Reactions of N 2 (a ′ 1 Σ u − ) with H 2 , CH 4 , and their isotopic variants: Rateconstants and the production yields of H(D) atomsHironobu Umemoto, Ryoji Ozeki, Masashi Ueda, and Mizuki Oku Citation: The Journal of Chemical Physics 117, 5654 (2002); doi: 10.1063/1.1502642 View online: http://dx.doi.org/10.1063/1.1502642 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/117/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Quantum instanton calculation of rate constant for CH4 + OH → CH3 + H2O reaction: Torsional anharmonicityand kinetic isotope effect J. Chem. Phys. 137, 214306 (2012); 10.1063/1.4768874 Reactive quenching of OH A 2Σ+ by O2 and CO: Experimental and nonadiabatic theoretical studies of H- and O-atom product channels J. Chem. Phys. 137, 094312 (2012); 10.1063/1.4748376 Reactive quenching of OD A 2Σ+ by H2: Translational energy distributions for H- and D-atom product channels J. Chem. Phys. 135, 144303 (2011); 10.1063/1.3644763 Production yields of H(D) atoms in the reactions of N 2 ( A Σ u + 3 ) with C 2 H 2 , C 2 H 4 , and their deuteratedvariants J. Chem. Phys. 127, 014304 (2007); 10.1063/1.2746851 Theoretical study of kinetic isotope effects on rate constants for the H 2 + C 2 H→H+C 2 H 2 reaction and itsisotopic variants J. Chem. Phys. 113, 4060 (2000); 10.1063/1.1288173

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Reactions of N 2„a8 1SuÀ… with H 2, CH4, and their isotopic variants:

Rate constants and the production yields of H „D… atomsHironobu Umemoto,a) Ryoji Ozeki, Masashi Ueda, and Mizuki OkuDepartment of Chemical Materials Science, Japan Advanced Institute of Science and Technology, Asahidai,Tatsunokuchi, Nomi, Ishikawa 923–1292, Japan

~Received 24 May 2002; accepted 3 July 2002!

The reactions of N2(a8 1Su2 , v50! with H2, CH4, and their isotopic variants were examined.

N2(a8, v50! was produced by energy transfer from N2(a 1Pg , v50!, while N2(a, v50) wasproduced by two-photon excitation of ground state N2. The rate constant for the deactivation ofN2(a8,v50! can be determined by measuring the decay profiles of N2(a, v50) under theconditions that equilibration between N2(a, v50) and N2(a8, v50) can be assumed. Thedetection of N2(a, v50) was accomplished by a laser-induced fluorescence technique by utilizingthe N2(b8 1Su

1 , v57! state as an upper state. The rate constants for the quenching of N2(a8, v50! by N2, H2, D2, CH4, CH2D2, and CD4 were determined to be~2.060.1!310213, ~2.860.1!310211, ~1.760.1!310211, ~2.960.2!310210, ~2.460.3!310210, and ~2.660.2!310210cm3 molecule21 s21, respectively. H~D! atoms were identified as reaction products by atwo-photon laser-induced fluorescence technique. The yields for the production of H~D! atoms fromCH4 and CD4 were both determined to be 0.760.2 under the assumption that the only exit forH2~D2) is the production of two H~D! atoms. No preferential production of H or D atoms wasobserved in the reaction with CH2D2, suggesting that the reaction proceeds via bound intermediatecomplexes. ©2002 American Institute of Physics.@DOI: 10.1063/1.1502642#

I. INTRODUCTION

When nitrogen gas is excited by electric discharges,metastable atoms and molecules, such as N(2D! andN2(A 3Su

1), are produced efficiently besides ground-stateatomic nitrogen, N(4S!. These excited species play importantroles in many processings, since N(4S) is rather inert in re-actions with stable molecules.1,2 Among the metastableatomic species, N(2D) is much more reactive than N(2P!and can be one of the initiators of reactive processes.3 Re-cently, we have studied the reaction dynamics of N(2D! withvarious hydride~deuteride! molecules by employing a two-photon dissociation technique to produce N(2D! and laserspectroscopic techniques to probe the reaction products.4–11

For example, in the reaction with CH4, the two major exitchannels were identified as the production of CH25NH1Hand CH31NH.8 The branching ratio was determined to be7:3. It was also revealed that the primary step for the pro-duction of these species is the insertion of N(2D! into C–Hbonds.

Among metastable molecular nitrogen, the lowest tripletstate, N2(A 3Su

1), has been studied most extensively.3 On theother hand, much less attention has been paid to other meta-stable states, such as N2(a 1Pg) and N2(a8 1Su

2).N2(a8 1Su

2) is the lowest singlet-excited state, whileN2(a 1Pg) is 1212 cm21 higher in energy. The overall rateconstants for the quenching of these singlet-excited stateshave been reported by several groups.12–19 However, theidentification of the chemical reaction products has not been

reported. In general, N2(a) and N2(a8) are more reactivethan N2(A), especially against H2 and alkane hydrocarbons,and can be the initiators of a series of reactions in activenitrogen. The lifetime of N2(a8) is ;23 ms,13,15 which isshorter than that of N2(A), 1.9 s, but long enough to playimportant roles in many processings.

In the present study, the reactive processes of N2(a8)were examined. Besides the overall reaction rate constants,the yields for the production of H~D! atoms were deter-mined. The kinetic H/D isotope effect was also examined.

II. EXPERIMENT

N2(a8 1Su2 , v50! was produced from N2(a 1Pg , v50!

by collisional relaxation. N2(a, v50) was produced bytwo-photon excitation of ground-state molecular nitrogen. Inthe presence of an excess amount of N2, N2(a), and N2(a8)can easily be equilibrated. The rate constant for the deacti-vation of N2(a8, v50) was determined by measuring thetemporal profiles of N2(a, v50) under thermally equili-brated conditions. H~D! atoms produced in the reactions ofN2(a8, v50) with hydride~deuteride! molecules were de-tected by employing a two-photon laser-induced fluorescencetechnique.

The experimental apparatus was similar to that describedelsewhere.8,20 A frequency-doubled output of a Nd:YAG la-ser pumped dye laser~Quanta-Ray, GCR-170/PDL-3! wasused to two-photon excite ground-state N2 to the N2(a 1Pg ,v50! state. The excitation wavelength was 289.9 nm.12,15,21

The laser pulse was;10 mJ in energy and focused with a200 mm focal length lens. The rate constants for the overallquenching of N2(a8 1Su

2 , v50! were measured in the pres-a!Electronic mail: umemoto@jaist.ac.jp

JOURNAL OF CHEMICAL PHYSICS VOLUME 117, NUMBER 12 22 SEPTEMBER 2002

56540021-9606/2002/117(12)/5654/6/$19.00 © 2002 American Institute of Physics

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ence of 10.7 kPa of N2, while the quencher pressure was keptbelow 160 Pa. Under such conditions, N2(a8, v50) caneasily be produced from N2(a, v50) by collisional relax-ation and the equilibration between them can be accom-plished within 0.1ms.12,15,18N2(a8, v50) is energetically1212 cm21 below N2(a, v50!.22 As will be discussed later,the population ratio of N2(a, v50) to N2(a8, v50) is1/184 under equilibrated conditions. Under such conditions,the time constant for the decay of N2(a, v50) agrees withthat of N2(a8, v50) and it is possible to evaluate the rateconstants for the quenching of N2(a8, v50) by measuringthe decay profiles of N2(a, v50).14,15 The N2(a, v50)density was measured by laser-induced fluorescence~LIF! byusing another Nd:YAG laser pumped dye laser~Quanta-Ray,GCR-170 and Lambda Physik, LPD3000E!. TheN2(b8 1Su

1 , v57! state was utilized as an upper state for theLIF detection. The excitation wavelength was 250.0 nm. Thetypical pulse energy of the probe laser was 0.5 mJ. The life-time of N2(b8 1Su

1 , v57! is 0.9 ns and this state mainlydecays radiatively to ground-state N2 withoutpredissociation.23 This N2(b8–X) fluorescence in thevacuum ultraviolet region, Birge–Hopfield bands, was moni-tored with a solar-blind photomultiplier tube~Hamamatsu,R6835!. Although the main part of the Birge–Hopfield bandsis shorter than 115 nm,24,25which cannot be detected througha MgF2 window, it was still possible to obtain enough signalto noise ratio. We have also tried to detect N2(a) by observ-ing the forbidden N2(a–X) emission, the Lyman–Birge-Hopfield bands, but the present LIF technique was found tobe much more sensitive. The pump–probe delay waschanged between 1 and 5ms.

H~D! atoms produced in the reactions of N2(a8, v50) with hydride~deuteride! molecules were detected by atwo-photon laser-induced fluorescence technique, the detailsof which have been described previously.8,9 The2 2S1/2– 1 2S1/2 transition was utilized. The laser system wasthe same as that for the detection of N2(a, v50). Thevacuum ultraviolet fluorescence~Lyman a) was collimatedwith a MgF2 lens and detected with a solar-blind photomul-tiplier tube through an interference filter. In order to deter-mine the yield for the production of H atoms from CH4,D-atom signal from D2 was used as a standard.8 The typicalpump–probe delay was 1ms. The N2 pressure was kept at10.7 kPa, while the quencher pressures were 27 Pa. 67 kPaof He was added to suppress the diffusional loss of H and Datoms. The wavelength was scanned around 243.2 nm to

cover both the absorption lines of H and D atoms. The typi-cal probe pulse energy was 0.4 mJ. In this measurement, it isimportant to check that the precursor of H~D! atoms isN2(a8) and not N2(a). This was accomplished by comparingthe temporal profiles of the densities of N2(a) and H. Thedetails will be presented later. Detection of NH radicals wasalso attempted with a laser-induced fluorescence techniqueby utilizing the NH (A 3P –X 3S2) transition.5,6 All themeasurements were carried out at 29363 K.

N2 ~Teisan, 99.999%!, H2 ~Takachiho, 99.99995%!, D2

~Sumitomo Seika, isotopic purity 99.5%!, CH4 ~NihonSanso, 99.999%!, CH2D2 ~Isotec, isotopic purity 98%!, CD4

~Isotec, isotopic purity 99%!, and He~Teisan, 99.995%! wereused from cylinders without further purification.

III. RESULTS

A. Decay of N 2„a 1Pg , vÄ0… in the pure N 2 system

The N2(a 1Pg , v50! density decayed biexponentiallyin the pure N2 system. Similar behavior has been observedby other investigators.12,16 The fast component correspondsto the decay process to produce N2(a8), while the slow onecorresponds to the decay of equilibrated N2(a) and N2(a8).It was difficult to measure both the fast and the slow com-ponents at one N2 pressure, because at a high pressure thedecay of the fast part was too fast, while at a low pressure theslow part was too weak to be measured. The decay profile ofthe fast part was measured between 300 and 1700 Pa, whilethat of the slow part was measured between 6.7 and 13.3kPa. In both cases, the reciprocal time constants increasedlinearly with the increase in the N2 pressure, as are shown inFigs. 1 and 2. The apparent rate constant deduced from theslope of the linear plot in Fig. 1 is 3.3310211 cm3 molecule21 s21, while that for Fig. 2 is 2.0310213 cm3 molecule21 s21. The intercept in Fig. 1 corre-sponds to the diffusional loss.

If the endothermic production of N2(a8 1Su2 , v51!

from N2(a 1Pg , v50! can be ignored, the rate constant forthe fast component must represent that for the quenching ofN2(a, v50), since the reproduction of N2(a, v50) fromN2(a8, v50) is slow.15 However, the endothermicity forthe production of N2(a8, v51! is only 295 cm21 and it isimpossible to ignore the production of N2(a8, v51! and itsreverse process to reproduce N2(a, v50) as has been dis-cussed by Katayamaet al.18 Then, quantitative determination

FIG. 1. Reciprocal time constants for the decay of the fast component ofN2(a, v50) as a function of N2 pressure.

FIG. 2. Reciprocal time constants for the decay of the slow component ofN2(a, v50) as a function of N2 pressure.

5655J. Chem. Phys., Vol. 117, No. 12, 22 September 2002 Reactions of N2 with H2 and CH4

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of the rate constant for the quenching of N2(a, v50) isdifficult. The pressure dependence of the diffusional loss ratemust also be taken into account. It should be noted, however,that the apparent rate constant obtained in the present work isin good agreement with those of van Veenet al., Marinelliet al., and Katayamaet al.,12,15,18 while in fair agreementwith that of Magneet al.16 These values may be used forsemiquantitative analysis. The rate constant obtained for theslow decay component corresponds to that for the quenchingof N2(a8, v50) by N2 as will be discussed below.

B. Overall rate constant for the deactivation ofN2„a8 1Su

À , vÄ0…

The overall rate constant for the deactivation ofN2(a8 1Su

2 , v50! can be determined by analyzing the decayprofiles of N2(a, v50) under equilibrated conditions. Inorder to equilibrate N2(a) and N2(a8), the N2 pressure mustbe much higher than that of the quenching molecule. Thereaction mechanism in the presence of a quencher HX can berepresented as follows:

N2(X, v50)12hn →N2(a, v50),N2(a, v50)1N2N2(a8, v50)1N2, k61,N2(a, v50)1N2→other products, k2,N2(a, v50)1HX →N21H1X, ak3,N2(a, v50)1HX →other products, (12a)k3,N2(a, v50) →N2 1hn8, k4,N2(a8, v50)1N2→products other thanN2(a, v50), k5,N2(a8, v50)1HX →N2 1H1X, bk6,N2(a8, v50)1HX →other products, (12b)k6,N2(a8, v50) →N21hn9, k7

Here, X represents H or CH3. The production of N2(a8, v51) is omitted just for simplicity. The rate constants arerepresented byki , while the quantum yields for the produc-tion of H atoms for N2(a) and N2(a8) are represented byaandb, respectively.

If k1 @N2# is much larger than any other pseudo-first-order rate coefficients, such ask21 @N2# and k3 @HX#, thedecay profiles of N2(a, v50) and N2(a8, v50) must bethe same except at the initial rapid relaxation region.14,15Theprinciple of detailed balance suggests thatk1 /k215184 atroom temperature when the rotational states are thermalizedcompletely. The rotational constants of N2(a, v50) andN2(a8, v50) given by Huber and Herzberg were employedto calculate the rotational partition functions.22 The recipro-cal of the radiative lifetime of N2(a8, v50), k7, is <60s21, while that of N2(a, v50), k4, is;1.83104 s21.13,14,16 The value ofk1 is in the order of10211 cm3 molecule21 s21, while that ofk5 is in the order of10213 cm3 molecule21 s21.12,13,15,18Then, the above condi-tion, k1 @N2# @other pseudo-first-order rate coefficients, canbe satisfied if the N2 pressure is chosen to be high enoughcompared to the HX pressure. Under such conditions, thereciprocal time constant for the decay of the slow componentof N2(a, v50) is represented by

k5@N2#1k6@HX#1k7,

which agrees with that for the decay of N2(a8, v50) aswell as that for the formation of H. Then, by measuring thereciprocal time constants as a function of HX pressure, it ispossible to determine the rate constant for the deactivation ofN2(a8, v50), k6. The situation does not change if the pro-duction of N2(a8, v51! is included, as far as the decayprofile of the slow component is analyzed. Under thermallyequilibrated conditions, the density of N2(a8, v51! is,0.1% of that of N2(a8, v50).

Figure 3 shows the linear relationships between the re-ciprocal time constants for the decay of the slow componentsand the H2~D2) pressure. Similar results were obtained forother quenchers. The time constants were independent of thepulse energy of the pump laser between 6 and 10 mJ. Thedifference observed between H2 and D2 can almost be as-cribed to the difference in collision frequencies. The rateconstant for the quenching by N2 itself, k5, can also be de-termined from the slope of the plots in Fig. 2. The results aresummarized in Table I, together with the results obtained byother investigators. The error limits are the standard errors.Piper employed a discharge flow technique to produceN2(a8), while the forbidden N2(a8–X) emission, Ogawa–Tanaka–Wilkinson–Mulliken bands, was employed for thedetection.13 The agreement between the present results andthose by Piper is good. The procedure employed by van Veenet al. is similar to the present one except that the vacuum-ultraviolet emission was employed to monitor N2(a) andN2(a8).12

FIG. 3. Reciprocal time constants for the decay of the slow component ofN2(a, v50) as a function of H2 ~d! or D2 ~s! pressure. The N2 pressurewas 10.7 kPa.

TABLE I. Rate constants for the deactivation of N2(a8 1Su2 , v50! in units

of 10211 cm3 molecule21 s21.

Reactant Rate constant Author

N2 0.023 van Veenet al.a

0.01960.005 Piperb

0.02060.001 This workH2 2.660.6 Piperb

2.860.1 This workD2 1.760.1 This work

CH4 3068 Piperb

28.862.1 This workCH2D2 23.962.5 This work

CD4 25.662.3 This work

aReference 12.bReference 13.

5656 J. Chem. Phys., Vol. 117, No. 12, 22 September 2002 Umemoto et al.

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C. Yields for the production of H „D… atoms

In the quantum yield measurement, it is necessary tocheck if the precursor of H atoms is truly N2(a8). In otherwords, it is necessary to confirm that the production of Hfrom N2(a) is much less than that from N2(a8). This waschecked by comparing the temporal profiles of the densitiesof N2(a) and H. In the presence of 10.7 kPa of N2 and 27 Paof H2, the fast component of N2(a) decays completelywithin 100 ns. On the other hand, the H-atom densityshowed an increase up to 4ms. Then, the precursor of Hatoms must be equilibrated N2(a) and N2(a8). Figure 4compares the decay profile of equilibrated N2(a) and N2(a8)and the growth profile of H. The solid lines are the calculatedones by using the rate constants obtained in the present work.The reciprocal time constant for the diffusional loss of Hatoms was assumed to be 33104 s21. The agreement be-tween the experimental and the calculated results is good.Similar good agreement was obtained for the N2/CH4 sys-tem.

If the production of N2(a8, v51! can be ignored, it ispossible to determine the rate constant for the quenchingof N2(a, v50) by H2 by analyzing the fast componentof the decay profiles at various H2 pressures. Actually,the production of N2(a8, v51! cannot be ignored andquantitative discussion is difficult. However, a semi-quantitative analysis is still possible. The apparent rateconstant for the quenching of N2(a, v50) by H2 wasevaluated to be 1.4310210 cm3 molecule21 s21. The rateconstant for the quenching of N2(a8, v50) is 2.8310211 cm3 molecule21 s21, which is 1/5 of that forN2(a, v50). The steady-state density of N2(a, v50) is1/184 of N2(a8, v50). Then, the contribution of N2(a, v50) for the production of H is less than 1/37 of that ofN2(a8, v50), since the rate constant for the quenching ofN2(a, v50) includes not only the production of H atomsbut also the physical relaxation to produce N2(a8, v50).Similar discussion is possible for CH4. In this case, the con-tribution of N2(a, v50) is still smaller since the differencein the quenching rate constants is small.

The absolute yield for the production of H atoms in thereaction of N2(a8, v50) with CH4 was determined bycomparing the laser-induced fluorescence signal intensitiesof H over D for two mixtures, N2/CH4/D2/He and N2/H2/D2/He. Figure 5 shows the two-photon laser-induced fluores-

cence spectra in these systems. The H2, D2, and CH4 pres-sures were 27 Pa, while the N2 and He pressures were 10.7and 67 kPa, respectively. D2 was added to normalize thesignal intensities and make it possible to compare theH-atom signal intensities for H2 and CH4. The direct com-parison of the LIF intensities is difficult since CH4 absorbsvacuum ultraviolet radiation. He was added to prevent selec-tive loss of the faster H atoms. The ratio of the relative signalintensities is given by the H-atom production yields,f~H2)andf~CH4), and the rate constants for the overall deactiva-tion, kH2

andkCH4:

~@H#/@D#!CH4

~@H#/@D#!H2

5kCH4

@CH4#

kH2@H2#

f~CH4!

f~H2!.

As is illustrated in Fig. 5, the ratio of the integrated signalintensity for H2 to that for D2 is 1.660.2, which is consistentwith the result of the overall reaction rate constants. Theerror limit is the standard error. The ratio of the signal inten-sity for CH4 to that for D2 is 6.360.6. These ratios wereindependent of pump–probe delay time between 0.5 and 2ms, N2 pressure between 8.0 and 13.3 kPa, quencher pres-sure between 27 and 54 Pa, pump laser energy between 6 and10 mJ, and probe laser energy between 0.3 and 0.6 mJ. NoH~D!-atom signals were observed in the absence of N2. Ac-cording to the present measurements, the rate constant forCH4, kCH4

, is 10.361.2 times larger than that for H2 , kH2. It

may be assumed that the dissociation to two H atoms,N2(a8)1H2→N212H, is the sole exit channel for H2, i.e.,f~H2)52. This point will be discussed later. Then, the yieldfor the production of H atoms in the reaction of N2(a8) withCH4 is determined to be 0.760.2. The yield for the produc-tion of D atoms from CD4 was determined similarly to be 0.760.2. The ratio of the H-atom yield to the D-atom yield inthe reaction with CH2D2 was determined to be 1.160.1. Atypical two-photon LIF spectrum for CH2D2 is shown in Fig.6.

The yields less than unity for CH4 and CD4 suggest thepresence of molecular hydrogen~deuterium! elimination pro-cesses. The production of CH1H1H2 or CH212H cannotbe expected energetically. The production of vibrationallyexcited ground-state CH4 molecules is unlikely, because thatis too exothermic. The most plausible exit is the production

FIG. 4. Temporal profiles of the decay of equilibrated N2(a, v50) andN2(a8, v50) ~d! and the growth of H atoms~s!. The pressures of N2 andH2 were 10.7 kPa and 27 Pa, respectively.

FIG. 5. Two-photon laser-induced fluorescence spectra of H and D atomsmeasured in the N2/CH4/D2/He ~upper! and N2/H2/D2/He ~lower! systems.The pump-probe delay was 1ms. The partial pressures of H2, D2, and CH4

were 27 Pa, while those of N2 and He were 10.7 and 67 kPa, respectively.

5657J. Chem. Phys., Vol. 117, No. 12, 22 September 2002 Reactions of N2 with H2 and CH4

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of CH21H2. CH2 must be in its singlet state,a 1A1, due tospin-conservation. If CH2(a 1A1) is produced, that may reactwith D2 to produce D atoms.26 However, CH2(a) will bedeactivated to the ground state by collisions with N2 beforereacting with D2.27–29The ground-state CH2 is unreactive.30

The production of two NH radicals, N2(a8)1H2→2NH, isslightly exothermic, but we failed to detect NH radicals byLIF. This reaction is four-centered and should have a largeactivation barrier. It should be remembered that, in the reac-tions of N(2D! with H2 and CH4, both H atoms and NHradicals could be detected without difficulties.8 The presenceof the H2 elimination channel is in contrast to the quenchingof Xe(3P0,2) whose energy is almost the same as that ofN2(a8). The production of CH31H is the sole exit for Xe.31

IV. DISCUSSION

In the quenching of N2(A 3Su1) by H2, the production of

repulsive H2(b 3Su1) is spin-allowed but the vertical excita-

tion to H2(b 3Su1) is energetically inaccessible.32 In the

quenching of N2(a8 1Su2), the production of H2(b 3Su

1) isenergetically possible but spin-forbidden. The production ofsinglet-excited H2 molecules is energetically inaccessible.Then, in both cases, the direct energy transfer model cannotbe applied. The reaction must proceed via bound intermedi-ate complexes, N2H2, which decompose to produce two Hatoms. N2H may be produced as an intermediate, but thatwill decompose to N21H within a short lifetime.33–35 Thegeometry of the complexes must be co-parallel. The perpen-dicular approach, where the N–N and H–H bond axes areperpendicular, is repulsive because the triply occupiedpu2porbital of N2 cannot accept an electron from the filledsg1sorbital of H2 both for N2(A) and N2(a8). Sperleinet al.carried outab initio calculations for the triplet case and con-cluded that the reaction proceeds adiabatically on the co-parallel3B2 surface.36 They have also shown that there is anactivation barrier of 49 kJ mol21. This explains why the rateconstant for the quenching of N2(A) by H2 is small; in theorder of 10215 cm3 molecule21 s21.32,37 The first step forthe quenching of N2(a8) by H2 should also be the formationof HNNH-type complexes, 1,2-diazene. The complex mustbe in the lowest singlet excited state,S1, and have an out-of-plane geometry.38,39 Under thisC2 symmetry, both theS0

and S1 states are represented by1A. In other words, nona-

diabatic transition from theS1 state to theS0 state may pro-ceed efficiently. TheS0 state correlates adiabatically to theproduct, N2~X!1H1H.

The situation should be similar in the N2(a8)/CH4 sys-tem, since the vertical excitation of CH4 is energeticallyinaccessible.40 In a localized bond orbital picture, the singlyoccupiedpg* 2p orbital of N2(a8) may donate electron to theempty s* orbital on the C–H bonds, while the filledsorbital of C–H may back donate to thepu2p orbital.HNNCH3-type complexes must be produced during the de-activation process. Dissociation will proceed on the groundstate potential energy surface, which may correlate to bothN2(X)1CH3(X 2A29)1H and N2(X)1CH2(a 1A1)1H2.

The lack of the preferential production of H or D atomsin the reaction with CH2D2 is noteworthy since H/D kineticisotope effects have widely been recognized in many sys-tems. For example, in the photodissociation of partially deu-terated hydrocarbons, the C–H bond scission is preferredcompared to the C–D bond scission by a factor of two ormore.41–47 Similar results have also been obtained forHOD.48,49 These isotope effects have been explained by anRRKM model, by the difference in nonadiabatic transitionprobabilities, or the difference in Franck-Condonfactors.45–48 Preferential production of H atoms over D at-oms has also been observed in the bimolecular reactions ofO(1D2) with partially deuterated methanes and propanes.50,51

An isotope-specific factor, which represents a kinetic isotopeeffect favoring H-atom elimination from activated com-plexes, has been reported to be 1.360.1, which is consistentwith an RRKM model.50,51 Similar results have also beenobtained in the reactions of N(2D! with partially deuteratedmethanes.8 The H-atom elimination takes place 1.3 timesmore efficiently than that of D atoms. In contrast, the pro-duction yields of H and D atoms agree within the error limitin the reaction of N2(a8) with CH2D2.

The lack of the isotope effect in the present N2(a8) sys-tem is consistent with the above model that HNNCH3-typeintermediate complexes are formed during the deactivationprocess. The H/D ratio for CH2D2, 1.160.1, agrees with theratio of the rate constants for the deactivation by CH4 andCD4, 1.160.2. This agreement suggests that the productionyield of H~D! atoms is controlled by the rate constant for theformation of HNNCHD2~DNNCH2D! complexes. If the life-time of the complexes is short, H atoms may be producedselectively from HNNCHD2, while only D atoms may beproduced from DNNCH2D. The C–H~C–D! bond scission,such as HNNCHD2→HNNCHD1D and DNNCH2D→DNNCHD1H, must be minor. There may be isotope ef-fects in the lifetime of the complexes, but the productionyield may depend little on the lifetime if the major fate of thecomplexes is the elimination of H or D atoms. In the bimo-lecular reactions of O(1D2) and N(2D!, these species insertinto one of the C–H~C–D! bonds to produce chemically ac-tivated complexes, such as HOCHD2 and HNCHD2.8,52,53

The production of both H and D atoms is allowed from suchcomplexes. In other words, there may be a competition be-tween H- and D-atom elimination processes. Similar compe-titions are expected in the photodissociation processes. On

FIG. 6. Two-photon laser-induced fluorescence spectrum of H and D atomsmeasured in the N2/CH2D2/He system. The pump-probe delay was 1ms.The partial pressure of CH2D2 was 53 Pa, while those of N2 and He were10.7 and 67 kPa, respectively.

5658 J. Chem. Phys., Vol. 117, No. 12, 22 September 2002 Umemoto et al.

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the other hand, no such a competition is expected in thequenching of N2(a8).

V. CONCLUSIONS

N2(a8 1Su2) is quenched efficiently by H2, CH4, and

their deuterated species. The rate constants for methanes aregas kinetic and much larger than those for N2(A 3Su

1). Theexit channels are chemical in all the cases. The eliminationof H~D! atoms was found to be dominant for CH4 and CD4,but the elimination of H2~D2) takes place as a minor channel.Since N2(a8 1Su

2) is the lowest metastable singlet-state mo-lecular nitrogen, this state should be one of the candidates forthe initiators of chemical reactions in active nitrogen. In thereaction with CH2D2, the production yields of H and D at-oms agree within the error limit. This lack of the isotopeeffect is consistent with a complex formation model in whichHNNCH3-type complexes are formed during the deactiva-tion process.

ACKNOWLEDGMENTS

This work was partially defrayed by the Grant-in-Aid forScience Research~No. 14540469! from the Ministry of Edu-cation, Culture, Sports, Science, and Technology of Japan.

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5659J. Chem. Phys., Vol. 117, No. 12, 22 September 2002 Reactions of N2 with H2 and CH4

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