Reactions of highly excited oxygen atoms O(2p 33s,5 S) with simple gasmoleculesT. Mori, K. Kanou, K. Mizuta, T. Kuramasu, Y. Ishikawa, and S. Arai Citation: The Journal of Chemical Physics 97, 9094 (1992); doi: 10.1063/1.463336 View online: http://dx.doi.org/10.1063/1.463336 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/97/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Excited Atoms and Molecules in High Pressure Gas Discharges AIP Conf. Proc. 680, 172 (2003); 10.1063/1.1619691 Formation of highly excited oxygen atoms O(2p 33p,5 P) and O(2p 33s,5 S) in electron irradiation ofmixtures of rare gases and O2 J. Chem. Phys. 96, 8258 (1992); 10.1063/1.462329 Evaluated Chemical Kinetic Data for the Reactions of Atomic Oxygen O(3P) with Saturated OrganicCompounds in the Gas Phase J. Phys. Chem. Ref. Data 17, 967 (1988); 10.1063/1.555810 Quenching and reactions of laserexcited I(52 P 1/2) atoms with halogen and interhalogen molecules J. Chem. Phys. 69, 641 (1978); 10.1063/1.436629 Rate Constants for the Reactions of Atomic Oxygen (O 3 P) with Organic Compounds in the Gas Phase J. Phys. Chem. Ref. Data 2, 467 (1973); 10.1063/1.3253125

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Reactions of highly excited oxygen atoms O(2p33s,55) with simple gas molecules

T. Mori, K. Kanou, K. Mizuta, T. Kuramasu, Y. Ishikawa, and S. Arai Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606. Japan

(Received 11 August 1992; accepted 31 August 1992)

The electron pulse irradiation of 10 atm He containing one of CO, CO2, NO, N02, or N20 at small amounts produced highly excited oxygen atoms 0(2p33s,5S), which subsequently disappeared by their reactions with parent molecules. The rate constants have been deter­mined from the absorption decay curves at 777.3 nm 0(2p33p,5p) <-0(2p33s,5S). The rate constants for the reactions of o (2p33s,5S) with Xe, H 2, N2, CH4, and C2H6 could be deter­mined in the irradiation of mixtures of He, O2, and these gases with electron pulses. The up­per limit of a quenching rate constant was estimated for Kr. The observed rate constants showed good correlation with the quenching rate constants of Kr( 4p 55s, 3 P2) or Xe(5p56s,3P2 ) by the same gas molecules.

I. INTRODUCTION

The formation of highly excited oxygen atoms 0(2l3s,5S) has been observed for electron impact of O2 (Refs. 1 and 2) and electric discharge of rare gases con­taining O2•

3•4 In the excited state one of the electrons in the

outest 2p shell is raised to the 3s orbital, and the energy level is 9.15 eV above the ground state 0(2p4,3p). Hereaf­ter, we describe 0(2l3s,5S) as OCSS), 0(2p33p,5p) as 0(5p), 0(2p4,3p) as Oep), 0(2p4, ID) as OeD), and 0(2p4, IS) as OeS), for simplicity. The lifetime of 0(5S) is expected to be significantly long, because the optical transitions to lower states are spin-forbidden. Our previous studies have shown that the electron pulse irradiation of high pressure He containing a small amount of O2 pro­duces both 0(5S) and 0(5p), which absorbs and emits the light at 777.3 nm, respectively.5.6 These excited oxygen at­oms are produced from the reaction of He!(2s 3};:) with O2, We have also observed the same absorption in irradi­ated He containing one of CO, CO2, NO, N02, or N20. These results are providing us a convenient method for the production of OCSS). The present paper describe mainly kinetic parameters ofO(5S) in its reactions with a number of simple gases.

II. EXPERIMENT

Details of experimental apparatus and procedures have been described in our previous papers.5.6 The source of 600 keY electrons was a Febetron 706, which generated elec­tron pulses with an output of - 10 J and a full-width at half-maximum (fwhm) of 5 ns. Both transient absorption and emission were detected spectrophotometrically using a technique employed in conventional pulse radiolysis. The host gas He apparently contained a trace of O2, However, we have determined rate constants under the condition where the effect of O2 on the decay ofO(5S) was negligibly small. The other gases purchased from Takachiho Shoji Co. were of the highest grade and used without purifica­tion.

III. RESULTS

Figures 1 and 2 present the absorption and emission decay curves at 777.3 nm obtained with mixtures of He plus CO, NO, CO2, and N20. The decay rates of OCSS) were mostly determined from absorption curves after the emission intensities decreased to negligibly small extents. In several cases, however, small corrections were made for the overlapping emission in absorption decay analyses. Our previous paper has demonstrated that the Lambert-Beer's law holds almost satisfactorily between a concentration of OCSS) and a transmittance at 777.3 nm in 7600 Torr He.6

Absorption decay curves always fit a first-order kinetic law; decay rates increase with increasing partial pressures of additive gases. Typical plots of first-order decay rates vs partial pressures of additive gases are shown for mixtures of He plus CO, CO2, NO, and N20 in Fig. 3. The slopes of straight lines correspond to the rate constants for the re­actions of 0(5S) with these additive gases. The rate con­stants determined are tabulated in Table I.

The rate constants for reactions of 0(5S) with mole­cules bearing no oxygen atom could be obtained with elec­tron irradiation of 7600 Torr He containing O2 and a gas to be examined. The first-order decay rate R of OCSS) in the system is expressed by Eq. (1)

(1)

where k3 and k4 are rate constants of reactions (3) and (4), respectively,

He,02-G(5S),

OCSS) +02-+0ep) +Oep) +Oep),

0(5S) +M-+disappearance of OCSS).

(2)

(3)

(4)

Figure 4 shows typical plots of R/[Oz] vs [M]/[Oz], where M's are CH4, Xe, H2, and N2• The rate constants k4's determined from slopes of such plots for several gases are also included in Table I. The intercepts correspond to k 3,

which have been found to be close to 2.2 X 10- 10

cm3 molecule-I S-I for reaction (3) in the present study.7

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Mori et al.: Reactions of O(2p33s,sS) atoms 9095

40~ 20

CO ......... 0 ~ 0 ..........

40~ NO c 0 20

.... 0 a. L...

40 ~ 0 CO2 U) 20 ..0 « 0

40 ~ N20 20

0

Time 1..-1 1 J..IS

FIG. 1. Absorption curves of O('S) at 777 nm obtained with electron pulse irradiation of 7600 Torr He containing 0.12 Torr CO, 0.066 Torr NO, 0.050 Torr CO2, and 0.088 Torr NP.

Kr must be inactive to OeS), because the energy level of Kr in the lowest excited state is higher than OeS) by 0.77 eV.8 We have compared the first-order decay rate ofO(5S) in the mixture of 7600 Torr He plus 0.1 Torr O2 with that in the mixture of 7600 Torr He, 0.1 Torr O2, and 0.5 Torr Kr. In the comparison, no appreciable difference between the mixtures with and without Kr could be observed in their decay rates. Considering an error limit of ~ 10% in measurement, the rate constant, if the quenching reaction occurs, should be <4X 10- 12 cm3 molecule-I S-I. On the other hand, Xe was found to quench OCSS) efficiently at a high rate. The reaction is exothermic, if XeeP2) is pro­duced in quenching, 8

O(SS) +Xe->Oep) + Xeep2)MI= -0.83 eV. (5)

........ CO ...... 0 C , ~ ::J

2 >- 0 NO '-

~ 0 , '- 2 ...... n 0

~ CO2

'- , 0 '-" 2 C 0 N20 0 ,

~ Ul (/) 2 E Time I~I W ljJS

FIG. 2. Emission curves of 0(' P) at 777 nm obtained with electron pulse irradiation of 7600 Torr He containing 0.12 Torr CO, 0.066 Torr NO, 0.050 Torr CO2, and 0.088 Torr N20.

"I 8 Ul

to • N20 0 7

'- 6

III 5 U") .......... 0 - 4 0

~ 3 --~ ~ 2

~ u CO ~ 0

0 0 0·, 0·2 0·3 0·4 0·5 0·6

Partial pressure of X I Torr

FIG. 3. First-order decay rates ofO(,S) vs partial pressures of CO, NO, N20, and CO2,

Excited Xee P2 ) has been reported to react with O2 at k6=2.2X 10- 10 cm3 molecule-' s-',9

(6)

The time dependence of a concentration of Xe(3p2 ), i.e. [XeeP2)lt is expressed by Eq. (7), where [0(5S)]t=0 and [XeeP2 )]t=0 denote the respective concentrations of OeS) and Xeep2) produced directly by the reactions of excited atomic or molecular species of He with O2 and Xe. The production processes may terminate immediately after pulse irradiation

TABLE I. Rate constants for reactions of 0(' S) with X's.

X Rate constant/em) molecule- 1 S-1

Hea (1.8±0.l)X 10- 1'

Nea (3.4±0.3) X 10-15

Ar' (2.9±0.2) X 10- 15

Kr <O.4X 10- 11 C

Xe (29±3)X 10- 11

N2 (3.5±0.4)x 10- 11

CO (8.3±0.8) X 10- 11

H2 (9.3±0.9)x 10- 11

NO (l9±2) X 10- 11

02b (22±2) X 10- 11

N02 (35±4)X 10- 11

CH4 (40±4)x 10- 11

Np (49±5)X 10- 11

CO2 (53±5) X 10- 11

C2H6 (60±6)x 10- 11

"Reference 6. bIn 7600 Torr He, we obtained a little larger value than the previous one of Ref. 6 in 1520 Torr He.

cUpper limit.

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9096 Mori et al.: Reactions of O(2p33s,5S) atoms

7 I" Vl

I" L.. 6 CH4 L-a ......

t-- 5 0 or-- 4 ,...., N

0 L...I 3 -~ ..... 0 L-

2

>-0 u ~

0 o o 6

FIG. 4. {First-order decay rate of OeS)/[O;J} vs ([M]/[O;J), where M's are N z, Hz, Xe, and CH4. See text.

[Xeep2) lc= [Xeep2)]t=0 exp( -k6[02])t

ks[Xe] rOeS) ]t=O

+ (k3 [02] +ks[Xe] -k6[02])

X {exp( -k6[02])t

-exp( -k3 [02] -ks[Xe] )t}. (7)

When [XeeP2)]t=0>[OCS)]t=0, XeCP2) decays almost exponentially at a first-order rate of k6[0z]. When [XeeP2)]t=0<[OCS)]t=0, the slow formation ofXeCP2) by reaction (5) affects considerably the decay behavior of Xeep2). Figure 5 illustrates the observed and calculated decay curves of XeC P2 ) for the mixture of 7600 Torr He, 0.1 Torr O2, and 0.01 Torr Xe, where the rate constants

'·2 >-..... III '.0 C ~ "0 0·8

0 .~ 0·6 ..... a. 0 0·4 ~ > ..... 0·2 0 ~ 0:: 0

0·0 ,.0 2·0 3·0 4·0 5·0

Time /10-6 5

FIG. 5. Calculated absorption curves of Xeepz)' (A) With the forma­tion ofXeepz) in reaction (5); (B) without the formation ofXeepz) in reaction (5); circles, observed absorption curve. See text.

25- J.P.

20r =

101-

Sf-

5p

I 55 _ I ----1\ J. P.

J.P. = _1- ~ /1

Ar2°

I , 0.0 ,-I

5p

~ J.P.

NO.O

5p

5S

5P 5p

5.. ~ I.P. f.i5.

C.O

I.P.

O~----------------------~

FIG. 6. Energy levels of excited rare gas atoms and diatomic molecules as well as energies necessary for the formation of Oep), OCSS), and O(sp) from Oz, NzO, COz, NO, and CO. An ionization potential of each gas is also presented.

previously determined have been used in the calculation (k3=2.2X 10- 10 cm3 molecule-I s-I, ks=2.9X 10- 10 cm3

molecule- i S-I, and k6=2.2X 10- 10 cm3

molecule- i S-I).7,9 Since k3=k6' the term {ks[Xe] [0(sS)]t=0}/(k3[0z]+ks[Xe]-k6[02]) of Eq. (7) is re­duced into [OCS)]t=o' We assumed [OCSS)]t=oI [XeCP2)]t=0= 1, which was estimated from the results ob­tained with mixtures at various ratios of [Oz]:[Xe]. The figure also shows the simulated decay curve of Xee P2) in the case that Xeep2) is not formed in reaction (5). We conclude from these results that the quenching of O(sS) by Xe gives rise to Xeep2).

IV. DISCUSSION

Figure 6 shows atomic and molecular energy levels of He, Ne, and Ar, and energies necessary for the production of 0(5p) and 0(5S) from O2, NP, N02, NO, and CO, together with their ionization potentials.6

,8,10 It is energet­ically possible to produce both 0(5p) and OCS) in the reactions of Her (2s 3~:) with the molecules examined here except for CO. At higher partial pressures of these gases, however, excited He atoms probably contribute di­rectly to the formation of both excited O(sS) and 0(5p) atoms. Since a bond dissociation energy of CO is as high as 11.10 eV, only highly excited He atoms can produce 0(5S) in a mixture of He and CO. Our previous study has dem­onstrated that OCS) originates from the reaction of a highly excited Ar atom with O2 in a mixture of Ar and O2 similarly to the formation of OeS) in He.6 However, the mechanism leading to the formation of OCS) is not im­portant for the determination of quenching rate constants of OCS) in the present study.

It has been well established that Oep) in the ground state reacts with O2 to form 0 3, after deactivation of a vibrationally hot initial adduct. However, 0(5S) seems not to form 0 3 because of its extremely large electronic energy.

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Mori et a/.: Reactions of O(2p33s,5S) atoms 9097

The main reaction of OCS) with O2 may be the dissocia­tion of O2 into 20 e P). The other possible exothermic re­actions are as follows8,1O:

0(5S) +02-0eD) +20ep),

OCSS) +02 ...... 0( ID) +Or(triplet excited),

(8)

(9)

0(5S) +02--+0(IS) + Or (triplet excited). (10)

The formation of Oep), OeD), and OeS) from OCS) plus O2 is slightly endothermic (0.14 e V). The decompo­sition of parent molecules may occur in the quenching of 0(5S) by CO2, NO, N02, and N20,8,10

0(5S) +C02--+CO(X I~+) +Oep) +Oep)

!llJ= -3.66 eV,

0(5S) +NO-+N(4S) +Oep) +Oep)

!llJ= -2.62 eV,

0(5S) +N02-+NOen) +Oep) +Oep)

!llJ= -S.97 eV,

0(5S) +N20-+N2(X I~+) +Oep) +Oep)

!llJ= -7.42 eV.

(11)

(12)

( 13)

(14)

There are possibilities that excitation transfer or atom transfer takes place in these reactions. However, the eluci­dation of reaction channels requires further experiments involving detections of products. Dissociation probably oc­curs in the reactions ofO(5S) with H2, CH4, and C2H6,8,10

0(5S) +H2--+HeS) +HeS) +Oep) !llJ= -4.63 eV, (1S)

0(5S) +CH4 -+CH3+HeS) +Oep) Ml= -4.6S eV, (16)

0(5S) +C2H6 ...... C2H5+HeS) +Oep) !llJ= -4.93 eV. (17)

The previous studies have observed visible emission due to N 2(B 3ng) or COCa 3n, at3~+, d 3a , and e 3~-) in quenching of Xeep2 ) by N2 or CO. ll The same excited species may be formed in the reaction ofO(5S) with N2 or CO,

0(5S) +N2 -+0ep) + Nr(triplet excited),

0(5S) +CO-+Oep) + CO* (triplet excited).

(18)

(19)

The excitation energies of OctD), OeS), 0(5S), Kr(4iSs,3P2 ), and Xe(Sp56s,3P2 ) are 1.97, 4.19, 9.1S, 9.92, and 8.32 eV, respectively.8 We have tentatively plot­ted quenching rate constants of 0(5S) by various gases against those of OeD),12-15 Kr(4p5Ss,3P2),9 and Xe(5p56s,3P2 ) (Ref. 9) by the same gases in Figs. 7, 8, and 9. There is a roughly linear relation with a slope of about unity between 0(5S) and KreP2) or Xeep2), although no correlation exists in OeD). We also failed to correlate rate constants ofOCS) with those ofOep) or OctS).12-17 These facts suggest that the energy content of an excited

X 10-11

100 I' III

50 I'

bI

:::::l 20 u bI 0 10 E

C") 5 E u -,....... 2

>< +

(j') L{)

0 .......... ..:lC

NzO - 0

Xe --0

NO J" 8 02 ---'

CO - 0

N2 - 0

o - C02 8 - CH4 L- N02

o - H2

1 2 5 10 20 50 100 X 10-11

k( 0(10) + X) I cm3 molecule-1 5-1

FIG. 7. Plots of k[OeS} +X] vs k[O( 'D) +X], where X's are quencher gases.

atom is a determining factor in quenching rate and the chemical properties of an oxygen atom are not important for deactivation of OCS). When the excitation energy is transferred from OCS) to a quencher, the oxygen atom appears not to affect the potential curves of a quencher, probably owing to a relatively large interaction distance between them. The excited quencher may decompose by its own potential leading to an exit channel. The excited level of a quencher must have an acceptable spin multiplicity. The density of acceptable levels at 9.15 e V or a little less than 9.15 eV is important for the quenching rate ofOCS). An ionic complex with an attractive Coulombic potential is presumed to be an intermediate in quenching mechanism of excited rare gas atoms.9 In OCS), however, a doublet­doublet complex Q+ -0- cannot be correlated with the intermediate ionic species produced from a singlet quencher and 0(5S). The complex Q--0+ (0+, quar­tet) is produced or 0- must be in the excited state with a

XlO- lt

100 ., III 50 ., bI ::l 20 u bI '0 10 E

(") 5 E u - 2

x +

c/) It)

(5 '-' ..:lC

Nz - 0

NzO --­

Xe -- 0 Oz --00 __ NO

Hz - 0 o-CO

5 10 20 50 100 XlO-11

X) I cm3 molecule-1 5-1

FIG. 8. Plots of k[OeS) +X] vs k[KrePz) +X], where X's are quencher gases.

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9098 Mori et al.: Reactions of O(2p33s,5S) atoms

XlO- 11

\" 100 III

\" 50 ~ :J U 20 ~

0 10 E M

E 5 u --........

2 >< +

(j') LO~

0 '-" ~

o CO

Nz 0

C2Hs C02 - gO N20.I()CH4

02 00 NO

1 2 5 10 20 50 100 XlO- 11

k( Xe(3P2) + X) I cm3 motecute-1 5-1

FIG. 9. Plots of k[OeS) +X] vs k[Xeepz) +X], where X's are quencher gases.

multiplicity of a quartet or sextet. We consider that OeS) merely plays the role of an energy donor at the beginning of the quenching. More kinetic studies on highly excited atoms other than rare gases are required for understanding of quenching mechanisms.

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