A cw laser absorption diagnostic for methyl radicals

J. Qumt. Spectrosc. R&at. Tranrfer Vol. 49, No. 5, pp. 559-571, 1993 Printed in Great Britain. All rights reserved

0022-4073/93 $6.00 + 0.00 Copyright 0 1993 Pergamon Press Ltd

A CW LASER ABSORPTION DIAGNOSTIC FOR METHYL RADICALS

D. F. DAVIDSON,? A. Y. CHANG, M. D. DI ROSA, and R. K. HANSON

High Temperature Gasdynamics Laboratory, Mechanical Engineering Department, Stanford University, Stanford, CA 94305, U.S.A.

(Received I April 1992; received for publication 20 October 1992)

Abstract-Absorption of narrow-line laser radiation by methyl radicals produced at high temperatures was studied in the Herzberg B, band near 216nm. cw radiation for these measurements was generated in a ring dye laser system using intracavity BBO frequency doubling. Methyl radicals were produced by the shock wave heating of five selected source compounds: azomethane, methyl iodide, tetramethyl tin, tetramethyl silane and ethane. The variation with wavelength of the CH, absorption coefficient between 210 and 225 mn was measured at 1625 K and 1.25 atm using ethane/Ar and methyl iodide/Ar gas mixtures. The variation of the absorption coefficient with temperature from 1350 to 2450 K was measured at 216.615 mn using four source compounds. Using this wavelength for CH, measurements resulted in a typical detectivity limit of 2 ppm at 1650 K, 1 atm, L = 10 cm, with a SNR of unity and a detection bandwidth of 500 kHz.

INTRODUCTION

The methyl radical is an important intermediate species in the pyrolysis and oxidation of aliphatic hydrocarbons such as methane and ethane. Accurate concentration measurements of CH3 in these systems, however, have been limited by the lack of a sensitive, selective, fast time-response optical diagnostic and by the ever-present interference to detection from other species with spectral features in the same wavelength regime. Despite these circumstances, CH3 U.V. absorption techniques have been applied to the measurements of rate coefficients in kinetically-simple high temperature chemical systems,lA and occasionally to the analysis of complex combustion systems59 6

One of the first comprehensive shock tube studies of methyl reactions using absorption was conducted by Gliinzer et al.’ These authors studied the recombination of methyl radicals to form ethane using absorption of relatively broadband 216 nm radiation. This radiation was generated using a high pressure xenon arc lamp, dispersed using a quartz prism and a monochromator and detected with a photomultiplier. Miiller et al,’ Hwang et al3 and Tsuboi4 used similar methods in their shock tube studies of methyl reactions. CH, absorption measurements were first done in a low pressure flame by Harvey and Jessen,7 also using a Xe lamp system. Tsuboi’ studied methane oxidation in a shock tube with several optical diagnostics, including CH, absorption at 216 nm using a xenon lamp. Gardiner et al6 repeated a portion of this latter study using a zinc lamp which has an atomic resonance line at 214 nm.

These earlier experimental methods have significant limitations. Interference absorption from combustion reactant or product species such as O2 and NO can complicate interpretation of the data, and significant absorption from certain species cannot be avoided with either the fixed narrow-linewidth Zn lamp system or with the tunable, but spectrally-broad, Xe lamp system. In addition, neither of these earlier systems takes full advantage of the larger magnitude of the peak of the CH3 absorption feature, nor do they provide the large spectral intensities generally associated with lasers. A spectrally narrower, tunable and more intense laser source would help overcome these limitations.

tTo whom all correspondence should be addressed.

559

560 D. F. DAVIDSON et al

Work in our laboratory has concentrated on the accurate and sensitive detection of combustion species by the use of cw ring dye laser absorption. Although narrow-line cw radiation at wavelengths as short as 144 nm has been generated through frequency tripling,’ cw power levels sufficient for use in transient experiments can only be obtained through either second-harmonic generation or sum-frequency mixing. Performing either technique intracavity leads to enhanced power levels.g Until recently, the shorter wavelength limit for intracavity doubling was 238 nm (achieved using LFM, Lithium Formate Monohydrate). ‘O The advent of a frequency-doubling technology based on /I-BaB,O, (BBO) has permitted access to wavelengths as short as 205 nm, and to even shorter wavelengths through frequency mixing. ‘I At wavelengths between 205 and 238 nm, strong absorption bands of several significant combustion species, including NO, 0, and CH,, appear. The use of this BBO laser system to detect NO and 0, has been described elsewhere.‘2*‘3 Here we describe the results of high-temperature shock tube/BBO laser system experiments which were performed to determine the optimal wavelength for a CHS absorption diagnostic and the temperature dependence of the absorption coefficient at this wavelength. A brief discussion of the effect of absorption by certain interfering species is also included.

METHOD

The apparatus, shown schematically in Fig. 1, consists of two primary components: the shock tube and the laser absorption system. High temperatures were generated behind incident and reflected shocks in a pressure-driven shock tube. The stainless steel shock tube driven section is 6.25 m in length with an i.d. of 14.3 cm. The driver section is 2.0 m long, with a 5.0 cm i.d., and has a diverging nozzle after the diaphragm/isolation-valve station. The vacuum system employs a turbomolecular pump and several zeolite-trapped mechanical pumps. The ultimate pressure of the system attained by pumping for 30 min between shocks is 8 x 10T6 torr; the combined leak and outgassing rate was less than 16 x 10V6 torr-min- ‘. Shocks were produced by bursting polycarbon- ate diaphragms (0.13 and 0.26 mm in thickness) against a rigid cutter. In all the experiments described, helium was used as a driver gas and 99.9995% argon (Matheson) was used as the driven carrier gas. Gas mixtures were prepared by partial pressures in a mixing tank with a large enough volume and at a great enough pressure to permit the use of the same gas mixture for six shock experiments. Mixing was accomplished with a magnetically-driven stirring vane. Temperature and pressure were calculated from the incident shock velocities using an ideal shock code. The shock

Vacuum Pump

Fig. 1. Schematic of the apparatus: WM-wavemeter; SI-scarming interferometer; Det.-photodiode detector; T.F. gauges-thin film gauges.

cw laser absorption diagnostic for methyl radicals 561

velocity was determined from shock arrival times measured using four platinum thin film gauges spaced at intervals of 600 mm. In separate experiments with this shock tube, the temperature, which was measured using the laser-scanning temperature monitor described in Chang et aLi4 and the pressure, which was measured using a piezoelectric transducer, were shown to vary from those values predicted using the local shock velocity by no more than the experimental uncertainty of 1.5% over the approx. l-2 msec of reflected shock test time available.

The ring dye laser (Coherent model 699) was run with stilbene 3 dye (Lambda-Physik) and pumped with 5-6 W of all-lines U.V. from an Ar+ laser (Coherent model Innova 25/5). A custom BBO angle-tuned intracavity-doubling system was used to obtain approx. 1 mW of U.V. radiation near 216 nm (most experiments were done at a fixed wavelength of 216.615 nm), with a typical spectral width of less than 0.001 cm-‘. The crystal was cut for Brewster-angle incidence and for Type I phase-matching at 432 nm. With this system, we were able to produce useful levels of frequency-doubled output from 209 nm to beyond 220 mn.

A portion of the visible (undoubled) laser output was passed into a wavemeter (Burleigh model WA-20) which measured the laser wavelength, and another portion was passed through a 2 GHz FSR scanning interferometer (Spectra-Physics model 470-2) to monitor laser mode quality. The U.V. beam was split into two approximately equal intensity components, one passing directly onto a detector to record the incident laser intensity (lo) as a function of time, and the second passing through uncoated fused silica windows, across the 14.3 cm dia of the shock tube 18 mm from the end wall and onto a second detector to provide the transmission measurement (I). Prior to each experiment, the detectors were optically balanced to obtain a null absorption signal (AZ = I0 - I) and the beams were centered on the detectors to minimize sensitivity of this null signal to shock tube vibration. The absorption signal (AZ) and the incident signal (lo) were recorded on a digital oscilloscope (Nicolet model 4094 with model 4562 vertical amplifiers) sampling at 0.5 psec per point. This dual-beam absorption method was efficient in rejecting common-mode noise leading to improved absorption detection limits relative to a one-beam absorption system. The detectors employed photodiodes (EG&G model WlOOBQ) and had electronic risetimes (I/e) of less than 0.5 /lsec.

DATA REDUCTION AND KINETIC MODELLING

Knowledge of the absorption coefficient combined with a radiation source of linewidth much narrower than the absorption feature width permits use of Beer’s law to directly interpret absorption data traces, i.e.

ln(Z,/Z) = k&xcn,L,

where I,, and Z are the incident and transmitted intensities at wavelength Iz, kn is the temperature- dependent absorption coefficient (atm-‘-cm-‘) at wavelength 1, PT is the total pressure (atm), xCHl is the mole fraction of the absorbing species CH,, and L is the absorber path length (cm).

In order to determine the absorption coefficient of CH3, the absorption time histories were first converted to values of kA x xCH, vs time, and then were compared to CH3 mole fraction histories computed using the reaction mechanism of Table 1. The best-fit value of kA, for the temperature of the experiment, is extracted from the best-known portion of CH, time history, usually the period immediately after the arrival of the reflected shock. Only mixtures and shock conditions which were predicted to have peak yields of methyl radicals near the theoretical maximum (i.e., complete conversion of source species to CH,) were selected for the actual calibration in order to minimize the uncertainty associated with fitting the data with a kinetic mechanism. The reaction mechanism used is a subset of the H/C reactions found in the mechanisms of Miller and Bowman” and Glarborg et al,16 with the addition of several source species reactions described in the next section. Computations were done using the Sandia CHEMKIN-II program” and the 1991 Sandia Thermodynamic Database.”

METHYL RADICAL SOURCES

The selection of a suitable methyl radical source for use in shock tube experiments is affected by many factors including absorptivity, decomposition products and mechanism, and background


Table 1. CH, reaction mechanism.

- No. -

1

2

3

4

5

6

I

8

9

10

11

12

13 -

uote:

Reaction

C&&=~CI-&+NZ CH31+h4=CH3+I+M

SiC,H,2+M=Si+4CH3+M

SnC,H,2=Sn+4CH3

C&+M=CH,+CH,+M

&H~+H=CH,+CH,

CH3+CH3=r$H,+H2

CH3+c!H3=CHH,+CH4

CH3+H=CHz+H2

CH.,+H=CH3+H2

CJ%+H=C&+H2

w6+cJ53=c1%~

CH3+M=CX&+H+M

A

8.OElO

3.OE14

2.OE12

3.OE12

1.3E13

l.OE14

2.1E14

2.1E14

9.OE13

2.2EO4

5.4EO2

5.5E-1

1.9E16

B

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0

0.0’

3.0

3.5

4.0

0.0

l-

EA Ref.

26cKlO. *

32OcKl. *

14000. *

47000. *

78400. *

0. 15

19200. 16

19200. 15

15100. 15

8750. 15

5210. 15

8300. 15

91600. 16

Rate coefficients are given in the form A ? exp(-E,/RT) (mol-cm~3-s~‘);

ZA (calories-mol.‘). * see the text for details.

absorption from source species or subsequent reactant products. Many possible CH, source species are liquids at room temperature and may absorb onto the walls of the mixing vessel and shock tube in unknown amounts. Because of this absorption, it was necessary to compare the methyl yields of several source species to ensure that known reproducible quantities of methyl radicals are formed. In the following section, the results from experiments with azomethane, methyl iodide, tetramethyl tin, tetramethyl silane and ethane are compared.

Azomethane (CH,N:NCH,) was originally chosen because it cleanly decomposes into two methyl radicals and N,. This is advantageous since, at the conditions of the present experiments, Nz plays no role in the chemistry. Azomethane is not commercially available and was synthesized using the method of Jahn.” This method is cleaner and simpler than the more frequently referenced method of Renaud and Leitch.” It also has the advantage of involving the production of a relatively stable intermediate which can be stored. Azomethane produced using the Jahn method may have trace water, ethane, CuCl, or acetal-based impurities.

When azomethane is used in highly diluted (50-100 ppm) mixtures with argon, calculated yields of methyl radicals in the reflected shock regime can reach 97% or greater (of the theoretical maximum yield of 2 methyls per azomethane molecule). At very low shock temperatures (below 1250 K), where there is an extended CH, formation time, the peak yields are reduced and are determined by the rate for the azomethane decomposition, reaction 1, and the rates for reactions 6, 7 and 8 (see Table I). At temperatures higher than this level, the absorption coefficient derived from comparing the absorption data to the detailed kinetic model predictions is insensitive to the actual rate coefficients, though improved long-time fit can be achieved by adjusting the rate coefficients for reactions 7 and 8. The rate coefficient listed for reaction 1 is 10 x larger than that reported by Miiller et al, and gives improved fits to the lowest temperature data.

An example data trace of the measured CH3 mole fraction vs time in an acceptable azomethane decomposition reflected shock experiment is shown in Fig. 2, along with calculated profile. In Figs. 2-6 the diagnostic wavelength used was 216.615 nm. The modelled peak CH, mole fraction is 94ppm, which represents a conversion yield of 98%; the estimated uncertainty in this calculated yield is +2%/- 10%. The time response of this trace is limited by the shock transit time across the probe laser beam, typically 1 psec. The noise, using the common-mode rejection scheme described earlier, is typically 0.2% of the lo signal. This corresponds to a CH3 detectivity limit of approx. 2 ppm. The spike near - 75 psec is a result of beam steering by the incident shock. The sharp absorption peak at time zero is a result of the combination of the slight absorption by azomethane, absorption from rapid methyl formation and beam steering by the reflected shock. The absorption fraction corresponding to 94ppm CH, in this trace is 17%; this was the mole


1445 K, 1.24 atm

48 ppm CHaN:NCHdArgon

Fig. 2. Azomethane decomposition. The reflected shock conditions are 48 ppm azomethane/Ar, 1455 K, 1.24 atm; (---) kinetic model; IcL = 110 atm-l-cm-‘.

fraction used to infer kn. Figure 3 shows the data of Fig. 2 plotted as l/x,-.,, vs time. The linearity of this plot supports the view that the decay is dominated, in the first 200 psec, by a second-order loss mechanism consisting of reactions 6,7 and 8. This is consistent with the calculated contribution factor analysis.

Azomethane has limited use as an instantaneous methyl radical source at high reflected shock temperatures experiments (> 1900 K) as it first decomposes rapidly in the accompanying incident shock region. Under these extreme conditions, incident shock temperatures are sutlicient to decompose the azomethane into two methyl radicals, and these radicals have sufficient time (> 150 psec particle time at 0.2 atm 1000 K) to recombine into ethane. Then, behind the reflected shock, this ethane acts as a slower-releasing reservoir for methyl radicals and the CH3 profile appears to have a plateau at early times, This effect was noticed first by Glanzer et al.’

Methyl iodide (CH,I) was considered as a source species because it decomposes rapidly above 1600 K, and the iodine-related chemistry was expected to be insignificant in the temperature and pressure regime of the present experiments. 2’ Methyl iodide does not absorb on the mixing vessel and shock tube walls as much as the other sources tested. Adsorption, which usually manifests itself as a lower source species concentration in the first shock of a gas mixture, was not evident in the

6

iZ E 6 i?

z

YZ 4 1445 1.24atm K,

46 ppm CH3N:NCH3/Argon 2

Fig. 3. Example data reduction. The data from Fig. 2 are plotted as l/xcH, vs time; (---) least-squares fit to data.


300 ,,,I,““,““,““,““,“”

1907 K, 1.09 atm

214 ppm CH#Argon

Fig. 4. Methyl iodide decomposition. The reflected shock conditions are 214 ppm CHJAr, 1807 K, 1.09 atm; (---) kinetic model; /cl = 70 atm-‘-cm-‘.

methyl iodide mixtures tested. The bimolecular rate constant for CH31 decomposition, reaction 2, was determined approximately by fitting the overall shape of the modeled profile to that of the experimental data. k2 was found to be 3 x 10L4exp( - 16,000/T) cm3-mol-‘-set-’ for the six data points from 1397 to 1807 K. Computed yields were in the range 50% (at 1397 K)--88% (at 1807 K). There was no initial absorption by methyl iodide which permitted a relatively clean interpretation of the early formation of methyl radicals. There also appeared to be no significant long-term absorption by iodine-related chemistry. A typical modeled xc+ profile and experimental data trace (matched at the peak to infer k,) are shown in Fig. 4. The narrow peaks in the signal at -65 and 0 psec are again the result of beam steering of the probe laser by the incident and reflected shock fronts respectively. The peak absorption in this trace was 19%.

Tetramethyl tin [Sn(CH,),], TMT, was also utilized; it also has high calculated conversion yields, and unlike azomethane, it is commercially available. A rate for reaction 6, TMT decomposition, 10 x larger than that found in Miiller et al’ was used. Modeled yields ranged from 50% at 1300 K to >97% at temperatures above 1700 K. There was evidence of significant absorption in the very dilute mixtures which were used. There was also systematic interference absorption at long times; this effect was greater at lower temperatures (1300 K). We hypothesize that this is related to the

500

1735 K, 0.68 atm

90 ppm TMT/Argon

0 . .

-100 0 100 200 300 400 500

Time UsI

Fig. 5. Tetramethyl tin decomposition. The reflected shock conditions are 90 ppm TMT/Ar, 1735 K, 0.68 atm; (---) kinetic model; kA = 53 atm-‘-cm-‘.


2374 K, 0.61 atm

63 ppm TMSlArgon

-100 0 100 200 300 400 500

lime [cls]

Fig. 6. Tetramethyl silane decomposition. The reflected shock conditions are 63 ppm TMS/Ar, 2374 K, 0.61 atm, (---) kinetic model; kA = 31 atm-‘-cm-‘.

formation of tin compounds. As with azomethane, high incident shock temperatures (above 1000 K with TMT) can lead to premature dissociation. Figure 5 shows an example TMT pyrolysis trace. Absorption by shock-heated TMT in the incident region (916 K, 0.17 atm) which has not yet decomposed is evident, The methyl radical yield in this case was calculated to be 96%.

The difficulties posed by the decomposition of TMT and azomethane in experiments with hot incident shock regions led to the suggestion that another source compound with a higher decomposition energy barrier might be more suitable for calibration at higher temperatures. Tetramethyl silane, TMS [Si(CH,),], was thus selected as a source species. Above 2000 K, the assumption of a one-step decomposition is probably adequate for the present work, and yields of >90% are predicted. At lower temperatures, the interpretation of the pyrolysis of TMS is not adequately described by a simple one- or four-step methyl-releasing decomposition, and a direct interpretation of the yield is difficult. There appears to be some interference absorption at later times, possibly related to the formation of other silane compounds. This interference increases with temperature, the inverse behavior of that found in the TMT mixtures. An example TMS decomposition data trace is shown in Fig. 6. Peak modeled CH, is 234ppm, with a corresponding absorption of 6% (used to infer k,). Interference absorption at times longer than 60 psec is evident.

Ethane was also considered as a methyl source species because of its direct high temperature decomposition to two methyl radicals and its ease of handling. The decomposition rate of ethane is, however, relatively slow, and estimated yields are sensitive to the rate coefficients and temperatures used to model the profiles. The rate coefficient for ethane decomposition, reaction 5, given in Table 1 is from a recent measurement in our laboratory. Estimates of the yields of CH3 vary typically from 12% at 1500 K to 83% at 1800 K. Ethane was suitable, however, for the relative measurement of the variation of kl with wavelength. Figure 7 shows an example ethane decomposition trace using 215.91 nm radiation. This data was running-mean averaged using a 2 psec window. The time of the peak absorption (which occurs at 90 psec) is very late compared to the values found in Figs. 2, 4 and 5, and hence the conversion yields are expected to be small. Thus, although ethane was a convenient source of CH, for studies of the relative variation of the absorption coefficient with 1, it was not judged suitable for extracting absolute values of k*.

From the above discussion, comparing the sources of CH3 utilized, we conclude that for reflected shock tube kinetics experiments azomethane or methyl iodide are the most suitable sources for rapid methyl radical production. Long time modelling of Sn(CH& or Si(CHj), is sufficiently complicated by Sn- or Si-related chemistry that it may preclude their use as sources of CH, in kinetic experiments.


400 ppm C*H$Argon

215.91 nm Absorption

Fig. 7. Ethane decomposition, absorption at 215.91 nm. The reflected shock conditions are 400 ppm ethane in Ar, 1600 K, 1.30 atm.

THE VARIATION OF ABSORPTION COEFFICIENT WITH WAVELENGTH

The spectroscopic descriptions of CH, and CD, have been given by Her&$* and Callear and Metcalfe.23 Spectroscopic data for CH3 and CD, are summarized by Jacox.” Strong predissociation gives the CH, spectrum a broad and smooth appearance. CD3, on the other hand is not so strongly predissociative and permits the spectroscopist to make an accurate interpretation of the spectral structure of both species. The CH3 R(*A ;)tX(*A;) transition at 216 nm [called the /3,(0-O) band by Herzberg] is composed, at room temperature, of two prominent features, an R-band peak at 215.7 run and a P + Q band peak at 216.4 nm. Accurate values for the peak wavelengths and for the absorption coefficients at higher temperatures, however, can be found only by measurement.

The variation of the absorption coefficient with wavelength, at 1625 K and 1.25 atm derived from shocks of 400 ppm ethane/Ar and 200 ppm methyl iodide/Ar mixtures, is shown in Fig. 8 along with the model calculation. An expanded detail of this figure near the 216 bandheads is shown in Fig. 9. The absorption coefficients were derived by fitting the calculated peak of the CH, absorption to that measured at the individual wavelengths.

225

Wavelength [nm] 220 215 210

Wavenumber [cm-‘]

Fig. 8. Variation of the absorption coefficient with wavelength. The reflected shock conditions are 1625 K, 1.25 atm; (-) calculated spectrum; ( . . . . .) linear addition of absorption to the symmetric top model.


217 Wavelength [nm]

216

i CH,dlagnosticwavelength

sot1 ’ ’ ’ ’ ” ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 11 46100 46300

Wavenumber [cm-‘]

Fig. 9. Detail of Fig. 8. (+) 200ppm methyl iodide/Ar data; (m) 400ppm ethane/Ar data.

The absorption spectra expected from the B(‘A;) +@A;) parallel band transition were calculated using a symmetric top model. The details of the development of this model for the parallel bands of CH, and CD, can be found in Her&# and Gltinzer et al.’ Added to the values of this calculation was a linear increase in baseline absorption from 44,000 to 48,OOOcm-‘. The slope of this increase was adjusted to supplement the baseline of the model calculation and n$ttch the trends of the absorption data.

The high temperature R-band peak at 216.0 nm can be distinguished from the P + Q band peak at 2 16.6 nm in Fig. 9. The calculated maximum of the P + Q band peak at 2 16.615 nm vat is slightly red-shifted from the room temperature maximum (at 216.4 ,).22 The symmetric top model of the (0,O) band duplicates the general structural features of the peak very well. However, the calculated peak absorption coefficient is sensitive to the dissociative broadening width, the oscillator strength and to any absorption from underlying features. A sufficiently good fit was found by using the dissociative broadening width of approx. 100 cm-’ (Gliinzer et al’ used 60 cm’), the oscillator strength of Callear and Metcalfe23 (&, = 0.0137) and the linear increase in baseline absorption described above. The P + Q band peak was chosen for the temperature dependence experiments as the calculated spectrum, near the P + Q band maximum, varies less rapidly with wavelength than the R band maximum and was less sensitive to small changes in temperature.

We are aware of three other measurements of CH3 spectra in this wavelength region at high temperatures, two of low resolution and one of high. The measurement of Harvey and Jensen’ is a high resolution flame absorption scan which resolves the P + Q and R peaks. Tsuboi4 and Glgnzer et al’ present low resolution scans at 1800 and 1400 K, respectively, which do not resolve the two peaks. The two low resolution (Al = 1.4 nm) studies inferred peak absorption coefficients which are approximately half the value found in the present narrow-line scan.

All authors find a secondary absorption band at 212.5 nm. iv4, 7*23 The higher temperature studies of GlZinzer et al,’ Tsuboi4 and the present work also show a very broad absorption feature which extends from beyond 216 nm to below 2 10 nm. The relative heights of the 2 16 and the 2 12 nm peaks are affected by this underlying very broad feature (modeled here by a linear increase in absorption). It is possible that this very broad feature is related to the 207.9 nm band found in the spectrum of Gliinzer et al and Callear and Metcalfe.”

THE VARIATION OF ABSORPTION COEFFICIENT WITH TEMPERATURE

The measured variation of the absorption coefficient with temperature at 216.615 nm vat (46,165.l cm-‘) is shown in Fig. 10. These data are taken from reflected shock wave experiments

D. F. DAVIDSON et al

3

1250 1500 1750 2ooa

Temperature [Kj

2250 2500

Fig. 10. Variation of the CH, absorption coefficient with temperature at 216.615 (+O.OOS) nm. Solid symbols, reflected shock wave experiments; open symbols, incident shock wave experiments; squares, data from tetramethyl silane mixtures; triangles, tetramethyl tin mixtures; diamonds, methyl iodide mixtures; circles; axomethane mixtures. The error bars are & 10% and * 30%. (-) Least-squares fit to the data; (---) high resolution (10 cm-‘) calculation of Ghinzer et al;’ (. . . . .) low resolution (340 cn-‘)

experimental data of Miiller et al.*

of the following mixtures in argon: 1.2 atm, 357 ppm and 214 ppm methyl iodide; 1.2 atm, 35 ppm, 59 ppm, and 0.7 atm, 39 ppm and 90 ppm tetramethyl tin; 0.6 atm, 63 ppm tetramethyl silane; 1.2 atm, 48 ppm azomethane, and from incident shock wave experiments conducted at 0.7 atm with methyl iodide. Data points derived from azomethane and methyl iodide mixtures have average uncertainties of + 10%; because of uncertainties in the kinetic mechanism and background absorption, the data points derived from the other sources have uncertainties which vary up to a maximum of + 30%. A least-squares fit of this absorption coefficient over the temperature range 1350-2450 K, using an expression which separates the effects of number density from those of the partition function and broadening, is given by

k, = (562,4OO/T)exp( - T/1087) atm-‘-cm-‘;

the scatter of the data has a least-squares deviation of + 12%. Because the high temperature contribution to the 216.615 nm absorption coefficient by the broad 212.5nm and very broad 207 nm features is uncertain, an extrapolation of the fit to higher temperatures will introduce larger uncertainties. This expression extrapolates well to lower temperatures, coming within a factor of 1.5 of the room temperature measurements of Callear and MetcalfeF3 There is no discernible variation in this data with pressure in the range 0.6-1.4 atm.

These data can be compared to those presented by Gllnzer et al,’ Tsuboi4 and Gardiner et a1.6 While there is good agreement between the temperature dependence of our data and those of Gl&mer et al, their magnitude of kA is smaller by about 50%. A similar situation exists with the data of Tsuboi. Both results may possibly be explained by the lesser maximum absorption inherent with the wider (1.4 nm) spectral resolution method they employed.

The Zn lamp narrow-line measurements at 214 nm done by Gardiner et al6 support our higher value for the absorption coefficient. They found a narrow-linewidth absorption coefficient at their wavelength which is twice the value found in the low-resolution scan reported by Glanzer et al’ and by Tsuboi.* The temperature dependence of the absorption coefficient at 214nm given by Gardiner et al varies more weakly than the other studies (k, N l/T), though this may be because the 214 nm data are away from the peak of the absorption feature.

EFFECT OF INTERFERING ABSORBERS

The use of a 216 nm absorption diagnostic for CH3 detection requires an understanding of the possible interference absorption from other species found in the chemical system under study. We will focus on four of these: 02, NO, CzHz and C2H4.


The dense discrete spectra of Schumann-Runge bands of oxygen extend from the vuv to beyond 216 nm.2s Beneath these lines there is a very weak continuum. Though the absorption coefficient at combustion temperatures for the peaks of most overlapping O2 line features is very small (of the order of k = 0.1-I atm-‘-cm-‘), the large concentrations of O2 found in many combustion systems can give rise to a large interfering absorption. The narrow-line laser absorption diagnostic can be tuned slightly away from these scattered Schumann-Runge features, however, with no significant change in CH, absorption coefficient. Using the computer code of Lee and HansotP to estimate the location and magnitude of absorption features, we find that between 216.52 and 216.58 nm and between 216.70 and 216.74 nm, the discrete absorption coefficient of O2 falls to less than 0.04 atm-r-cm-’ at 2000 K. Even in the region between these bands at 2OOOK, the peak absorption coefficient never exceeds 0.5 atm-‘-cm-‘.

Efforts to correct for this interference in lamp systems by directly subtracting the interfering absorption from the total absorption signal, especially at early times behind shock heated mixtures, can be complicated by the vibrational relaxation of 02. The Schumann-Runge bands near 216 nm are from the higher vibrational levels (a” = 4-7) of the O2 X3X:- state, and absorption is related to the population in these upper vibrational states. Shock tube measurements made in this laboratory of 3% O,/Ar mixtures at 2000 K, 1 atm, indicate that the time to populate these higher vibrational states may be as long as 50-70 psec. A similar thermal effect was also noticed by Gardiner et a1.6

The situation with NO is substantially simpler. The NO A cX (1,O) band extends no further into the red than 215.8 nm. The (0,O) band, centered near 225 nm, degrades rapidly towards the vuv, and at 2000 K there is insignificant absorption at wavelengths shorter than 218 nm. Absorption measurements between 215.8 and 218.0 nm should thus not be affected by interfering NO absorption.

At room temperature, neither acetylene nor ethylene produces significant absorption at wavelengths longer than 203 nm. 26*27 At high temperatures, however, both ethylene and acetylene appear to have weak broad absorption in the wavelength region of the CH, peak. Absorption coefficient measurements at several wavelengths and temperatures are shown in Fig. 11 for acetylene and in Fig. 12 for ethylene. These measurements are from 1 atm, 1000 ppm mixtures in argon. Wavelengths for the acetylene measurements were chosen from the relatively strong room temperature features near 216 nm given by Ingold and King,2* though we found no evidence of discrete features at these wavelengths at high temperatures. In the acetylene shock wave experiments, an absorption plateau forms immediately after the reflected shock from which the absorption coefficient can be calculated. At longer times, a strong interference extinction occurs,

To 2.5 - Acetylene Absorption: ::::::::::: 216

0 nm, Koike and Modnaga

- -- 230 nm, Koike and Morinaga . . . .

‘; . ..’

6 2.0 - ...‘....... 214 nm, Gardiner et al

. ..’ - . ..’

..”

-e- 216.65 nm ,..’ E .P,

--v- 220.67 nm

g 1.5- -+- 223.76 nm

$ 0 s l.O-

‘L:

i n 0.5 - a

Fig. 11. Variation of the acetylene absorption coefficient with temperature and wavelength. (-_O-) Absorption data and fit at 216.65 nm using acetylene mixtures, error bar f 10%; (-.V.-) 220.67 mn; (-- 0 --) 223.78 nm; C. .) 214 nm absorption data taken from Gardiner et al;6 (: : : : :) 216 nm absorption

data taken from Koike and Morinaga;29 (= = -) 230 nm data from Ref. 29.


3.0 I 8 I I , I I I I , I I I I , I I I ,,.‘I ,.: _

Ethylene Absorption: ,:’

2.5 - (...

::::::::::: 216 nm, Koike and Modnaga == = 230 nm, Koike and Morinaga

2.0 -

1.5 -

.......‘... 214 nm, Gardiner et al --8- 216.61 nm --b-. 220.67 nm

:’

1.0 - ,:’

,:’ __.’

__.’ __:

0.5 - ._.’

. ..’

. ..” _..’ . ..’

0 “ls’llll’llll’lll’ 500 1000 1500 2000 2500

Temperature [Kj

Fig. 12. Variation of the ethylene absorption coe5cient with temperature and wavelength. (-_O-) Absorption data and fit at 216.61 mn using ethylene mixtures, error bar f20%; (-.V.-) 220.67 nm; (. . . .) 214nm absorption data taken from Gardiner et ah6 (: : : : :) 216 mn absorption data taken from

Koike and Morinaga;” (= = =) 230 nm data from Ref. 29.

likely from sooting. In the ethylene mixtures, the absorption coefficient is derived from the initial absorption at the time of the reflected shock arrival. Immediately after the arrival of the reflected shock, ethylene begins to decompose, acetylene is created, and finally, soot begins to form.

The absorption coefficients for ethylene and acetylene increase, albeit weakly, at shorter wavelengths and higher temperatures. There is agreement in this trend, and with the absolute values found, between the present data and the narrow-line data of Gardiner et al6 and the spectrally- broad data of Koike and Morinagazg measured using a D2 lamp and a monochromator.

CONCLUSIONS

The cw ring dye laser diagnostic technique offers the significant advantages (relative to previous sources) of tunability to optimize the absorption coefficient and minimize interferences, and of narrower spectral resolution and lower detectivity limit, due largely to the high spectral intensity. The common mode noise rejection scheme used with the present CH, diagnostic has a signal detection limit of less than 0.2% of I,,. This noise rejection scheme combined with the narrow spectral linewidth and tunability of the ring dye laser, which permit the use of the peak of the CH, absorption feature, should offer an approximately 5- to IO-fold improvement in the CH3 detectivity limit over lamp systems.

This new CH3 diagnostic technique should permit more direct investigation of several combustion reactions. Of particular current interest is the branching ratio of the reaction CH3 + 0,, whose products are hypothesized to be either CH30 + 0 or CHzO + OH.M Experiments, which will include measurements of both CH3 and OH via laser absorption, are planned to determine these rate coefficients. Methane and ethane pyrolysis kinetics are another area of interest, as the primary decomposition reaction rate coefficients still have large uncertainties. The CH3 absorption diagnostic should be well suited to examine this problem.

Acknowledgements-This work was sponsored by the U.S. Department of Energy, Gtlice of Basic Energy Sciences, Division of Chemical Sciences, The authors would like to thank E. J. Chang and L. S. Zelson for their assistance in running the experiments.

REFERENCES

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