Laser Induced Fluorescence

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Laser Induced Fluorescence

• Zhenjun Zhang

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Definition of LIF

• Laser-induced fluorescence (LIF) is the optical
  emission from molecules that have been excited to
  higher energy levels by absorption of
  electromagnetic radiation.

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What Can LIF Do?

• studying structure of molecules

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What Can LIF Do?

• flow visualization and measurements

Measure the velocity of the liquid by taking a
series of pictures
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Advantages of Laser-induced Fluorescence

• 1. to get two- and three-dimensional images
  since fluorescence takes place in all directions
• 2. signal-to-noise ratio of the fluorescence
  signal is very high
• good sensitivity
• 3. to distinguish different species, since the
  lasing wavelength can be tuned to a particular
  excitation of a given species which is not shared
  by other species.

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Viable Candidate for LIF Detection

• 1. molecule/atom have appreciably populated
  states and this can be achieved by the laser we
  are using
• 2. the spectrum of interest must be sufficiently
  analyzed to permit unambiguous identification
• -Some molecules will be unsuitable because of
  poor fluorescence yields owing to predissociation
  in the excited state

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What Kind of Laser Could be Used

• Tunable dye lasers
• Nd-YAG pumped dye laser
• produce 1024nm light source
• then doubled, tripled or
• quadrupled

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What Kind of Laser Could be Used

• Excimer lasers
• XeF(351nm),XeCl(308nm) etc are usually used
• spectral width - 0.3nm
• pulse width 10ns

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LIF V.S Absorption Measurements
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Detection of Selective Species

• Formaldehyde is an important intermediate in the
  oxidation of hydrocarbons
• In methane combustion
• The decomposition of the methoxy radical
• 1. CH3O.M
• 2. CH3O.H.
• 3. CH3.O, O2, OH. or HO2.

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Other Techniques Utilized for Detecting
Formaldehyde

• Mass spectrometry-difficult
• Can not easily distinguish formaldehyde from
  ethane at m/e30 in hydrocarbon flames
• Gas chromatography-works
• Time consuming gas sampling required
• Analysis procedure is not easy

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Previous Study of Formaldehyde

• Dieke and Kistiakowsky-1934
• A1A2-X1A1 electronic transition
• First rotational analysis of polyatomic molecule
• C2V symmetry
• 6 normal vibrational modes
• it is widely studied since then

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Why Formaldehyde is Widely Studied

• 1. Large rotational constants
• 2. Give significant geometry change upon
  excitation
• So the rotational structure of the A-X transition
  is surprisingly open
• The spacing between the individual J,K
  transitions a few tenths of a wavenumber
• So it can be resolved using conventional pulsed
  dye lasers

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Basic Idea about LIF
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Basic Idea about LIF

• A-X 401 gives the strongest fluorescence band
• Could be detected in the region between 352-357 nm

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Measurement of Formaldehyde in High Temperature
-8 mm wide slot for methane -two16 mm slot for
air PMT-photomultiplier tube Laser
source-Molelectron MY35-10 Nd3YAG laser focused
with 100 mm spherical lens Create 355nm
radiation(15-60 mj/pulse)
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Measurement of Formaldehyde in High Temperature
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Dispersed Fluorescence Spectrum
-Methane raman transition -polycyclic aromatic
hydrocarbons (PHA) fluorescence broadening
-formaldehyde bands
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• A1A2-X1A1410 vibronic band is not be able to view
  by using this laser
• 2nm bandwidth is too large to observe PAH
  fluorescence independently
• mixed with 410 vibronic band which cause
  broadening
• better resolution is needed

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High Resolution Laser

• A dye laser is utilized to excite formaldehyde
  between 350 and 355nm

0.2-0.9 mJ/ pulse Bandwidth 0.5 cm-1 Two
filters Corning GG-395 long pass Corion LS-550
short pass
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410 Vibronic Band Fluorescence Spectrum
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410 Vibronic Band Fluorescence Spectrum

• RR3(9)
• ?K1, ?J1
• K3 , J9
• This band(28370.5 cm-1) provides maximum
  fluorescence intensity for on resonance data
  collection
• 28371.5 cm-1 gives the best profile for off
  resonance data collection

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Formaldehyde LIF Profiles
Each points is the average of 20 laser shots per
position In every 0.2 mm a point is taken
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Formaldehyde LIF Profiles

• From the study above, it turns out
• the strongest formaldehyde LIF signals are
  observed at the low level of the flame
• PAH background LIF signals are small
• PAH signal is easy to saturate using even a few
  mj/pulse
• Formaldehyde signal is linearly increased

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Two factors considered for correction Collisional
quenching Partitional function which correspond
to the Boltzmann population
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Conclusion

• The 401 vibronic band was observed via the laser
  induced fluorescence
• It shows promise for diagnostic applications for
  combustion conditions
• The fluorescence is quite strong for formaldehyde
  and it is reasonable to think it might be
  detectable in other hydrocarbon flames

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Question

• What is the advantage of LIF compare to
  absorption measurement?
• 1.higher sensitivity
• 2. low background

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Acknowledgement

• Dr.Cedeno
• Audience