, the capture rate coefficients are always indicated by k, whether they represent Langevin limiting values, k ~or, values calculated +
4 @
Charles University. Leopold Franzens Universitat. Abstract published in Advance ACS Abstmcts, October 1, 1995.
according to the A D O - t h e ~ r y , ~the ~ , *more ~ recent models of Su and C h e ~ n a v i c h an , ~ ~adiabatic capture theory (see review by Claryz8), or statistical capture t h e o r i e ~ . ~In~drift , ~ ~ tubes, K&M can be increased to values up to a few electronvolts, where theories which predict thermal energy capture rate coefficients are no longer applicable. Reaction rate coefficients measured in drift tubes at elevated energies are often observed to exceed the kc calculated for 300 K. This phenomenon has been discussed theoretically in a recent paper of S . C. Smith et al.,31where the capture rate coefficient is predicted to increase as P5(Le., as at suprathermal energies, e.g., at those acquired in drift tube experiments. To avoid complicated calculations, we introduced (in the recent papers refs 20 and 23) an empirical correction function /3 (dependent only on KEcM)derived from experimental data for many ion-molecule reactions. This function is equal to unity at near thermal energy and increases with increasing KECMand has a value of approximately -2 at K&M= 2 eV. We assume that the actual capture rate coefficient k = P ~ owhere , ko is the capture rate coefficient for a particular reaction at low KECM(limiting value for KECMapproaching thermal energy). For details see refs 20 and 23, where values of the function P are also given. In order to describe the kinetics of the ion-molecule reactions investigated here, we consider the following reaction scheme (Here we partly follow the explanations given in our recent work22 in order to obtain formulas used for analysis of the present experimental data.):
A+
+ B A (AB+)* %c++ D k-
I
(1)
where kl is the bimolecular rate coefficient for the formation of the excited intermediate complex (AB+)*and k-1 and k2 are unimolecular rate coefficients for dissociation of the excited intermediate complexes into the reactant and the product, respectively. We assume that the excited intermediate complex is formed at the capture rate, k, Le., kl = kc = /3kc0. A similar assumption concerning relation of kl and k is very common (e.g., in the discussion of vibrational quenching by F e r g ~ s o n ~ ~ ) .
0022-36541992099-15890$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 43, 1995 15891
Reactions of S+(4S) with CH4, C2H2, C2H4, and C3H8 By applying simple reaction kinetics and applying the steadystate approximation to (AB+)*, the following expression is obtained for the overall rate coefficient k of reaction 1:
It is evident from this expression that the ratio (k-llk2) and its dependence on K&M governs the energy dependence of the overall reaction rate coefficient k, as long as the assumption concerning the complex formation is valid. On the basis of statistical arguments,33in analogy with the ideas widely discussed for three-body association reaction^^,-^^ and on the basis of our studies of the kinetics of many ionmolecule reactions which proceed via formation of long-lived intermediate c o m p l e x e ~ , ~ we+ assume ~ ~ . ~ ~here that the ratio (k-l/ k2) will vary with KECMin a power-law fashion: (k-llk2) (K&M)".Then expression 2 can be rewritten as
-
(3)
where m and are constant parameters (K&M~ is the value of KE.cMat which Wg has decreased to l/2 of its maximum value and hence for which k-1 = k2). The parameters ko, KECMI, and m can be obtained from the fit of the experimental data (see, e.g., refs 19 and 20). If KECM>> =MI, then eq 3 can be written in the form A
(I(EcM)I -
kco KECMl
B(KEcM)-"
-
(4)
at near thermal energies, where # % I 1, k (K&M)-". This form of the dependence of a rate coefficient on K&M (or temperature) has been observed many times for three-body association reactions (e.g., refs 39-43), where the value of the parameter m was coupled to the number of rotational degrees of freedom in the reactants and products. We will use the parameter m to characterize the energy dependencies of the studied reactions. In order to characterize the energy dependence of a particular reaction process, it is reasonable to remove the energy dependence of the collisional rate coefficient from the energy dependence of the overall reaction rate coefficient. It is evident from eq 3 that the function l8 = kJa is characterizing the energy dependence of the process following the formation of the collisional complex. In the following we will call function l8 the "/3-corrected" reaction rate coefficient. "/?-corrected" values of reaction rate coefficients will be indicated by a superscript
"P. II. Experimental Section
of the drift tube, the average collision energy of the ions with the buffer gas atoms was kept below 0.1 eV. The reactant and product ions were monitored downstream with a second quadrupole mass filter as a function of the flow rate of the added neutral reactant. Established methods of analysis were used to derive reaction rate constants and product distributior~s.~~~~~ In order to determine the internal state of the S+ ions in the reaction region of the drift tube in the present experiment, H2 was added as a reactant gas. The reaction of the ground-state S+(,S) with HZis endoergic and very slow whereas the reactions of the excited states S+* (namely the ,P and 2D states) with H2 are fast (5.0 x 1O-Io cm3 s-I), yielding the product SH+.I3,l4 Because we observed the production of SH+ when H2 was added as a reactant, we concluded that part of the injected S+ ions is in an excited state. In order to quench excited ions injected from the ion source, N2 was added to the He buffer gas. A flow of N:! of -1% of the He flow was high enough to quench all excited S+ ions before they enter the reaction region. The degree of the quenching was detected by the reaction with H2. K&Mwas derived by means of the Wannier e x p r e ~ s i o n . ~ ~ The mobility of S+ ions in He was measured in the present experiment, and it agrees well with the mobility data obtained by Dotan et al." The accuracy of the measured rate coefficients is f30%, as is usual for swarm experiments of the SIFDT type.
III. Results and Discussion The reaction rate coefficients and product ion distributions were determined for the reactions of S+(4S) ions with the organic molecules CHq, C2H2, C2H4, and C3H8 over the energy range of K&Mfrom near thermal to about 2 eV. All these reactions are fast (with rate coefficients of a few times to 1 0 - ~ cm3 s-l) at near thermal energies, and their rate coefficients have a pronounced negative energy dependence over the whole or part of the energy range covered. A. Reaction of S+(4S) with C&. The reaction of S+ with CHq has been the subject of many studies, partly because this reaction may be a source of H2CS observed in interstellar c l o ~ d s . ~In, ~the, ~early ~ ICR experiments6 the product SCH3+ was observed and the measured reaction rate coefficient was 1.4 x 1O-Io cm3 s-l. In the SIFT study of Tichy et all3thermal reaction rate coefficients of the ground state S+(4S) and excited states S+(2D) and S+(2P) with CHq have been determined. cm3 s-l for and 1.3 x Obtained values are 3.5 x ions in their ground state and excited state (2D or 2P), respectively. Only the product SCH3+ has been reported for the reaction of ground-state ions. In the SIFT study of Smith et a1.2-'2the reaction rate coefficient 4 x cm3 s-l and production of SCH3+ with a minor contribution of SCH+ have been reported. In the present SIFDT study reaction rate coefficients and product distributions have been measured for ground-state ions S+(,S) in the energy range from near thermal up to 2 eV. Observed reaction channels are
S+
+ CH, - SCH3+ + H
(5a)
+ H,
(5b)
Measurements of the energy dependencies of the reaction rate coefficients and product ion distributions were carried out using the Innsbruck selected ion flow drift tube (SIFDT) of conventional design, which has been described in detail e l s e ~ h e r e . ~ ? ~ ~ The reactant ions S+ were produced from H2S in a high-pressure electron-impact ion source. The ions were subsequently mass selected in a quadrupole mass filter and injected into the flow tube via a venturi-type inlet. In the upstream part of the drift tube, prior to the reaction region, low E/N was used ( E is the electric field strength in this part of the drift tube and N is the buffer/canier gas number density). In this "thermalization" part Data obtained for the reaction
-
SCH,' SCHC
SCH+
+ H, + H
(5c) (54
rate coefficient of the overall
Zakouiil et al.
15892 J. Phys. Chem., Vol. 99, No. 43, 1995
t n
k,,=1.2~lOe
S’+CH4
1
A
5
--
c A
.e
*In
E Y x
-
10.10
A
. m r
+
. .
5.10.”1
m A
9.21 Torr 0.15 Torr 0.40 Torr SIFT ICR
8
8
I
* * . * I
I
0.1
1
I
KE, lev] Figure 1. Variation of the reaction rate coefficient, k, with I U h for cm3 s-’). Different the reaction of St(4S) with CH4 ( k=~1.2 x buffer gas pressures are indicated by different symbols. Previous data from SIFT experiments are 4 x 1O-Io cm3s-l 2 ~ 1 and 2 3.5 x cm3 cm3 s - ’ . ~ SKI l 3 and from the ICR experiment are 1.5 x in.
E
t
2 .- 05 -
B
4L
I n .
a
tj 0.4 3
-
P a
-
0
.
02
-
SCH,’ 0
SCH,‘
A
SCH,‘
V
CH’,
0
SCH’
0,o
1
KE, [evl Figure 2. Product distribution of the reaction of Si(4S) with C&.
reaction (eq 5 ) are presented in Figure 1. Previous ICR and SIFT data are also included in the plot. The previous thermal SIFT data2,12,13 are in very good agreement with present data. The ICR data6 are too low, and we assume that they were taken at higher collisional energy than the one corresponding to 300 K. Note in Figure 1 that the reaction rate coefficient is decreasing with increasing K&M (negative energy dependence) at low K&M. For KECM 0.5-1 eV the reaction rate coefficient has a minimum, and for K&M > 1 eV the reaction rate coefficient is increasing with increasing K&M(positive energy dependence). The product distribution obtained is plotted in Figure 2. SCH3+ is the dominant product (90%) at near thermal energies with slightly decreasing population with increasing KEcM. SCH2+ is a minor product at near thermal energies. Traces of association product SC&+ were also present, and the reaction rate coefficient at low KECMincreases by a few percent with increasing buffer gas pressure from 0.21 to 0.40 Torr. This indicates that long-lived collisional complexes are formed in the collisions of S+ with C&. These three product channels (eqs 5a, 5b, and 5c) are exothermic (see Table 1 for AH” and ASo). At higher production of ions SCH’ and CH3+ has been observed also. Production of SCH+ and CH3+ in reactions 5d and 5e is endoergic by 7.8 and 7.4kcall mol, respectively. For KECM> 0.3 eV the production of CH3+ is increasing very rapidly with increasing WM, and at K&M > 1 eV channel 5e is dominant. We assume that the increase of the overall reaction rate coefficient at KECM> 1 eV (see
-
m~,
Figure 1) is connected with this channel. For K&M> 1 eV also a small increase of the production of SCH+ was observed. The fact that CH3’ ions were not produced at near thermal energies is proof of the absence of excited S+* ions in the reaction region (excited S+* ions react with C&, producing CH3+ 13). Production of dissociated neutral products in channels 5b, 5d, and 5e is too endoergic to be considered in the present SIFDT experiment. The data obtained after “0-correction” of the measured reaction rate coefficients are presented in Figure 3 (see Introduction for details). From the reaction rate coefficient, @, and from the product distribution the reaction rate coefficients for the particular reaction channels were calculated. The “0-corrected” rate coefficients of the two major reaction channels (eqs 5a and 5e) are presented in Figure 4. Reaction channel 5a, dominant with product ion SCH3+ has a negative energy at low EM, dependence of the reaction rate coefficient with clear powerlaw dependence on K&M(see discussion in Introduction and eq 4). Note the good linearity of the log-log plot over one order of magnitude in K&M. We assume that this reaction channel is proceeding via formation of the intermediate longlived collision complex, as discussed in the Introduction. Reaction channel 5e, dominant at high WM, with product ion CH3+ has a positive energy dependence of the reaction rate coefficient, which is typical for endoergic reactions or reactions with an activation energy barrier. We assume that the two dominant channels (5a and 5e) are independent and do not influence each other. Indexes I and I1 will be used to indicate these “low”- and “high”-energy channels, respectively. We assume that the low-energy process proceeds via formation of a long-lived complex; the kinetics of this type of process was already discussed and can be described by eqs 2 and 3. We assume that the “high”-energy process is the endoergic reaction which can proceed only at higher energies and that the dependence of its reaction rate coefficient on KECMis of an Arrhenius type, involving an activation energy, E,. Over the whole energy range covered, kis can be approximated by the expression:
here k1, is a “proportionality constant” and k11~is the “limiting value” of @ at high KEcM. Because K&M>> KECMI, the first term in eq 6 is of the form given by eq 4 and only the value of kIO(K&Ml)m and parameter m can be determined from the experiment. Formula 6 has been used to fit the data (dotted line in Figure 3); obtained parameters of the fit are listed in Table 1. B. The Reaction of S+(4S) with CzHz. The reaction of S+(4S) with C2H2 has two channels (measured with N2 added to the buffer gas): a pressure independent binary channel (with the reaction rate coefficient k ~ with ) products SC2H+ and H and a pressure dependent three-body association channel (with the reaction rate coefficient k ~ with ) product SCzH2+. S+
+ C2H2--.SC2H++ H
(74
-
(7b)
kef,
kB
k,
SC,H2+
When excited states of Sf were not quenched by N2 in the buffer gas, production of CzH2+ has also been observed. From the product distribution, it was estimated that at least 20% of the
J. Phys. Chem., Vol. 99,No. 43, 1995 15893
Reactions of S+(4S) with CH4, C2H2, C2H4, and C3H8
TABLE 1: Summarized Data for the Reactions Included in This Study ~~
reaction
products
s++ cH4
AW
+
SCH+ H2 CH,+ + SH
-29.2 (-1.27) -47.2 (-2.05) -74.9 (-3.25) 7.8 (0.34) 7.4 (0.32)
SCzH+ + H SC2Hz+
1.6' (0.07) -117.5 (-5.1)
SCH3+ H SCH2' + H2
scH4+
+ +H
S+
+ C2H2
s++ c2H4
S+
+ C3Hs
ASo
>0.4
1.2
m 0.6
1.o
1.1
1.1
0.22
2.1
0.65
kroC
1.5
kLc
Ead
=MId
0.8
27.6
-24.7 1.3
+s + CH3 SCzH3++ H C2H3' + SH C2H2+ + H2S C2H2' + + S C3H7' + SH C3H5' + SH + H2 C2H5+ + SCHs C2H3+ + products SCH3+ + C2H5
3.5 (0.15) -81.7 (-3.5)
C2H4f HCS'
6.0 0.67
-103.5 (-4.5) -18.2 (-0.8) -5.1 (-0.22) 66.1 (2.9)
H2
2.87f 1.4
-55.7 (-2.42) -20.7 (-0.9) -35 (-1.5) '15 (>0.65) -46 (-2.0)
0.87
3
0.8
0.098
1.7
0.5
a AW is given in kcal/mol; the values in parentheses are given in eV. Data for calculation of AHO and ASo are taken from refs 15, 52, 53, 54, 55, 56. Calculated only for reactions where structural data are available; ASo is given in cal/(moldeg)(=eu). Rate coefficients are given in cm3 s-l x and E, are in eV. Calculated value by Banientos et al. at the PMP3 level (see discussion in ref 15). f Arrhenius activation energy E, is equal to the endothemicity of this reaction channel (see text and Figure 12).
S++ CH,
I
c
1
AA,,A
'4 ,.A
c '4
0 0
0
10'0 A
?#
0.21 Torr 0.15 Torr 0.40Torr SIFT
e.?A
c .tA, ~ ~ . . ) 8.0...' 0 1
0.1
\ .
I
1 KECM
'
[ev]
0
101'
1
1
..
1
0.1
KECM
iev]
Figure 3. Variation of the "@-corrected"reaction rate coefficient, @, with for the reaction of S+(4S) with CK. Dotted line is the best fit curve according to eq 6. Different buffer gas pressures are indicated by different symbols.
Figure 4. "@-Corrected"overall reaction rate coefficient of the reaction of S+(4S)with C& and reaction rate coefficients for reaction channels 5a and 5e producing SCH3+and CH3+,respectively. Solid line indicates the apparent linear fit, corresponding to m = 0.7.
S+ ions in the reaction region were excited. C2H2+ production
reaction rate coefficient is constant and the branching ratio between binary and three-body channels has a pronounced dependence on tem~erature.~' Data obtained for the effective binary rate coefficients (h) of this reaction are presented in Figure 6. &fi values are plotted as functions of KE~cMfor different pressures of the buffer gas. The reaction rate coefficients for channels 7a and 7b were obtained from the pressure dependence of the overall reaction rate coefficient kff.We assume that the three-body channel is proceeding via formation of the long-lived intermediate complex and that stabilization of this excited complex takes place in collisions with buffer gas atoms (He):
(endothermic from the ground state) was not observed with N2 in the buffer gas. Product distributions as functions of K&M for two different pressures are plotted in Figure 5 . The branching ratio, at a given pressure, is constant up to K&M 0.4 eV (within the experimental accuracy), and only at higher K&Mis the association channel restrained. A constant branching ratio between the three-body channel and the binary channel was observed for many ion-molecule reactions having threebody and two-body channels simultaneously. In the reaction of CH3+ with N H 3 the reaction rate coefficients of the threebody and binary channels decrease with increasing K&M, but the branching ratio stays constant for =M from thermal values up to 0.2 eV (see ref 50). In the reaction of Si+ with C2H4, which was studied recently in our laboratory, the branching ratio between the binary and three-body channels is approximately constant for K&Mfrom thermal values to 0.08 eV. For KECM =. 0.08 eV the branching ratio depends on K&M.24 On the other hand, in the reaction of Br- with w F 6 the effective (total)
where, similarly as in reaction scheme 1, k l is ~ the bimolecular rate coefficient for the formation of the excited intermediate complex (SC*H2)+*, k-IT is the unimolecular rate coefficient
Zakoufil et al.
15894 J. Phys. Chem., Vol. 99, No. 43, 1995
. . . . . ..
in
.
. .
.
. . . .,
S++ C2H2 0.8
A
‘ A t A
b
,
a
A , A
0
v)
&AAP
A A
A
A
A
A
~
~
SC,H,’
SC,H*
0
0.0
0..
I
I
0.1
1
KEC, [evl Figure 5. Product distribution of the reaction of S+(4S) with C2H2. The solid and open symbols correspond to the pressures 0.18 and 0.43
0
A
Torr, respectively.
0
OlOeV 020eV 040eV 070eV
OlBeV 030eV A 055eV 0 090eV 0
0
I
S+ + C2H2.--) SC2H* + H
I
00
01
02
03
04
05
06
07
P rrorrl Figure 7. Pressure dependence of the effective reaction rate coefficient,
+SC2H2*
/teff, of the reaction of S+(4S) with C2H2 at constant KEcM.Data corresponding to different K E ~ M are indicated by different symbols.
0
Y
F
0.62 Torr 0.43 Torr 0.31 Torr 0.18Torr 0.13Torr SIFT I
0.1
I
1 KEC,
Ievl
Figure 6. Effective binary reaction rate coefficient, kff,of the reaction of S+(4S) with CzH2. Different buffer gas pressures are indicated by different symbols. The SIFT value is taken from a previous study by Smith et aL2
I 0.1
for dissociation of the excited intermediate complex into the primary ion S+ and primary neutral CzHz, and ~ Z isT the binary rate coefficient for stabilization of (SCzH2)+* in collisions with He (into SCzH2+). The subscript T indicates rate coefficients of the intermediate steps of the ternary channel (eq 7b). Application of the steady-state approximation to (SCzHz)+* in the mechanism described by eq 8 yields
(9) and for the three-body rate coefficient k3,
where [He] is the number density of the I-Ie atoms. In the lowpressure limit, where !-IT >> k2~[He],k3 klTk2T/k-lT, and k~ is proportional to the helium pressure (p):
-
The overall effective rate coefficient of reaction 7 can be
1
KE ,, WI Figure 8. Three-body reaction rate coefficient of the association channel (eq 7b) of the reaction of S+(4S) with C2H2.
expressed in the form
where the constant a is relating p and [He]. The plots of keff versus pressure at constant &M are presented in Figure 7. Note the good linearity of the plots. The values for kB have been determined as the extrapolation of keff to zero pressure, and the values k3 have been calculated from the slope of the plots. The obtained k3 is plotted in Figure 8. The three-body rate coefficient k3 is nearly constant for KE.cM .e 0.4 eV and decreases with increasing &M for K&M> 0.4 eV; in this energy region the energy dependence of k3 can be characterized by a power law with the exponent m 2.5: k3 = k 3 0 ( K E C h 1 ) - ~ . The obtained independence (very small dependence) of k3 on KECMin the low-energy region is expected when applying a simple model used by N. G. Adams and D. SmithI2 and considering that the only energy brought to the reaction (dependent on K&M)is the translational energy, because the reactant ion is atomic and in a SIFDT the neutral reactants have a constant intemal energy corresponding to 300 K (see the discussion of the application of this model for SIFDT in ref
Reactions of S+(4S) with CH4, C2H2, C2H4, and C3H8
-
J. Phys. Chem., Vol. 99, No. 43, 1995 15895
-
S++ CH ,,
2 1
m
I-
‘”*
X..?
,,k:
..
1
. . . . I
SC,H+ + H
kc,
i
a, “ “ ; ! I
, 0.13 Torr
.
k!, , 0.18 Torr
,p ?VL W.
@%Zrr
SIFT
rrv I
0.1
i
1I
?# SIFT
1
[eVI Figure 9. “,!?-Corrected”reaction rate coefficient of the binary channel (eq 7a) of the reaction of Sc(4S) with C2H2. keg values taken at low pressures of the buffer gas, where the three-body channel is negligible, are also included. The SIFT data are taken from a previous study by Smith er aZ.* The best fit of the data (k#) is indicated by the dotted line. KE,
20). A similar result was obtained in our recent study of the dependence of the rate coefficient for the three-body association reaction of Si+ with CzH4.24 The binary channel (eq 7a) of reaction 7 is dominant over the pressure range covered (up to 0.6 Torr). For He pressurep < 0.2 Torr the three-body channel is negligible. The obtained reaction rate coefficient kB of the binary channel, together with the measured k,ff at pressure below 0.2 Torr, was ‘IS-corrected” (as is discussed in the Introduction). Obtained “B-corrected” values ke@and k& are plotted in Figure 9. Because of the fast decrease of the three-body reaction rate coefficient k3 at K&M higher than 0.5 eV, kB@= %# for KECM> 0.8 eV. At near thermal energies (for KECM< 0.1 eV) values of the binary reaction rate coefficient kB are close to collisional rate coefficients (70-80%). Our data are in very good agreement with the thermal SIFT data of Smith et al.: which are also included in Figure 9, and with the near thermal data of Anicich and HuntressS8 Note in Figure 9 that with increasing KECM the reaction rate coefficient of the binary channel is decreasing over one order of magnitude. The decreasing part of the plot can be fitted for K&M> 0.4 eV by a straight line, indicating a powerlaw dependence of the rate coefficient on K&M. This is a typical behavior observed for many fast ion molecule reactions proceeding via formation of a long-lived intermediate complex, as observed in our recent SIFDT studies, including also the already discussed reaction 5 of S+(4S)with CH4. For example reactions of C+, CH+, and CHZ+ with HCl and C02,19320 reactions of Si+ with HC1, HzO, NH3, and H z S ? ~etc. have such an energy dependence of the reaction rate coefficients. At energies KECM> 2 eV there is a deviation of the plot from linearity probably due to opening of a new channel which is not accessible at lower energies. In order to parametrize the dependence of k ~ pon K&M, obtained data were fitted by function 6 ; for K&M> 0.8, where kB@has not been determined, the values k,& = kB@were used (see discussion above). From the small deviation it is very inaccurate to determine Arrhenius parameters, so only parameters corresponding to the first term (kI) of function 6 were determined. On the basis of the present data we cannot comment on the small increase of ks at high =M. The parameters of the fit are listed in Table 1. The best fit of the data (k#) based on eq 6 is plotted by a dotted line in Figure 9. The binary reaction (eq 7a) is thought to be the important step in the synthesis of the SCZ observed in dense interstellar
-
C,H;
+ neutrals
(13e)
The obtained overall reaction rate coefficient as a function of K&Mis presented in Figure 10, and in Figure 11 the product distributions are plotted as dependent on K&M. Previous data obtained by Smith et al. in a SIFT experiment2are also included in Figures 10 and 11. Note the good agreement of the present and previous data. At low &there are two dominant product ions: HCS+ (-65%) and SC2H3+ (-30%). The population of these sulfur-bearing ions is decreasing, and the population of “fragment ions” is increasing with increasing K&M. CZH3+ ions are present over the whole energy range covered; the population of this ion is slightly increasing with increasing K&M (from 10 to 20%). CZ&+ ions are present for K&M> 0.15 eV, when their production is energetically accessible; at =M PZ 1 eV production of C2&+ reaches its maximum (-50%).
Zakoufil et al.
15896 J. Phys. Chem., Vol. 99, No. 43, 1995 1
.-u
=,,t
e
e
e
0
CH ,‘,
0
A
HCS‘ SC,H,’
T
C,H,’
+
C,H,’
S++C3H8
A
v
+ m
0.1
1 KEcM
[ev]
Figure 11. Product distribution of the reaction of Sf(4S) with C2H4. The SIFT data are taken from ref 2.
S*+ C2H4
[evl
Figure 13. Reaction rate coefficient of the reaction of S+(4S) with C&. Different buffer gas pressures are indicated by different symbols. * indicates the SIFT data of Smith et aL2 1.0,
I
I
I
I
S++ C3H8
10”
t
*
0.18Torr 0.16Torr 0.13 Torr SIFT
10‘0 KEcM
I
1
A
1 eV.
IV. Concluding Remarks Atomic sulfur cations have been produced in their ground state S+(4S) by electron-impact ionization, and the kinetics of their reactions with simple hydrocarbon molecules (CH4, C2H2, C2H4, and C3H8) has been studied in a SIFDT experiment. The studied reactions are fast at low (near thermal collision energies), and the reaction rate coefficients decrease rapidly with increasing KECM(average center-of-mass kinetic energy). It
where and m are constant parameters different for different reactions. This is a similar energy dependence as has already been observed for several fast binary ion-molecule reactions, e.g., reactions of Si+ with HCl, H2S, H20, NH3, C2H2, C&, et^.^^,^^ These energy dependencies of the reaction rate coefficients can be interpreted on the assumption that the reactions proceed via formation of long-lived intermediate complexes (the assumption is supported also by observation of traces of adduct ions). The overall reaction rate coefficient of the reaction proceeding via formation of a long-lived complex is given by the ratio k-I/ k2 (see eq 2 ) of the unimolecular reaction rate coefficient for decomposition of the complex to the reactants (k-I) and to the pioducts (k2). Observed dependencies of k on KECMindicate that the dependence of the ratio k-llkz on K&M can be approximated by a power function: k-1lk2 (KECM~MI)". This "power-law" variation with K&M is underlining the similarity between the mechanisms (between rate-determining steps) of the studied binary reactions and the three-body association reactions, a similarity which, we expect, is connected with the energy dependence of the lifetime against decomposition of the formed excited collisional complex.
-
Acknowledgment. P.Z. is thankful to Osterreichischer Akademischer Austauschdienst for a Scholarship, which allowed his stay at Institut fiir Ionenphysik, Universitat Innsbruck. J.G. and P.Z. wish to express their thankfulness for the hospitality during a visit at the Instituit fiir Ionenphysik. This work was supported in part by the Fonds zur Forderung der Wissenschaftlichen Forschung under Project No. 10014, in part by the Grant Agency of the Czech Republic under Project No. 0449, and in part.by Charles University under Project GAUK-294. References and Notes (1) Dedicated to Professor Zdenek Herman on the occasion of his 60th birthday, in recognition of his pioneering contributions to the dynamics of ion-molecule reactions and in appreciation of his friendship with all of us. This paper was originally submitted for the Zdenek Herman Festschrift [J. Phys. Chem. 1995, 99 (42)]. (2) Smith, D.: Adams. N. G.: Giles, K.: Herbst, E. Astron. Astrouhvs. . . 1988, 200, 191. (31 Millar,T. J.; Herbst, E. Astron. Astrouhvs. 1990,231, 466. (4) Smith, D. Chem. Rev. 1992,92, 1473. (5) Millar, T. J.; Adams, N. G.; Smith, D.; Lindinger, W.; Villinger, H. Mon. Not. R. Astr. SOC.1986, 221, 673. (6) Huntress, W. T.; Pinizzotto, R. F. J. Chem. Phys. 1973, 59, 4743. (7) Laudenslager, J. B.; Huntress, W. T., Jr. Int. J. Mass Spectrom. Ion Phys. 1974, 14, 435. (8) Anicich, V. G. J. Phys. Chem. Ref:Data 1993, 22, 1469. (9) Fehsenfeld, F. C.; Ferguson, E. E. J. Geophys. Res. 1973, 78, 1699. (10) Liddy, J. P.; Freeman, C. G.; McEwans, M. J. Astrophys. Lett. 1975, 16, 155. (11) Dotan, I.; Fehsenfeld, F. C.; Albritton, D. L. J. Chem. Phys. 1979, 71, 4762. (12) Smith, D.; Adams, N. G.; Lindinger, W. J. Chem. Phys. 1981, 75, 3365. (13) Tichf, M.; Rakshit, A. B.; Lister, D. G.; Twiddy, N. D.; Adams, N. G.; Smith, D. Int. J. Mass Spectrom. Ion Phys. 1979, 29, 231. (14) Stowe, G. F.; Schultz, R. H.; Wight, C. A.; Armentrout, P. B. Int. J. Mass Spectrom. Ion Processes 1990, 100, 177. (15) Armentrout, P. B. Isotope Effects in the Reactions of Atomic Ions with Hz,Dz. and HD. In Isotope Efects in Gas-Phase Chemistry; Kaye, J. A., Ed.; American Chemical Society: Washington, DC, 1992. (16) Barrientos, C.; Largo, A. J. Phys. Chem. 1992, 96, 5808.
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