A diagnostic technique for monitoring the concentration of NO using absorption of CO laser radiation was developed and applied in a study of the decomposition kinetics of NO. Simultaneous measurements of infrared emission by NO at 5.3 microns were also made to validate the laser absorption technique. The data were obtained behind incident shocks in NO-N2O-Ar (or Kr) mixtures, with temperatures in the range 2400-4100 K. Rate constants for dominant reactions were inferred from comparisons with computer simulations of the reactive flow

Flower, W. L.

Hanson, R. K.

Kruger, C. H.

Monat, J. P.

English

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DECOMPOSITION OF NO STUDIED BY INFRARED
EMISSION AND CO LASER ABSORPTION
R. K. Hanson, W. L. Flower, J. P. Monat and C. H. Kruger
Mechanical Engineering Department
Stanford University, U.S.A.
ABSTRACT
A diagnostic technique for monitoring the concentration of NO using
absorption of CO laser radiation has been developed and applied in a study
of the decomposition kinetics of NO. Simultaneous measurements of infra-
red emission by NO at 5.3 microns were also made to validate the laser
absorption technique. The data were obtained behind incident shocks in
NO-N20-Ar (or Kr) mixtures, with temperatures in the range 2400-4100
0 K.
Rate constants for dominant reactions were inferred from comparisons with
computer simulations of the reactive flow. i
INTRODUCTION H CW
The anticipated availability of tunable continuous-wave infrared a
lasers offers prospects for important new quantitative spectroscopic ot
techniques well suited for shock tube kinetics studies of infrared-active =
gases. One obvious advantage of tunable laser spectroscopy is high
specificity for a given chemical compound so that negligible interference
is produced by other constituents present in a mixture.
ot
To investigate the potential of tunable lasers for shock tube appli- M 0
cations we have constructed a discretely tunable CO laser and used it for m o
absorption measurements of NO, taking advantage of the near-coincidence of W H M
some NO vibration-rotation absorption lines and CO gas laser lines (refs. 4A 0
1, 2). In this paper we describe the experimental apparatus and compare 
H
simultaneous measurements of NO concentration obtained using laser absorp- =q
tion and infrared emission techniques. Some results for rate coefficients
relevant to NO decomposition kinetics are also presented. Z
EXPERIMENTAL I o
The experiments were done behind incident shock waves in a conven- 2 J >
tional, pressure-driven shock tube (15-cm internal diameter). Shock speeds
varied from about 1.3 to 1.8 mm/lsec, producing frozen-chemistry.temper-
atures of 2500-41000K and postshock pressures between .0.3 and 0.5 atmo-
spheres. Test gases consisted of NO in an excess of either Ar or Kr, with
a trace amount of N20 added in some experiments. The mixtures were pre- iw
pared manometrically and mixed immediately before each run.
A schematic of the experimental setup is shown in Fig. 1. The CO
laser was operated on a single vibration-rotation line at v=1935.48 cm
- 1  ia
(V=7-*6, J=12-13) with a typical output power of about 0.1 W. The absorp- n
tion line of NO is centered at V=1935.49 cm-1 (213/2, V=0, m=39/2) (ref. 1). o -I
Transmitted radiation is directed onto a photovoltaic InSb detector U L
'(liquid-nitrogen cooled). The data is displayed on an oscillogram, as
shown in Fig. 2a. The data displayed include the transmit intensity
I(t), the zero signal (laser blocked off), and the ab =
Io-I(t), where t is the time after shock arrival.
C.D-M L. Jc  -P, .
https://ntrs.nasa.gov/search.jsp?R=19750007125 2020-03-23T01:57:03+00:00Z
The governing equation for absorption is the Beer-LaiLert Law
I I o exp(-BL[NO]) (1)
where Io and I are the incident and transmitted light fluxes and L is 
the
path length of the beam through the sample. The absorption coefficient 
B
is a function of temperature, pressure and composition and is essentially
zero before the shock arrives. For the small absorptions to 
be reported
here, the absorption is given approximately by
A(t)/A* = B[NO]I/(B[NO])* [NO]/[NO]* (2)
where the asterisk denotes some convenient reference condition. Here we
have elected to neglect the small changes in which occur on the time
scale of interest after the shock. Thus, for present purposes we can plot
A/A* and compare this directly with calculations of [NO)/[NO]*.
The infrared emission system has been described previously (refs. 3,4).
The emission from the fundamental vibration-rotation band of NO (at 5.3
microns) is isolated from N20 emission (4.5-micron band) using a filter
passing wavelengths greater than 5.0 microns. The detector is 
coupled to
an amplifier whose output is displayed on an oscilloscope. A typical
record is.shown in Fig. 2b.
For an ideal system, the output voltage of the amplifier V is propor-
tional to the intensity of spontaneous emission I, which in turn is pro-
portional for an optically thin gas to the product of the NO concentration
and the vibrational energy per unit concentration of NO, ev(T). For the
present we choose to neglect the influence of small temperature 
changes so
that V is linearly proportional to [NO] and we may conveniently plot
V/V* = [NO]/[NO]* (3)
for comparison with both the laser absorption data and computer calcula-
tions of the reacting flow.
RESULTS
Results based on the data of Fig. 2 are presented in Fig. 3. Here we
have plotted absorption and emission data and kinetic calculations of
[NO]/[NO]max, the maximum NO concentration providing a convenient reference
for normalization of the calculations and the absorption data. Because of
initial formation of additional NO during the decomposition of N20, the
peak value of [NO] occurs about 1 sec after shock arrival. Unfortunately,'
the emission data at 1 lsec is distorted by two nonideal effects, NO vi-
brational relaxation and a slight transient interference from N20 emission.
To minimize these effects, we have normalized the emission data to match
the kinetic calculations at 5 psec.
Kinetic calculations were made with the NASA Lewis kinetics Program
(ref. 5) using a nine-reaction scheme developed in a previous study (ref.3).
After the rapid decomposition of the initial N20, the rate-limiting step
for NO removal is the reaction
NO + 0 + N + 02.
The N20 was added in many of the experiments.-to serve as a quick source of
oxygen atoms, thereby enhancing the role of reaction 1 in the 
overall NO
decomposition scheme. The calculated NO profile shown in Fig. 3 was
obtained by choosing thevalue of k1 so as to yield the best fit to the
data. All other rate constants were held fixed. Calculations testing
sensitivity to reasonable variations in other rate constants substantiated
this approach for present conditions. Results for k i and other rate con-
stants inferred from fits to the data from all the experiments will be
presented in the complete paper.
The good agreement in Fig. 3 between the two independent measurements
of [NO] serves to validate the laser absorption technique and also provides
some indication of the accuracy with which [NO] can be determined using
these methods. The interference in the NO emission data due to N20 emis-
sion, even in this relatively simple mixture and with a carefully'chosen
filter, emphasizes the merit of tunable laser spectroscopy.
Measured values for the absorption coefficient g were in the range
5-11 x 104'cm2 /mole, decreasing with increasing temperature. Detailed
results will be presented in the full paper.
We gratefully acknowledge support of this work by the National
Science Foundation and the National Aeronautics and Space Administration.
REFERENCES
1. C. Chackerian, Jr. and M. F. Weisbach, Opt. Soc. America 63, 342 (1973).
2. R. T. Menzies, App. Optics 10, 1532 (1971).
3. R. K. Hanson,.W. L. Flower and C. IH. Kruger, Comb. Sci. and-Tech.,
Dec. 1974 (in press).
4. W. L. Flower, R. K. Hanson and C. H. Kruger, 15th Symp. (Intern.) on
Combustion (Tokyo, 1974).
5. D. A. Bittker and V. J. Scullin, NASA TN D-6586 (1972).
InSb Detector. Sapphire
Windows
Fig. 1 Schematic of experiment..
Slit
BaF
Window
Tunable
CO 'Laser
InSb Detector.
%". 
,
Fig.2 Typical data: CO laser
absorption (Fig. 2a) and NO IR
emission (Fig. 2b). V=1.53mm/
Ii.... isec, P 1=6.94 Torr (N20/NO/Kr =
0 ,.. ;,,,; 2.2/9.8/88), T2frozen = 4048
0 K.
In Fig. 2a the vertical sensi-
tivities are 50 and 5mV/div;
in Fig. 2b the sensitivity is
10mV/div.
(2a)
0- -- ---- , t e --1 , 2..0 psec
(2b)
- ime k-b-4 20 isec
1
REPRODUCIBATLITY OF TH
0 
..z_ .4'
.2 O
0 20 40 60 
80
LAB TIME, JSEC
Fig. 3 Absorption and emission data and kinetic calculations of [NO]/[NO]max.
Shock conditions are the same as Fig.2: --- , Kinetic calculations with kl=
2.7xl09T exp(-38640/RT)cm3/mol-s;A, absorption data for 
A(t)/Amax;O, emis-
sion data for V/Vmax renormalized to match kinetic calculations at 
5 psec.


Decomposition of NO studied by infrared emission and CO laser absorption

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