Formation of NH(A 3Π, c1Π) by the electron impact dissociation of ammonia

Chemical Physics 52 (1980) 47-53 @ North-Holland Publishing Company

FORMATION OF NH(A3H, OF AMMONIA Ikuo TOKUE Department

and Masaru

ofChemistry.

c ‘II) BY THE ELECTRON IMPACT DISSOCIATION

IWAI

Faculty ofScience,

Niigara Unicersity, Igorashi, Niigm

950-21,

Japan

Received 26 February 1980

Emission spectra of the NH(A%rX%) and NH(c ‘n-a ‘h) systems were observed by the electron impact of ammonia from the threshold energies up to 120 eV. The formation of NH(A) and NH(c) by the dissociative excitation of ammonir was investigated. The ratio of the vibrational populations, P( u’ = l)/P(u’ = 0) for NH(A ‘II) is 0.4-0.6, depsnding on the impact energy. The rotational energy distribution of the U’= 0 level of the c ‘n state can be approximated by the effective Boltzmann temperature of 1690*100 K. The rotational excitations of the of=0 and u’= 1 levels of the A 3il state are much higher and their rotational populations deviate from Boltzmann distributions. showing slight dependence on the impact energy. The ratio of the formation rate of NH(A) to that of NH(c) is estimated from the threshold to 92 eV.

1. Introduction An emission spectrum attributable to the NH(A 3ff-X 32) and NH(c ‘n-a ‘A) transitions has often been observed in a discharge in a medium containing nitrogen and hydrogen atoms. Mechanisms of formation of the excited NH radicals have been studied by vacuumultraviolet photolysis [I-5], metastable atom impact [6], and electron impact [7-g]. In the photolysis of ammonia [3,5], NH(c ‘l-I) is formed in a primary process, while the emission of the NH(A-X) transition appears only in secondary processes. According to the spin conservation rule, the primary formation of NH(c) should mainly result from a singlet repulsive excited state of ammonia. On the other hand, spin-forbidden transitions can occur by electron impact through an electronexchange excitation. Fukui et al. [9] measured the appearance potential and the excitation function of NH(A) and concluded that a triplet precursor mainly contributes to the formation of NH(A ?I) near the threshold.

As for the rotational distribution of NH(c) produced by the electron impact on ammonia, Bubert and Froben [8] represented the distribution by the Boltzmann temperature of 1730 K from the measurement of the line intensities of the c ‘II, u’ = 0 -*a ‘A, v” = 0 transition. The temperature was found to be independent of the energy of the impinging electrons. Thereafter, no population analysis of the NH(A-X) and NH(c-a) transitions produced by the electron impact on ammonia has been reported. In order to make a further study of the mechanism of dissociative excitation and energy transfer, we have analyzed precisely the rotational and vibrational structures of the NH(c ‘II-a ‘A) and NH(A 311-X ‘H) transitions by computer simulation of the band envelopes.

2. Experimental The apparatus in operation at the University of Tokyo and the experimental details have

I. Tokue,h4. Iwai / Formationof NHCA‘n, c ‘II)

4%

mported by Nishiyama et al. [lo]. The and the collision chamber were o oil diffusion pumps

3. Results and discussion

roan ~urce

gas was introduced through a nor.le and condensed on a liquid Tbc pressure of the sample gas in gian during the emission was controlled to 10-3-10-4

Torr

potential of 10-120 eV was applied a) transitions were with a trap current of 100-300 PA, high intensity and a good S/N ratio ired to analyze the rotational and ~~~~~a1 structures. The light signal of 250 to 600 nm was detec1 m Cserny-Turner scanning the first order of a 1200 Gr/mm

t 300 nm. A photomultiplier IMTY R!%S) and a photon-counting system WW~mcd for high resolution measurements. bath a slit width of SO pm a spectral band pass d ME5urn fwhm was obtained. The relative ~~t~~;y response of the detection system WMcalibrated by means of a halogen ~~rn~~lI]. Ttr cxcirstion functions of the Q-heads of tbc Wt. l-1 bands of the NH(A-X) transition nosajl ThorQ.hrench of the (I-0 band of the NH(c~8~~~~~~t~~~n were measured with a beam currrrrt r~tnttollcd to 100-251) PA. The energy WDICtctr the tnctdcnt clcctron beam was cali~+~atedWII~Y,rhc cxcitatkm function for the $1 (11hnnd al the N:(C’ ‘11,-B ‘n,) emission by t trtrt tt a1 [ 121 Nttrogcn was mixed with .~rrr~~c:r.

antt tf,clr

emissions

were measured

UXWJ~. Shgfit discrepancies were wed in the steepness of the excitation

~~~~~~~Enfhc threshold region and in the shift #W ~~p~~t cncrgy to higher energy, probably au@ rrf the energy spread of about 1 eV and gofer kdrtm current.

3.1. Observed spectra

The intense O-Oband of the NH(c ‘II-a ‘A) system and the intense O-Oand 1-I bands the NH(A 311-X32) system were observed.

of

Since the observed 1-O and O-1 bands of the A-X system were very weak, they were not used for further analysis. Lines of the O-O bands of the c-a system were assigned by comparison with those observed by Dieke and Blue [13] and Shimauchi [14]. The wave numbers observed by Dixon [lS] were used for assignment of the lines of the O-Oband of the A-X system. The emission spectrum which covers the region 324-348 nm is built up from two heavily overlapping bands corresponding to the O-O transition of the c-a system and the O-O and l-l transitions of the A-X system (fig. 1). Only the Q-branch of the c ‘II, LJ’=O+ a ‘A, u” = 0 transition can be resolved for N’ up to 11. In fig. 2 log (Zo/S,~N+3) is plotted as a function of N’(N’+ l), where Z,, and &W are the observed line intensity and the line strength with the quantum number N’, respectively. The plot is a straight line (fig. 2); the slope represents the effective rotational temperature of 1690*60 K. Beyond this point no such analysis was performed because serious errors may arise from overlapping of different vibrational transitions and from overlapping of different e!ectronic transitions in the rotational structure. 3.2. Analysis of band envelopes The observed spectrum is compared with a band envelope simul .ted on a computer. Because all transitions contributing to the spectrum are taken into account in the calculated spectrum, even the lines which are overlapped with other lines can be analyzed. The analysis of the intensities of these bands provides information about the rotational distributions, P(W), and the vibrational population ratios P(u’)/P(u’ = 0) of the NH molecule produced from ammonia in the A 311and c ‘II states independently. Furthermore, the analysis pro-

49

vides information about the formation raft0 P(A %)/P(c ‘11) after the dissociativeexit&an of ammonia. The Franck-Condon factors for the 0-O and l-l bands of the NHtA-XI tranitim and tk 0-O band of the NH(c-a) transition were taken from ref. [ 161. The electronic transition

!_

E~=Y=U,tI3andco~tot~nnine dkmxi for the “lL,,%band. For the NHIA ‘I?,) state the tramition from HumI. coupr&rgcse bal to case lb) is very rapid with itrmeasittg mtatioml quanrum number, and !smcc she retmining 18 satellite km&es arc very weak except for the loWest ?c: vah~5. The fzalmfatd freqwncies of the O-O band agreed within 2 cm -’ with the observed vaipaes[Is] for N‘ np UI 30. The line strengths strong. bmwk

1.0

100

momentsfar the 0-O and 1-I bmds of the NHIA-XI tza&&~ were rccaicnfated frum the lifetime data mca~red by Smith et al. [ 171. The r-cermoid for tbc A-X tmtsition was calculated fram a propam QlB] based on the RKR potential hy Fleming and Rae [PI]. Among th sis hnnlclles obtained for the t ’ IJ-a “3 tratiripns, two O-branches are about twice as intense as the other four branches. The c&dated fr~que&es af the O-0 band apeed within 2 cm ’ with those obscmd by Dieke and B1uc[ t 3] and by Shimauchi [ I-l] for 5’ up f 1 15. l%e lmc saren@zs cri the c-a syem a2rrc c&ulated according to the formulae pen I?! Hnnl and Lrsndan [IO]. Thcrc are 27 branches III the A ‘11,-N ‘1 tzanrd~~~~Vc’snrcpfthem arc the mam tvrlnchc\

4 206

Fig.2. Plot of logllnn/S,v rC”i$. m 3” artxtrarg zcak. vrtvus N’(N’+ 1I for the0-Obilnd of the F*‘Htc-at emnszon. *. lines superimposed by the R-bnnch 04 lbe NHIA-XI emission.

so

I. Tokue;M. hail

Formation of NH(A’ll.

of the A-X rotational transitions were calculated according to the formulae described, ,by Bud6 [21]. Two branches with particularly weak line strengths were excluded from the bandenvelope analysis. Band envelopes were computed by convolution of the calculated intensities of the six branches for N’ up to 30 for the O-O band of the c-a system and 25 branches for N’ up to 35 of the O-O and l-l bands of the A-X system with a slit function, for which a gaussian function with a bandwidth parameter is assumed. The bandwidths assumed were 0.05 nm fwhm for the spectra at the impact energy of 42 eV and 0.1 nm fwhm for all the other spectra to account for the observed spectral resolution. The P(N’), P(u’)/P(u’ = 0) and P(A 311)/P(c ‘II) values were adjusted until the best visual tit was achieved. The observed spectrum at the impact energy of 42 eV is compared in fig. 3 with the synthetic band enveIopes.

325

c ‘n)

From the results of the above-mentioned simulation, the rotational distributions of the c ‘II state were found to be represented by the effective Boltzmann temperature of 169Ozt 100 K, being independent of the impact energy. This result is consistent with the abovementioned analysis (fig. 2) of the line intensities of.the Q-branch and is also in good agreement with 1730 K reported by Bubert and Froben [S]. On the other hand, the rotational distribution of the A 311state could not be represented by a simple Boltzmann temperature. The detailed distributions, P(N’), for the u’ = 0 and u’ = 1 levels of NH(A) used in the best-fit simulated spectra are shown in figs. 4a and 4b, respectively. In both of the t)’ = 0 and u’ = 1 levels the peaks of the distributions as well as the mean angular momenta are nearly identical for electron energies higher than 42 eV, but they shift slightly downwards near the threshold.

335

3&O Wovtkngth/nm

Fig. 3. Top trace is a synthetic spectrum generated by a computer_ Bottom trace is the observed spectrum at the impact energy of 42eV with 0.05 nm fwhm resolution. The experimentat spectrum of the bottom trace is identical with that shown in fig. 1.

I. Tokue. hf. hail

52

Formarion of NHIA

‘II. c ‘I71

NHI~NH(ctn,+H:cX’r;I.

o !ct’.

11,

NH3-. NHlc ‘IJI+ ZHI’S), 13.8et’.

a2a

NH3* NH/A ‘fl) + H&t ‘Z;,.

ak

NH3+ NH(A ‘IT)+ 2Hh The

lhresbofd

7.45 rV.

122 eV. cxcSW0a

that confribwtion ptmwm small. the of rpia ruk, pra&#WW Wfect inthedissociationfrom(I)shouldkr~ state. Likewise, the preauwr form&gNHfiAt by dissociation (3) should be a triplrr pie AI for process (41, however. the prv can hr singlet and/or triplet SMC. AWot# a gpmca state is also permissible. direct ezcifazxm fnnn the ground state does not seem lo be paaMc From the present measurrmcm, r*a di&ti channels for the prod&on of NHtA? appm ta exist. The thresholds for NH(A %I. 8 0 and 12.3 eV correspond IO proca~ 131 and 44,. respectively. The rate of formatJon d &c u * I level of NH(A) by way of (4) is wmM tbur channel is

10

20

15 Electron

energy(cV)

Fig. 5. Square root of the intensity of (a) the O-O band of rhc NH(A-X) emission, (b) the l-1 band of NH(A-X), and (c) the O-O band of NH(c-a) versus corrected electron energy. The arrow marks the lowest electron energy where the observed spectra could be analyzed by band envelope simulation.

According to the measurement by Fukui et al. [9]. only one threshold at 12.2 eV was observed. The existence of two thresholds has been obscured in their plot of the emission intensity with a linear scaling, because the excitation function of the NH(A-X) transition has two gentle slopes. The head of the O-O band of the N2(C-B) transition, 337.13 nm, is accidentally overlapped with the head of the l-l band of the NH(A-X) transition. The contribution of the O-O band of the Nz(C-8) transition from residual air to the emission intensity of the l-l band of the NH(A-X) transition is estimated to be less than 4% at the impact energy of 14 eV, where the excitation function of the O-O band of the NI(C-8) transition has a sharp maximum. J.4 Formations of NH(c) and NH(A) By comparison of the excitation threshold energy with thermodynamic [22] and spectros<+it 123j data, the dissociation processes ‘ltrrning NH(c) and NH(A) from ammonia can rcprcscnted as follows:

that through (3) (fig. 5). Thea &c ewmtoa, function suggests that a triplet pprzntrrnr from (3) and singlet and/or triplet prccuzurrr from (4) can contribute to the format~tn d NH~AI This process can proceed unimokcu!arlr m t-b steps, i.e. NH3+NHf+H-NH+tH H--et no excited state of NH* corrclntrng to UH prod H seems to have been observed From the rotational analp rpi NH:.~.. thz angular momentum convUttd to rbe rourwwtpl energy of NH(A) is found to vary only moderately from near the thresWd up lo 92 eV. Unfortunately, the con?ribur&m frcu.n process (3) may be m&ed by m Gli. because contributions from the two pnrcer;cicl

are blended even at tie ioweu u&&on m (17 eV) to which the present band-cavekrpr analysis of the observed spectra o&d be applied.Atripletprecursorisexpcctedrobeoedyo minor contributor at an impact energy a~ ttr@ as 92 eV, because excitation 10 a yipkt precursor involves an eicciron~~

procnr

The formation rate of NHt A I is nrart~ equal ?a that of NH(c) near the threshaid and rr b&I rhat