Investigation of Two-Photon Laser-Induced Fluorescence Detection ...

Investigation of Two-Photon Laser-Induced Fluorescence Detection of
Carbon Monoxide for Applied Combustion Diagnostics
M. Richter, Z. S. Li * and M. Aldén
Division of Combustion Physics, LTH, P.O. Box 118
Lund University, S-221 00 Lund, Sweden
Abstract
The properties of two-photon laser-induced fluorescence (LIF) signals from carbon monoxide have been studied in
order to investigate the applicability of this technique for measurement and visualization of CO molecules in applied
combustion environments. Spectrally and spatially resolved measurements were carried out in a test cell, in a flame
and in an engine. In the cell measurements, the two-photon fluorescence signal from CO was studied as a function
of laser flux and ambient pressure. In addition, measurements with buffer gas at pressures from 0.1 up to 10 bars
were carried out. In the flame measurements, problems from spectral interference due to undesired C2 molecules
produced by photodecomposition of fuel and fuel fragments were investigated at different stoichiometries. Two
dimensional LIF imaging of CO was also performed in an engine. The interference from C2 and photolysis of hot
CO2 was excluded from the spectral and spatial resolved measurements.
.
Introduction
Detection of Carbon monoxide has always been a
important task for laser combustion diagnostics.
Various laser diagnostic techniques have been
developed during the last decades and they have
become of utmost importance for a deepened
understanding of combustion processes. This
statement is to a large degree due to the outstanding
features like non-intrusiveness in combination with
high temporal and spatial resolution for
measurements of species concentrations,
temperatures, velocities and particle characteristics,
see e.g. [1-3]. The technique that has attracted most
attention is Laser-Induced Fluorescence (LIF), see
e.g [4]. Most of the combustion diagnostic
experiments using LIF have been directed towards
single photon excitation. There are, however, several
species of combustion interest that do not have easily
accessible excitation wavelengths in the UV/visible
part of the spectrum, e.g. the light atoms O, N, H, C
and molecules like H2, H2O, NH3, CO. Many of these
species have their main electronic resonances in the
vacuum ultraviolet region of the electromagnetic
spectrum. In order to overcome the problems of
generating and using VUV laser beam and the
problems associated with excitation in this spectral
region, multi-photon excitation processes have been
utilized for the probing of these species.
One of the species mentioned above, CO, as an
indicator of the combustion efficiency and as a major
pollutant, is one of the most important molecules in
combustion processes. In combustion environments
CO has been measured commonly by exciting the
molecule from its ground state, X 1 Σ + , to the excited
* Corresponding author: zhongshan.li@forbrf.lth.se
Associated Web site: http://spartacus.forbrf.lth.se/
Proceedings of the European Combustion Meeting 2005
state, B 1 Σ + , via two-photon excitation at 230.1 nm
and observing the fluorescence from the excited level
to the lower lying A 1 Π level by detecting the
fluorescence in the spectral range between 451 nm
(v’=0, v’’=0) and 662 nm (v’=0, v’’=5), see Figure 1
where the collision induced triplet transitions are also
indicated. Using this excitation scheme several flame
measurements have been performed [5-9]. Quenching
measurements in flames using pico-second excitation
[10, 11] have also been reported. In order to achieve
a quantitative interpretation of the two-photon LIF
signal, substantial efforts have been put in measuring
important parameters such as the two-photon
excitation cross-sections [12], photoionization crosssections
and ac Stark shift [13], collisional
broadening and shift [14], species-specific and
temperature-dependent quenching cross-sections [15,
16]. Photochemical effects from vibrational hot CO2
[17] has been investigated as an interference for twophoton
LIF CO detection in atmospheric pressure
flames. Among other alternative spatial-resolved CO
detection techniques, Linow et al. reported the
comparison between the excitation scheme
mentioned above and excitation from the ground
state, X 1 Σ + , to the excited state, C 1 Σ for two photon
LIF measurements [18]; infrared LIF detection of CO
has been reported by Hanson’s group [19, 20], by
probing infrared active ro-vibrational overtone bands;
two-photon polarization spectroscopic was
investigated by Nyholm et al. for spatial resolved CO
detection [21].
Despite the relatively large number of
publications including flame measurements of CO
detection using two-photon LIF, so far to our

knowledge no investigation at high pressure and
engine applications have been reported. The aim of
the present work was to investigate the applicability
of CO detection with two-photon LIF in combustion
engine. In order to understand the behaviour and
achieve a quantitative interpretation of the CO LIF
signal from the combustion engine, where both high
pressure and high temperature presented, a pressure
dependent measurement in a static cell and a flame
measurement with different stoichiometries have
been performed. Single shot, two dimensional CO
LIF imaging in a test engine was finally
demonstrated with exclusion of the potential
interference from C2 and from photolysis of hot CO2.
Figure 1. Schematic energy level diagram for
carbon monoxide. Depicted also the two-photon
excitation, the fluorescence decay and the collisioninduced
triplet state transitions.
Cell and flame measurements
In order to investigate the potential for
quantitative LIF measurements of CO in general and
at elevated pressure in specific, it is necessary to
control a range of properties affecting the relation
between laser power and the generated signal level
and the signal dependences on pressure and possible
photolytic creation of C2 radicals. When probing CO,
in addition to the more conventional quenching
phenomenon (marked with a Q in Figure 1),
properties like two-photon absorption, stimulated
emission and ionization effects have to be taken into
account, which are all dependent on the laser
intensity. The laboratory experiments served to
investigate these properties in simulated combustion
environments, i.e. to investigate the dependencies on
laser energy, pressure, temperature and
stoichiometry, as well as, the influence from the
presence of other species.
A Nd:YAG (Continuum NY82-S)) pump dye
laser (continuum ND60) system are utilized in the
cell and the flame measurements. The dye laser (with
a mixture of Rhodamine 590 and Rhodamine 610
dissolved in methanol) was pumped by the second
2
harmonic from the Nd:YAG laser, producing
radiation at 587.2 nm. Frequency doubling in a KDPcrystal
resulted in 293.6 nm. By mixing this with the
fundamental 1064 nm from the Nd:YAG laser,
approximately 4.5 mJ of the required radiation at
230.1 nm was generated.
The investigated gas mixtures were kept in a
stainless steel high pressure vessel equipped with
quartz windows. The laser beam was focused with a
single spherical lens through the cell with the focus
point centred in the middle. The interaction region
between the CO molecules and the laser beam was
imaged with an image intensified CCD (ICCD)
camera (Princeton Instruments, ICCD-5765S)
perpendicular to the excitation laser beam. A long
pass filter was placed in front of the ICCD to block
the scattered light from the 230 nm excitation laser.
Figure 2. Plot showing the CO LIF signal measured
at twelve different pressures ranging from 0.1 to 3.0
bar.
Shown in Figure 2 is the resulting CO LIF signal
versus pressure when the cell was filled with pure
carbon monoxide and the pressure varied from 0.1
bar to 3 bar. The laser was propagating from left to
right and the focus point located at 0 mm. The
absorption effect of the excitation laser as it
transverse the cell was obvious as indicated by the
decrease of the LIF signal along the laser beam and
the absorption increase with the increased CO
pressure in the cell. Note that even for a fix laser
energy output the laser flux density will vary along
the focal line, i.e. it will decrease with the distance to
focus. As can be seen this results in a variation of the
fluorescence intensity along the beam. The ionization
potential of the CO molecule is 14.0139 eV. This
gives that when the B 1 Σ + level is excited, absorption
of one additional photon would be sufficient to ionize
the molecule. The dip at the focus location is
probably due to the increased influence of photo
ionization. To understand this better, spatial resolved
CO LIF measurement with 400 mbar pure CO in the
cell was performed with varied excitation laser pulse
energy. Shown in Figure 3 is an example of such a
measurement with the laser pulse energy varied from
0.2 to 2.4 mJ. The CO LIF emission is highly

dependent on several parameters, like laser intensity,
ionization, non-linear absorption which all contribute
to the question of correct signal interpretation in an
applied combustion situation. In order to understand
the phenomena taking place and the interaction
between these phenomena a simulation based on a
four level rate equation system including photoionisation,
quenching, two-photon absorption, and
spontaneous emission was applied to calculate the
spatial distribution of the CO LIF signal [22]. The
thick line shown in Figure 3 is a simulation of the
measured curves with 2.4 mJ per pulse the laser
intensity. A general similarity between the simulated
profile and the measured one was achieved, which
indicates that the model are proper in prescribe the
observed the phenomenon.
Figure 3. The variation of the CO LIF intensity along
the laser beam. The gas pressure of CO in the cell was
held constant at 400 mbar, while the energy of the laser
pulse was varied from 0.2 to 2.4 mJ. The thick line
represents a simulated fluorescence signal along the
laser beam with 2.4 mJ per pulse.
Signal intensity (counts)
10 4
10 3
k = 2
k = 1
0.1 bar
0.2 bar
0.5 bar
1.0 bar
2.0 bar
3.0 bar
106 107 108 109 Laser fluence (W/cm2) Figure 4. Plot showing the measured LIF
signal vs excitation laser fluxes at different CO
pressures.
Shown in Figure 4 are the LIF signal intensities
as the laser energy was increased for different
pressures of CO in the cell. Normally, the
3
fluorescence signal, S, in a two-photon excitation
processes depends quadratically on the incident laser
intensity, I, i.e. S ∼ I k , where k = 2. However, at
higher laser energies photo-ionization of the CO
molecule becomes significant. Moreover, the
transition starts to saturate and also at higher laser
energy two-photon absorption decreases the signal
intensity. Hence, the fluorescence signal scales with k
= 2 in the low laser intensity limit, and turns into k is
close to one when the laser intensity is increased. If
the laser intensity is further increased the signal goes
through a maximum until the signal actually
decreases as the laser intensity is increased further.
Figure 5. LIF signal dependence on the buffer N2 pressures (100 mbar CO, N2 pressure varied).
In order to estimate the delectability of using
two-photon LIF at elevated pressure, e.g. in engines,
measurements in the cell with and without buffer
gases were carried out at pressures from 0.1 bar up to
8 bar. It is more realistic to investigate the CO LIF
signal intensity while adding a buffer gas to a
constant pressure of CO in the cell. Shown in Figure
5 is the signal distribution from CO at a pressure of
100 mbar while the pressure of the buffer gas, N2,
was varied from 0 to 10 bar. As expected the signal
decreases as the pressure is increased. Also the shape
of the signal distribution changes as the pressure
increases.
In the flame investigations, the behaviour of
spectral interference from non-resonantly excited C2
molecules, produced by photo-decomposition of fuel
and fuel fragments, were studied at various
stoichiometries and laser intensities. The first flame
measurement was performed in a laminar, premixed,
methane/air flame. During the flame measurements
the cell was replaced by a premixed Bunsen-type
conical burner, which has a prolonged intake duct in
order to assure a laminar flow. Through a
spectrometer (Acton, SpectraPro-150) equipped the
ICCD camera, a spectral resolved CO LIF spectrum,
as shown in Figure 6, was recorded for a Φ = 1.5
methane/air flame with 50 laser shots averaged. As
shown in the Figure, some C2 interference lines are
clearly shown.

C 2 interferences
(c 1 Π g – b 1 Π u)
(d 3 Π g – a 3 Π u)
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
Distance (mm)
400
450
500
550
600
650
Wavelength (nm)
Figure 6. A CO fluorescence spectrum from a
methane/air (φ=1.5) burner is shown above. The C 2
lines around 474 nm and 520 nm are visible.
Engine measurements
The measurements in the cell and the premixed
flame were followed by measurements in a small
single-cylinder four-stroke Briggs & Stratton engine
[23]. The engine featured a side-valve design with the
spark plug located above the exhaust valve. The
original crankcase, cylinder liner and piston remained
unaltered, while the cylinder head was modified to
allow for optical access to the combustion chamber.
Three windows were mounted in the cylinder head
for this purpose. Two vertical windows, one on the
side of the combustion chamber, made it possible to
have a horizontal laser sheet passing through it. The
induced fluorescence was imaged through a third
window mounted horizontally above the two valves.
A picture taken from above, through the top window,
is shown in Figure 7. In this image the intake valve
can be seen to the left and the exhaust valve to the
right. The spark plug electrodes are also visible in the
lower right corner. The field of view for the ICCD
camera is also shown in this figure, where the outer
white frame illustrates the area covered during the
flame chemiluminescence imaging and the inner
frame illustrates the area covered during the CO LIF
imaging.
Isooctane was used as the fuel for the engine
measurements. Shown in Figure 8 are single-shot
images of the flame chemiluminescence. These
images are recorded at subsequent crank angle
positions to cover the different parts of the engine
cycle. The exposure time for the ICCD camera was
set to 5 µs, which corresponded to a crank shaft
rotation of less than 0.04 crank angle degrees (CAD).
At Top Dead Center (TDC), a developing flame
kernel originating from the vicinity of the spark plug
can be seen; 5 CAD after TDC, the flame is more
spread out from the spark plug; the flame then
continues to expand as expected and as shown by the
images recorded at 15 and 30 CAD after TDC.
4
Figure 7. Vision of the engine chamber from the top
window. Outer write frame indicate the chemiluminescence
collection area; inner write frame
indicates the LIF imaging area.
300 600 900 400 1000 1800 2400
TDC
15 TDC
5 TDC 30 TDC
Figure 8. Flame chemiluminescence emission taken
with 5 microsecond gate time.
During the engine LIF experiments a singlemode
Nd:YAG laser (Spectra Physics, PRO 290-10)
pumping a OPO laser system (Spectra Physics,
MOPO 730-10) was utilized to produce the required
230 nm laser radiation. When pumped with 550 mJ
per pulse at 355 nm, the OPO delivered 75 mJ per
pulse at 460 nm. After frequency doubling in a BBO
crystal, laser radiation at 230 nm with a pulse energy
of 15 mJ was obtained. For this setup the 230 nm
laser has an estimated linewidth of ~ 0.3 cm -1 . The
230 nm beam was formed into a 12 mm wide laser
sheet with a thickness of approximately 300 µm. The
laser sheet was sent through the combustion chamber
right above the valves as indicated in Figure 9. The
ICCD camera was used for detection of the LIF
through the top window perpendicular to the laser
sheet. A long-pass filter was used for suppressing the
scattered laser radiation while transmitting the redshifted
fluorescence. The engine was run at 1200 rpm
corresponding to a firing frequency of 10 Hz
matching the repetition rate of the laser system.
Setting the engine as master, a trigger signal for each
engine cycle, locked at a selected crank angle
position, was sent to a pulse-delay generator
(Stanford, DG535), which was used to synchronize
the laser system and the ICCD camera. This enabled

LIF images to be recorded at any desired crank
angles of the engine cycle. The integration time for
the ICCD camera was set to 50 ns in order to
minimize the flame chemiluminescence background.
200 300 400 500 600 700 800 900 1000 1100 1200
TDC
5 TDC
10 TDC
20 TDC
30 TDC
Figure 9. CO TPLIF images at different crank angles.
Single-shot CO PLIF images from the engine were
recorded at subsequent crank angles, examples are
shown in Figure 9. At TDC just before the ignition, a
faint CO LIF image was recorded. This small amount
of CO originates from the exhaust gas of the previous
cycle, remembering this is a side valve engine with
high amounts of residuals. At 5 and 10 CAD after
TDC, the increased intensity and spread out CO LIF
distribution reveal the flame location. At 20 and 30
CAD after TDC, the CO PLIF images show a more
intense signal in the left part of the combustion
chamber (furthest away from the spark plug) and
only a weak signal in the right part indicating the
fading away of the reaction zone of the flame.
Compared with the flame chemiluminescence images
shown in Figure 8, one can find that although a
strong flame chemiluminescence was observed at 30
CAD after TDC the CO LIF had already decreased. It
should be kept in mind that the recording of
chemiluminescence is a line-of-sight technique
integrating across the entire thickness of the
combustion chamber whereas the PLIF technique
monitors a thin slice in the middle of the combusting
volume. The results indicate that the CO
concentration reaches the highest value in the flame
reaction zone in this high pressure engine combustion
environment. This is in agreement with the expected
scenario where CO is formed as an intermediate
species when the fuel is decomposed and then
consumed as the temperature increases. At high
5
temperature and pressure, the interference from
photolysis of hot CO2 molecule [24] can be a
problem in the CO LIF detection. From the fact that
only weak CO LIF was detected at 30 TDC, where
high temperature are revealed from the
chemiluminescence measurement, one can judge that
the hot CO2 photo-fragmentation are of little concern
in this experiment. This might be due to the high
nascent CO concentration in the engine flame.
Intensity (arb. Units)
Intensity (arb. units)
Intensity (arb. units)
Intentisty (arb. units)
6 x104
5
4
3
2
1
0
7 x104
6
5
4
3
2
1
0
6 x104
5
4
3
2
1
0
14 x104
12
10
8
6
4
2
scattered 460 nm laser light
emission from C2
5 degree ATDC
15 degree ATDC
scattered 460 nm laser light
25 degree ATDC
scattered 460 nm laser light
a
b
c
20 degree BTDC
exhaust cycle
scattered 460 nm laser light
d
0
450 500 550 600 650 700
Wavelength (nm)
Figure 10. Spectra of TPLIF of CO from engine
chamber.
In order to clarify any other possible
interference in the recorded CO LIF images, spectral

investigations of the fluorescence emission from the
engine were performed. The CO LIF was sent
through a spectrometer (Acton, SpectraPro-150) and
detected with the ICCD camera. Shown in Figure 10
are CO LIF spectra from the engine recorded at
different crank angles. A spectral line at 460 nm,
which is residuals from the OPO signal beam, is
evident in every spectrum. The relative intensity of
this line can be used as a measure of the CO LIF
intensity. The emission from C2 can introduce strong
interference in CO LIF measurements especially in
fuel rich flames. In Figure 10 (a), the strongest C2
line showed only a very small peak, which indicates
that the C2 interference were negligible in the
presented measurement. Shown in Figure 10 (d) is a
CO spectrum recorded at 20 CAD before TDC in the
exhaust stroke, hence, the detected CO LIF signal
originates from the CO in the exhaust gases. The low
temperature and low pressure at this crank angle may
explain the relatively intense CO LIF spectrum
observed there, as indicated in Figure 5 there is a
strong pressure dependence of the LIF signal. One
might also be encouraged to use CO LIF technique to
study engine exhaust gas recycling (EGR)
distributions by performing single-shot 2D EGR
measurements.
Summary
Two-photon LIF detection of carbon monoxide
has been performed in a cell at elevated pressure, in
flames with different stoichiometric and in an engine.
The dependence of the LIF signal on pressure and
excitation laser power density was investigated.
Despite the complex involving photo-ionization,
quenching, absorption etc, a general understand of
the behaviour of the CO LIF signal was achieved. In
flame measurements, C2 interference was clearly
observed especially in rich flames. Finally in the
engine measurements, single-shot, two dimensional
CO imaging was achieved. With the spatial- and
spectral- resolved measurements, interference both
from C2 and from hot CO2 photo fragmentation are
excluded. To the best of our knowledge, this
represents the first single shot CO imaging in
combustion engines.
Acknowledgments
This work is supported by the Swedish Science
Council (VR) and The Swedish Energy
Administration (STEM).
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