The composition of the CH4 plasma

1 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Review THE COMPOSITION OF THE CH, PLASMA

G. DRABNER,

A. POPPE* and H. BUDZIKIEWICZ

Institut ftir Organische Chemie der Universitiit zu K&z, Greinstr. 4, 5000 Kiiln 41 (F.R.G.)

(First received 25 September 1989; in revised form 12 November 1989)

ABSTRACT In this review the CH, plasma will be discussed as it is encountered in chemical ionization mass spectrometry. Topics are the formation by electron impact on CH4 of positive and negative ions, as well as of radicals, and their subsequent conversions by ion/molecule and ion/radical reactions and by ion/electron, ion/ion and radical/radical recombinations. The importance of these processes for the abundances of the various species will be considered using thermodynamic and kinetic data (calculated or measured). The dependence of the relative abundances of positive ions on experimental parameters (temperature, pressure, repeller potential and electron energy) will be described. Ion/radical reactions and surface-catalyzed reactions (which have not been considered so far) will be proposed to explain features difficult to understand by other reaction mechanisms.

I. INTRODUCTION

Initiated by the pioneering work of Munson and Field [l], CH, has been used widely both as a reagent gas for positive chemical ionization [PCI(CH,)] [2] and as a moderating gas for the production of thermal electrons for negative chemical ionization [NCI(CH,)] [3]. In PCI(CH,) ionization of a substrate is achieved by fast ion/molecule reactions between reagent gas ions and substrate molecules, whereas in NCI(CH4), because of the minimal abundance of negative plasma ions, (dissociative) resonance capture of thermal electrons is the dominating process. The reactions leading to an ionization of the substrate molecules as well as their relative preponderance depend on the composition of the reagent gas plasma, which in turn will be determined by experimental parameters such as temperature or pressure. In view of the substrate ionization in PCI(CH,) the ionic composition of the plasma and its variation caused by experimental parameters are of paramount importance. In the CI(CH,) spectra of several compounds ions with masses larger than those of the molecular ions have been observed [4] whose genesis cannot be * Present address: Landesamt fur Wasser und Abfall NW, Auf dem Draap 25,400O Dusseldorf 1, F.R.G. 0168-l 176/90/$03.50

0 1990 Elsevier Science Publishers B.V.

2

explained by ion/molecule reactions. It has been suggested that they are formed by reactions of the substrate molecules with reactive neutral species (alkyl radicals, *CH) before the ionization. As hardly any experimental data are available on such neutral components of the methane plasma, their identity can frequently be deduced from indirect arguments only. In Section II, the nature of the primary products of electron impact ionization of CH4 and their formation will be discussed. In Section III, kinetic and thermodynamic data will be presented for those ion/molecule reactions in the plasma which lead to secondary and tertiary plasma ions. Also, the results of a recent systematic study [5] on the relative abundance of these ions and their dependence on the reagent gas pressure, the ion source temperature, the repeller potential and the electron energy will be summarized. In Sections IV-VI, reactions will be surveyed which are initiated by reactive neutral particles in the CH, plasma formed either during the electron impact ionization or by secondary reactions of positive ions. II. PRIMARY

PRODUCTS

OF THE ELECTRON

IMPACT IONIZATION

OF CH,

Inelastic collisions of CH4 molecules with electrons which have an energy of 5OeV or higher yield ions and neutral particles at about the same rate [6]. With higher electron energies the ion pair formation increases, whereas at lower energies bond cleavages without charge separation prevail. The nature of the charged and neutral particles formed at an electron energy of 100 eV has been identified by Melton and Rudolph [7] using a mass spectrometer with a dual electron-beam ion source (see Table 1). The ions formed by electron impact were retained in the first chamber by a suitable repeller potential, and the neutral particles were permitted to diffuse into the second chamber, where they could be ionized and subsequently mass analyzed. In an analogous experiment it could be shown that H2 and ‘CH3 are formed in vibrationally excited states (0.8 and 0.3 eV, respectively) [lo]. The absolute density of ‘CH, radicals formed by r.f. discharge in CH, (0.5-20mtorr) was determined as (l-4) x lO”cm- 3. In the afterglow of pulsed r.f. discharges (in the absence of ions and electrons) they are lost by recombination in the gas phase rather than by surface reactions (see ref. 128). The cross-sections r~for the formation of the various particles (see Table 1) depend on the electron energy. The integral cross-section c+ for the formation of positive ions shows a maximum at 85 eV [6] (70eV: Q~ = 4.04 x 10Pr6, 80eV: 4.08 x 10-16, 90eV: 4.08 x 10-16, 1OOeV: 4.04 x 10-‘6cm2 [S]). The primary products of the electron ionization of CH, listed in Tables 1 and 2 are formed by ionization, dissociative ionization, dissociation or electronic excitation followed by dissociation of CH4 [7] or of ‘CH, [l 11. Neutral particles are formed especially by dissociative processes [lo]. In some

3 TABLE 1 Primary products of the electron impact ionization of CH4 (100 eV, 8 x 10U4Pa) (from ref. 7) Ion (a) Positive ions HC H:’ C+’ CH+ CH: CH: CHa+

(b) Neutral particles ‘H H2 C ‘CH :CH, ‘CHx

(c) Negative ions HC-’ CHCH; CH;

% C particles

Formation

0.47 0.24 0.59 1.65 3.29 17.65 21.18 45

u (10-‘6cm-2) 0.04 (0.11 [9]) 0.02 (0.019 [9]) 0.05 (0.043 [8], 0.035 [9]) 0.14 (0.109 [8], 0.097 [9]) 0.28 (0.260 [8], 0.21 [9]) 1.5 (1.82 [8], 1.26 [9]) 1.8 (1.81 [8], 1.45 [9]) 3.8 (4.04 [8], 3.08 [9])

28.24 9.41 0.001 1.18 2.35 14.12 55 8.242 8.38 6.6 2.5 9

x x x x x

1O-4 1O-5 10-5” 1O-5 lo-’ 1.0 x 10-j

cross-section

u (10-‘6cm-2) 2.4 0.8 0.1 0.2 1.2 G Q (lo-*’ cm2) 8 0.81 0.8” 0.24 0.09 9.9

‘Because of the straightforward correlation between % I: and Q, one of the figures in ref. 7 has to be in error (either 6.6 x lo-’ and 0.64 or 8.3 x lo-’ and 0.8 are the correct values).

cases, they could be identified directly by their UV emission when formed in electronically excited states (H* [20-241, C* [20,21,23], CH* [20]). H* formed in high Rydberg states can ionize CH, to give CH:’ as well as CH: and could thus be detected in an indirect way in a divided ionization chamber [25]. Several important reactions responsible for the formation of primary products have been assembled in Table 2. For their vertical appearance energies (AE) listed there, AE 3 AH, (AHR is the standard reaction enthalpy). The numerical values for these two terms differ when the reaction products are formed with excess internal or translational energy at the threshold and/or when the back-reaction has an activation energy (cf. the discussion by Mathur [13]). As an example, the AEs of ‘CH #A) formed according to Eqs. 13 and 14 (see Table 2) differ by more than the dissociation energy of H2 (4.5 eV; see

4 TABLE 2 Primary reactions of CHI CH,+e-+ CH$‘+2eAE(CH:‘) < 12.71eV [13] AE(CH: ‘)adiab= 12.61 +_ 0.01 eV [12]

(1)

CH4 + e- CH: + H’ + 2eAE(CH:) = 14.25 f 0.05eV [13]

(2)

CH4 + e- + CH: + H2 + 2e- [13] AE(CH:‘) = 15.20 + 0.05eV [13]

(3)

CH4+e--+CH:‘(ZA,)+e+ CH+ + Hz + H’ + 2eAE(CH+) = 22.3 _+ 0.1 eV [13] CH4 + em + C+’ + 4H’ + 2eAE(C”) = 25.2eV [14] CH:‘(*A,)+e+ ‘CH, + H+ + 2eAE(H+) = 22.17 + O.leV [15] = (22.7 + 0.5eV [16])

[14]

(4)

[16,17]

(5)

[15]

(6)

CH,+e----,

CH4 + e- -F H: + C + 2H’ + 2eAE(Hl ‘) = 27.9 f 0.5 eV [16] CH4 + e- -t

CH; + H+ + e-

[16]

(7)

[18]

(8)

CH, + e- + CH;’ + 2H’ [16] AE(CH; ‘) = 9.3 f 0.2 eV [18]

(9)

CH, + e- + CH- + ‘H + H, [19] AE(CH-) = 9.7 + 0.2eV [19]

(10)

CH, + e- -f C-’ + Hf + 3H’ + eAE(C-‘) = 27.4 + 0.6eV [16]

[16]

(11)

CH, + e- -L H- + ‘CH, [16] AE(H-) = 8.3 * 0.3eV [18]

(12)

CH, + e- _r’ :CH(A’A) + H2 + ‘H + eAE[‘CH(A*A)] = 13.4eV [20] CH, _t eT + ‘CH(A*A) + 3H’ + eAE[‘CH(A*A)] = 20.8 eV [20]

[20]

[20]

CH, + e- + :CH, + ‘H + ‘H(2p) + eAE[‘H(2p)] = 27.3eV [21]

[21]

(13) (14) (15)

ref. 28), i.e. by 7.4 eV, as a result of the translational excitation of the reaction products (Eq. 14) [20]. III. SECONDARY

AND TERTIARY

IONS

A. Modes of formation Provided the reagent gas pressure is sufficiently high, the mean free paths of the primary ions formed from CH, become smaller than the dimensions of the ion source. Thus before leaving the latter or before being neutralized by reaction with slow electrons they may show fast ion/molecule reactions, leading to secondary and tertiary ions. Because a primary ion during its residence time in a CI source will collide with fewer than a hundred molecules [I], only those ion/molecule reactions will be observed where almost every collision will lead to products. The rate constants of these reactions are about lop9 cm3 s-‘, which is in good agreement with the values calculated according to the theory of Langevin (for a compilation of calculated rate constants see ref. 32). Because of the thermalization of the colliding particles (relative kinetic energy $1 eV), the observed reactions have to be exothermic or at best slightly endothermic [33]. Kinetic and thermodynamic data for the most important ion/molecule reactions encountered in the CH, plasma are given in Table 3. In addition to the products from reactions of primary and secondary ions with CH,, products of reactions with impurities present in the ion source (H20, O,, N,, CO, CO,) may be observed. Radical recombination can lead to new products such as 2’CH, (which is formed, e.g. in reaction 31) giving C,H,, which in turn will be protonated by CH,+ to give C,HT [34]. It is remarkable that no reactions of plasma ions with plasma radicals have been discussed so far. It may be that they have not been considered in cases where the expected product ions could also be obtained from ion/molecule reactions. Not much is known about the rate constants of ion/radical reactions (for k < lo-” cm3 s-’ they could not be observed in the conditions prevailing in the ion source). Kinetic measurements are available only for reactions of H’ with H; ’ and some hydrocarbon ions [26]. In these experiments, reactions were observed with H: ’ (charge exchange, k = 6.4 x 10-‘0cm3s-‘) and with CH+ (H abstraction, k = 7.5 x lo-” cm3 s-l), but not with CH:, CH:‘, CH:, C,H:, C,H4f’, C,H: and C,Hz (k < lo-” cm3 s-‘). For several other reactions of hydrocarbon ions with ‘CH3, :CH, and ‘CH rate constants have been calculated according to Langevin [35] (see Table 4). These ion/radical reactions have been included successfully in astrophysical model calculations to describe the formation of interstellar molecules [35-391. For the composition of the CI(CH,) plasma these ion/radical reactions are probably of minor importance, as only a small

3

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

no.

Eq.

and standard

reaction

H

enthalpies

-) C2H:’ + Hz + ‘H + CH4--, C2H: + ‘H -C,H:’ + H, +C2H: +H2+‘H + C2H:‘. + 2H2 CH: + CH,C2H: + Hz CH:’ + CH,+ CH: + ‘CHx

CH;’

Reaction

Rate constants

TABLE

(eV) a

- 0.99 - 3.80 -2.95d - 2.82 - 1.15 -4.19 - 4.04 -2.71 - 4.43 - 0.08 - 1.95” - 2.65 +0.11 - 0.04 - 1.07’ -0.17f

A@

of ion/molecule

0.18 0.82 < 0.03 0.37 0.60 0.25 0.75 0.05 0.84 0.11 0.23 0.42 0.22 0.13 1.oo 1.00 + 10%

+ 20% f 20%

1.2(-9)

1.2(-9) 1.5(-9)

300 300

298

300

1.3(-9)

f 20%

296

1.3( - 9) _+ 30%

298

3.q - 9) f 10%

T(R)

300

b f 10%

4.5(-9)

Rate constant

in the CH4 plasma

Distribution of products

reactions

SIFT SIFT

26 26

26

26

SIFT

ICR

26

26

26

Ref.

SIFT

ICR

DT

Method’

CH: + CH, + productsg C,H:’ + CH,--, CrH:’ + H, + CIH: + ‘H CSH: + CH,--, C3H: + H, C2H4+ + CH4 + no reaction CIH: + CH4+ C,H; + H, C2H: + O2 + no reaction CH: + 02+ no reaction CH:’ + O,--rO;’ + CH, CH: + H*O-+ H,O+ + CH, C2H: + H*O--* H90+ + CzH4 CH: + CO+ HCO+ + CH, CH: + CO2 - HCO; + ‘CH3 CH: + CO,+ HCO; + CH, HCO: + CH,--* CH: + CO2 1.00

1.00 1.oo 1.oo 1.oo 1.oo 1.00 1.oo

- 0.44 e

-0.54 - 1.87’sh - 0.45’” - 0.62 -0.13’*’ < f0.01 e,i > - 0.01 e,i

1.00

0.21 0.79 1.00

- 3.25 -0.93’ - 1.17

300 298 300 300 300 483 483 300 297 298 296 300 300 300

3.0( - 11) * 30% 8.4(- 10) +_ 10% 2.0( - 10) + 20% < l.O(- 13) l.O(- 14) + 10% <2.0(-11) <5.0(-11) 4.4( - 10) f 20% 3.7( - 9) + 25% 1.4( - 9) & 25% 9.9( - 10) f 20% 1.2(-9) f 20% 3.2( - 11) f 30% 7.8( - 10) f 20%

26 26 26 26 26 27 27 26 26 26 26 26 26 26

ICR ICR FA SIFT DT HPMS HPMS SIFT FA FA FA SIFT FA FA

a Standard reaction enthalpies AH? calculated from the standard enthalpies of formation AH? [28]. be.g. 4.5( - 9) means 4.5 x 10m9cm3 s-l. ‘DT, Drift tube; FA, flowing afterglow; HPMS, high-pressure mass spectrometry; ICR, ion cyclotron resonance mass spectrometry; SIFT, selected ion flow tube. “AHr*(CH:) calculated from AH 2, Eq. 31. ‘298K. ‘Ref. 29. gIsotopc exchange reaction. hPA(CH4) = 5.46eV, PA(H20) = 7.33 eV [30]. ‘Ref. 31.

32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

8

TABLE 4 Rate constants calculated for iron/radical reactions which cold occur in a CH4 plasma (from ref. 35) Eq. no.

Reaction

AH~ (eV)a

Rate constant (cm3S-- 1 )

47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

H+ + H+ ÷ H÷ + H+ + C +" + C+ +

-2.96 -3.13 - 3 18 -3.76 - 1.89 -0.62 -0.84 --3.00 -4.14 -2.28 - 3.39 -4.63 -4.67 -3.73 -3.76 - 1.90b -2.97 b -2.07 -3.42

1.9 × 1.4 × 1.4 × 3.4 X 3.8 x 3.8 x 5.2 x 5.2 x 1.3 × 7.4 × 1.0 x 7.2 × 1.0 X 7.1 × 9.9 × 6.9 × 9.6 X 6.4 x 8.8 X

'CH ----, CH + + "H CH + + H2 :CH2 ----* C H f + 'H " C H 3 "----* CH~ + "H "CH ~ C~-' + "H "CH ~ CH + + C C+'+ :CH2 ""-* CH~- + C C + + :CH2 ---* C2 H + + "H C + ' + "CH3 ----* C2H+" + "H CH + + "CH ----* C ~ ' + H 2 CH + + :CH2 -----* C2H+ + H 2 CH~-" + "CH ---* CzH+ + "H C H f ' + :CH: ~ C2H~- + "H CH~ + "CH ---* C2H~-+H2 CH~ + :CH2 ~ C2H~- + H2 CH5~ + "CH ---* CHf" + CH4 CH~ ÷ : C H 2 ~ CH¢ + C H 4 C 2 H f + "CH ----0 C3Hf" + 'H C z H f + :CH2 ----* C3H ¢- + "H "CH2 ~

10 -9

10 - 9 10 -9 10 -9

10-l° I0 -l° 10-l° 10-l° 10 -9

10-l° 10-9 10-l° 10 -9 10 -10

10-1° 10-l° 10 -10

10-l° 10 -10

"Standard reaction enthalpies AH~ calculated from the standard enthalpies of formation AHfe [28]. bAH~ (CH;) calculated from AH~, Eq. 31.

p o r t i o n o f C H 4 will be ionized [40] a n d c o n s e q u e n t l y a large a m o u n t o f u n - i o n i z e d CH4, c o m p a r e d with the radical c o n c e n t r a t i o n , will be present in the ion source. C o n s e q u e n t l y , the m o s t i m p o r t a n t a n d m o s t a b u n d a n t ions in the C H 4 p l a s m a will originate f r o m reactions with CH4, w h e r e a s i o n / r a d i c a l reactions m a y be responsible for ions o f low a b u n d a n c e , such as C 2 H + (Eqs. 54 a n d 57), a n d for some H - d e f i c i e n t larger ions (C4-C7, see T a b l e 6). T h e c o n c e n t r a t i o n o f negative ions in the C H 4 p l a s m a is extremely low (the total ion c u r r e n t is lower b y a f a c t o r o f 104 t h a n t h a t o f the positive ions [4]). T h e f o l l o w i n g relative a m o u n t s have been r e p o r t e d [4]: at 5 0 P a a n d 100°C: 6 3 % H - , 18% C - ' , 8 % C H - , 8 % C H ~ - ' , 3 % C H 3 ; a t 2 5 P a a n d 250°C: H n o t registered, 2 % C - ' , 4 % C H - , 15% C H 2 , 7 9 % C H 3 . T h e negative ions are f o r m e d m a i n l y d u r i n g the electron i m p a c t i o n i z a t i o n o f C H 4. F o r m a t i o n f r o m the c o r r e s p o n d i n g n e u t r a l particles b y r e s o n a n c e c a p t u r e o f t h e r m a l electrons is possible t h e r m o d y n a m i c a l l y as their electron affinities (EA) exceed zero { E A C H ) = 0.756 + 0 . 0 1 3 e V [28]; EA[C(3p)] = 1.263eV [41];

9 TABLE 5 Conversion of pressure gauge reading into real pressure values [5,47] Gauge reading (mtorr)

10 20 40 60 80 100 150 200 300 400 500 600 700 800 900 1000

Real pressure Ion source temperature 50°C

Ion source temperature

175°C

(mtorr)

(pa)

(mtorr)

(Pa)

5fl 9fl 21 f 1 31 f 1 40 f 1 48 * 1 72& 1 94 +_ 1 131 + 2 165 + 2 197 + 2 227 f 2 245 f 2 270 f 2 287 _+ 1 312 + 2

0.7 f 0.1 1.2 f 0.1 2.8 + 0.1 4.1 + 0.1 5.3 f 0.1 6.4 _+0.1 9.6 + 0.1 12.5 f 0.1 17.5 * 0.3 22.0 f 0.3 26.3 f 0.3 30.3 f 0.3 32.7 +_0.3 36.0 f 0.3 38.3 _+0.1 41.6 f 0.3

5fl II f0 21 f 1 32 + 0 41 +o 51 _+ 1 71*2 95 f 0 133* 1 167f 1 197 * 1 228 f 2 247 + 3 271 +2 289 &-2 312 + 1

0.7 ‘I 0.1 1.5 + 0 2.8 * 0.1 4.3 * 0 5.5 f 0 6.8 _+0.1 9.5 * 0.3 12.7 &-0 17.7 f 0.1 22.3 f 0.1 26.3 + 0.1 30.4 * 0.3 32.9 + 0.4 36.1 f 0.3 38.5 _+0.3 41.6 f 0.2

EA[‘CH(211)] = 1.238 + 0.008 eV [41]; EA[:CH,(3B,)] = 0.652 ) 0.006eV [41]; EA[‘CH,) = 0.08 f 0.03 eV [41]}. However, the rate constants are extremely small (k < 1.9 x lo-l5 cm3 s-’ [42]) and, consequently, reactions of the radicals with thermal electrons are not observed in the CH4 plasma [43]. The negative ions do not react with CH, (for a reaction of CH; with CH,, k $ 10-“cm3 s-’ has been determined [44]), but with traces of O2 and H20 (C + O2 --) O-’ + CO, H- + H20 + OH- + H, [45]). In the CH, plasma, negative ions are lost mainly by autoneutralization and by reactions with positive ions (ion/ion recombination [46]). B. Dependence on experimental parameters

This section is based on a recent comprehensive study [5] performed with a Finnigan (Finnigan-MAT, D-2800 Bremen, F.R.G.) 3200 instrument which allowed registration of masses 2 12 u only. All intensity values (% X,2) have been corrected for “C contributions. The standard deviation c lies between 1% and 3 % . The gas pressure values given in the figures are gauge readings. (For a transformation into absolute values, see Table 5; for further experimen-

163-16 100 200

LOO

600

800

1000

WmTorr)

Fig. 1. Percentage of total ionization of m/z 12 (C’ ‘), 13 (CH+) and 14 (CH: ‘) as a function of the CH4 pressure (ion source temperature 5O”C, electron energy lOOeV, repeller potential SV).

tal details, see ref. 47.) Pressure and temperature dependence of the various relative ion intensities agree well with the results of earlier studies [48-501. I. Pressure dependence of the positive ion currents

Figures l-3 show the dependence of ion currents of the primary ions C+’ (m/z 12), CH+ (m/z 13), CH:. (m/z 14), CH: (m/z 15), and CH: ’ (m/z 16) on the CH, pressure between 10 and 1OOOmtorr. With increasing pressure the number of collisions between the primary ions and CH, molecules increases. Consequently, reactions 2 l-3 1 lead to an increase of the portion of secondary ions. CHC and CH: ’ each give rise to only one secondary ion, i.e. C,Hc (m/z 29) and CH: (m/z 17), respectively (see Figs. 2 and 3). Studies of the reaction mechanisms may be found in ref. 51 and in refs. 29, 52 and 129, respectively. The ions C*H,f and CHC can also be formed according to Eqs. 18 and 26, but because of their minor importance and hence the resulting experimental errors these methods of formation cannot be separated from the main paths. Collisions of CH, with C+’ (for the mechanism see ref. 53), CH+ and CHZ’ lead to C,H,f * (m/z 26), C,H: (m/z 27), C, Hi ’ (m/z 28), and C,H5+ (m/z 29) (see

11

,r----++-_

---_+,

J

+

+ \: +

I

1Kl 200 I

I

I

LOO

,

I

600

8

I

800

m/z17

I

I

1000

Fig. 2. Percentage of total ionization of m/z 16 (CH:‘) and 17 (CH:) at ion source temperatures 50°C () and 175°C (----) as a function of the CH., pressure (electron energy 100 eV, repeller potential 5 V).

PcH (mTorr) 4

Fig. 3. Percentage of total ionization of m/z 15 (CH: ) and 29 (C,H: ) at ion source temperatures 5OT () and 175T (----) as a function of the CH4 pressure (electron energy 100 eV, repeller potential 5 V).

“&ZJ!?& I

la*‘! 100 203

LW &L(mTorr)

600

ml

1000

;

I

lal

,

2cc

,

,

,

Lo3

,

600

800

lccQ

F&(mTorr)

Fig. 4. Percentage of total ionization of m/z 27 (C,H:) and 28 (CzH:‘) at ion source tem) and 175T (----) as a function of the CH4 pressure (electron energy peratures 50°C (100 eV, repeller potential 5 V). Fig. 5. Percentage of total ionization of m/z 26 (CzH:‘) and 39 (C,Hc) at ion source temperatures 5OT () and 175“C (- - --) as a function of the CH, pressure (electron energy 100 eV, repeller potential 5 V).

Figs. 4 and 5). Some of the secondary ions may react again with CH, (Eqs. 33-35). This means that the ion current of the reactive secondary ions first increases with increasing CH, pressure, reaches a maximum and then decreases again [see Figs. 4-6 for the ions C2H: ’ and C2H3f, which give the tertiary ions C, H3+(m/z 39), C, Hl* (m/z 40) and C3Hc (m/z 41)]. The reaction of C2H: with CH, leads to an excited transition complex (C,H: )* which can be deactivated by unreactive collisions with CH, [54]. For this reason the portion of C,HT (m/z 43) in the total ion current increases with increasing CH, pressure (see Fig. 6). C3H: stems from C2Hz’, but it is not formed by a reaction with CH, (in SIFT and ICR experiments only the product ions C,H,+’ and C,H,+ have been recorded [26]). It is probably the result of a reaction of C,H:’ with C,H, [I 11;it has been postulated that the formation of C,H: occurs on r.f. discharge in CH, [l 11. C,H, is formed when C2Hc reacts with H,O (Eq. 42), and during the dissociative recombination of C2H: with thermal electrons (see Section IVA). The secondary ions CHC , C,H:. and C,HT react with CH, under CI conditions either at low rates or not at all (see Eqs. 32, 36 and 37). The primary, secondary and tertiary ions discussed so far yield 95.5% of the total ion current at, e.g. 400mtorr. The rest is made up from higher hydrocarbon species and from ions containing oxygen (these stem from impurities in the ion source). Hydrocarbon ions have been registered with masses up to m/z 125 (see Table 6), with intensities between lop4 and 10-20h and a total of 0.12% at 300mtorr and 0.20% at 1OOOmtorr. Higher-order

13

Fig. 6. Percentage of total ionization of m/z 41 (C,H:) and 43 (C,HT) at ion source tem) and 175°C (----) as a function of the CH4 pressure (electron energy peratures 50°C (100 eV, repeller potential 5 V).

endothermic reactions of C+’ , CH+ and CH:’ with CH4 have been suggested for the formation of these ions [48,50,125], but exothermic ion/radical reactions as insertions of :CH, may be even more likely (cf. Section IIIA). Oxygencontaining ions are formed by reactions of plasma ions with traces of H,O, 02, CO and CO,. These impurities stem from residual air in the mass spectrometer and (with the exception of CO) may also be introduced with the CH,. H,O is usually the main component of the impurities and can be protonated (H,O+) in fast reactions by CH: or C,H: (Eqs. 41 and 42). The intensity of H,O+ (m/z 19) has been found to be 4.38% of the total ion current at 400mtorr and it increases with increasing pressure. The intensities of the ions produced from reactions with 02, CO or CO, are much lower (less than 10-20h). 0, does not react with the main plasma ions CH; and C2H: (Eqs. 38 and 39); however, it can be ionized by charge exchange with CH:’ (0:‘) m/z 32). CO can be protonated by CH: (Eq. 43) and by CH:’ to give CHO+ (m/z 29), but it does not react with C2H,f [55,56]. The signals of CHO+ and of C2H,+ coincide under low-resolution conditions. CO, can also be protonated by CHZ’ (Eq. 44) or in an equilibrium reaction by CH: (Eqs. 45 and 46) to give HCOZ (m/z 45). Reactions of CO2 with C,Hz have not been observed [31,56]. N, does not show any fast reactions with ions of the

14 TABLE

6

Relative intensities m/z Ion

12 13 14 15 16 17 18 19 25 26 27 28 29 30 31 32 33 34 35 37 38 39 40 41 42 43 44 45 46 47 51 53 55 56 57 58 59 60 61 63 65

Cf. CH+ CH: CH; CH:’ CH; H,O+’ H,O+ CzH+ C2H:’ C,H; C2H: C2H$ C*H,f’ C2H; 0:’ CH,. CH: ? CH4 * H,O’ C3H+ C3H:’ C,H; CJH:’ C3H: C3H,t’ C3H: C3H,f’ C3H; CTH&‘? C3H;? C4H: C4H: C4H; C4H,t’ C,Hg+ C4H:,’ C,H:, C:’ C5H+ C,H: C,H:

(as percentages)”

of positive

ions in a CH, plasma

Wexler and Jesse [48] 140 eV, 27 Pa, (120 f 3yc

Field et al.

0.0061 0.010 0.088 0.78 0.20 32.6 _c _ 0.022 0.079 11.2 5.0 39.0 1.2 0.26 _ 0.32 0.42 7.5 0.041 0.61 0.29 0.063

Field and Munson [SO] 150 eV, 45 Pa, (210 * 1oyc

Poppe 151 100 eV, 22 Pa, 50°C 175OC

0.03 b 0.06 0.2 3.4 1.9 54.4 0.1 0.2 _

0.0077 0.022 0.077 0.61 0.40 35.1 -

0.0054 0.016 0.052 0.83 0.89 43.3 _

0.015 0.038 0.13 1.45 1.44 46.7

4.38

0.2 5.9 1.9 29.2 0.02 0.004

0.047 3.9 3.2 44.0

2.21 0.0005 0.076 3.42 2.68 36.5

[491 70 eV, 28 Pa, x 210°c

0.22 _

0.035 1.32 3.23 37.9 _

0.085

0.016 0.0024 0.0036 0.0007 0.013 0.0084

_

_ -

_ 0.08 0.09 1.0

-

0.47

0.32

0.15

0.0019 0.008 0.0029

0.005 0.0003 0.0013

-

0.0029 0.022 0.0016 0.0041

_ -

_ 0.002 0.001 0.002

_ -

-

0.0012 0.029 0.0030 0.023 0.0007 0.0015 0.0005 0.0005

0.063 _ 0.055 _ -

0.0005 0.11 0.18 4.84

0.059 0.19 7.30

0.022 0.029 0.016 0.039 0.012 0.063 0.010

-

0.16 0.29 8.6 -

0.06

0.011 0.0052 0.0015 0.0002 0.0002

_

0.0013 0.0086 0.0006 0.0055 0.0002 0.0012 0.0005 -

15 TABLE 6 (continued) m/z Ion

67 69 71 72 73 74 75 76 77 79 81 83 85 87 89 91 93 95 97 111 121 125

C5H: C5H,+ CSH:, C;’ C6H+ C6H: CsH: CsH: CsH: CsH: CsHg+ CsH:, C,H+ C,H; C,H: C,H: C,Hg+ C,H:, C,H:3 CsH; C,H:, CgH:,

Wexler and Jesse [48] 140 eV, 27 Pa, (120 + 3yc

Field et al. [491 70 eV, 28 Pa, x 210°C

0.0029 0.00041 0.00061 0.0016 0.0029 0.0061 0.012 0.0061 0.0012 0.0012 0.0031 0.016

-

Poppe 151 100 eV, 22 Pa, 5o”c 175OC

-

0.0020 0.012 0.012 0.016

Field and Munson [50] 150 eV, 45 Pa, (210 * lO>oC

0.0010 0.0009 0.0006

0.007 0.006

0.002 0.001 -

-

0.0003 0.0004 0.0002 0.0004 0.0002

0.0005 0.0004 -

-

-

-

0.0002

0.001 0.001

0.0002 0.0003

-

0.0007 0.001 0.002 0.0007 0.0007

-

-

-

a Corrected for “C contributions. bIons with intensities less than 0.001% not reported. ’ -, Not observed or reported.

CH, plasma [32]; nitrogen-containing tered.

ions have, therefore, not been encoun-

2. Temperature dependence of the positive ion currents The ion/molecule reactions which take place in the CH, plasma (see Table 3) are fast and exothermic or slightly endothermic, and need only a very small activation energy or none at all. In addition, CH, does not have a permanent dipole moment [57] and hence, theoretically, the rate constants of these reactions should be temperature independent [58,59]. This is in contrast to the observation that the pressure dependences of the positive ion currents are significantly different at 50 and 175°C. At an ion source temperature of 175OC the ion currents of the primary ions C+ ’ , CH+ , CHZ ’ , CH,f (Fig. 3) and CH,f ’

16

(Fig. 2) decrease less sharply with increase of the CH, pressure than at 50°C and they approach the 50°C value only at about IOOOmtorr. This observation may be interpreted as an indication that increase of the ion source temperature reduces the probability for secondary and tertiary reactions. Consequently, the ion currents of the latter are lower at 175OCthan at 50°C, especially at a pressure of 200mtorr. Also, the maxima of the ion currents of the reactive secondary ions are observed at higher pressures (see Figs. 2-6). A temperature increase has the following effects on the ion currents of the plasma ions: (1) The kinetic energy of the ions increases. With a constant influx of CH, into the ion source the residence time of the ions in the source is reduced and, consequently, the probability of ion/molecule reactions decreases. (2) An increase of the kinetic energy of the particles enhances the probability for collisions, and the probability for the occurrence of ion/molecule reactions grows. (3) The rate constant of an ion/molecule reaction which is collision controlled at room temperature may become temperature dependent starting from a certain temperature if the reaction includes transition complexes [60,61]. The reason is that the formation of the transition complexes is impeded by the higher rotational energies of the colliding particles. It follows that the rate constant decreases with increase in temperature. Kinetic measurements for the ion/molecule reactions in a CH, plasma are not available [26]. It is known that reaction 31 includes a transition complex [29,52,129] whereas reaction 30 does not [51]. The relative influence of the three factors mentioned on the ion/molecule reactions in the CH, plasma is not known. A more detailed discussion of the temperature effects is, therefore, not warranted. 3. Dependence of the positive ion currents on the repeller potential An increase of the repeller potential increases the acceleration of ions towards the exit slit and thus reduces their residence time in the ionization chamber [62, 631,which in turn results in a reduced collision probability. At the same time, the kinetic energy of the ions is increased; during collision with neutral species this energy can be transformed into internal energy [63-651. This opens the possibility for plasma ions to undergo endothermic reactions with CH4. In Figs. 7-9 the dependence of the ion currents of the main plasma ions on the repeller potential is shown for various pressures. It can be seen that up to 5 V the influence of the repeller potential is rather small, as the ionization chamber has also a small positive potential (2-5 V). At higher values, the percentage of primary ions increases and the percentages of unreactive secondary (CH: , C,Hi ’ , C,H5+ ) and tertiary ecrease because of the reduced reaction ions(C,H,+,C,H:‘,C,H,+,C,H,+)d

17

.-_.-. FJgg* / I 0 5

m/z14

m,z 27 _-_‘___+___‘__,_---~ .-.-

repeller

/ IO

/

I 15

I 20

potentId

(V)

I 25

I 30

25

30

% 212

0

5 repeller

10

15

potential

20 (V)

Fig. 7. Percentage of total ionization of several CH4 plasma ions as a function of the repeller potential (pCH480mtorr, ion source temperature SOT).

rate. The percentage of the reactive species (C,H:’ , C,H:) does not show a constant trend, because with increasing repeller potential the probabilities of their formation and of reactions leading to tertiary ions decrease. Deviations from the described behavior are observed at high pressures (300 and 700 mtorr) and a repeller potential above 15 V. Most striking is the increase in the relative portions of the ion currents of CH: and CH,f ’ and the strong increase of that of C2H3+ (see Figs. 8 and 9). This behavior can be explained by an increasing importance of the endothermic reactions 66 [6669] and 67 [33,69,70] CH:’ + CH,d

CHC + H’ + CH, - 1.37eV

(66)

CH: + CH,--,

C,Hl

(67)

+ 2H2 - 1.6OeV

These reactions compete with reactions 30 and 31. An increased repeller potential therefore reduces the production of the ions CHZ and C,Hf .

0

0

5 repeller

10

15

potential

20

25

30

repeller

(V)

potentiai

“/. 212

PCH 3 7mmTorr

1.0 I

f ,: / .;

,0,6-l

20 repeller

potential

(V)

I

25

(V)

mh.43

i

,‘;nlZ39

30 repeller

potential

(V)

Fig. 8. Percentage of total ionization of several CH, plasma ions as a function of the repeller potential (pCH4300 mtorr, ion source temperature SOT). Fig. 9. Percentage of total ionization of several CH4 plasma ions as a function of the repeller potential (pCH4700mtorr, ion source temperature 5OT).

4. Dependence of the positive ion currents on the electron energy

Different energies of the ionizing electrons result in different rates for the formation of the primary ions of CH, (see Table 7), which in turn influences the portion of secondary and tertiary ions present in the CH, plasma. During electron impact ionization, the electron beam passes through the entire ion source and only part of it interacts with the sample molecules. Under CI conditions the electrons are deprived of most of their energy shortly after entering the ion source and never reach the exit slit [71,72]. As the depth to which the electrons enter the source dependson their original energy the

19 TABLE I Energy dependence of the ionization cross-sections (units lo-l6 cm2) during the electron impact ionization of CH, (from ref. 8) and relative rates of formation normalized to 100 for each energy

E W)

a(C’.)

a(CH+)

a(CH;‘)

a(CH: )

o(CH$‘)

CT

20 50 100 150 200

0.00 0.028 0.043 0.038 0.031

0.001 0.091 0.109 0.093 0.077

0.026 0.238 0.260 0.232 0.208

0.40 1.65 1.82 1.71 1.56

0.51 1.69 1.81 1.68 1.52

0.94 3.69 4.04 3.75 3.40

E WI

(C”)

WI+)

O-G’)

(CC )

(CH:

20 50 100 150 200

0 0.8 1.1 1.0 0.9

0.1 2.5 2.7 2.5 2.3

2.8 6.4 6.4 6.2 6.1

42.6 44.7 45.0 45.6 45.9

54.3 45.8 44.8 44.8 44.7

-

100 100 100 100 100

region of ion formation will be determined by that energy [71,72]. Consequently, the electron energy in connection with electric lield gradient (in ref. 5 this results from an interplay of extraction, repeller and ionization chamber potentials, and has a minimum in the middle of the chamber) which prevails at the region of ion formation influences the residence time of the ions in the source. In Figs. lo-12 the ion currents of the common plasma ions are given as functions of the electron energy at CH, pressures of 100, 300 and 700mtorr. It can be seen that the largest effects were observed between 20 and 50eV. Beyond 50 eV the influence of the electron energy is rather small. In accordance with the data given in Table 7, at 50eV the portions of the ion current of c+., CH+ and CH: ’ and those of the secondary ions C,H: ’ , C,Ht and C,H:. derived from them are larger than at 20 eV, and those of CH: (except for 100 mtorr) and of CH$ ’ (and consequently of CH: derived from it) are smaller. The same dependence holds for the tertiary ions. The dependence of CH: and C*H,f on the electron energy, especially at 100 mtorr (see Fig. IO), cannot be reconciled with the ion intensities given in Table 7. For these anomalies the space effects mentioned above are responsible. At 20eV, ion formation occurs in the vicinity of the electron entrance slit, and because of a steeper gradient of the electric field caused by the geometry of the ion source [5] they reach the ion exit slit faster than ions which are formed closer to the middle of the ion source (50 eV). The thus increased residence time at 50 eV offers a higher probability for reactions of the primary ions. Hence, the fraction of CH: (and consequently also of C,H:) is larger at 20eV than at

20 % ~12 ,50

PCHL~100 reTort

Pc~&300 mTorr

60

40 30-

/ * - ~

/ 20 -

m/z 29 ......

% Fm/z 15 F m /z 27

j"~\

0

~ ~-~

i 2O

q 5O e(ectron

~m_/z 16

i i lO0 150 energy (eV)

20

I 2OO

Fro/z28

/

i 101 \m/zl5 0L

,~-

20 °/o if-12

PCH~ 100 mTorr

\

vm/z27

~ \

\ _\

L

50 etectron

~rn/z 16 \ m/z/,t

,

~

100 150 energy (eV)

%'~12 0,8 q6 q4

- " 4

q

PCHt.-&300 reTort

m~z~ ,~X_._ ~ n ~. . . . . . . " ~ ~mtz3g . __~__n',/zl2 . . . .

20

,

200

50

electron

100 150 energy (eV)

200

m! 0~10-]

/

0/

/

ml~'~ I

20

m/z 39 ...................................... I I I

50 electron

100 150 energy (eV)

~ / z 26 I

200

Fig. 10. Percentage of total ionization of several C H 4 plasma ions as a function of the electron energy (PcH4 100mtorr, ion source temperature 50°C, repeller potential 5V). Fig. 11. Percentage of total ionization of several CH4 plasma ions as a function of the electron energy (pcH4 300 mtorr, ion source temperature 50°C, repeller potential 5 V). 50 eV despite the lower rate Of formation (see Table 7). Certainly, the fractions of C ÷ ", CH ÷ , CH~-" and CH~-" are influenced by the locus o f formation also, but that factor is outweighed by the direct influence of the electron energy, as the change of formation rates is much larger than for CH~- when going from 20 to 50 eV. At a pressure of 100 mtorr the primary ions (except for C ÷" and CH -~) reach a minimum and the secondary and tertiary ions a maximum at an electron energy of 100 eV. Again, space effects can be invoked (formation by electron impact at a most distant point from the exit slit and hence a maximal residence time, which results in a high probability o f reaction with CH4). IV. NEUTRAL COMPONENTS OF THE

CH 4

PLASMA

A. Formation of neutral particles The concentration of neutral particles such a s H 2 or CH3 in the C H 4 plasma exceeds that of the ions by a factor of between 7 and 80 [73]. They are formed not only during the electron impact ionization of CH4 (see Section II) or by ion/molecule reactions (see Table 4), but also by neutralization of ions on the

21 %212

P,g700

mTorr

60 I\

I

/ 0

<

20

-I-+-+

50

I 100

electron energy % 112

m/z26 I 200

I 150 (eV)

fL~700rnTorr .I

,I

~1~~ 20

50

100

150

Fig. 12. Percentage of total ionization of several CH4 plasma ions as a function of the electron energy (f+ 700mtorr, ion source temperature SOT, repeller potential 5 V).

of the ion source (Eq. 68) [74] and by dissociative recombination positive ions and electrons [75-781 walls

C2HSf + e-a

‘C,H,

of (68)

According to an estimate by McEwen and Rudat [73], 80-90% of the ions formed originally in the CI source are lost by dissociative recombination. This is in accordance with the high rates of these reactions (see Table 8), which depend on the energy content of the reacting particles and on that of the reaction medium [76]. These energies are usually expressed in units of kinetic temperature (E = k, T, where k, is Boltzmann’s constant). When the rate constants were determined experimentally two cases were distinguished [76]: TE= T, = TG(TE, TIand TGare the kinetic temperatures of the electrons, the ions and the gas; rate constant is aT) and TE> T,, TG(rate constant is aE) (see Table 8). Knowledge of the electron energy distribution has, therefore, a

22

TABLE 8 Rate constants electrons

(cm’s_‘)

for the dissociative recombination

Ion

Rate constant

CH+ CH; CH; CH: ’ CH; CH; CH: C,H;’ C,H: C,H: GH:

CIE= GIE= c$ = aE= tlE = cr, = L?+= c+ = I+ = UT= dlT=

3.0 x lo-’ x (3OO/T#4 5 x lo-’ x (300/T,)“,5 7.0 x lo-’ x (300/TE)o,5 7.0 x IO-’ x (300/TE)o,5 7.0 x lo-’ x (300/TE)o,5 1.5 x 1o-6 1.1 x 1o-6 5.4 x lo-’ x (300/TE)‘.’ 9 x lo-’ x (300/T,)“.5 7.4 x lo-’ 3.5 x lo-’

of positive plasma ions and

Energy range (K)

Ref.

75

TE < TE < TE < TE = TE = TE < T, < TE =

2000 1000 1000 95

300 1000 1000 300 T,=300

75 75 75 75 76 76 75 75 78 78

fundamental importance in evaluation of the processes in the CH4 plasma. Electrons which enter a CI gas are thermalized quickly, and the consequence of this is approximately a Boltzmann distribution of their energies [79-811. The exact energy distribution depends on the energy of the primary electrons, the reagent gas [80,81], its pressure, the ion source temperature and the electric fields in the ion source [82]. Crow could show [83] that in a CH, plasma (760 mtorr, primary electron energy 100 eV, source temperature 13O”C, emission 0.2 mA) 53% of the electrons had an energy less than 0.15 eV (1740 K) and 65% had an energy between 0.035 and 0.45 eV (406 and 5221 K). Gregor and Guilhaus [84] found for 45% of the electrons energies less than 0.03 eV (348 K) and for 95% energies less than 0.10 eV (1160 K) (100 mtorr, 50 eV, 230°C 0.5 mA). Obviously, the majority of electrons in a CH, plasma possess energies which are only slightly above the thermal energy of the medium. The rate constants for dissociative recombination for the various ions in a CH, plasma do not differ greatly from each other (see Table 8) and show only a small energy dependence (e.g. for CH: k(300 K) = 7.0 x lop7 and k(lOOOK) = 3.8 x 10P7cm3s-‘). Their average value (roughly 5 x lo-‘cm3s-‘) is about two orders of magnitude higher than that of ion/molecule reactions. Nevertheless, dissociative recombination is of importance only for ions which do not react with CH, (essentially CH: and C,H: , the main plasma ions), as only a small part of the plasma is ionized and hence there are many more CH, molecules than electrons [40,85] (dissociative recombination and ion/molecule reactions are both pseudo-first order; therefore ion/molecule reactions are more likely for [CH,] $ [e-l). There are no experimental data available on the products formed during dissociative recombination of positive ions. Herbst

23

[86] and Bates [77] suggested that products which require the fewest bond cleavages are formed preferentially. According to Bates [77], the electron is incorporated into an antibonding orbital and another electron is promoted into the same orbital. CH: may serve as an example. The CHC species is an (ion-induced) dipole complex between CH: and H, (CH: *HZ). Recombination with an electron abolishes the ion-induced dipole attraction and the complex decomposes into :CH, + ‘H + H2 (see Eq. 74). Analogous considerations lead to the proposal of Eqs. 69-83 for the dissociative recombination of a series of hydrocarbon ions [77].

CH+ + eCH:’ + eCH: + eCH:‘(E

CH: *H) + e-

CH: (E CH: *HI) + e-

+C + + + + + --, +

C,H: (E H,C=CH+)

+ e---t + +

C,Hc (E H,C-CH:)

+ e- + + + +

+ ‘H

‘CH :CH, :CH, ‘CH3 :CH1 CH, ‘CH3

+ ‘H + ‘I-I +‘H+‘H

(main reaction) (side reaction) + ‘H (main reaction) f ‘H + H, (side reaction) + ‘I-I (side reaction) +H, (main reaction) H,C=C: + ‘H (main reaction) + ‘CH :CH, HCrC’ (side reaction) i- HI(2.H) (main reaction) H,C-CH: + ‘H (main reaction) :CH, ‘CH3 + (side reaction) H,C=CH’ + I-W=) HCECH + ‘H + H,(2’H) (side reaction)

(69) (70) (71) (72) (73) (74) (75) (76) (77) (78) (79) (80) (81) (82) (83)

According to Eq. 80, the product of dissociative recombination of C2H: is methyl carbene (CH,-CH:). Ab initio calculations show that this species is neither stable nor metastable in the gas phase because it isomerizes quickly by a 1,2-hydrogen shift to give ethylene. The activation energy for this isomerization is rather high (53 kcalmol-’ [87]) starting from the triplet ground state (3H3C-.CH: + 3H,C=CH2), but there is no energy barrier for the same reaction in the singlet state [88]. As the energy difference between 3H3C-CH: and ‘H,C-CH: is only 10 kcalmol-’ [89], the metastable triplet methyl carbene also isomerizes readily to ethylene by intersystem crossing (3H3CCH: + ‘H,C-CH:) [89]. The “classical” structure of C,Hc is in an equilibrium with an H-bridged structure which may form CH,=CH, directly during dissociative recombination (Eq. 84) [90]

24

H,C-CH:

eH,C

. . . . CH2 e-

H,C=CH*

+ H’

(84)

An indication of the formation of C2H, is the presence of C, Hl in the CH, plasma (cf. Section IIIBl), which is formed by reaction of C2HT ’ with C2H4 but not by reaction of CH, plasma ions with CH,, as detailed studies have shown [26]. It should be mentioned that C2H, may also be formed by reaction of ‘CH with CH, (see Section IVB) and of C2H: with H,O (see Section IIIBl). (Triplet) vinylidene CH,=C: (see Eq. 77) could be generated and identified by neutralization-reionization mass spectrometry [ 1261 {calculated singlet (ground state)-triplet separation is 49.6 kcal mol-’ [127], and calculated barriers for isomerization to acetylene are 4 kcalmol-’ or lower and 55 kcal mol - ’ , respectively [ 1261). B. Reactions of neutral particles

The neutral particles of the CH, plasma (see Section WA) may participate in ion/molecule reactions (see Sections IIIA, IIIBl and IVA), and in ion/radical reactions (see Section IIIA), as well as in radical/molecule reactions (see Table 9) and in radical/radical reactions (see Table 10). Radical/molecule reactions which could occur in the CH4 plasma-with the exception of the diffusion-controlled reaction of ‘CH(X’II) with CH, according to Eq. 89-are slow processes (k < lo-i2cm3 s-’ for reactions of C, :CH, and ‘CH, with CH,; Eqs. 85-87 and 90-93). Hence they are probably of minor importance only. Also, radical/radical reactions are slow (k N 10-l’ cm3s-‘, see Table 10) and hence of little importance as far as the composition of the CH, plasma is concerned. Recombination of 2 ‘CH, yields C2H6, which reacts with CHC to give C2H: [34] (see Section IIIA)---a minor component of the CH, plasma (see Table 6). V. RADICAL

ADDITIONS

TO SUBSTRATE

MOLECULES

Possible reactions of radical species present in the CH, plasma have been studied by McEwen and Rudat [73,102], using tetracyanoquinodimethane (TCNQ) as a radical trap. They observed several series of ions, such as [M + RI- and [M + R - CN”’ as well as [M + R + HI+‘, [M + R - CN]+ mainly R = H, CH3, C2H,), for the formation of which and[M+R+2H]+( they invoked reactions of TCNQ-a well-known radical trap-with radical species before ionization by electron capture or protonation (see Scheme 1). They could corroborate their suggestion by showing that the reactions of

[93]

k = 4.6 x 10-‘9cm3sm’ (500 K)

[97]

[98]

(300K)

k = 3.5 x 10-‘9cm3s-’

[96]

(300K)

(300K)

k = (1.21 + 0.36) x IO-“cm3s-’

[96]

[94]

“The ground electronic state is C(3P). b ‘CH(A’A) is the first excited electronic state of the ‘CH radical, but less reactive than the ground state ‘CH(X’lT) [95]. ‘The ground electronic state is :CH2(X3B’).

CH, + ‘CH,

‘CH, + ‘CH3

:CH2(X3Bl) + CH,+

‘CH3 + CH,+

s-’ (300 K)

k = (5.93 f 1.71) x IO-“cm3s-’

:CH,(X3B,) + CH4

‘CH3 + ‘CH,

(4 ‘CH, /W

k = (2.3 + 0.15) x lo-“cm3

[92]

[91]

(‘C2H5*)+ C2H4 + ‘H [Ml ‘C2H5 k = (0.96 + 0.08) x 10-‘0cm3s-1 (297K)

:CH2(I’A,) + CH,--r

(c) :CH2/CHdC :CHr(a’A,) + CH,+

‘CH(X’l-I) + CH, -

[91]

(300K)

(300K)

k < 10m’2cm3s-’ (300 K)

CH(X*lT) + CH4

products

(b) ‘CH/CH, b ‘CH(A2A) + CH,+

C(S) + CH4+

k < 5 x 10-‘5cm3s-’

reactions which could occur in a CH4 plasma

+ H2 k = 3.2 x IO-“cm3s-’

C,H,(?)

C(‘D) + CH4+C2H2

C(3P) + CH,--,

Rate constants of radical/molecule

TABLE 9

(93)

(92)

(91)

(90)

(89)

(f-w

(87)

(86)

(85)

26

21 -CN. [TcNQI - -

[TCNQ+Rlm/z 205, 219, 233

[TCNQ+R-CNI-. m/z 179, 193, 207

e-

e-

e-

(l0-'cm~~s-~l

(lo-'cm3

m/z

204

T

(lo-'cnP~s-')

T (10-'0cm3

TCNQ

+

-CN.

-

R.

T

.s-'1

-

[TCNQ+RI-

[TCNQ+R-CNI

1

li’

i

es-'1

1

B’ (lO-~ca3.s-')

(10-9cnP*s-')

Ii’ (lo-9cmJ*s-'1

-CN. w [TCNQ+R+HI+. m/z 206, 220, 234

[TCNQ+Hl+ m/z 205

1

Cli4 (lcf-~cm3es-1

[TCNQ+R+2Hl

l

+

[TCNQ+R+H-CN]* m/z 180. 194, 208

(7))

-CHa

m/z 207. 221, 235

Scheme 1.

TCNQ with alkyl radicals are very fast (k N 10-‘0cm3 s-‘) and can be observed also outside the ion source [73]. The reaction sequences depicted in Scheme 1, which require in part three successive collisions, have been modified (Scheme 2), as it could be shown that TCNQ can be reduced to TCNQH, by hydrogen adsorbed on metal surfaces in the ion source (which stems partially from the reagent gas and partially from H,O as demonstrated by experiments with D,O and CD, [47]): ions as [M + R + Hz]+ apparently originate from TCNQH, . (lo-*cm”

[TCNQI-. m/z 204

+

-

R-

-CN-

*s-‘)

LTCNQ+Rlm/z 205. 219. 233 245. 247

[TCNQ+R-CNI‘. m/z 179, 193, 207 219, 221

e(lo-'cm~*s-') T

Hr. wall -

TCNQ

I-

(10-9cnP~s-'~ H'or R'

TCNQHn

-

[TCNQ+R+SHI* m/z 207. 235. 247

n*

(lo-~cm

LTCNQ+HI’

m/z 205

*S-I

) (lo-‘cm”

+

R-

_?*

15-l)

-CN[TCNQ+R+Hl'. m/z 206, 220, 234

IR'ITCNQ+R'+R+HI* m/z 221, 235, 249, 263

Scheme 2.

[TCNQ+R+H-CNI. m/z 180, 194, 208

28

The mechanism proposed by McEwen and Rudat [73,102] leaves open the question of why the formation of [M + RI- ions can compete with that of M-’ despite the high electron affinity of TCNQ [103,104], which should result in much faster addition of an electron (k N 10-7cm3s-‘) [105] than radical addition with subsequent ionization (k N lo-“cm3 s-‘) (see Scheme 1). This discrepancy can be resolved if radical addition subsequent to the ionization is taken into account. Similarly, the formation of the positive adduct ions can be explained more readily by ion/radical (k N 10-9cm3s-‘) rather than by molecule/radical reactions (k N lo-” cm3 s-‘) (see Scheme 2). In this context it should be mentioned that for certain aromatic sulfur compounds convincing evidence has been offered that [M + HI- is formed by a reaction of M-’ with CH, and not by a reaction with H’ before or after the ionization [ 1241. [M + C,HJ and [M + CnHIn+,]+ ions observed with aromatic compounds will be discussed in the following section. VI. SURFACE-CATALYZED CI(CH,) CONDITIONS

REACTIONS

OF SUBSTRATE

MOLECULES

UNDER

Undesirable side reactions of substrate molecules subjected to chemical ionization with CH, are observed occasionally; these are caused by metalsurface-catalyzed processes, the most prominent of which are hydrogenation (phenazines [ 1061,quinones [4], TCNQ-see Section V [47]), dehydrogenation (quinoline derivatives [ 1061, hydroquinones [4]) and oxidation [4,107,108]. [M + 14]-’ and [M + 15]+ ions are a special problem. Compounds which have an allylic or benzylic CH, group can be oxidized to give [M - 2H + O]- ’ by formation of a CO group [4,107]. In a detailed study [log] it could be shown that the [M + 14]-’ species observed in the CI(CH,) spectrum of fluorene is identical with M-’ of 9-fluorenone formed by a surface-catalyzed oxidation (see also [108] regarding [M - 2H + O]+’ in the CI(O,/N*) spectrum of fluorene). For a series of polycondensated aromatic hydrocarbons (PAHs), [M + 13]- ([M - 3H + O]-) [llO], [M + 15]- ([M - H + O]-) [llO,lll], [M + 30]-’ ([M - 2H + 20]-‘) [107] and [M + 32]-’ ([M + O,]-‘) [107] ions have been mentioned. In specific cases it could be shown [107] that [M - 2H + O]-’ ions are formed by a surface-catalyzed oxidation (e.g. anthracene + anthraquinone) and subsequent ionization by electron capture, whereas [M - H + O]- ions are formed in the gas phase by reaction of the neutral molecules with O-’ (from traces of 0,). It is likely that [M + O,]-’ ions observed in the CI(CH,) spectra of some PAHs are also formed in the gas phase [ 1071. These oxidation products have been mentioned because they have been observed in CI(CH,) spectra. Without exact mass measurements or evidence from isotope labeling studies they may be confused readily with species

29

stemming from an addition of hydrocarbon moieties. Thus, homologous series [M + 14n]-’ identified as [M + C,H,,]-’ have been reported for PAHs [l 1 l-l 151,heteroaromatics [106] and transition metal complexes mostly with aromatic electron donor ligands [116]. Their positive counterparts [M + CnHzn+,]+ have been observed for PAHs [ill], I-cyano-naphthalene [117] and for those heteroaromatics which yield [M + C,H,,]-’ ions [106]. Two mechanisms, one comprising an addition/abstraction (Eq. 98) and the other an insertion/addition sequence (Eq. 99) have been proposed [106,112]:

-

M + ‘CJL+,

(M + CnHzn+,)’ (M

+

C,H,,)

-H’

W + WGJ

(984

5

[M + WL-’

(98b)

H+

M+‘CH

-

[M + CnHzn+,l+

(98~)

(M + CH)’

G-I&~-I -

(M + C,Hzn)

(99a)

(M + C,H,,)

5

[M +

CnH,nl-’

@9b)

[M

Wbn+,l+

(99~)

H+ +

Both mechanisms can be reconciled with results obtained when CD, was used as a reagent gas [106,112,114,116]. There are, however, two problems, at least as far as N-heterocycles are concerned: the intensity of higher members of the series [M + C,H2,]-’ is much higher ([M + C7H,Jis about 3% of [M + CH,]-’ [106]) than the relative intensity of C,H,+,+, ions in the plasma -provided the C, Hi,, , radicals are formed from the corresponding ions by electron capture (wall or gas phase; see Section IV). In addition, it has been shown that these compounds do not react at all (or at best very slowly) with alkyl radicals in the gas phase [ 1181. One might consider, therefore, a one-step mechanism which is also more feasible for kinetic reasons. It would consist of an insertion of singlet :CH, or of its higher homologs (H,C-CH:, (H3CJZC:, etc.) into activated C-H or O-H bonds of the substrate molecules with subsequent ionization by electron capture or protonation. The drawback of this mechanism is that H,C-CH: is neither stable nor metastable in the gas phase (see Section IVA). If one assumes gas-phase reactions one can explain the formation of only [M + CH,]-’ and of [M + CH,]+ in this way [112,118]. It is, however, a different story if one considers surface reactions. :CH, and its higher homologs adsorbed on surfaces have been discussed as reactive intermediates for various hydrocarbon reactions, such as, e.g. :CH, in the Fischer-Tropsch process [ 1191, or the sequence depicted in Eq. 100 for the catalytic hydrogenation of CH,=CH, [ 1201 (H,C=CH&,

-

(H, C-CH&,

-

(H, C-CH:),,, -

(H, C-C’ )ads

(100)

30

:CH, and H,C-CH: are relatively stable p2-ligands in transition metal clusters [ 12 l-l 231. They are formed, among other means, by intramolecular hydrogen migration from the corresponding alkyl complexes (p-CH, + p2 -CH, [121], &,H, -+ p2-CH-CH, [122]. Reactions 101-103 seem, therefore, possible on metal surfaces in the ion source CH; , CH: -

‘CH, +

(‘CH3),d,--+ (:CH2),d,

‘CH, (from ion/molecule reactions) C2H; -

‘C,H,-

(‘C2HS)ads-

(‘CH,),,, -

(H,C-CH:),,,

(101) (:CH2),,,

(102) (103)

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