Fluorescence spectroscopy of aromatic species produced in rich premixed ethylene flames

Article abstract of DOI:10.1016/S0045-6535(00)00258-7

The fluorescence spectra of the condensed species (CS) collected in the soot inception region of a rich premixed laminar ethylene/oxygen flame have been measured by excitation in the UV at 266 and 355 nm excitation wavelength. The contribution of the most abundant polycyclic aromatic hydrocarbons (PAH) to the CS fluorescence has been evaluated in order to attribute the CS fluorescence at different emission wavelengths to specific aromatic structures. The fluorescence peaks detected in the UV region of the CS fluorescence spectrum was found to be mainly due to a typical PAH like fluorene, that is, the most fluorescent one among the PAH analyzed in the CS by chromatographic analysis, The CS exhibited the larger emission in the visible where the PAH contribution has been shown to be negligible and other fluorescing aromatic species, not identified by chromatographic analysis of the CS, have to be considered responsible for the visible fluorescence. Laser induced fluorescence (LIF) flame measurements excited at 266 nm and detected at two selected wavelengths (310 and 410 nm) have been performed along the flame axis and compared with the CS fluorescence intensity. The LIF and CS fluorescence signals show quite similar axial trends demonstrating that the LIF signals are related to CS fluorescence. In particular, the LIF fluorescence signals detected in the UV could be attributed to the PAH fluorescence whereas the unidentified species contained in the CS can be followed by LIF detection in the visible region.

Full text of DOI:10.1016/S0045-6535(00)00258-7

Chemosphere 42 (2001) 835±841
Fluorescence spectroscopy of aromatic species produced in rich
premixed ethylene ¯ames
a,
a
b
a
b
A. Ciajolo *, R. Ragucci , B. Apicella , R. Barbella , M. de Joannon ,
a
A. Tregrossi
a
Istituto di Ricerche sulla Combustione, CNR, Pl.e V. Tecchio 80, 80125 Napoli, Italy
Dipartimento di Ingegneria Chimica, Universitaꢀ di Napoli Federico II, Pl.e V. Tecchio 80, 80125 Napoli, Italy
b
Abstract
The ¯uorescence spectra of the condensed species (CS) collected in the soot inception region of a rich premixed
laminar ethylene/oxygen ¯ame have been measured by excitation in the UV at 266 and 355 nm excitation wavelength.
The contribution of the most abundant polycyclic aromatic hydrocarbons (PAH) to the CS ¯uorescence has been
evaluated in order to attribute the CS ¯uorescence at di€erent emission wavelengths to speci®c aromatic structures. The
¯uorescence peaks detected in the UV region of the CS ¯uorescence spectrum was found to be mainly due to a typical
PAH like ¯uorene, that is, the most ¯uorescent one among the PAH analyzed in the CS by chromatographic analysis.
The CS exhibited the larger emission in the visible where the PAH contribution has been shown to be negligible and
other ¯uorescing aromatic species, not identi®ed by chromatographic analysis of the CS, have to be considered re-
sponsible for the visible ¯uorescence. Laser induced ¯uorescence (LIF) ¯ame measurements excited at 266 nm and
detected at two selected wavelengths (310 and 410 nm) have been performed along the ¯ame axis and compared with the
CS ¯uorescence intensity. The LIF and CS ¯uorescence signals show quite similar axial trends demonstrating that the
LIF signals are related to CS ¯uorescence. In particular, the LIF ¯uorescence signals detected in the UV could be
attributed to the PAH ¯uorescence whereas the unidenti®ed species contained in the CS can be followed by LIF
detection in the visible region. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Fluorescence; PAH
1. Introduction
haust gases, etc. However, these are time-consuming and
intrusive techniques and the current trend in diagnostic
research aims to the implementation of non-intrusive
on-line high spatial and high temporal resolution
multi-species detection methods. Optical diagnostics
techniques have the potentialities to satisfy these re-
quirements and in particular, methods based on ¯uo-
rescence spectroscopy techniques seem to be the most
promising ones for PAH analysis.
Polycyclic Aromatic Hydrocarbons (PAH) hold a
twofold importance: as micropollutants and as possible
precursors in soot formation (Haynes and Wagner,
1981; Barfknecht, 1983). Probing and o€-line determi-
nation of PAH represent at the moment the more sen-
sitive and speci®c techniques suitable for the
quali®cation and quanti®cation of single PAH in hostile
combustion environments such as ¯ames, furnaces, ex-
Broadband ¯uorescence has been measured by laser
induced ¯uorescence (LIF) in rich sooting premixed
¯ames (Haynes et al., 1980) and in di€usion ¯ames in the
spatial regions closer to the regions where soot is present
(Smith et al., 1985). This ¯uorescence has been often
attributed to PAH since they are, indeed, the most
* Corresponding author. Tel.: +390-81-768-2252; fax: +390-
81-593-6936.
E-mail address: ciajolo@irc.na.cnr.it (A. Ciajolo).
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 0 0 ) 0 0 2 5 8 - 7

836
A. Ciajolo et al. / Chemosphere 42 (2001) 835±841
¯uorescing species typically detected in rich ¯ames.
Combined optical and sampling techniques applied in
di€usion ¯ames corroborated this attribution indicating
the coincidence of the PAH concentration peak and
¯uorescence intensity (Barbella et al., 1982; Prado et al.,
1984).
2. Experimental
CS were isokinetically sampled by means of a stain-
less steel water-cooled probe (i.d. ˆ 2 mm) on an atmo-
spheric-pressure rich premixed C2H4/O2 ¯ame (C/
O ˆ 0.8; v0 ˆ 4 cm/s). The ¯ame was stabilized on a
water-cooled sintered bronze burner (d ˆ 60 mm) sur-
rounded by a shroud of nitrogen.
The correlation between ¯uorescence intensity and
PAH concentration has been also found in premixed
benzene ¯ames whereas in ¯ames of aliphatic hydro-
carbons the ¯uorescence intensity could not be
straightforward related to PAH presence since ¯uores-
cence signal increases downstream of the ¯ames much
more rapidly than the PAH concentration (Haynes et al.,
1980).
CS collected on the probe wall, in a te¯on ®lter and in
an ice trap placed before the ®lter were extracted by
dichloromethane (DCM) in order to separate the CS
that are soluble in DCM from soot.
PAH up to 300 u were quanti®ed in the CS by gas-
chromatography±mass spectrometry (GC±MS) on a
HP5890 gas-chromatograph coupled with an HP5970
mass spectrometer.
Most of the early work on LIF in rich ¯ames has
been done by using excitation with visible light and the
LIF signals have been attributed to the PAH usually
detected in ¯ames even though most of these PAH do
not ¯uoresce in the visible region (Berlman, 1971). Some
experiments on acenaphthylene and pyrene ¯uorescence
in ¯ames have further indicated that these PAH, and in
particular acenaphthylene at high temperature, can be
responsible for the broad band ¯uorescence in the visible
(Coe and Steinfeld, 1980; Coe et al., 1981).
UV±visible spectra of CS dissolved in DCM were
measured on an HP 84523 diode array spectrophotom-
eter.
Emission ¯uorescence spectra of standard PAH and
CS dissolved in DCM were measured on a Perkin±Elmer
LS-50B spectro¯uorimeter connected to a personal
computer. A xenon discharge lamp equivalent to 20 kW
for 8-ls duration was used as excitation light source.
The detection device was a gated photomultiplier with
modi®ed S5 response for operation to about 650 nm.
Monochromators were Monk±Gillieson type that covers
the following ranges: excitation 200±800 nm, emission
200±900 nm. The wavelength accuracy was Æ1:0 nm and
the wavelength reproducibility was Æ0:5 nm. Instru-
mental parameters were controlled by the Fluorescence
Data Manager Perkin Elmer software.
More recently, the ¯uorescence measured at di€erent
UV and visible excitation wavelengths has provided
more structured spectra whose shapes have been related
to di€erent speci®c aromatic compounds (Beretta et al.,
1985; Petarca and Marconi, 1989). Moreover, the use of
laser sources at 266 nm of excitation wavelength has
provided bimodal ¯uorescence spectra with a sharp peak
located in the UV ( 320 nm) and a broad emission in
the visible (D'Alessio et al., 1992).
All the standard compounds used were commercially
available (Aldrich) and used without further puri®ca-
tion. The ¯uorescence measurements of standard PAH
and CS were performed on very dilute DCM solutions
(10 lg/l) in order to avoid concentration quenching and/
or other phenomena that could distort the spectral shape
a€ecting the ¯uorescence intensity and quantum e-
ciency.
Fluorescence-based diagnostic for the determination
of PAH species presence and, possibly, of their con-
centration relies on the knowledge and availability of as
much as possible precise correlation between the pres-
ence of a speci®c PAH species and spectroscopic features
observed in practical combustion systems. Exploitation
of both optical and sampling techniques in a combined
way can greatly contribute to increase such knowledge.
In this paper, the ¯uorescence features (spectrum
shape, intensity and quantum eciency) of the con-
densed species (CS) collected in the soot inception region
of a rich premixed ethylene/oxygen ¯ame are reported.
The contribution of the most abundant PAH to the CS
¯uorescence has been evaluated by measuring the ¯uo-
rescence of single PAH, analyzed by gas-chromatogra-
phy of the CS, in order to attribute the CS ¯uorescence
at di€erent emission wavelengths to speci®c aromatic
structures.
LIF signals have been measured in ¯ame using as
exciting radiation the fourth harmonic of a Nd±YAG
laser (k ˆ 266 nm). The optical diagnostic used was a bi-
dimensional implementation of a simple 90° LIF col-
lection scheme. To this aim the laser beam was shaped as
a thin (200 lm) sheet by using a cylindrical telescope and
a focussing lens. The laser sheet (1.5 cm tall) was focused
in the center of the burner where typical laser ¯uence
ranged between 10 and 10 W/cm2. The probe vol-
ume was de®ned as the intersection of the laser sheet and
the image (magni®ed 1:2) of the entrance slit of a
polychromator. The slit width was ®xed in all the text at
200 lm, which resulted in a reasonable trade-o€ between
level of collected signal and spectral resolution. The
holographic grating used was a 300 g/mm one. This
produced on the ICCD detector 256 spectra made of
6
7
The comparison of the CS ¯uorescence with the LIF
signals measured in ¯ame at 266 nm of excitation
wavelength has been performed to explore the possibil-
ities of interpreting and attributing LIF signals to spe-
ci®c aromatic structures.

A. Ciajolo et al. / Chemosphere 42 (2001) 835±841
1024 pixels over a 260-nm wavelength range. The real
837
800
600
400
200
0
Condensed Species
spectral resolution of the system was limited by the in-
tensi®er resolution and was evaluated to be lower than 1
nm. Spectral and spatial calibration was performed by
dividing collected images by a reference image built up
using reference lamps and correcting for the laser sheet
intensity pro®le.
λ
= 266 nm
exc
composed
(yPAH=1)
For each test run 500 images were collected in the
same experimental condition and averaged in order to
smooth away laser pulses energy oscillation and to in-
crease the signal-to-noise ratio of measured spectra.
Spectra at a de®ned spatial position were obtained by
averaging 10 adjacent spectra around the chosen posi-
tion. This resulted in a lower spatial resolution, but both
greatly reduced uncertainties in system positioning and
increased signal quality. The ®nal spatial resolution
achieved by using this technique was lower than 0.6 mm
which is by far better than that typical of sampling
systems.
measured
composed (y
350
PAH=0.29)
300
400
450
500
Wavelength, nm
Fig. 1. Measured and composed ¯uorescence spectra
(kexc ˆ 266 nm) of CS collected at z ˆ 4 mm HAB of a premixed
ethylene/oxygen ¯ame (C/O ˆ 0.8). The composed spectra have
been evaluated by considering the real mass contribution to CS
(yPAH ˆ 0:29) and assuming that CS are totally constituted of
PAH (yPAH ˆ 1).
3. Results and discussion
3.1. Fluorescence spectra of condensed species
tions of each PAH to the CS ¯uorescence have been
summed in order to obtain a spectrum representative of
the total contribution of two- to seven-ring aromatic
systems to the CS ¯uorescence and this spectrum will be
referred to as ``composed spectrum''.
The ¯uorescence spectra of the CS collected along a
premixed rich ethylene/oxygen ¯ame (C/O ˆ 0.8, v0 ˆ 4
cm/s, Tmax ˆ 1720 K at 1.5 mm from the burner) have
been measured at two di€erent UV excitation wave-
lengths (266 and 355 nm) that are typical outputs of a
Nd±YAG laser.
For the CS sample collected at 4 mm height above
the burner the total mass contribution of PAH to the CS
mass was found to be 29 wt.% and the relative composed
spectrum is also reported in Fig. 1, denoted with
yPAH ˆ 0:29. It can be noted that the CS measured
spectrum presents the larger emission in the visible re-
gion whereas the composed PAH spectrum exhibits the
larger emission region in the 300±350 nm wavelength
region. Overall, the composed PAH spectrum partially
contributes to the CS spectrum and the PAH contribu-
tion is lower as higher the emission wavelength is, i.e., in
the visible region.
The shape of the ¯uorescence spectra of the CS col-
lected in the soot inception region of the ¯ame (from 3 to
7 mm height above the burner) did not signi®cantly
change along the ¯ame. This has been already shown in
previous work (Ciajolo et al., 1998) and just a relative
enhancement of the visible portion of the emission
spectrum at higher ¯ame heights could be observed. For
this reason the ¯uorescence spectra reported in the fol-
lowing are relative to the CS sample collected at 4 mm
height above the burner chosen as a representative
sample.
Assuming that CS be totally constituted of PAH
having the same distribution as analyzed by GC±MS,
the composed spectrum has been also evaluated and
reported in Fig.1 denoted with yPAH ˆ 1. It can be ob-
served that even with this hypothesis, the visible features
of the measured spectrum are not reproduced indicating
that other aromatic species should be responsible for the
visible ¯uorescence. On the opposite, in the UV region
the composed spectrum largely overcomes the measured
one.
Fluorescence emission excited in the UV region
(k ˆ 266 nm) of the CS collected at 4 mm height above
the burner (the spectrum is relative to a CS concentra-
tion of 0.1 lg/l in DCM solution) is shown in Fig. 1 in
the 280±500 nm wavelength region. The ¯uorescence
emission spectrum of CS exhibits a broad shape and
some peaks emerge on the two main emission regions:
the ®rst one located between 300 and 350 nm and a
second larger one extending from 350 to 500 nm.
The contribution of each PAH to the ¯uorescence of
the CS has been evaluated by multiplying its mass con-
tribution to the CS, as analyzed by GC±MS, by the
relative emission spectrum measured dissolving standard
PAH solutions at known concentration. The contribu-
The reliability of the spectra composition procedure
above described has been veri®ed on a synthetic mixture
prepared by mixing standard solutions of the most
abundant PAH detected in the CS that include 18 PAH

838
A. Ciajolo et al. / Chemosphere 42 (2001) 835±841
species from naphthalene to coronene. The ¯uorescence
spectrum of this mixture has been measured and re-
ported in Fig. 2. The same composition procedure has
been carried out for this mixture starting from the ¯u-
orescence spectra of standard solutions of the single
PAH and the composed spectrum relative to this mix-
ture is also reported in Fig. 2.
ture. These e€ects cause the reduction of the UV ¯uo-
rescence. For instance, it has been previously found that
a radiative energy transfer mechanism (Parker, 1968)
involving ¯uorene and some PAH (pyrene, benzo(e)py-
rene, etc.) can occur that causes the reduction of the
¯uorene emission when admitted in mixture with other
PAH in solution (Latz et al., 1978; Katoh et al., 1980).
Moreover, the e€ect of heavy atom (chlorine) present in
the solvent used for the measurements (DCM) (Lloyd,
1974) is known to a€ect the ¯uorescence of PAH in
solution (Lakowicz, 1983). The occurrence of these ef-
fects on the UV ¯uorescence of CS samples has to be
taken into account in the comparison of CS ¯uorescence
with LIF spectra measured in ¯ame.
First of all, it can be observed that the measured
spectrum of the synthetic PAH mixture is well structured
and does not show the broad features observed in the CS
spectrum (Fig. 1). It is also evident that the composition
procedure works quite well in the visible range where
four-, ®ve-ring PAH give their contribution whereas the
composed spectrum overcomes the measured one in the
UV portion of the spectrum.
As the excitation wavelength shifts towards the visi-
ble, the contribution of the PAH to the CS ¯uorescence
becomes practically negligible. This is shown in Fig. 3
where the ¯uorescence emission of the CS excited at 355
nm (typical output of the Nd±YAG laser) is reported
along with the spectra composed considering the real
mass contribution of PAH (yPAH ˆ 0:29) and consider-
ing the CS totally composed of PAH (yPAH ˆ 1). Two-,
three-ring PAH do not signi®cantly ¯uoresce in this re-
gion and it is also noteworthy that the ¯uorescence in-
tensity measured by excitation at 355 nm is lower than
that measured by excitation at 266 nm, even though the
intensity of the excitation wavelength was quite con-
stant. The ¯uorescence emission intensity at 355 nm has
been found lower also for the synthetic PAH mixture
whose measured and composed spectra excited are re-
ported in Fig. 4. It can be also noted that the measured
By evaluating the single contribution of PAH to the
total ¯uorescence, it has been assessed that the emission
in the UV region is typically due to light, mainly two-
ring, PAH that are known to be strongly ¯uorescing in
this region (Berlman, 1971). In particular, it has been
found that ¯uorene is the main PAH contributor to the
UV ¯uorescence of both the PAH mixture and CS
samples. This is due to the very high quantum eciency
of ¯uorene (0.76 as measured in this work) with respect
to the other light PAH like naphthalene, acenaphthene,
¯uorene, biphenylene, present in the PAH mixtures that
typically have a quantum eciency of about 0.1 (our
measurements and Berlman, 1971).
The overestimation of the ¯uorescence peak in the
UV region when recomposing the PAH mixture spec-
trum (Fig. 2) is due to the fact that the composition
procedure, starting from the ¯uorescence of the single
PAH solutions, does not take into account the complex
e€ects occurring between PAH when admitted in mix-
Condensed Species
120
λ
= 355 nm
exc
measured
2500
PAH mixture
λ
= 266 nm
exc
80
40
0
2000
1500
1000
500
0
composed
composed
(yPAH=1)
composed (yPAH=0.29)
400
450
500
550
600
Wavelength, nm
measured
Fig. 3. Measured and composed ¯uorescence spectra
(kexc ˆ 355 nm) of CS collected at z ˆ 4 mm HAB of a premixed
ethylene/oxygen ¯ame (C/O ˆ 0.8). The composed spectra have
been evaluated by considering the real mass contribution to CS
(yPAH ˆ 0:29) and assuming that CS are totally constituted of
PAH (yPAH ˆ 1).
300
350
400
450
500
Wavelength, nm
Fig. 2. Measured and composed ¯uorescence spectra
(kexc ˆ 266 nm) of a synthetic standard PAH mixture.

A. Ciajolo et al. / Chemosphere 42 (2001) 835±841
839
sured in ¯ames by LIF measurements at 266 nm of ex-
citation wavelength (D'Alessio, 1994). This discrepancy
could be due to the reduction of the quantum eciencies
in high temperature ¯ame environment or to the pres-
ence in ¯ame of other absorbing, but not ¯uorescing
compounds like soot unsaturated light hydrocarbons
etc. that are not included in the CS.
800
600
400
200
0
PAH mixture
λ
= 355 nm
exc
composed
3.2. Pro®les of CS ¯uorescence and LIF intensity in the
soot inception region
Axial pro®les of the LIF intensity measured in ¯ames
have been considered to be related to the PAH con-
centration pro®les (Haynes et al., 1980; Prado et al.,
1984; Barbella et al., 1990). The approach used in this
work is the comparison of the LIF ¯uorescence intensity
measured along the ¯ame with the ¯uorescence intensity
pro®les due to the CS collected along the ¯ame. The
signal intensity pro®les have been measured by excita-
tion at 266 nm and detection at 310 and 410 nm emission
wavelengths. The CS ¯uorescence intensity has been
evaluated by multiplying the ¯uorescence intensities of
the CS samples by the solvent volume in which the in-
tensities are measured and dividing by the volume of
combustion gases sampled to obtain that CS amount.
The CS ¯uorescence intensity at 310 nm that as
mentioned before corresponds to a typical ¯uorescence
peak of ¯uorene is reported in Fig. 5 as a function of the
¯ame height. It can be noted that the ¯uorescence signal
steeply increases between 2 and 4 mm thereafter reach-
ing an almost constant value. The CS ¯uorescence in-
tensity signal detected in the visible at 410 nm, that is,
another typical ¯uorescence peak of both CS (Fig. 1)
and PAH (Fig. 2) is also reported in Fig. 5. Di€erent
measured
400
450
500
550
600
Wavelength, nm
Fig. 4. Measured and composed ¯uorescence spectra
(kexc ˆ 355 nm) of a synthetic standard PAH mixture.
and composed spectra of the synthetic PAH mixtures
are coincident.
Overall, the comparison of the ¯uorescence spectra,
measured by excitation with 266 and 355 nm wave-
lengths demonstrates that the better response in terms of
number of ¯uorescing species and of signal intensity is
obtained by using excitation more shifted toward the
UV. This statement can be extended also to LIF mea-
surements.
The quantum eciency of DCM solutions of the CS
has been measured by comparison with a standard
species (9,10-diphenyl anthracene) having a known
quantum eciency (Berlman, 1971) and the results are
reported in Table 1. It can be observed that the quantum
eciency of the CS is very low at the beginning of the
¯ame and rapidly increases along the ¯ame reaching an
almost constant value. It is worth underlining that the
quantum eciency of the CS is lower than that of PAH
that typically varies from 0.1 to 0.8 in dependence of the
speci®c PAH. However, the CS quantum eciency is
comparable to the quantum eciency evaluated by LIF
¯ame measurements at 488 nm of excitation wavelength
(Muller-Dethlefs, 1979), but is higher than that mea-
15E+5
Condensed Species
λexc= 266 nm
λem=410 nm
10E+5
Table 1
Quantum eciency of the CS at various height above the
burner
5E+5
λem=310 nm
z, mm
kexc ˆ 266 nm
kexc ˆ 355 nm
2
2
2.5
3
3.2 ´ 10
6.8 ´ 10
7.4 ´ 10
7.6 ´ 10
8.0 ´ 10
6.1 ´ 10
5.4 ´ 10
9.3 ´ 10
1.0 ´ 10
1.1 ´ 10
1.1 ´ 10
9.5 ´ 10
2
2
2
1
0
3.5
4
6
8
0
2
4
2
1
2
1
Height Above the Burner, mm
5
2
2
6
Fig. 5. Axial pro®les of CS ¯uorescence intensity detected at
310 and 410 nm of emission wavelength.
2
1
7
7.9 ´ 10
1.3 ´ 10

840
A. Ciajolo et al. / Chemosphere 42 (2001) 835±841
4. Conclusions
LIF
0.4
The ¯uorescence features of the CS collected in the
Premixed Flame
C/O=0.65
soot inception region of a rich premixed ethylene/oxygen
¯ame have been measured. The contribution of the most
abundant PAH to the CS ¯uorescence has been evalu-
ated by measuring the ¯uorescence of single PAH, an-
alyzed by GC±MS of the CS in order to attribute the CS
¯uorescence at di€erent emission wavelengths to speci®c
aromatic structures.
v =4 cm/s
o
0.3
0.2
0.1
0
λ
=410 nm
em
The better response, in terms of number of ¯uoresc-
ing species and of signal intensity, is obtained by using
excitation more shifted toward the UV and this state-
ment can be extended also to LIF measurements.
It has been found that the ¯uorescence peaks de-
tected in the UV region of the spectrum, where typically
two-ring PAH ¯uoresce, are mainly due to ¯uorene that
is, among the ¯ame measured PAH, the most ¯uorescent
one.
λ
=310 nm
em
0
2
4
6
8
Height Above the Burner, mm
Fig. 6. Axial ¯ame pro®les of LIF signals detected at 310 and
410 nm of emission wavelength.
The CS exhibit the larger emission in the visible
where the PAH contribution has shown to be negligible
and other ¯uorescing aromatic species, not identi®ed by
GC±MS analysis of the CS, have to be considered re-
sponsible for the visible ¯uorescence.
from the signal detected at 310 nm of emission wave-
length, the emission signal detected at 410 nm steeply
rises along the ¯ame even though a change of the slope
can be observed at soot inception occurring after 4 mm
of height above the burner.
LIF ¯ame measurements excited at 266 nm and
detected at two selected wavelengths (310 and 410 nm)
have been performed and compared with the CS ¯u-
orescence intensity. The LIF and CS ¯uorescence sig-
nals show quite similar axial trends demonstrating that
the LIF signals are related to CS ¯uorescence. In
particular at 410 nm, the unidenti®ed heavy species
contained in the CS seem to be the main responsible of
LIF signals.
The LIF signals at 310 and 410 nm are reported as a
function of the ¯ame height in Fig. 6. Similar to the CS
¯uorescence signals, the LIF signal at 310 nm shows a
small increase at the beginning of the ¯ame reaching a
quite constant value downstream of the ¯ame. Also the
LIF signal intensity detected at 410 nm shows a similar
trend to that measured for the CS steeply increasing
along the ¯ame.
The similarity of the trends of LIF signals with the
¯uorescence measured on the CS corroborates the hy-
pothesis of the attribution of the LIF signals to the CS.
In particular, at 310 nm the LIF ¯uorescence signal
could be attributed to the PAH ¯uorescence since it has
been previously shown that at this wavelength the CS
¯uorescence is mainly due to a typical PAH formed in
¯ame like ¯uorene. At 410 nm it has been shown that the
contribution of PAH to the CS ¯uorescence is negligible,
consequently other aromatic and ¯uorescing species
should be responsible for the visible ¯uorescence and
these species could be detected by following the LIF
signal at 410 nm.
References
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Overall, the similarity of LIF and CS intensity axial
pro®les indicates that CS formed along the ¯ame are the
main contributors to the LIF ¯ame signals. It is note-
worthy that this similarity occurs in spite of the di€erent
temperature, concentration, and chemical characteristics
of the environment in which the CS ¯uorescence (am-
bient temperature, solvent presence) and the LIF ¯uo-
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