Exciplex-Based Vapor/Liquid Visualization Systems Appropriate for Automotive Gasolines

L Y N N A. M E L T O N Department o/Chemistry, University o[ Texas at Dallas, Richardson, Texas 75080-0688

This paper reports the development of exciplex-based vapor/liquid vi- sualization systems based on exciplexes formed from tertiary amines and fluorine-substituted benzene and/or toluene. These systems are ex- pected to be virtually coevaporative with solvents (fuels) boiling in the temperature range 70 to l l0°C and thus are expected to track the va- porization of automotive gasoline effectively. A system consisting of 10% triethylamine/0.5% fluorobenzene/89.5% hexane should be coevapora- tive with a normal boiling point of 69°C. A system consisting of 10% n-propyldiethylamine/0.5% 4-fluorotoluene/89.5% isooctane should be coevaporative with a normal boiling point of approximately 100°C. Al- though the coevaporation of these systems is excellent, the exciplexes revert to varying extents to excited monomer at temperatures near 100°C. Thus there is considerable cross talk from the liquid into the vapor spectral region. The tertiary amines generally require excitation at wave- lengths below 250 nm; the fluorobenzene or 4-fluorotoluene can be ex- cited at 266 nm. Monomer emission peaks at 290 nm; exciplex emission peaks at 350 nm.

Index Headings: Fluorescence; Instrumentation, imaging; Analytical methods; UV-visible spectroscopy.

INTRODUCTION

Exciplex-based vapor/liquid visualization (EBVLV) systems, in which exeiplex-forming dopants are used to provide spectrally separated fluorescence emissions from the vapor and liquid phases of an evaporating fuel spray, were introduced in the early 1980s by Melton et al. TM Exciplex-based vapor/liquid visualization has since been used in a variety of engine-related investigations. ~9 These visualization systems, the most successful of which uses naphthalene and N,N,N',N'-tetramethyl-p-phenylene- diamine (TMPD), contain fluorescent components whose normal boiling points are in the range 200 to 300°C. The fuel, which is presumed to be a hydrocarbon solvent, has no substantial fluorescence of its own, and thus the evap- oration of the fuel is tracked by the vapor-phase fluo- rescence of one of the exciplex-forming components. Quantitative measurement of the fuel vapor concentra- tion, or even good semiquantitative estimation, requires that the fluorescent marker evaporate at nearly the same rate as the bulk fuel. These initial systems, while appro- priate for diesel and gas turbine fuels, whose [)oiling ranges are roughly 200-300°C, are not appropriate for studies of fuel sprays involving automotive gasolines, whose boiling range (80-150°C) is much lower. In par- ticular, a low-volatility marker in a high-volatility solvent will cause a significant underestimation of the initial fuel evaporation rate, a key parameter in engine ignition and performance.

This work describes the development of exciplex-based vapor/liquid visualization systems appropriate for the

Received 21 December 1992.

volatility range of automotive gasolines. The systems are built around the photophysics of tertiary alkyl amines, which can act as the fluorescent vapor-phase marker and which can form exciplexes with various electron accep- tors. Because the photophysics of the tertiary amine is localized at the nitrogen atom, the volatility of the amine can be varied, without significant variation of the pho- tophysics, by varying the length of the alkane chains attached to the nitrogen atom. Thus volatility matching of these exciplex-based vapor/liquid visualization sys- tems is readily achieved.

PHOTOPHYSICS

Exciplex-Based Vapor/Liquid Visualization Systems. These visualization systems are based on the following reaction

M * + G ( M ) ~ E*

in which M* is the first excited singlet state of a fluo- rescent exeiplex-forming molecule M, the monomer. The fluorescence emitted from M* is used to track the vapor. G, which may be M, is a ground state exciplex-forming molecule, and E* is the exciplex (excited state complex). The emission of E* is necessarily red-shifted with respect to that of M*, sometimes by 100 to 150 nm. In the liquid, the concentration of G may be made sufficiently high for the reaction to be driven to the right; i.e., the dominant emission from the liquid phase is from E*. In the vapor phase, E* is not stable, and the dominant emission is from M*. Thus the fluorescence at longer wavelengths tracks the liquid, and the fluorescence at shorter wave- lengths tracks the vapor. The solvent for the system is the fuel (F), which is presumed to be one or more (pho- tophysically inert) alkanes.

In practice, exciplex-based vapor/liquid visualization systems usually are more complex than the ideal system described in the above paragraph. There may be a flu- orescent excited state G*, which provides an additional pathway for the formation of E* in the liquid and which makes the interpretation of the vapor-phase fluorescence more complex. It may not be possible to drive the ex- ciplex-forming reaction all the way to the right, so that there is significant residual M* emission from the liquid phase; this cross talk can make the detection and/or interpretation of vapor-phase fluorescence, which is gen- erally weaker than liquid-phase fluorescence, more dif- ficult. The exciplex-forming reaction is temperature de- pendent, and increasing the temperature may result in a shift of the reaction back to the left, i.e., towards sig- nificant M* emission from the liquid phase. Finally, if M does not evaporate at approximately the same rate as

782 Volume 47, Number 6, 1993 0003-7£,28/93/4706-078252.00/0 APPLIED SPECTROSCOPY © 1993 Society for Applied Spectroscopy

TABLE I. Selected photophysical properties. Emission Fluorescence Quantum

Compound a wavelength (nm) Phase lifetime (ns) yield

TEA 278 Vapor 61 0.98 ~° (Xo. > 240 nm)

TEA 282 Solution (n-hexane) 31 0.69 H TEA Solution (cyclohexane) 54 0.44 ~ NPDEA ...b FBZ 275,282 Solution (cyclohexane) 0.1313 FBZ Vapor 9.4 0.22 ~4

(X~. = 265 nm) 4FT Vapor 13.9 0.35 ~4

(X~x = 265 nm) 4FT Vapor 10.1 0.1614

(h~x = 248 nm) TEA/FBZ exciplex Solution (isooctane) 5.5 ± 0.5 c NPDEA/4FT exciplex Solution (isooctane) 7.3 ± 0.5 c

" Abbreviations for compounds: TEA, triethylamine; NPDEA, n-propyldiethylamine; FBZ, fluorobenzene; 4FT, 4-fluorotoluene. ~' "As the three alkyl chains are extended, the fluorescence quantum efficiency seems to hold constant at about 0.70 while the lifetime gradually

increases; for example, ~'l for tri-n-dodecylamine is 38.0 ns. Also, the fluorescence spectra of methyl-substituted amines have maxima at ~280 nm, while the spectra of other tertiary amines have maxima at 282 nm. ''~1 This work. Laser excitation was with an Nd:YAG laser at 266 nm (8 ns FWHM). Because the lifetimes are comparable to the laser pulse width and because equilibration between monomer and exciplex is probably significant, these numbers should be used only as phenomenological decay times.

F, then the fluorescence f rom M*, however accurate ly measured , does not t rack the overall fuel vapor concen- t rat ion. In this work the first three effects are significant l imi ta t ions to the proposed vapor / l iquid visualization systems. The fourth effect, volatil i ty differences, has been rendered vir tual ly insignificant.

Photophysies of Components. The dopan t s in the pro- posed sys tems are te r t ia ry alkyl amines, t r i e thy lamine (TEA) and n -p ropy ld ie thy lamine (NPDEA), and elec- t ronegat ively subs t i tu ted benzenes, f luorobenzene (FBZ) and 4-fluorotoluene (4FT).

For all the te r t ia ry alkyl amines, absorp t ion of light s tar ts at app rox ima te ly 260 nm and arises smooth ly to a m a x i m u m at abou t 210 nm. Thus , exci tat ion of fluo- rescence f rom these compounds generally requires light at wavelengths subs tant ia l ly below the 266 n m obtain- able f rom the four th harmonic of the Nd:YAG laser. Fluorescence f rom the te r t ia ry alkyl amines rises f rom its origin at approx ima te ly 270 nm, peaks a t 290-305 nm, and falls to zero by 380 nm. The photophys ics of te r t ia ry amines has been s tudied by Ha lpe rn e t al . 1°,11 and by Beecrof t and Davidson. 12

For f luorobenzene and 4-fluorotoluene, absorpt ion s tar ts a t 260-280 nm and rises irregularly at shor ter wavelengths. The fluorescence f rom these molecules peaks a t abou t 290 nm. The photophys ics of these compounds has been s tudied by Ber lman 13 and by Breuer e t al . 14

Exciplexes fo rmed f rom fluorobenzene and diethyl- me thy lamine have been s tudied by Ha lpe rn and Frye. I5

Since bo th the te r t ia ry alkyl amines and the substi- t u t ed benzenes can potent ia l ly absorb the exciting light, since bo th fluoresce, and since the exciplex can be formed via exci tat ion of e i ther molecule, the choice abou t which should be designated as " m o n o m e r " (marker for the va- por) and which as "ground s ta te r eac t an t " is somewhat arbi t rary . For exci ta t ion a t wavelengths sufficiently long to ensure t ha t the te r t ia ry alkyl amines do not absorb, the subs t i tu ted benzenes are the " m o n o m e r . "

Tab le I summar izes re levant in format ion abou t the photophys ics of the componen t s of these new exciplex

sys tems and abou t the photophys ics of the exciplexes themselves.

The a lkane solvents are photophysica l ly inert; pho- tophysical results ob ta ined in one a lkane solvent are ex- pec ted to change insignificantly in going to ano ther sim- ilar boiling a lkane solvent.

E X P E R I M E N T A L

Chemicals and Solvents. n - P r o p y l d i e t h y l a m i n e (NPDEA) was purchased f rom Pfal tz and Bauer. Tr i - e thy lamine (TEA) was purchased f rom Aldrich. T h e y were dr ied over po tass ium hydroxide, distilled, and then s tored over fresh po tass ium hydroxide prior to use to e l iminate moisture. The spect rograde so lven t s - -hexane (HX), 2-methy lhexane (2MH), hep tane (HP) , isooctane (IO), and n-oc tane (OC) (not spec t rog rade ) - -were pur- chased f rom Aldrich and were used as received. Fluo- robenzene (FBZ) and 4-fluorotoluene (4FT) were pur- chased f rom Aldrich and were distilled pr ior to use. Since all the componen t s are liquids at room tempera tu re , all solutions were p repa red (and are repor ted) on a volume basis.

Apparatus. Spect ra were t aken on a S P E X D M I B spect rof luor imeter with D A T A M A T E da ta sys tem in ra- tio mode, which corrects for f luctuat ions in the exci tat ion intensity. Raw spec t ra were s tored on disk and were la ter corrected for the spectra l response of the spectrof luorim- eter.

The spectra l response of the spect rof luor imeter was de te rmined by insert ing a ca l ibra ted spectra l i r radiance deu te r ium lamp (Optronics Laborator ies , Model UV-40) in the sample chamber and measur ing its emission spec- t rum. T h e rat io of this exper imenta l spec t rum to the cal ibrat ion spec t rum provided with the l amp resul ted in the i n s t rumen t spectra l response function, which was s tored in the da ta sys tem memory . The response correc- t ion becomes increasingly significant for wavelengths be- low 300 nm.

The fluorescence cell was designed specifically for these

APPLIED SPECTROSCOPY 783

A

(a)

to vacuum system ~ ~ , ~ ~ Stopcock

~ L~!) ~ ~eH:a::nc gu pbll: c k A

~ N ~ -~ - - - Window

B Sample c e l l - ' ~ ' ~ " Thermocouple

FIG. 1. (A) Fluorescence cell with freeze-pump-thaw sidetube at- tached; (B) heater block for fluorescence cell.

measurements. The components of these systems are vol- atile, and it is necessary to exclude atmospheric oxygen in order to avoid uncertainties due to fluorescence quenching. In addition, the fluorescence must be mea- sured as a function of temperature. The resulting ap- paratus is shown in Fig. 1A. A liquid sample is added to the upper tube (a), the cuvet apparatus is attached to a vacuum line, and a series of standard freeze-pump-thaw cycles are used to degas the liquid thoroughly. At this time the entire cuvet apparatus is evacuated. After the final cycle, the sample is allowed to melt and is poured from the freeze-pump-thaw tube into the cuvet (b). The cuvet stopcock is then closed and the cuvet portion is detached for use in the spectrofluorimeter. This appa- ratus allows thorough degassing and maintenance of that degassed condition during acquisition of spectra. The separate Pyrex ® freeze-pump-thaw tube is necessary since standard quartz fluorescence cuvets crack under freeze-pump-thaw conditions. The pouring operation al- lows transfer of the solution without differential distil- lation.

Figure 1B shows the apparatus used to vary the tern- perature of the fluorescence cuvet and stopcock. The

TABLE II. Ratings of tertiary amine EBVLV systems."

Sys - tern Component NBP (°C) SS ES/MC M/FT

1A 10% TEA 89 3.5 3 3 0.5% FBZ 85 89.5% 2MH 90 Fig. 2

1B 10% TEA 89 3.5 3 2 0.5% FBZ 85 89.5% HP or IO 98 Fig. 2

1C 10% TEA 89 3.5 3 4 0.5 % FBZ 85 89.5% HX 69 Fig. 2

2A 0.5% TEA 89 3.5 3 3 10% FBZ 85 89.5% 2MH Fig. 3

2B 0.5% TEA 89 3.5 3 2 10% FBZ 85 89.5% HP or IO 99 Fig. 3

3A 10% NPDEA 108 3.5 2 2 0.5% 4FT 116 89.5% IO 99 Fig. 4

3B* 0.5% NPDEA 108 3.5 0 2 10% 4FT 116 89.5 % IO 99

" Note: (*) Not recommended. Composition is percent by volume. Ab- breviations for compounds: TEA, triethylamine; NPDEA, n-propyl- d ie thy lamine ; FBZ, f luorobenzene; 4FT, 4-fluorotoluene; 2MH, 2-methylhexane; HP, n-heptane; IO, isooctane; HX, n-hexane. NBP = normal boiling point; SS = rating of spectral separation of monomer and exciplex bands; ES/MC = rating of exciplex stability and/or ex- tent of monomer fluorescence appearing in the liquid phase spectrum at solution NBP; M/FT = rating of monomer/fuel tracking (coevapo- ration). Ratings: 0 unacceptable; 1 marginal; 2 acceptable; 3 good; 4 excellent; 5 ideal.

cuvet is inserted into a square slot in the lower aluminum block. The temperature of the lower aluminum block is monitored by a thermocouple (Type J, Omega Model 199). The stopcock is contained within an upper alu- minum block whose temperature is monitored by a ther- mocouple (Type J). The two aluminum blocks are heated independently by two sets of two cartridge heaters (15 W) connected to separate variacs. Both aluminum blocks are well insulated with solid ceramic insulation scav- enged from an old gas chromatograph oven. In typical operation, the upper block is kept approximately 1-2°C higher than the lower block in order to ensure that the vapor composition is at the temperature measured for the liquid.

The lower aluminum block has openings at three sides in order to allow absorption and right-angle or front-face fluorescence measurements. The cuvet can be moved ver- tically by approximately 1.5 cm in order to position either the liquid phase or the vapor phase on the optic axis. Both liquid- and vapor-phase fluorescence measure- ments were taken in the front-face configuration.

Fluorescence lifetimes of exciplex systems were mea- sured with excitation at 266 nm with the fourth harmonic of Nd:YAG (8 ns, FWHM) and capture of the fluores- cence decay on a Tektronix DSA 602 storage oscilloscope.

RESULTS AND DISCUSSION

Criteria. An ideal exciplex-based vapor/liquid visual- ization system should produce strong, interpretable flu-

784 Volume 47, Number 6, 1993

1 1

0.9

0.8

0.7

0.6

o.5

O.4-

0.3

0.2-

0.1-

o 250 350 350 46o 460 ~00

Wavelength (nm)

Fro. 2. Corrected fluorescence spectra of 10% TEA/0.5% FBZ solu- tion in isooctane, kex = 266 nm; T = 98°C. Vapor (dash) and liquid (solid) emissions are normalized to same peak value.

0.9-

0.8-

0,7

0.6

0.5

0.4

0.3- i o. :f 0.1

0 250 300 350 460 450 500

Wavelength (nm)

FIe. 4. Corrected fluorescence spectra of 10% NPDEA/0.5% 4FT solution in isooctane. Xe. = 266 nm; T = 98°C. Vapor (dash) and liquid (solid) emissions are normalized to same peak value.

orescence from both the monomer in the vapor phase and from the exciplex in the liquid phase with the mono- mer and exciplex bands sufficiently well separated to allow separate visualization of the two phases. Because the temperature of the fuel droplets is expected to be near the boiling point of the fuel solution (at the ambient pressure), an ideal system should show liquid-phase flu- orescence from only the exciplex at temperatures near its boiling point. In order for the fluorescence from the monomer to track accurately the vapor-phase concen- tration of the fuel, the monomer (M) and the fuel (F) should be virtually coevaporative; further, it is desirable (but not required) that the ground-state exciplex-form- ing molecule (G) be coevaporative with the fuel (F) in order that the exciplex equilibrium in the droplet remain as designed during the lifetime of an evaporating droplet; i.e., the liquid-phase composition does not change.

Recommended Systems. Table II lists the exciplex- based vapor/liquid visualization systems which are rec- ommended for potential use in automotive gasoline fuel

spray characterization. Note that system 3B is not rec- ommended; it is included for completeness. Corrected fluorescence spectra relevant to these systems are shown in Figs. 2-4. Figure 5 shows corrected fluorescence spec- tra for FBZ; the corrected fluorescence spectra for the other three monomers (TEA, NPDEA, and 4FT) are virtually identical to Fig. 5.

Discussion. Examination of the spectra in Figs. 2-4 shows that, in general, the spectral separation of the monomer and exciplex bands will allow choice of wave- lengths at which (1) exciplex emission can be measured with negligible monomer contribution (for liquid-phase visualization) and (2) monomer emission can be mea- sured with modest exciplex contribution (for vapor-phase visualization).

At temperatures of 90 to 100°C, near the normal boiling point of the solutions, and in some cases even at room temperature, there is a significant shift of the monomer/ exciplex equilibrium back toward the monomer. The ex- ciplex is not sufficiently strongly bound with respect to

c a)

0.9

0.8

0.7

0.6-

0.5

0.4-

0.3-

0,2-

0.1

0 25O 360 350 460 450 500

Wavelength (rim)

FIG. 3. Corrected fluorescence spectra of 0.5% TEA/10% FBZ solu- tion in isooctane, ho, = 266 nm; T = 98°C. Vapor (dash) and liquid (solid) emissions are normalized to same peak value.

~D EE

0- 250 360 3~0 46o 4~0 50o

Wavelength (nm)

FIG. 5. Corrected fluorescence spectra of 10% FBZ in isooctane. Xo. = 266 nm; T = 98°C. Vapor (dash) and liquid (solid) emissions, nor- malized to same peak value, are virtually identical.

APPLIED S P E C T R O S C O P Y 785

separated M* and G to overcome the dissociation caused by increased temperatures. Systems 1A, 1B, 1C, 2A, 2B, and 2C show this effect, but it is at a sufficiently modest level to ensure that one should be able to correct for cross talk from the liquid into the vapor measurement spectral band in those regions of the spray in which vapor and liquid are intermixed in significant quantities. In system 3A, the effect is sufficiently large for correction for liquid/ vapor cross talk to probably be questionable. In system 3B, the monomer/exciplex equilibrium shifts so strongly toward the monomer that distinction of the liquid from the vapor, in intermixed regions of the fuel spray, would be difficult. System 3B is definitely not recommended for use in vapor/liquid visualization studies.

The primary difference between systems 1A, 1B, and 1C and systems 2A, 2B, and 2C is that the former group contains high amine and low substituted benzene while the proportions are reversed in the latter group. The performance of the systems, as given in Table II, does not vary significantly with this shift in composition. The significance of these composition shifts, not noted in Ta- ble II, is that FBZ can be excited at 266 nm, the fourth harmonic of Nd:YAG, and thus systems 2A, 2B, and 2C are probably easier to use. In general, the vapor.-phase signals are weaker than the liquid-phase signals, and thus systems with higher liquid-phase concentration of the monomer exhibit higher vapor-phase concentrations of the monomer and thus higher fluorescence signals, pro- vided that optically thin conditions are maintained.

The ratings of the extent to which the evaporation of M tracks the evaporation of the whole droplet involve an added dimension in the development of exciplex-based vapor/liquid visualization systems, the thermodynamics of evaporating droplets, which will be treated in another paper2 G In prior work, 1~4 closeness of the normal boiling points of the system components has been taken as an adequate criterion for coevaporation. This criterion, which neglects differences in the heats of vaporization and the possible nonideality of the solutions, was used in the selection of components for the systems presented in Table II. Systems 1C and 2C show that this simple criterion is not adequate. Primarily because the solutions show significant deviations from ideal solution behavior (FBZ and TEA have activity coefficients significantly greater than unity in alkane solutions), more nearly co- evaporative behavior is obtained when the alkane is re- placed by a component boiling some 20°C lower than the other components. Thus systems 1C and 2C have the highest overall rating. In an unexpected result, they are now coevaporative but at a temperature somewhat lower than the boiling range of automotive gasolines.

Systems 3A and 3B were developed in order to match more closely the volatility of isooctane and/or n-octane, two common automotive reference fuels. The best re- sulting system, 3A, is marginal in its overall performance.

Additional Concerns. The tertiary amines in general have very low toxicity, 17 but they do smell somewhat like

dead fish. The hazards associated with fluorobenzene and 4-fluorotoluene are probably comparable to those asso- ciated with benzene and toluene, respectively.

Felton has noted recently that the relevant absorption properties for the vapor-phase marker M are those at the temperature of the ambient gas, which in the pre- ignition phase of an engine cycle may be several hundred degrees centigrade. He found that the absorbancies of TMPD and FBZ increase significantly with tempera- ture2 s The liquid- and vapor-phase fluorescence quan- tum yields may also be strongly temperature depen- dent. 19 Thus considerable caution should be exercised in using room-temperature or even 100°C photophysical properties to make quantitative estimates of the amount of monomer present in the vapor.

ACKNOWLEDGMENTS

Support through the Energy Research Applications Program of the Texas Higher Education Coordinating Board, Contract #27, is grate- fully acknowledged. Ms. Jan Shaffner contributed greatly through her excellent technical work.

1. L. A. Melton, U.S. Patent 4,515,896, issued May 7, 1985, assigned to United Technologies Corporation.

2. L. A. Melton and J. F. Verdieck, in Proceedings of the Twentieth Symposium (International) on Combustion (The Combustion In- stitute, Pittsburgh, 1984), pp. 1283-1290.

3. L. A. Melton and J. F. Verdieck, Combust. Sci. and Tech. 42, 217 (1985).

4. L. A. Melton, "Quantitative Use of Exciplex-Based Vapor/Liquid Visualization Systems (A Users Manual)," Final Report, Army Re- search Office Contract DAAL03-86-K-0082, January 1, 1988, NTIS Order # ADA191649.

5. M. E. A. Bardsley, P. G. Felton, and F. V. Bracco, SAE Interna- tional Congress and Exposition, Detroit (1988), Paper 880521.

6. M. E. A. Bardsley, P. G. Felton, and F. V. Bracco, SAE Interna- tional Congress and Exposition, Detroit (1989), Paper 890315.

7. R. Diwakar, T. D. Fansler, D. T. French, J. B. Ghandhi, C. J. Dasch, and D. M. Heffelfinger, SAE International Congress and Exposi- tion, Detroit (1992), Paper 920422.

8. J. Senda, Y. Fukami, Y. Tanabe, and H. Fujimoto, SAE Interna- tional Congress and Exposition, Detroit (1992), Paper 920579.

9. R. Shimizu, S. Matumoto, S. Furuno, M. Murayama, and S. Kojima, SAE Fuels and Lubricants Meeting, San Francisco (1992), Paper 922356.

10. A. M. Halpern and T. Gartman, J. Amer. Chem. Soc. 96, 1393 (1974).

11. A.M. Halpern, in The Chemistry o/Amines, and Nitro Compounds and Their Derivatives (Part 1), S. Patai, Ed. (John Wiley and Sons, New York, 1982), pp. 168, 171.

12. R. A. Beecroft and R. S. Davidson, J. Chem. Soc. Perkin Trans. II, 1063 (1985).

13. I. B. Berlman, Handbook of Fluorescence Spectra o/Aromatic Molecules (Academic Press, New York, 1971), 2nd ed.

14. G. M. Breuer, P. A. Hackett, D. Phillips, and M. G. Rockley, J. Chem. Soc. Faraday Trans. II, 1995 (1972).

15. A. M. Halpern and S. L. Frye, J. Phys. Chem. 92, 6620 (1988). 16. L. A. Melton, "Evaluation of Vapor Tracking in Exciplex-Based

Vapor/Liquid Visualization of Evaporating Droplets," unpublished data.

17. Material Safety Data Sheets supplied by Sigma-Aldrich Corpo- ration; correspondence with Pfaltz and Bauer, Inc.

18. P. G. Felton, private communication. 19. A. M. Halpern and D. K. Wong, Chem. Phys. Lett. 37, 416 (1976).

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