Reaction of C+ with CH4 in the 2–200eV Energy RangePatricia Sullivan Wilson, R. W. Rozett, and W. S. Koski Citation: The Journal of Chemical Physics 52, 5321 (1970); doi: 10.1063/1.1672781 View online: http://dx.doi.org/10.1063/1.1672781 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/52/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photoabsorption and S 2p photoionization of the SF6 molecule: Resonances in the excitation energyrange of 200–280 eV J. Chem. Phys. 134, 174311 (2011); 10.1063/1.3583815 Complex formation in the C+(2 P)+CH4 reaction at 0.1 to 10 eV J. Chem. Phys. 65, 3665 (1976); 10.1063/1.433554 Negativecharge production by hydrogen atoms in Ar, Kr, Xe, CO, CO2, and CH4 in the energy range80–2000 eV J. Chem. Phys. 58, 5111 (1973); 10.1063/1.1679101 Energy Dependence for Single and Double Ionization by Electron Impact Between 250 and 2200 eV J. Chem. Phys. 48, 478 (1968); 10.1063/1.1667949 Reactions of Photochemically Produced Hot Hydrogen Atoms. II. Reaction and Moderation Probabilitiesfor 3eV H Atoms in D2, CD4, and C2D6 and for 3eV D Atoms in H2, CH4, and C2H6 J. Chem. Phys. 40, 3007 (1964); 10.1063/1.1724941

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CHEMICAL KINETICS OF ENERGETIC ATOMS. IV 5321

23 M. D. Kostin, J. Chern. Phys. 46, 1316 (1967). 24 P. Laasonen, J. Assoc. Computing Mach. 5, 32 (1958). 25 L. Lapidus, Digital Computation jor Chemical Engineers

(McGraw-Hill, New York, 1962). 26 M. Karplus, R. N. Porter, and R. D. Sharma, J. Chern. Phys.

43,3259 (1965); 45, 3871 (1966).

THE JOURNAL OF CHEMICAL PHYSICS

27 E. A. Mason, J. T. Vanderslice, and C. J. G. Raw, J. Chern. Phys. 40, 2153 (1964).

28 L. I. Schiff, Phys. Rev. 103,443 (1956). 29 E. P. Wigner and J. E. Wilkins, U.S. At. Energy Comm.

Rept. AECD 2275 (1944).

VOLUME 52. NUMBER 10 15 MAY 1970

Reaction of C+ with CH 4 in the 2-200-eV Energy Range

PATRICIA SULLIVAN WILSON, R. W. ROZETT,* AND W. S. KOSKI

Department of Chemistry, The Johns II opkins University, Baltimore, Maryland 21218

(Received 12 January 1970)

The reaction of C+ (2 P) with C~ has been investigated in the 2-200-eV energy region using a tandem mass spectrometer. All possible ions containing one or two carbon atoms were observed except C2~+. The behavior of the cross sections as a function of the kinetic energy of the incident ion was used to infer reaction mechanism. Evidence was obtained for the presence of a complex, stripping and binary collision mechanisms.

INTRODUCTION

With the exception of a number of charge transfer reactions!·2 very few ion-molecule reactions of C+ ions, especially in the low-energy region, have been reported. Recently Pohlit et al.3•4 irradiated solid targets of ben­zene with monoenergetic 14C+ ions in the O.l-lS-keV energy region. In recent studies Lao et al.5 studied the reactions of C+ ions with N2 and O2 in the gas phase covering the energy range from 2-200 eV. In view of the importance of this ion as a precursor in a number of organic reactions involving hot carbon atoms and since the study of ion molecule reactions of C+ is of interest in its own right, we have investigated the reactions of this ion with methane in the 2-200-eV energy range using a tandem mass spectrometer. All possible product ions containing one or two carbon atoms were observed with the exception of C2H4+' The variation of cross sec­tion for the production of such ions as a function of the kinetic energy of the incident C+ ion energy was deter­mined and used to infer reaction mechanisms.

EXPERIMENTAL

The tandem mass spectrometer used in this study has been described previously.6.7 Briefly, it consists of two magnetic mass spectrometers in series. The first is a 1800 magnetic sector instrument with a 1-cm radius of curvature. The secondary mass spectrometer which analyzes the ionic products extracted from a reaction chamber is a 600 magnetic sector instrument with an 8-in. radius of curvature. The pressure in the reaction chamber is measured with an MKS Barytron Type 77H-1 pressure meter. Detection is made by counting the ions with a 17 stage electron multiplier. The methane that was used in these studies was Phillip's research grade and the carbon monoxide was Mathe­son's C.P. product.

RESULTS AND DISCUSSION

When an ion beam is passed through a reaction chamber containing a gas whose pressure can be varied, it will be attenuated with increasing pressure in an exponential manner. If there is more than one state of the ion present, each state will be attenuated exponen­tially in a manner characterized by its reaction cross section. If there are two states present, the intensity at any pressure is given by the equation

1= (l-j)lo exp( -nQil)+j10 exp( -nQjl).

I and 10 are the beam intensities with and without gas being present in the reaction chamber, respectively. j is the fraction of j-type ions in the beam. Qi and Qj are the cross sections for loss of the i- and j- type ions, re­spectively, and n is the number density of the gas and I is the reaction path length. A semilog plot of 1/10 vs gas pressure gives one a curve which can be resolved into its components in a manner similar to that used in the resolution of radioactive decay curves. This method has been used by Turner et al.s and Lao et al.5 for esti­mating the fraction of excited ions present in an ion beam. In the case of C+ ions produced by electron bom­bardment of CO, Lindholm! reported 60% 2 P state and 40% 4p state as the composition of the beam. Lao et al.5 reported 70% and 30%, respectively, for 2 P and 4 P states in C+ produced from 70-eV electron bombard­ment of CO. Turner9 recently found that under similar conditions in his instrument he obtained only 6% C+ in 4p state. A study of the situation showed that the amount of 4p state produced was a function of the gas pressure in the ion source of the primary mass spectrom­eter and by maintaining the CO pressure high enough one could get a C+ beam practically en tire! y in 2 P sta teo This effect has also been reported by Mathis et al.lO in the case of NO+. Fig. 1 gives the attenuation curve for

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5322 \VILSON, ROZETT, ;\K1) KOSKI

1.0

0.8

0 H .....

0.6

0.4 '-------------------'

PRESSURE N2 (microns)

FIG .... 1. Attcnuation~'o(C+ in nitrogen as a function of pressure.

C+ in N2. The linearity of the curve and its intercept indicate that to within our experimental error all of the C+ ion beam is in the ground state. These conditions in the ion source of the primary mass spectrometer were maintained throughout this study. Hence, this report is concerned with the reaction of C+ (2 P) with methane.

For the purposes of this discussion, we will divide the observed reactions in two classes. The first set of reactions will be those that produce ions with a single carbon atom. The second set of reactions produce ions containing two carbon atoms. The first category of reactions are those represented by the following equa­tion

C+ (2P)+CHc~CH4+, CH3+, CH2+, and CH+.

The cross sections vs beam energy are plotted in Fig. 2. The recombination energy of C+ (2 P) which is most pertinent for this discussion is 11.26 eV. This is sig­nificantly lower than the ionization potential of CH4

so the cross section for the charge transfer reaction to produce CH4+ would be expected to appear above its threshold energy (2.45 eV) and rise with kinetic energy. Figure 2 shows that the CH4+ cross section appears just below 10 eV, and rises to over one square angstrom at 200 eV, so one can assume that the CH4+ ion is being produced by a nonresonant charge transfer process.

One might, at first thought, assign CH3+ formation to a dissociative charge transfer reaction. However, such a reaction is more endothermic (3.05 eV) than the CH4+ reaction (1.4 eV), yet CH3+ appears earlier than the methane ion in the energy scale. CH3+ appears at the lowest kinetic energy measured, below the threshold for the appearance of CH3+ by dissociative charge transfer (5.34 eV). Over most of the energy range studied the ClI.-!+ abundance is constant while the CH4+ cross section rises. It is unlikely, therefore, that the CH3+ is the product of the dissociation of CH4+. Furthermore, the CH3+ cross section at low kinetic energies is typical of an exothermic ion molecule reac­tion. Presumably the neutral produced is CH, and the heat of reaction is exothermic by 0.46 eV (Table I).

The simplest process producing CH3+ and CH is H­transferY As we shall see, below 20 eV the hydride ion

transfer could take place predominately within a com­plex. At higher energies a more direct sort of reaction, hydride ion pickup, is probable. In H- pickup the re­sulting CH3+ ion is the spectator and, as such, would be expected to have very little forward momentum. This is experimentally confirmed by our measurements since in our instrument product ions with significant amount of momentum in the beam direction tune differently with repeller voltage than ions that do not have for­ward momentum.6•7

The situation involving the CH2+ ion is complex since it can be formed by dissociative charge transfer, H2 pickup, H2- pickup or by dissociation of an excited CH3+. Dissociative charge transfer can be ruled out by the same arguments that ruled out this mechanism for the CH3+ reaction. CH2+ appears before CH4+ on the energy scale, and below the threshold energy for this highly endothermic reaction (6.93 eV). See Table r. The CH2+ cross section varies independently of the CH4+ cross section.

The simplest explanation is hydride ion transfer followed by dissociation into CH2+ and a hydrogen atom. This would offer a mechanistic ground for the parallel behavior of the CH3+ and CH2+ cross sections over the upper end on the energy range. However, the CH2+ appears below the threshold for dissociative hy­dride ion transfer (8.70 eV), consequently some other mechanism must be at work at lower energies. Neither H2 pickup nor H2- pickup has a threshold. Either pro­cess is endothermic by 0.57 eV. (Table I). If one in­vokes the extreme version of the pickup process, the spectator mechanism, neither H2- pickup nor H- pickup has an upper limit, a critical energy since we observe the spectator product. H2 pickup, on the other hand, has a critical energy of 56.4 eV. But since both frag-

TABLE 1. Heats of reaction for C+ with C~.·

Products

C2H+ C2H3++H C2H2++H2

C2H++H C2++2H2

CH.++C CH3++CH CH3++C+H CH2++CH2

CH2++CH+H GV+C+H2

CH++CH3

CH++CH+H2

CH++C+H2+H

a Calculated from Ref. 14.

Reaction energy (eV)b

6.96 4.01 4.18

-1.63 -2.67 -1.40

0.46 -3.05 -0.57 -4.97 -3.96 -0.83 -5.57 -9.08

b Po~itive values for exothermic reactions.

Threshold energy (Lab)

2.85 4.67 2.45

5.34 1.00 8.70 6.93 1.45 9.75

15.9

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REACTION OF C+ WITH CH, 5323

FIG. 2. Cross section for the produc­tion of CH,+ (0), CH.+ (0), CH,+ (D.), and CH+ (e) as a function of C+ ion kinetic for the reaction C+ ('P)+C~.

7.0 ~

6.0

5.0

;;- 4.0 o<t

b 3.0

2.0

~)O-=-~-o-. ----C;._......[]---e.-;--~ -~~ 7

1.0

OL-~~~L-~-L~ __ L--L~ __ L-~~ __ L-~-L~ __ ~-L~~

o 20 40 60 80 100 120 140 160 180 200

ments of the reaction are polyatomic, the critical energy may be extended to higher energies due to excitation of both the spectator and the nonspectator. While sim­plicity and the parallel behavior of the cross sections at higher energies lead one to postulate dissociative hydride ion pickup at the upper end of the energy scale, one cannot decide between H2 and H2- pickup at the lower end.

b

10.0.0..-------------------,

10. 10.0. 100N ENERGY (eV)

10.0.0.

FIG. 3. Log cross section for the production of C,H, + (e), C,H.+ (0), and C,H+ (X) as a function of C+ ion kinetic energy for the reaction C+ ('P) +CH4• The sum of the cross sections of all ions produced from this reaction is shown by (0) and the solid line gives the Gioumousis and Stevenson cross section.

ION ENERGY (eV)

The CH+ ion may arise from three reaction paths, dissociative charge transfer, H atom pickup and disso­ciative hydride ion pickup. The threshold for dissocia­tive charge transfer (15.9 eV) is not violated, nor does CH+ precede CIL+, nonetheless the CH+ cross section does not parallel the CH4+ abundance. The strikingly similar constancy of the CH3+, CH2+, and CH+ cross sections from 40--200 eV, leads one to suppose a com­mon mechanism, hydride ion pickup, followed by dis­sociation of the CH3+ in the latter cases. The threshold of 9.75 eV is less demanding than dissociative charge transfer, but more than H atom pickup (1.45 eV). It is not possible to rule out H atom transfer at low energies.

The second category of reactions observed contained ions with two carbon atoms and can be represented by the following equation

C+ (2P)+CH4~CCHx++(4-x)H,

where x= 3, 2, 1, and O. The cross section behavior of CCH3+ and CCH2+

were those typical of exothermic ion-molecule reactions, whereas the CCH+ and CC+ ion yields indicated endo­thermicity. !lIn view of the fact that cross section dependence on energy of a number of ion-molecules is of the form fFr"V E7' the data is presented in a log-log plot in Fig. 3. The CC+ ion is omitted since its cross section was very small. The reaction to produce mass 27 can be written as

~H=4.01 eV.

The reaction is exothermic by 4.01 eV if it is assumed that CCH3+ has the same heat of formation as the mass 27 ion formed by photoionization of ethylene. The reac-

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5324 WILSON, ROZETT, AND KOSKI

tion to form CCHz+ is

C+ (2P)+CHr--tC-CHz++Hz, MI=4.1S eV,

if the mass 26 ion has the same heat of formation as that of the same mass produced by direct ionization of acetylene and molecular H2 is formed. The higher cross section for the formation of mass 26 in contrast with 27 probably reflects the higher exothermicity of the reaction producing C-CH2+' The reaction to produce CCH+ is

MI=-1.63 eV,

assuming that the molecular hydrogen is a product and the CCH+ ion has a structure like the corresponding mass ion produced from electron bombardment of acetylene. These expected exo and endothermicities are in harmony with the experimental observations. C2H4+ does not occur because of its high exothermicitYi the order of the abundances of the other ions is in the order of their exothermicities at low energies.

The prediction of the Gioumousis and Stevenson ion-induced dipole mechanism is shown as a straight line in Fig. 3. The sum of the cross sections of all ions, whether containing one or two carbon atoms, is shown as circles in the same figure. Until about 30 eV the sum of cross sections follow the Gioumousis and Stevenson model surprisingly closely.

Below 30 eV, therefore, one might be tempted to conclude that the abundant ions, at least, are formed through complex formation. However, a 1/v dependence of the cross section is indicative of the presence of a 1/r4 ion-molecule potential. It is probably not a neces­saril y indicative of complex formation. Above that energy, the cross section deviates from the slope of the G--S curve. The cross-section levels out to approxi­mately 5.5 A2. This type of limiting behavior has been attributed by Boelryk and Hamill12 to head on collisions between the ion and the target molecule. Beyond 30 eV the main contribution to the sum of cross sections comes from the single carbon atom ions (CHa+, Cffi+, CH2+, and CH+) with CHa+ being the main contributor. Our observations support the view that in this high energy region these ions, with the exception of Cffi+, arc formed by hydride ion pickup leaving the CHa+ ion in various state of exciation to produce CH2+ and CH+.

Some comment is in order as to the existence of ionic products such as CzHa+ and C2Hz+ in the ion kinetic energy region above 10 eV in the laboratory system. Such ions regardless of whether they are formed by complex or stripping mechanisms would possess more than enough internal energy to undergo dissociation. Examination of Fig. 3 shows that for ion kinetic ener­gies in excess of 6 eV for CzHa+ and 15 eV for C2H2+ the cross sections fall much more rapidly with energy than in the respective lower energy regions. This change in slope of the cross section vs energy curves is indica­tive of a mechanistic change. The very steep cross sec­tion dependence on energy suggests that a binary collision type of mechanism suggested by Bates et al.1a

may be in operation. This matter is being investigated further.

All heats of reaction were calculated using the com­pilation of Franklin et al. 14

* Present address: Department of Chemistry, Fordham Univer-sity, Bronx, New York.

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