Absolute rate constant for the CH + O reaction

Volume

74, number

1

ABSOLUTE

RATE CONSTANT

I. MESSING, Deparrrnenr Recelvcd

CHEhlICAL

T. CARRINGTON,

of Chemxrry,

28 Apnl 1980.

PHYSICS

LETTERS

15 August 1980

FOR THE CH + 0 REACTION

S-V. FIL.SETH and C.M. SADOWsKI

York Unwernty.

Dormsslew,

Ontano M3.I IP3, Canada

m final form 2 June 1980

CH (>(’ fl) was produced by multiple mfrared photon dlssoclatlon of CH30H kapour m the presence of excess atomic oxygen Tune-resolved measurements of relative CH concentrations were made at 298 K \\xth a tunable dye laser. From the dependence of CH decay rate on atom conccntratlon, an absolute rate constant (9.4 = 2) X lo-” cm3 molecule-’ 5-l aas deduced for the CH + 0 reactlon.

The

reactlon

between

CH and 0 was ongmalIy sugas the prmclpal source of prunary chenn-ions m hydrocarbon combustion. Recently, Vmckier [l] measured the rate constant for the chermiomzatlon channel at 295 K as 2.4 X 10-I” cm3 molecule-l s-1. The comparative mefficlency of this channel IS not surpnsing m view of the requrred curvecrossmg between surfaces of neutral and singly ionIsed HCO. Muumal state correlation diagrams 111C, symmetry show several doublet and quartet surfaces wkch connect CH(X ZfI) and O(3P) with ground state OH and with ground or excited state CO. The latter channel was proposed by Clough et al, [2] as the ongm of the 5 m emlsslon m CO which was observed U-Ithe reaction of atomic o.xygen with acetylene. The same reactlon has been suggested by Lm [3] as the source of 5 pm laser action m the SO,/ CHBr, photochermcal system. Thus It 1s to be expected that other reactlon channels wdl dommate over that leadmg to HCO’. The purpose of this work was to measure the overall absolute rate constant for reaction of CH \lth 0 and to denve thereby a value for the branchmg ratlo for productlon of HCO+. The expenmental method involved m this work has been described previously [4]. Multiple mfrared photon dnsoclation of 0.5-2 mTorr CH,OH vapour was employed as the source of CH(X ‘II, u” = 0). A multunode TEA CO, laser operatmg at 1046.85 cm-l produced l-2 J pulses which were brought to a focus inside a reaction cell kvlth a 20 cm Ge lens. Relative gested by Calcote

56

CH concentration measurements were made with a nitrogen laser pumped dye laser tuned to inlvidual rotation lmes m the Q branch of the CH(A ‘A--X ‘II) system near 430 nm. The resultmg laser-mduced fluorescence (LIF) was accumulated in a gated counter by single photon methods for typically 60 CO, laser pulses at each of several selected intervals following the CO, laser pulse. Reactive removal of CH was studled under pseudofirst-order conditions in the presence of excess atomic oxygen. Oxygen atoms were produced by the wellknown NO titration reaction for atomic mtrogen wkch was itself produced m a microwave dlscharge through a ddute mixture of nitrogen containing 60 ppm SF, in argon. Concentrations of atomic o_xygen were determined by titration Lvlth ddute rmxtures of NO, m AI at the centre of the reaction vessel. These concentrations were m agreement with N atom concentrations determmed by NO titration when the two titrations were carned out at adJacent Inlets. These procedures have been described previously [5]. In the absence of atormc oxygen, CH loss rates were determmed by diffusion out of the excltatlon volume defmed by the lameter of the probe dye laser beam. This lffusive loss was minunized by carrying out the measurements m Ar buffer gas at pressures between 5 and 10 Torr. Under these con&tions the diffuswe rate law was observed to be close to first order for the range of delay tune (20-200 ps) over which reactive depletion of CH was studied. The

Volume 74, number

1

CHEMICAL

PHYSICS

LETTERS

15 Auyzst 1980

which is obtained from an unweighted least-squares fit to the data in fig. 1 is (9.4 f 1.1) X IO-l1 cm3 molecule-l s-l at 298 K. The error estimate is a 90% confidence interval derived from the goodness of fit of the points in fig. 1. Estimates of systematic errors are necessanly subjective, but we feel that a range of F2 X lo-l1 cm3 moleculeS1 s-l is reasonable. When this rate constant is compared with the value obtained by Vinckier for the channel leading to HCOf, a branching ratio for chemi-ionization of 2.5 X lo4 is Fe. 1. Dependence of the decay rate of the LIF sgnal on the partial pressure of atomic oxygen for nu\Wrcs contammg S-10 Torr Ar and 0.5-l mTorr CHsOH.

obtained. This work is part of a broader programme

of research on the reactions of CH with atoms. More extensive results for the CH + 0 and CH f N systems, including the kmetics of chemilurninescent reactions, wdl be presented in a future publication.

average diffusive decay constant at 5 Torr buffer gas pressure was 0.4 X IO4 s-l. Decay rates attnbuted to reaction of CH with atomic oxygen were as large as 7 X 104 s-1 and LIF signals displayed single exponential behavior over approxunately three decay constants. The decay rates were calculated from the hnear portions of the decay curves by an unweighted least-squares method over a

References

range of atomic oxygen concentration of 25 mTorr. The LIF signals were corrected for background chenu-

[l] C. Vmcluer, J. Phys. Chem. 83 (1979) 1234.

luminescence and the decay rates have been individually corrected for the contnbution due to diffusive loss and reaction with 0, [4] formed by atom recombination. The dependence of the decay rates on atom concentrahon is shown in fig. 1. The absolute rate constant for the CH + 0 reaction

The work reported here was supported by York University and by the Natural Sciences and Engineering Research Council of Canada.

[2] P.N. Clough, SE. Sch\rartz

and BA. Thrush, E’roc. Roy. sot. A317 (1970) 575. [3] KC. kn, Intern. J. Chem. Kinetics 6 (1974) I. [4] I. hlessmg, S.V. Filseth and C.M. Sadowski, Chem. Phye

Letters 66 (1979) 95. [S] B. Gelemt, S.V. Fiketh and T. Caningon,

J. Chern.

Phyr 6.5 (1976) 4940.

57