Reactions of Ar2+ ions with neutral molecules

International Journal of Mass Spectrometry and Ion Processes, 66 (1985) 109-119 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

REACTIONS

OF Ar;

IONS WITH NEUTRAL

109

MOLECULES

A.B. RAKSlT Baker Laboratory of Chemstry, (First received

15 November

Cornell Unic!ersity, Ithaca, NY 14853 (U.S.A.) 1984; in final form 3 January

1985)

ABSTRACT A selected-ion drift chamber mass spectrometer was used to measure rate coefficients and product channels for the reactions of Arc with H,, CO, O,, C02. N,O and SO,. In most cases, the reactions were found to proceed by charge transfer. Compared with the reactions of ions react quickly with these molecules, except for I-I,. The rate Ar+ ions, the Arl coefficients for the reactions of Ar: with oxygen were found to be dependent on argon pressures in the drift chamber. The rate coefficients are compared with data from other authors where possible and are discussed with respect to reaction efficiencies.

INTRODUCTION

Recently. we reported the rate coefficients and product channels for the reactions of Ar+ with several neutral molecules [I]. During these studies it was found that at a high argon pressure of 3.4 torr in the reaction chamber, a significant Ar: ion signal appeared in the mass spectrometer. The Ar: ions are formed by a termolecular association process of Ar+ ions with argon atoms with a rate coefficient of 1.5( - 31) cm6 s-r, so they can be generated either in high pressure experiments at room temperature or in low pressure environments at low temperature. Some of these Arc reactions have been studied by Collins and Lee [2] using optical absorption in the high pressure afterglows, and by Adams et al. [3] and Bohme et al. [4] who used a flowing afterflow apparatus operating at 200 K. All these authors found similar rate coefficients within experimental uncertainty. Former authors gave no information regarding product ions from these reactions and later groups reported only possible main products because of the presence of other impurity ions. To resolve such discrepancies, an additional measurement using a different technique seems worthwhile. These measurements were done using a selected-ion drift chamber mass spectrometer. The rate coefficients are compared to the ion/molecule capture rate coefficients predicted by the 01681176/85/$03.30

0 1985 Elsevier Science Publishers

B.V.

110

Langevin [5] and the ADO collision rate theory [6] and also with the data obtained by different techniques. Although the present paper is concerned mainly with reactions of Ar: ions, the reaction of Arf with oxygen is reported because of its unusual behavior at high argon pressure in the drift chamber. EXPERIMENTAL

A detailed description of the selected-ion drift chamber mass spectrometer [7] and the production of Ar+ ions in the reaction chamber has been given previously [l]. Only a brief description will be given here. In the present study, primary argon ions were generated by electron impact at an electron energy of 30 eV. The argon ions were mass-selected and then injected into the drift chamber which was filled with argon at a pressure of 3.4 torr. Near the injection orifice, the primary argon ions suffer symmetric charge transfer whereby nearly thermal Ar + ions are produced. The secondary argon ions are attached to another argon atom by a termolecular association process and Ar: ions are formed. At this pressure, about 50% of Ar+ ions are converted to Ar: ions and we had an opportunity to study ion/molecule reactions between Arl ions and some neutral molecules. Due to the presence of Ar+ ions, these experiments were done at low pressure (4 1 torr) where Ar: was only 10% of the total intensity in order to distinguish product channels from the Ar+ reactions. A weak electric field (10 V cm- ‘) drives these ions and any product ions resulting from reactions with added gases towards the rear plate of the chamber where they are sampled for mass spectrometry analysis. The residence time of the argon ions in the chamber was determined separately by means of a double pulse gating technique [7] and reported previously [l]. For Arl tons, t the mobility value obtained by Beaty [8] in pure argon, p0 = 1.83 cm2 V’ s-r, was used in these studies. All experiments were done at 300 K. Rate coefficients are given in units of cm3, molecule and seconds: the power of ten is indicated in parentheses. Determination

of rate coefficients

Experiments were performed at a drift chamber pressure of 3.4 torr. At this pressure, the Ar + ions become partially attached to argon atoms by a termolecular association process, whereby Ar: ions are formed. The Ar+ ions react with water vapor which is present as an impurity. As a consequence, the analysis of experimental data must take into account the reactions Ar++2Ar-+Arl Ar++

+Ar

H,O 4 Products

(1) (2)

111

Ar+ + R + Products

(3)

Ar:

(4)

+ R + Products

where R denotes the reactant admixed to the argon flow. The number densities of Ar, H,O and reactant molecules in the drift chamber are denoted by n, n, and n,, respectively. If one neglects losses due to diffusion, the number densities of the reactant ions in the tubular region connecting entrance and exit apertures follow from kinetic equations, viz.

dn40 v403-

=

d%o

-= 080 dl

-(k,n*+k,n,+k,n,)

kln2n40--k4n,nxo

Here, the individual ion intensities n, and the associated drift velocities v, are identified by their mass numbers, M(Ar+) = 40 and so on; I is the length of the drift chamber and the ks are the rate coefficients for the reactions involved. Similar equations describe the behavior of the product ions resulting from reactions (2)-(4). Integration of these equations over the entire drift length, I = d, will, nevertheless, give the correct individual ion intensities emerging from the chamber exit aperture provided these are normalized to the sum of all ion intensities. Integration of the kinetic equation yields the normalized ion intensities of the reactant ions

The normalized

intensities

l

d)

= 4o

u40n40(l=

are represented

here by

etc

Zv,n,(l=d)

.

f = 0.95 is the fraction of Ar + ions resulting from the initial charge transfer process and 740= d/VA0 = 4.604 X 1O-4 s is the residence time of At-+ ions in the drift chamber for an electric field strength of 10 V cm-’ and P = 3.4 torr. The other parameters are A = k,n2 + k,n,

+ k3n,

B = k,n,& PSO

Values for k, and k,n, are obtained from the ion intensities in the absence of added reactant R. The rate coefficients k,, k, and k, are taken from our earlier results [l], and slight adjustment was necessary to represent the

112

experimental data. The rate coefficients k, are obtained by fitting the equation for Z,, to the observed ion intensities as a function of n,. Rate coefficients derived in this manner are then used to calculate the normalized intensities of the product ions resulting from reactions (2)-(4). Reaction of Ar + ions with oxygen Reaction of Ar+ and Ar: ions with oxygen was the only one which showed unusual behavior at high argon pressures in the drift chamber. Therefore, it needs a detailed explanation especially as a function of Arl ion intensities. Figure 1 shows the normalized ion intensities observed when oxygen is added to argon at a total pressure of 3.4 torr in the drift chamber. A greater number of product ions are evident. The initial ions in the absence of oxygen are Ar+, Arc, H,O+ and ArH+. The first is the primary ion while

1

z

5;

i5 g 5 i

0.1

-

r; P coo.1 9

0.001

Fig. 1. Normalized ion intensities observed 85 a function of oxygen partial pressure at a total -. Calculated as described in the text argon pressure of 3.4 torr in the drift chamber. assuming the occurrence of reactions (I)-( 11) and the rate coefficients given in Table 2.

113

the last three ions result from the reaction with argon atoms and water vapor, which is present as impurity. All four ions decay exponentially when reactant oxygen is added and, subsequently, the product ions arise. Thus one must consider the following reactions to represent the Arl ion intensity Ar++

0, + (ArOc)*

(ArOT)*

(5)

+ Ar A Arc + 0, A ArOc

(ArO:)

+ 0;

(6)

+ Ar

+ Ar

(7)

The Ar: ion intensity increases at low flow rates of oxygen according to reaction (6). In addition, one should also consider losses of Ar:, H,O+ ArH+ and ArO: ions Ar:

+ 0, + 0:

H,O++

+ 2 Ar

0, -+ 0;

ArH++O*-+HOT ArOc

+ 0, + 0:

(4)

+ H,O

(8)

+Ar

(9)

+ Ar

(IO)

The HO: ion intensity formed by reaction (9) indicates that it enters into further reaction but actually it does not. It decays because its precursor ions decay. The 0: ion intensity formed by reaction (10) accounts for more than the observed ion intensity indicating that it follows the reaction 02

+Ar-+O:

+O,+Ar

(II)

Equations for the intensities of the various ions appearing as reactants or products in the mechanism of reactions (l)-(11) can again be obtained by integrating the appropriate kinetic equations. Since our interest is concerned primarily with reaction (4), we give here only the equations for the normalized intensities of the ions Ar+, Arc and ArOc. The kinetic equations have been deduced by assuming the reaction proceeds via an excited intermediate complex ion and has a long lifetime rather than by unimolecular decomposition. Thus it is effectively stabilized by argon atom collisions before it can dissociate. The normalized ion intensities of the principal ions are I,, =f eeXT*l Z,, = f

k,n2 + i

I 12

k&c?, k_,

+ k,n + k, i

[e-%

_ e-%1]

e -q

114

Here, n, n,, n r and rdO have the same significance other parameters are defined as

as mentioned

earlier. The

X= keffn, + k,n2 + k,n, Y=k

n hf.!!? 4 ‘l-%0

where pT2, the mobility for ArO: , was calculated in a manner similar to that described earlier [15] and ~40/~L,, and p40/p72 values of 0.8281 and 0.8634, respectively, were used. k

_ k&,n +k7) eff - k_, + k,n + k,

k, and k,n, are obtained from the initial intensity distributions of At-: and H,O+ and ArH+ combined when oxygen is absent. The rate coefficient deduced for reaction (l), k, = 1.56( - 31) cm’ s-t is in good agreement with data obtained previously by other authors [16]. The number density of water vapor present as an impurity was determined from the observed product k,n, = 350 and the known value of k, = 1.6( - 9), the average of data given in the literature [17]. The number density obtained, n, = 2.210” cmP3, is similar to that found in the other experiments [15]. The rate coefficient for

01 0

I

At

I

I

1 2 3 PRESSUREitorr)

Fig. 2. Effective rate coefficient pressure in the drift chamber. -,

4

for the reaction Ar+ + 0, Calculated as described

as a function in the text.

of total

argon

115

reaction (3) is obtained directly from the decay of Arf ion intensity. We found that in this system the rate coefficient k, is pressure dependent. Figure 2 is a plot of k,(eff) vs. argon pressure in the drift chamber. Extrapolation to zero-flow of argon gives a value l( - 11) for the bimolecular reaction and the slope of the curve gives 1.24( -31) cm6 s-l for the termolecular rate coefficient. The solid line was calculated using Eq. (12) assuming the occurrence of reactions (5)-(7). The unknown rate coefficients for reactions (l)-(13) were obtained in a stepwise procedure until the ion intensities measured as a function of reactant pressure were matched by the calculated intensities. The rate coefficients thus obtained were used subsequently to calculate the intensities of the product ions. Table 2 summarizes the reactions observed or inferred in the present studies and the associated rate coefficients. RESULTS AND DISCUSSION

Table 1 summarizes the rate coefficients for reactions (3) and (4) and the product ions derived in the present investigation. Rate coefficients k, are given here for comparison with our previous results [l]. The table also contains the rate coefficients predicted from Langevin [5] and ADO collision theory [6] and by other authors using different methods. Our results for the reactions of Ar: with O,, CO, and N,O are in good agreement with the values obtained by Adams et al. [3] and Bohme et al. [4] and also in acceptable agreement with the values obtained by Collins and Lee [2] who used optical absorption in a high pressure experiment. The rate coefficients for the reaction Arl + H, and Ar: + CO are lower by a factor of 2 and 3, respectively, from the results reported by other authors [2,4]. Collins and Lee did not report the product ions from these reactions. Hydrogen and carbon monoxide were observed to react with Arc ions to produce Ar,H+ and ArCO+ which may arise by H-atom abstraction and switching, respectively. Bohme et al. predicted similar product ions from these reactions. The Ar: ion was observed to react with O,, CO,, N,O and SO, predominantly by charge transfer. Although the formation of mixed molecule ions, ArAB+, e.g. ArCO: , from these reactions is more exothermic, they were not observed. The ArCO+ ion was found to react with CO by the reaction ArCO’+CO+CO+.CO+Ar with a rate coefficient of 2.2( - 10). This value is lower by than data reported by Bohme et al. [4]. Consider the other possibility for the formation of ArO: al. [3] have observed ArO: ions in the Arf+ 0, reaction afterglow at 200 K and suggested the ArO: ions may be

03) a factor

of two

ions. Adams et in the flowing formed by the

TABLE 1 Observed

product

channels

and rate coefficients

for reactions

of Ar+ and Ar:

Reactant

Product from Ar+

k, a.b

k, (calcd.)

R.E. e

Product Arl

H*

ArH+

5.4( - 10)

1.5( - 9)

0.36

N* co

N: co+

O&1.0( - 11) = 3.6( - 11)

7.6( - 10) 8.1( -10)

0,

0;

5.5( - 11)

CO, N,O so,

co: NzO+ so;

4.7( - 10) 2.2( - 10) 1.35( - 9) d

with the neutral

molecules

at 300 K

k,

k,(calcd.)

R.E. ’

Other f

Ar,H+

2.0(-10)

15-9)

0.13

5.9( - 10) [2], 4.9( - 10) [4]

0.01-0.013 0.04

No reaction ArCO+

4.8(-10)

6.8( - 10) 7.2(-10)

0.67

7.0( - 10)

0.06

0;

l.O(-IO)

6.1(-10)

0.16

8.2( - 10) 8.9( - 10) 1.57( - 9)

0.67 0.27 0.86

co; N,O+ so;

l.O(-10) 7.0( - 10) 1.8( - 9)

7.1(-10) 7.7( - 10) 1.3( - 9)

1.41 0.91 1.38

6.9( 8.5( 1.2( 1.2( l.l( 5.9(

from

a These rate coefficients can be compared with previously reported data [I]. b Rate coefficients are given as a( - b) to represent a x 10-b and are in units of cm3 s- ‘. ’ For details see ref. 9. d Not reported previously. e R.E. (reaction efficiency) is defined as the ratio of observed rate coefficient to the calculated ’ Only for Arc + R reactions.

value.

- 10) - 10) - 10) - 10) -9)[4] - 10)

[2], [4] [2], [4] [2]

117 TABLE 2 Reactions

observed

in the Ar+ + 0,

system and associated Rate coefficients

Reactions

Reaction no.

rate coefficients a

Others

This work

Calcd.

k,=9(-11) k_, = 3.8(7) SK’

7.0( - 10)

k, = 5.0( - 10)

5.q - 10)

+Ar +H,O

k, = 5.0(6) s-’ k, = 2.0( - 10)

8.7( - 10)

+ HOT +Ar

k, = 3.5( - 10)

6.8( - 10)

k,,=7(-11) k,, = 3.0( - 13)

6.3( - 10) 6.0( - 10)

5

Ar+ + 0, + (ArOc ) *

6

(ArO;)*+Ar+Ar:

94%

+O, 6% -*ArOT

7 8

(ArO:)*+O: H,O++O,+O;

9

ArH++02

10 11

ArO: + 0, + 0: + Ar 0: +Ar-,O: +0,-t-Ar

a Rate coefficients

+Ar

are given as a( - b) to represent

a

X

4.6( - 10) [lo], 2( - 10) [ll] 1.29( - 9) [12] 4.1( - 10) [13], l( - 10) [14] 2.5( - 11) [3]

10Fb and are in units of cm3 s-‘.

reactions Ar++O,+Ar+ArOl

+Ar

(14)

0;

+ 0,

(15)

+ 0, + Ar -+ ArOc

involving a termolecular association process. The inclusion of reactions (14) and (15) in our model calculation of the intensities of ArO: and 02 ions does not, on the one hand, represent experimental data nor, on the other hand, the formation of Ar: ions from the reaction ArO:

+ Ar - Ar,f + 0,

(16)

which Adams et al. assumed would be endothermic if ArO: is formed by reaction (14) and (15). Another more probable production process for ArO: ions, which Adams et al. also assumed, is the reaction Ar:

+ 0, -+ ArOc

+ Ar

(17)

since Ar: ions are present in significant amounts which increase with low flow rates of oxygen and decrease with high flow rates. Formation of ArOl by reaction (17) makes reaction (16) endothermic again. However, Bohme et al. have shown that the reaction of Arl with 0, proceeds exclusively by charge transfer at 200 K. Our results agree with those of these authors. The observed rate coefficient for the reaction Ar’+ SO, is higher by a factor of 3 than the results reported by Dotan et al. [18].

118

Inspection of Table 1 makes evident that, where other data are available for comparison, the rate coefficients obtained are in satisfactory agreement and provide further substantiation of the suitability of the drift chamber technique for such measurements. Mason et al. [19] have reported that charge transfer occurs by two mechanisms: (i) an electron transfer without momentum transfer which they suggest occurs at relatively large ion/molecule separation, and (ii) formation of a complex between the molecular ions formed and the argon atom participating in electron transfer resulting in momentum transfer. The electron transfer at large ion/molecule separation should lead to higher rate coefficients than calculated from collision models. The observed rate coefficients are either less or equal to collision rate coefficients within the experimental error. Table 1 shows that reaction efficiencies which involve the ratio of the observed and calculated rate coefficients, for reactions of Arl with H, and 0, are only 13-15s whereas reactions with CO, CO,, N,O and SO, are more efficient and vary from 67-100s. When comparing reactions of Ar+ and Arl ions, one finds smaller efficiencies for reactions of Ar+ with O,, CO, CO,, N,O and SO,. Efficiency is still low for reaction with 0, in both cases. Replacing Ar+ by increases the reaction efficiencies for 0, from 6 to 15%. Similar Arl observations were made for reactions of CO: and CO,. CO: with 0, [lo]. The reactions of Ar+ with H, is reasonably efficient (36%) while that of Ar.j+ is not (13%). Here, a similar situation has been observed previously for the reaction of Nl ions with H, [20]. A reaction efficiency less than unity suggests that there is a possibility of energy resonance only with the vibrational and rotational energy levels of the lowest electronic state of the product ions. For the Arc + 0, reaction, the energy resonance is 0: (X2II,, u = 10). CONCLUSIONS

The measurements reported here show that Arc ions react with simple molecules in the gas phase at room temperature in a variety of ways which include charge transfer, hydrogen atom transfer, and switching. Charge transfer was observed with molecules having ionization energies less than 13.79 eV but not with molecules having ionization energies greater than 14.01 eV. These observations suggest values for the ionization potential of Ar,, IP(Ar,) 13.90 k 0.15 eV at 300 K. There appears to have been no previous measurements of IP(Ar,). Payzant and Kebarle [21] reported the dissociation energies D(Ar+-Ar) in the range - 1.47 to - 1.99 eV. With these values and the ionization energy for Ar (15.76 eV), the ionization energy of Ar, was found to be in the range 14.29-13.77 eV. A final comment should be made on the comparisons with the results of

119

the previous FA study of Bohme et al. [4] and Collins and Lee [2]. The rate coefficients obtained with the present experiments are systematically lower than those reported earlier (see Table 1, column 7). ACKNOWLEDGEMENT

The author is grateful to Prof. P. Warneck Chemistry, Mainz, F.R.G., for his suggestions and interest in the problem.

of Max-Planck Institut for and stimulating discussion

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