Large ions in premixed benzene-oxygen flames

Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 355-362

LARGE

IONS

IN

PREMIXED

BENZENE-OXYGEN

FLAMES

S. LOFFLER A~D K. H. HOMANN Institut fiir Physikalische Chemic der Technischen Hochschule Darmstadt, Petersenstr. 20, D-6100 Darmstadt

The large, non-aliphatic ions in benzene-oxygen flames can be subdivided into three groups: polycyclic aromatic hydrocarbon ions (PAH ions), oxygenated PAH ions (oxo-PAH ions) and polyhedral carbon ions. They occur both positively and negatively charged. Oxo-PAH ions of either sign and negative PAH ions are not formed in acetylene flames. The oxo-PAH appear first and occupy a mass range up to 500 u, those with an even number of C atoms dominating the mass distribution. They show a weak growth but mainly decompose to PAH-. The even-numbered oxo-PAH- give odd-numbered PAH- which do not grow any further. Positive oxo-PAH ions peak later, occupying a mass range up to about 450 u. Among these the even-numbered species prevail. They decompose to PAH § which continue to grow up to more than 103 u. The positive even-numbered oxo-PAH ions can be considered as protonated phenol and its PAH analoga, while the odd-numbered oxo-PAH § are regarded as oxo-phenalenylium and its higher benzo-condensed analoga. The oxo-PAH- are taken as phenolate plus higher analoga and as phenalenate plus analogous species, respectively. Negative polyhedral ions (C~o, C~o) have recently been detected in non-sooting benzene flames. An additional source for small negative polyhedral ions other than small soot particles is discussed. It is concluded that negative polyhedral ions are formed from neutral species and gain their charge later. No such conclusion can be drawn for the positive polyhedral ions as long as the respective neutral molecules are unknown.

Introduction

tion to oxygen-containing PAH ions and to the polyhedral carbon ions. 9

Polycyclic aromatic hydrocarbon ions (PAH § play an important role for the ionic structure of fuel-rich and sooting pre-mixed hydrocarbon flames. Up to now mainly acetylene has been studied along these lines.l-3 It is known from investigations of neutral flame gas composition and of soot formation in benzene flames that there are some characteristic differences in the structure of acetylene and benzene flames, 4'5 although qualitatively the intermediates and products are largely similar. Within the oxidation zone of a fuel-rich benzene flame the overall PAH concentration is much larger than in an acetylene flame forming the same amount of soot. 6'7 The positive ion concentration in the oxidation zone is largely governed by chemi-ionization, while in the soot formation zone thermal ionization of soot is important. In benzene flames these two flame zones overlap to a certain extent, 8 so that mutual interference of the two ionization mechanisms can be expected, particularly in the zone where soot formation begins. Therefore, we undertook a mass spectrometric investigation of positive and negative flame ions up to masses of about 103 u in low pressure benzene-oxygen flames, paying special atten-

Experimental Low-pressure fiat premixed benzene-oxygen flames, burning at a pressure of 2.67 kPa on a cooled sintered disk burner of 75 mm diameter, were investigated. The burning conditions of the flames are given in the captions to the respective figures. The probing of the flame and the time-of-flight mass spectrometer have been described e a r l i e r f 1~ in short: A Pt-coated quartz probe was used to form a supersonic free jet, from which a molecular beam was formed employing a skimmer. The ions were extracted by a 10 kHz pulse of proper polarity, and accelerated to an energy of 2.6 keV in a flight tube of 520 mm length. For this investigation the detector plate from which secondary electrons were emitted was covered with CsI, thus increasing the detection sensitivity by a factor of about six.

Results In this paper we focus on three different groups of large ions: oxygenated PAH ions (oxo-PAH ions), 355

356

PREMIXED FLAMES

ions of polyeyclic aromatic hydrocarbons (PAH ions) and ions of polyhedral carbon molecules (polyhedral ions). They all occur positively and negatively charged. Positive PAH and oxo-PAH ions in ben-

zene flames have first been reported by Olson and Calcote. 1 The molecular formulae assigned to the ions are mainly based on their masses. The hydrogen con-

TABLE I Positive oxo-PAH ions. ~r-bonds in the ring structure are omitted.

m/u

ION

PROPOSEDSTRUCTURE

9s

%H70"

169

C12HgO*

181

C13H90 ~

~

193

C14HsO*

~

205

C15HsO*

219

CIsHuO*

231

C17H~10"

2/,3

C18Hl10*

255

CIgHuO" [ ~

267

C2oH11 O•

279

C21HI10"

m/u

ION

317

Cz4H~30"

0H2

329

C25H130"

{~H + 0Hz

341

C26H130"

353

C27H130"

365

C28H13 O'~

OHz

377

C29H13 O§

0H

391

C30HlsO §

/-03

C31H15 O+

415

C32H150"

/,2?

C33H~sO"

/.39

C34HlsO§

( ~ (~Hz

OH 0H2

@.

0Hz

V

V

PROPOSEDSTRUCTURE

V

,+ 293

C22H130"

OH2

303

C23HuO* [ ~

0H

IONS IN BENZENE-OXYGEN FLAMES

357

TABLE II Negative oxo-PAH and PAH ions. Structures for oxo-PAH ions are only drawn, if a respective positive ion could not be detected or if the ring structure of the phenolate derivative is not the same as that of the respective protonated phenol derivative.

m/u

93

ION

PROPOSEDSTRUCTURE

C6H50-

107

C7H70-

lt,3

CloH?O-

155

CllH70-

I~CH3 ",~ 0r'-k

<~ O-

m/u

ION

m/u

ION

279

C21Hl10

65

C5H 5-

291

C22HI10-

115

CgH?-

303

C23Hl10-

139

CtlH?-

315

C2t,HllO

189

C15H9CllH 9-

-

167

C~2H70-

329

C2sH~30-

213

181

C13H90-

339

C2sHuO-

237

CI9H9-

193

C1~H90-

353

CzTH~30-

287

023HII-

205

C15H90-

363

C2BHI10-

311

C25HI1-

217

ClsHgO-

377

C29Hi30-

231

C17H11 O-

389

C30H130-

241

ClsHs0-

403

C31H150-

255

C19HuO-

413

C32H130-

265

C20H90-

~

0-

~C;,o

tent of many ions was known from experiments with benzene-d6. 11 Furthermore, the change in concentration of individual ions with height in the flame and with burning conditions gave additional information about their nature.

C12H 11.5 mm

Oxo-PAH Ions: Oxo-PAH ions which occupy a mass range between 90 and about 450 u are listed in Tables I and II. Generally, for larger aromatic systems, highly condensed structures including those with fivemembered rings are the most probable.12 As PAH in flames are of this type 13 corresponding structures have been assigned to the oxo-PAH ions. The proposed structures for the larger species give only the general types of oxo-PAH ions for a given number of rings, while possible isomeric structures are disregarded. Only one 0 atom per ion was found. For reasons of thermal stability, heterocyclic ring systems have not been considered. A mass spectrum consisting mainly of oxo-PAH § is given in Fig. 1 (7.5 mm). Ions of an even number of carbon atoms (even-numbered ions) are much more frequent than the odd-numbered species, particularly in the lower mass range of the spectrum. Most of the even-numbered oxo-PAH § and oxoPAH- with the same number of C atoms differ by two H atoms, cf. Table I and II. They can therefore

••

10.5 mm

I' DOUBLET

7.5 mm I

100

I

300

!

500

'

900

mill

)

FIG. l. Mass spectra of positive PAH, oxo-PAH and polyhedral ions at different distances from the burner C / O = 0.80, v, = 42 cm/s, p = 2.7 kPa

358

PREMIXED FLAMES

be considered as the respective protonated and deprotonated phenol (naphthalenol etc.) derivatives. The only exception was C14H90 which occurred both positively and negatively charged. The same is not the case for odd-numbered oxoPAH ions. From C13H90 upwards, positive and negative ions have the same molecular formula.

lets," CmH, and Cm_lHnO-, in the mass spectrum of negative ions, see Fig. 2 (5.5 and 7 mm). However, the mass difference is four units and the "doublets" could only be distinguished for evennumbered oxo-PAH-. This means that only oddnumbered negative PAH ions were present (see Table II). In acetylene flames negative PAH ions were missing completely, lo

PAH Ions: A close inspection of the mass spectrum at 7.5 mm reveals that most of the mass peaks are "doublets" differing by two mass units. The lower peak is always due to a PAH § (Cm+IHn+2+) while the other peak is caused by an oxo-PAH+ CmH,O+). Surprisingly, these PAH + do not always have the same molecular formulae as in acetylene flames. In benzene flames several PAH § with the same number of C atoms have two additional H atoms. Furthermore CI3Ho+ and CagHn+, which are prominent ions in acetylene flames, could not be detected, cf. Ref. 1. Mass spectra of negative PAH and oxo-PAH ions, are shown in Fig. 2. The even-numbered oxo-PAHalso dominate this spectrum. There are also "doub-

~ 10

200

5.5 mm /* 0

600

1000

1 O0

m/u*

FIG. 2. Mass spectra of negative PAH, oxo-PAI-I and polyhedral ions at different distances from the burner C/O = 0.76, v= = 42 cm/s, p = 2.7 kPa

Variations in the Aromatic Ion Concentrations: The change of concentrations is demonstrated by the way in which the mass spectra develop with increasing distance from the burner (Figs. 1 and 2), and by the profiles of some individual ions (Fig. 3). These profiles show that positive and negative oxoPAH ions were formed and consumed completely within the oxidation zone. Negative ions peaked at lower heights above the burner than positive ions of the same number of C atoms. With increasing mass of the oxo-PAH § the position of the profile maxima moved, on average, to larger distances from the burner. However, within a smaller mass range this shift was not monotonous but was noticeable only when differences in mass of 100 u or more were examined. At the maximum overall concentration of oxoPAH § (Fig. 1, 7.5 mm) the spectrum of odd-numbered oxo-PAH+ and of even-numbered PAH + form a quasi-continuous, Gauss-like distribution with the center of mass at about 300 u, while the even-numbered oxo-PAH + tower above. Two mm downstream the latter have largely decayed. The distribution now consists mainly of PAH+ with the center at about 400 u. At 10.5 mm all oxo-PAH+ have almost disappeared and the concentration of PAH § in the range 600-850 u has increased further, thus shifting the center of mass to 480 u. The polyhedral ions C ~ a n d C~o begin to appear followed by C~o, + C~-4and larger C~,. They dominate the spectrum at 11.5 mm while the PAH § do not grow any further. This is in striking contrast to sooting acetylene flames where the growth of PAH § could be followed up to the formation of positively charged soot particles. 14 The maximum concentration of oxo-PAH§ and PAH § with respect to mixture ratio occurred beyond the threshold of soot formation (0.73) at C/O = 0.79. At lower C/O (not shown) the average mass of the oxo-PAH+ was smaller and their overall concentration lower. There was no extension of the distribution to larger PAH § ions at C/O > 0.79. The development of the negative ions with distance from the burner is very different (Fig. 2). At 5.5 mm the mass distribution of the oxo-PAH- peaks at C6H50- and then decreases towards larger masses, with a weak quasi-continuous tail at masses > 400 u. At this early stage, far from the soot forming zone (beginning at about 10 mm), Cffo al-

IONS IN BENZENE-OXYGEN FLAMES ARBITR. UNITS

,C12H9 O+ /

/

026H130§

i

t

~ I

/~

i

9

"., I

;i ! I.~/c~H70

~..~.~"

I

b) i \/06H50- x 0'5

I

I:. i

ARBITR. UNITS

a)

CmH110*

! i~

~: I

359

C 16Hl10

\ ~, ,

I

,

- ,4"

,

12 16 DISTANCE FROM BURNERImm

I

/.

I

~6H90

I~..,~

I

B

-

I

12

DISTANCE FROM BURNERImm

FIG. 3. Concentration profiles of positive (a) and negative (b) oxo-PAH ions C / O = 0.80 (a), C/O = 0.78 (b), v= = 42 cm/s, p = 2.7 kPa ready makes its first appearance, isolated from all lower-mass PAH and oxo-PAH ions. At 7 mm the smaller oxo-PAH- have decayed while the quasicontinuous mass distribution of larger oxo-PAHmixed with that of PAH- comes out more clearly without, however, extending to much larger masses than before. It stays separated from the first group of negative polyhedral ions and ends where the latter begins (C44). A comparison of Figs. 1 and 2 clearly shows that the growth of positive PAH ions exceeds the mass range occupied by the oxo-PAH

ions. Negative PAH ions do not grow at all. Their disappearance must therefore be caused by decomposition, oxidation or charge transfer.

Polyhedral Carbon Ions: With the improved detector it was possible to find polyhedral ions even in flames slightly below the soot threshold. In Fig. 4 profiles of C6o in two non-sooting flames are compared to one in a sooting flame. For all flames there is a maximum in the

ARBITR. UNITS

..~ __ . ...

//

~/'-.... !

,'. -/2

.,-~s 1,j 6

C/O : 0.83

A

----.. 0.70

t 10

0.72 ................

-I---I--

, 1/.

i 18

i

i ~ - i I 22 26 DISTANCE FROM BURNER/ram

FIG. 4. Concentration profiles of C~o at different C / O ratios v, = 42 cm/s, p = 2.7 kPa

PREMIXED FLAMES

360

oxidation zone. However, while the concentration of Cffo decreases as the burning proceeds under nonsooting conditions, it increases to a much higher second maximum when soot formation starts. The further smooth increase of Cffo in the burned gas of sooting flames is limited to this specific ion: the concentrations of all other polyhedral ions decrease after the second maximum.- 9 As reported previously, 15 the final mass distribution of negative polyhedral ions shows two modes, one centered at Cffo and the other peaking around C82 with C~o in between (Fig. 2, 9 ram). The lowermass mode mainly forms within the oxidation zone (7 and 8 mm) giving rise to the first maximum of negative polyhedral ions, while the second more intense mode develops in the soot forming zone (9 mm). Since there is a considerable overlapping of the oxidation zone and the soot formation zone in benzene flames, the growth of the two modes also overlaps to a certain degree. Larger ions than those shown in Fig. 2 were not present. The formation of positive polyhedral ions, however, is not observed outside the soot formation zone and their mass distribution does not show the bimodal form. 9

Discussion

Formation and Decay of oxo-PAH Ions: Positive PAH ions are formed both in acetylene and benzene flames. Since oxo-PAH§ with m > 145 u are absent from acetylene flames, their formation in benzene flames must be independent of the existence of PAH § Therefore, oxo-PAH ions are formed via a mechanism which is not possible or at least very slow and inefficient in acetylene flames. The fact that oxo-PAH ions are among the first ions to be detected in benzene flames, while PAH ions of comparable mass are formed later, is another indication that formation by oxidation of PAH ions does not play a role. A thorough molecular beam/mass spectrometric investigation of a nearly sooting low-pressure benzene-oxygen flame suggests that neutral ox0-PAH are not formed in appreciable amounts within the primary oxidation zone. 4 This would exclude the possibility that oxo-PAH § are generated by proton transfer to the respective uncharged species. As no chemi-ionizing reaction is known that may directly produce an oxo-PAH +, uncharged phenol is considered to be a key molecule for the formation of ~ositive oxo-PAH ions. A mole fraction of 1.0.10- of phenol in low-pressure benzene flames4 is certainly large enough for a rapid proton transfer, for example

C6HsOH + HCO + --->CsHsOH2 + + CO

hrH = -227 kJ/mol

The mere existence of larger positive oxo-PAH ions leads to the conclusion that protonated phenol is able to undergo condensation reactions with benzene or other unsaturated hydrocarbons. In principle there are two ways in which large negative oxo-PAH ions might be formed. Either large neutral PAH react with small oxygen-containing ions, such as OH- or O2-, or phenolate grows by condensation reactions. The absence of oxo-PAHfrom C2H2----O2 flames is an indication for the latter mechanism. Positive and negative oxo-PAH ions up to masses of about 300 u are formed almost simultaneously, the growth of the negative ions even preceding that of the positive species. Rapid condensation reactions are known for positivet6 but not for negative oxo-hydrocarbon ions. The ability of negative oxoPAH ions to undergo condensation reactions is attributed to the fact that the negative charge is located at the oxygen atom and not delocalized within the aromatic ring system. This would also explain the inability of negative PAH ions to add hydrocarbon material, since in this case the negative charge is probably delocalized within a cyclopentadienyl anion configuration (see below). The dominance of the even-numbered oxo-PAH ions may have several reasons. Unlike protonation at the benzene ring (I) a positive charge at the O atom (II) has little influence on the aromatieity of the ring system. The high concentration of the oddnumbered C13H9+ in acetylene flames, for example, is attributed to the relative stable phenalenylium structure (III): +

111

(111}

+

H

H

{Ill

{IV}

However, if C13H9O+, whose concentration lies well below that of C1~H90 +, has the structure of a protonated phenalenon (IV) its high reactivity and therefore low concentration is explicable. Similar considerations also apply to other even- and oddnumbered oxo-PAH. Growth and decomposition of oxo-PAH are parallel reactions in these flames. Elimination of CO from phenol to give cyclopentadiene is an important step in a mechanism proposed by Bittner and

IONS IN BENZENE-OXYGEN FLAMES Howard for the combustion of benzene. 4 In analogy to the decay of phenol it may be assumed that elimination of CO is also an important path for both positive and negative oxo-PAH ions. In this way, negative PAH ions could be formed which contain five-membered rings:

-----

~

*

CO

This decomposition explains the formation of the negative PAH ions, as for example: Cloa70- ~ CgH~ + CO C12H70- ~ CuHT- + CO etc. The cyclopentadienyl anion configuration in larger PAH- is regarded as important for their stabilization. Since there are no oxo-PAH- at all in acetylene flames this accounts for the absence of negative PAH ions. So far, only odd-numbered PAHhave been detected. In the case of odd-numbered oxo-PAH-, the O atom may be bound to an aromatic (V) or to a non-aromatic (VI) ring:

(V}

~0- (Vl}~0-

But only in the first case is it possible to generate an even-numbered PAH- cyclopentadienyl anion configuration. Furthermore, the concentration of the odd-numbered oxo-PAH- is relatively low. Similarly, positive oxo-PAH ions decompose thermally to positive PAH ions which continue to grow as demonstrated in Fig. 1. Within this group the even-numbered PAH + are more abundant. The apparent absence of phenalenylium (C13Hff) and benzo[cd]-pyrenylium (C13H ~1), § which are prominent ions in acetylene flames, and of other oddnumbered PAH § can be understood if PAH ions in benzene flames are mainly formed by decomposition of oxo-PAH § An elimination of CO from C14H90 § cannot yield phenalenylium

I~l +§CO without a drastic rearrangement of the product ion. Analogous arguments apply to explain the missing benzo[cd]pyrenylium.

361

The Formation of Polyhedral Ions: Previously we have proposed that polyhedral carbon ions are formed from nascent carbon particles.9 A main argument was that these ions were not observed below the threshold of soot formation. The detection of Cso and C6o in non-sooting benzene flames requires an additional source for polyhedral ions. There are only two classes of carbon-rich hydrocarbons in the flame: PAH, polyynes and their ions. In benzene flames there are also oxo-PAH ions. The fact that no mass growth of PAH- could be detected excludes them as direct precursors of the negative polyhedral ions. A weak growth of larger negative oxo-PAH ions was detected but it did not extend into the mass range of polyhedral ions. Furthermore, the appearance of Cffo, totally isolated from the oxo-PAH- (Fig. 2), makes it most improbable that oxo-PAH- play an important role. Negative polyyne ions 11 are disregarded because of their very low concentration in benzene flames and their small masses compared to Cffo. Consequently, C44 and Cffo are not formed from smaller negative (oxo)-hydrocarbon ions but from large uncharged hydrocarbons, gaining their charge afterwardsl The intensity of positive and negative ion peaks with masses >400 u was below the limit of detectability at 5.5 mm in Fig. 2 except for Cffo. There are two possibilities for its formation: 1) The growth of positive PAH ions at m >400 u is not representative for that of neutral PAH which grow faster. This would mean that at the appearance of C60 there might be neutral PAH which have grown up to masses >400 u. 2) Bimolecular reactions of very many different PAH of lower mass, beyond the limit of detectability for their respective ions, all lead to the formation of CBo which does not react further at this height in the flame and is thus much more accumulated than any other species of comparable mass. The second mode of the negative polyhedral ions appeared later, showing a quasi-continuous distribution towards the high-mass end. This mode was not observed before soot formation started in the flame. There is no new evidence that would contradict the hypothesis that the second-mode ions are formed from the first soot particles. Previous work has shown that there is no negatively charged soot in this flame zone. 17 One must therefore assume that the larger polyhedral species originate from uncharged soot, and that they acquire their charge during formation or afterwards. A comparison of Figs. 1 and 2 shows that positively charged polyhedral ions are formed still later than negatively charged ones. In contrast to negative ions, they appear to emerge from the bulk of growing PAH § when these have reached masses larger than those of the positive polyhedral ions.

PREMIXED FLAMES

362

This different behavior, and the difference between the mass distributions of positive and negative polyhedral ions, is even more difficult to discuss without having knowledge about the uncharged polyhedral species in the flame. On the one hand, positive polyhedral ions might be formed from certain PAH+ of similar mass. On the other hand, their formation simultaneous with that of the first soot particles suggests that this is the source, although the absence of the high-mass positive species (second mode) is surprising. In both cases their mechanism of formation would be different from that of the first mode of negative ions. In benzene flames the situation is further obscured by the absence of positively charged soot particles which constitute the largest fraction of the positive charge in the burned gas of acetylene flames.iS This is attributed to a different mechanism of growth of soot particles of a few thousand mass units in both flames, but details are not yet known. A number of questions remains which probably cannot be answered before large uncharged PAH and polyhedral carbon molecules are investigated, a task which we shall concentrate on in the near future.

4. BITTNEB,J. D. AND HOWARD,J. B.: Eighteenth Symposium (International) on Combustion, p. 1105, The Combustion Institute, 1981. 5. HOMANN,K. H. ANDWAGNER,H. Gg.: Ber. der Bunsenges. Phys. Chem. 69, 20 (1965). 6. PRADO, G., WESTMORELAND,P. R., ANDON, B. M., LEARY,J. A., BmMANN,K., THILLY, W. G., LONGWELL, J. P. AND HOWARD,J. B.: Fifth International Symposium on Polynuclear Aromatic Hydrocarbons, p. 189, Batelle Press, Columbus, Ohio, 1980. 7. BOCKHORN, H. AND WENZ, H. W.: Ber. Bunsenges. Phys. Chem. 87, 1067 (1983). 8. HOMANN, K. H., MORGENEYER, W. AND WAGNER, n . Gg.: Combustion Institute European Symposium (F. J. Weinberg, Ed.), p. 394, Academic Press, London, 1973. 9. GERHARDT, Ph., LOFFLER, S. AND HOMANN, K.

10. 11. 12.

Acknowledgments This work was supported by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie which is gratefully acknowledged. We also thank Ph. Gerhardt for his interest and helpful discussions.

13.

14.

REFERENCES 1. OLSON, D. B. AND CALCOTE, H. F. : Eighteenth

Symposium (International) on Combustion, p. 453, The Combustion Institute, 1981. 2. MICHAUD,P., DELFAU, J, L. AND BARASSIN,A.: Eighteenth Symposium (International) on Combustion, p. 443, The Combustion Institute, 1981. 3. GERHARDT,Ph.: Massenspektrometrie positiver und negativer Ionen in brennstofl~eichen EthinSauerstoff-Flammen, Dissertation, Teehnisehe Hochschule Darmstadt, 1988.

15. 16.

H.: Twenty-Second Symposium (International) on Combustion, p. 395, The Combustion Institute, 1989. GERHARDT,Ph. AND HOMANN K. H.: J. Phys. Chem., 1990, in press. LOFFLEB, S.: Dissertation, Technische Hochschule Darmstadt, 1990. HITES, R. A. AND SIMONSICK, W. J., JR.: Calculated Molecular Properties of Polyeyelie Aromatic Hydrocarbons, Physical Sciences Data 29, Elsevier, 1987. LONGWELL, J. P.: Nineteenth Symposium (International) on Combustion, p. 1339, The Combustion Institute, 1983. GERHARDT, Ph., HOMANN, K. H., LOFFLER, S. AND WOLF, H.: AGARD Conf. Proceed. No. 422, p. 22-1, PEP Symp. Chania, Crete, October 1987. GERHARDT, Ph., LOFFLER, S. AND HOMANN, K. H.: Chem. Phys. Lett. 137, 306 (1987). GARDNER,M. P. ANDVINCKIER,C.: Int. J. Mass. Spectrom. Ion Processes 63, 187 (1985).

17. HOMANN, K. H. AND STROFER, E.: Soot in Combustion Systems and its Toxic Properties (J. Lahaye and G. Prado, Eds.), p. 217, Plenum Press, New York, 1983. 18. GERHARDT, Ph. AND HOMANN, K. H.: Comb. Flame, 1990, irr press.