L.F. Phillips- Intensities of Emission Lines in Flames of Metal Halides with Active Nitrogen

8/3/2019 L.F. Phillips- Intensities of Emission Lines in Flames of Metal Halides with Active Nitrogen

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INTENSITIES OF EMISSION LINES IN FLAMES OF METAL HALIDESWITH ACTIVE NITROGEN

L. F . PHILLIPSChemistry Department, University of Canterbury, Ch~istchurch, ew Zealand

Received April 3, 1963ABSTRACT

Th e intensities of spect ral lines emitt ed by flames of a nu mber of metal halides with acti venitrogen have been found to var y as the square of th e nitrogen atom concentratio n. \\Then thetota l energy required for simulta neous dissociation of th e halide and excita tion of th e metalato m is less tha n abo ut 200 kcal/mole the energy transfer process is too efficient to be a ttri -bute d to th e termolecular reaction of a halide molecule with a pair of nitrogen atoms. Theobservations are consistent with the hypothesis th at in this case energy is transferred to thehalide molecule during collision with a nitrogen molecule in the 2 ,+ tate. Possible excitationmechanisms a re discussed for less intense lines which would requir e up t o 276 kcal/mole forsimultaneous dissociation and excitation.I NTRODUCT ION

In a previous con~munication 1) it was shown th at in flames of thallous halides withactive nitrogen the int ensity of the 5350 A thallium line is proportional to the first powerof the halide concentration and to the square of the nitrogen atom concentration. Th e ra teof the overall reaction, calculated from the absolute intensity of the 5330 line and alsofrom the ra te of consumption of halide, was approximately 2X cm6molecule-2sec-~,whereas the largest value to be expected for a termolecular reaction is about cm6mole-~ile-~sec-l 2 ) .

Thallium lines were observed photographically down to 3230 '4, for which the energyrequired in-the case of T l F is 221 kcal/mole. T he results were discussed in terms of thetransfer of energy to the halide du ring a collision with an electronically excited nitrogenmolecule (equation [I]), he excited molecule being produced during nitrogen atom re-combination. In the steady st ate the concentration of Ng* is proportional to the squareof the nitrogen atom concentra tion. I t was suggested tha t th e excited molecule involvedwas in the %,+ stat e, since the amount of energy required appeared to be too great to beprovided by other metastable species.

In the present paper the intens ity measurements are extended to all of the thalliumlines and to flames of active nitrogen with some halides of other metals . For all of theemission lines the in tensity varies as the square of t he nitrogen atom concentration, pro-portionali ty to the halide concentration also being observed over the range of part ialpressures used. The lines may be divided into three classes, depending on whether thetota l energy required to dissociate the halide and excite the metal atom is less than about200 kcal/mole, between 200 and 225 kcal/mole, or greater t han 225 kcal/mole. For linesin the first class the efficiency is in general too high for the termolecular mechanism [2]

X = halogen atom, lLI = metal at om of valency nand the argumen ts of the previous paper (I ) may be applied, leading to t he conclusionth at energy is probably transferred from a nitrogen n~olecule n the 52g+tate. For linesCanadian Journal of Chemistry. Volume 41 (1963)

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PHILLIPS: EMISSION LINE INTENSITIES 2061in the second class either of these mechanisms may operate . For lines in the third classboth mechanisms may be ruled out on energetic grounds, while a termolecular mechanisminvolving one ground state and one excited nitrogen atom may be eliminated on groundsof efficiency because of th e small proportion of excited a toms in discharged nitrogen (3).There are two other alternative s for this case, one involving a higher metastable sta te ofS2 nd t he other using consecutive excitation processes similar to [I ] or [2] to overcornethe high energy requiremetlt. Of these alternatives the la tte r is favored since it leads morereadily to a dependence of intensity on the square of t he atom concentration .

This work is essentially a continuat ion of the series of observations of St ru tt (4 ) onflames supported by active nitrogen, with t he advantage t ha t in the intervening 50 yearsa reasonably complete theory of act ive nitrogen has developed (5) . In the current theorymost of t he activ ity of discharged nitrogen is att ributed to the presence of ground-statenitrogen at oms; however, the role of metastable nitrogen molecules has been stressed b ya number of invest igators (6-9, 1) . Mulliken (10) has listed most of the known or pre-dicted s tat es of N2, ogether with their theoretical dissociation products and importantspectroscopic constants, including the known or predicted energy of the ground vibrationallevel. Table I, selected from Mulliken's data, contains those states which are expected tobe metastable or long lived. As pointed out by Kenty (11), a s tat e may be slightly aboveanothe r to which transition is allowed yet have a long lifetime because of smallness ofthe factor v 3 in the expression for the transition probability. Where a state is expected tobe long lived for this reason the nearby state to which a transition is allowed is given inthe right-hand column of Table I. As in Mulliken's table, square brackets indicate'\re-dicted sta tes or values, parentheses indicate doubtful values.

TABLE ILong-lived electronic s tat es of Nz

Energy (v = 01, Do, Dissociation AllowedSt ate kcal/mole kcal/mole products transition-- -

142 83 43+43 -11721 11071 4S+2D -+B3ng t 170 kcal196 . 138 2D+2D -(202) (132) 2D t 2 D allIna t 196 kcal(3 )222 4Sf4S -[(280)1 [ (H z) ] ~ S + ( S ~ ~ ~ . ~ S ) ~ PI& ] a t [(279)] cal(Possibly othe r state s, of high multiplic ity, above this)

EXPERIMENTALThe reaction vessel and flow system were as previously described (1) except that an improved heating

jacket enabled the reaction vessel to be heated t o 550 C. This higher temperature was necessary for flames\yith sodium iodide, a temperature of 400+10 C being used with th e other halides. Th e reaction vesselconsisted of a length of 34 mm 0.d. qua rtz tubing having one end window a t right angles to th e length ofthe tub e and two parallel side windows. In t he present work photometric observations were made throughthe side windows. Active nitrogen from a microwave discharge was pumped through the reaction tubewith a linear velocity of 1.2 m/sec a t 3.0 mm Hg, t he majority of the experiments being carried out a t thispressure. Meta l halides were carried from a small, independently heated oven by a stre am of argon, an dentered the reaction vessel through a 0.2 mm jet a t the end of a 3 mm 0.d. quartz tube. This jet could bemoved over a range of 6 cm along the axi s of t he reacti on vessel. Xitrog en ato m flows were determined byvisual titration with nitric oxide (12).Typical partial pressures of atomic nitrogen and metal halide were0.05 mm and 0.0005 mm respectively.Thallous halides were as used previously. The other halides were A.R. grade or else the best reagentgrade available. The process of vacuum s ublimat ion involved in producing a flame appeared t o be sufficientto elim inate th e effects of most impur ities, especially since the g reatest source of t his kind of e rror \vould b e


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2062 CANADIAN JOURNAL OF CHEMISTRY. VOL 41, 1963th e presence in th e reaction vessel of t races of halides from previous experiments. Prolonged cleaning of t hevessel with 20% hhydrofluoric acid was no t sufficient to ent irely free th e walls of metals or meta l halides.Measurements were carried out i n an order which would minimize the effect of t his cont amin atio n; forexample T1F was studied before TlI , the int ensity of emission at all wavelengths being greater for th e latt er.In every case the background intensity was measured with the argon flow cut off and a suitable correctionapplied:Photometric measurements were carried out using a Hilger medium quartz spectrograph fitted with anE720 scanning unit and a 1 P28 photomultiplier. T he ou tput of th e photomultiplier was taken to a O-250 pa meter through a c atho de follower. Th e variation of photometer sens itivity with wavelength wasdetermined from the response to a tungsten strip filament lamp of known temperature and emissivity (13).

RESULTSTh e four thallous halides, lead (11) chloride, bromide, and iodide, cuprous iodide, and

sodium iodide were studied . Th e spectral features observed were thallium lines a t 5350,3776, 3530, 3519, and 3230 A, lead lines a t 4058, 3683, and 3639 A and visible bands ofPb X, copper lines a t 3247 and 3274 with visible bands of C uI , and sodium lines a t5890, 5896, and 3302 A with the respective halides. Flames of this so rt have often beenused a s sources of M X bands for spectroscopy (14) . Flames of sodium and thallous halideswere short and tended to be spherical a t low halide flows. Those of the other halides weremuch more elongated and sometimes extended well beyond the heated portion of t heflow system. This suggests tha t the reactions leading to consumption of the la tter halidesare comparatively slow, and perhaps proceed in stages. Dark-colored deposits, similarto those noticed previously with thal lous halides, were formed in cool parts of the reactionsystem with all of the halides except sodium iodide, where a white deposit was formed.Hydrofluoric acid removed the metallic appearance of the deposits , leaving solids whichwere similar to the original halides and which could be removed with hot water.

The effect of varia tion of nitrogen atom concentration on the intensi ty of the emissionlines is shown in Fig. 1. All of the observed lines vary in intensity as the square of thenitrogen atom concentration. The M X bands th at appeared with halides of lead andcopper were superimposed on some moderately intense afterglow emission; however itwas noticed that the total intensity varied more skowIy than [N]2. Since the intensity ofthe afterglow alone is proportional to [NI2 (8) this suggests tha t the MX-band intensitydepends on a lower power of the nitrogen atom concentration.

For the measurements of Fig. 1 the power supplied to the discharge was varied, theatom concentrat ion a t each power level being determined by NO titration in absence ofhalide. Th e end point of the titration is difficult to locate at high temperatures, apparentlybecause of reduced efficiency of the light-emitting reactions, so the calibrations of powerlevel in terms of a tom concentration were carried out near room temperature. The amountof nitric oxide required to extinguish the halide flame at the high tempe rature was in-variably the same as that required to titrate the nitrogen atoms at the low temperature.

In order t o determine the relat ive efficiencies of the various excitation processes theintensit ies of emission lines were measured f or each flame under as nearly the same con-ditions as possible. Observations were made a t 2 cm from the halide inlet jet and t heintensities were corrected to the same partial pressures of halide and atomic nitrogen.The same argon flow (12.1 pmoles/sec), nitrogen flow (265 pmoles/sec), total pressure(3.0 mm Hg), and temperature (400&1O0C) were used in every case except tha t of sodiumiodide where the temperature was 550' C.

The results of the intensity measurements , normalized to a value of 200 for the ra teof production of Tl(7 .2S+)toms f rom TII, are given in Table 11. On this scale the nominalupper limit for the rate of a termolecular reaction is unity and emission lines which have


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PHILLIPS: EMISSION LINE INTENSITIES

Atomic Nitrogen ~mo l e s l s e cFIG.I. Variatio n of in tensity of metall ic emission l ines with a tom ic nitrogen flow. Molecular nitrogenflow rat e 265 moles/sec. Total pressure 3. 0 m m Hg. Descr ip t ion of graphs , reading f rom top to bot tom:T1 5350 A, from TIC1; Na 58934, aCI ; Pb 3683 A, PbIz; T1 3230 A, T I F ; Cu 3247 A , Cu r ; Pb 4058 A,PbC1, . Intensit ies in arbitrary units.

relative intensities which exceed thi s by an order of magni tude are almost certainly excitedby a different kind of process. Also listed in Tab le I1 are the exci tation energies E of theemission lines (15) and the dissociation energies D of the halides (16, 17). For lead halidesthe energies for dissociation into a toms a t 400' C were calculated from a thermodynamiccycle using da ta from Lewis, Randall, Pitzer , and Brewer (18).These dissociation energies-will be slightly high because heats of fusion of the halides were neglected and heats ofvaporization were calculated directly from vapor pressure data (19) without allowancefor the possible presence of polymeric species in the vapor. Th e results are en tered inTable I1 in order of increasing D +E.

DISCUSSIONTh e results in Table I1 show a st eady trend with increasing value of the sum (D +E ),

the only marked exception being found with cuprous iodide. I t is well known t ha t in thi scase the vapor contains a large proportion of dimer Cu212 and extrapolation of the vaporpressures of the two species (19) indicates that a t 400' C the rat io of dimer to monomeris of the order of 100, which is sufficient to explain the low intensity.

Th e criterion that the intens ity in Table I1 should be more than about 10 for a processto be too efficient to be accounted for by the termolecular mechanism [ 2 ] leads to an


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2064 C A N A D I A N J O U R N A L O F CHEMISTRY. VOL. 41. 1963

TABLE I1Relative intensities of metal lines at same partial pressure of halide

Atomic line E, kcal/mole Halide D, lical/mole D+E IntensityXaI 70TI1 68CuI 69TIC1 89T1I 6sTI1 68T1F 110TlCl 89TIC1 89TIF 110T1F 110PbI2 125PbIz 125PbBrz 152PbC1, 175PbC12 175KOTE:E = excitation energy of atomic line. D = dissociation energy of halide (at 0" K for all except leadhalides, where they are calculated for 400" C ; see text).

approximate lower limit for this mechanism a t D +E = 200 kcal/mole. 'There is no theo-retical reason to expect a lower limit for process [ 2 ] ;however, it is still not possible tor-ule out mechanism [2] for D f E greater than 200 kcal. This is not necessarily an upperlimit for the bimolecular process [I ],and there is nothing to imply that the same mechanismdoes not operate over the whole range of D+E below 222 kcal/mole. Apart from the5 2 , + sta te the only metastable stat es of Kzwhich might be present in appreciable concen-trations and which could supply 200 kcal/mole are the a and a' states (Table I ) . Withboth of these states the available energy is on the low side, and a further point againstthem is th at the sta te is probably required to be their precursor if they are to beformed from ground-state atoms. Unfortunately little is known about t he relevant crossingprobabilities, bu t the st rength of the Lewis-Rayleigh afterglow suggests tha t most 58,+molecules prefer to cross into the B state . The observed quenching of the afterglow in thepresence of metal halides also favo rs the choice of 52,+ s the st ate of the excited N z inequation [I].

With lead halides the energy required for simultaneous dissociation and excitation ofthe emission lines is equal to or greater t han 225 kcal/mole, which is the maximum whichcan be supplied by recombination of ground-state nitrogen atoms. Another kind of mechan-ism is therefore required. As mentioned in the introduction, a termolecular reaction ofthe halide with one ground state and one excited nitrogen atom would not be efficientenough to account for the observed intensities because of the small proportion of excitedatoms in active nitrogen (3) . I t is also difficult to see how this mechanism could lead t o adependence of int ensi ty on the square of the (ground-state) nitrogen atom concentration.In this respect the results of Fig. 1 were surprising because it had been anticipated th atwith lead halides the intensity would vary a s some higher power of the a tom concen-tra tion. The bimolecular mechanism [ I] might explain the observations if the excitednitrogen molecule were in the predicted l2,+ state, but here again it is difficult to seehow a dependence on [NI2 ould arise. The only remaining possibility is th at of excitationby consecutive energy transfer steps. This mechanism can lead to a dependence of intensity


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PHILLIPS: EMISSION LINE IKTENSITIES 2065on [NI2,as is shown below, taking lead chloride as an example. In this mechanism the

PbCl z + N2* -+ PbCl* + C1 + N 2 [3]PbCl* + N2* -+ Pb* + C1 + N z 14IPbCl* + Nz 4 PbCl + N2 151PbCI* -+ PbC l + hv 161PbCl + N2* -+ Pb* + C1 + N 2 [71

Pb* -+ P b + hv' 181part of N2* could equally well be taken by a pair of uncombined nitrogen atoms. Recombi-nat ion of PbCl and C1 has been neglected because of the low halide partial pressure, whilequenching of excited lead atoms should be negligible because of their shor t lifetime (ca.

sec). Halogen atoms are expected to be removed a t the walls of the reaction vessel(20). At the steady state

[PbCl*] = k~[PbCl~] [N~* l / (k~[N~* lks(Nz1 +KG) [91[PbCl] = (k,[N,] f k,) [PbCl*]/k.i[Nz*] [lo]

[Pb*] = k3[PbClz][Nz*]/ks. [I l lTh e intensity of PbCl emission bands is given by k~[PbC l*]. he observation tha t theintensity of these bands varies more slowly than [XI2suggests that k4[Nz*]s comparablein magnitude with the other terms in the denominator of equation [9]. Th e total i ntensityof Pb lines is given by ks[Pb*] and is equal to th e rat e of the primary process [3] whenrecombination and quenching of Pb* are not taken into account. I t is probable th at in t hepresent experiments a completely stationary st ate was not reached in the short time spentby the reactants in the reaction vessel, since it was found that the Pb-line intensityincreased steadily with increasing distance from the halide inlet jet.

A similar consecutive excitation mechanism for thallium halides is ruled out by theobservation of finite spherical flames a t low halide flows. Thallium nitride-is unknownand thallium metal has been found as a reaction product (I ), so from the fact that no T1emission occurs in the region immediately following the flame, where both thallium andactive nitrogen are certainly present, i t follows tha t t he efficiency of excitation of metallicthallium by active nitrogen under t he conditions of the present experiments is very low.This conclusion was tested by mixing thallium vapor with active nitrogen in a simplereaction vessel which had a fixed inlet jet and a single end window and which was ableto be t aken to temperatures approaching 1000" C. At a cherry red heat (ca. 800" C) th evapor pressure of t he metal is similar to th at of thallous iodide at 400' C. The emissiona t 5350 A with the metal a t this temperature was less tha n 1% of th at which was observedin a subsequent experiment with the iodide a t 400" C. Th e behavior of lead should besimilar to tha t of thallium in this respect , and in the mechanism just given no allowanceis made for the possibil ity of excitation of lead a toms by Nz*.

ACKNOL47LEDGhIENTSTh e author is grateful to the New Zealand University Research Grants Committee forfinancial sup por t and t o Mr. C. G. Freeman for assistance with many of the measure-ments. In addition i t is a pleasure to acknowledge the contribution of discussions andcorrespondence with Dr. A. N. Wright of McGill Univers ity to both this and the previouspaper.


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2066 CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963REFERENCES

1. L. F. PHILLIPS. Can. J. Chem. 41, 732 (1963).2. S. W. BENSON. The foundations of chemical kinetics. McGraw-Hill, New York. 1960.3. Y. TANAKA, . JURSA, nd F. LEBLANC.The threshold of space. Edi ted by M . Zelikoff. PergamonPress. New York. 1957. D. 89.4. STRUTT(~ or d ayleigh). P ~ O C .Roy. Soc. Ser. A , 85 , 219 (1911); 86 , 56 (1911); 88 , 539 (1913); 91,303(191.5\., ----, .5. K. R. JEXNINGSnd J. W. LINNETT. Quart. Rev. (London), 12 , 116 (1958).6. A. N . WRIGHT, . L. NELSON, nd C. A. WINKLER. Can. J. Chem. 40, 1082 (1962).7. C. KENTY. J. Chem. Phys. 35, 2267 (1961).8. A. N. WRIGHT nd C. A. WINKLER. J. Phys. Chem. 67, 172 (1963).9. K. D. BAYES. Can. J. Chem. 39, 1074 (1961).10. R. S. MCLLIKEN. The threshold of space. Edi ted by M . Zelikoff. Pergamon Press, New York. 1957.n 169.r -11. C. KENTY. J. Chem. Phys. 37, 1567 (1962).12. F. KAUFRXANnd J. R. KELSO. J. Chem. Phys. 27, 1209 (1957).13. J . C. D ~ v o s . Thesis, Amsterdam. 1956.14. R. \V.B. PEARSE nd A . G. GAYDON.The identification of molecular spectra. Chapman and Hall,London. 1950.15. C. E. MOORE. Natl. Bur. Std. (U.S.), Circ. 467, Vol. I (1949); Vol. I1 (1952); Vol. I11 (1958).16. E. M. BULEWICZ, . F. PHILLIPS, nd T. M. SUGDEN. Trans. Faraday Soc. 57, 921 (1961).17. A. G. GAYDON.Dissociation energies. Chapman and Hall, London. 1953.18. G. N. LEWIS,M. RANDALL, . S. PITZER, nd L. BREWER. Thermodynamics. McGraw-Hill, NewYnrk l QG l-A A-. - - - .

19. LLANDOLT-BORNSTEIN.abellen. Zweiter Band, Teil 2, Bandteil a. Springer-Verlag, Berlin. 1960.p. 49.20. E. A. OGRYZLO.Can. J. Chem. 39, 2556 (1961).

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L.F. Phillips- Intensities of Emission Lines in Flames of Metal Halides with Active Nitrogen