LASER INDUCED PHOTOACOUSTIC SPECTROSCOPYLIPAS OF RARE EARTH IONS

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LASER INDUCED PHOTOACOUSTIC SPECTROSCOPY(LIPAS)
OF RARE EARTH IONS IN AQUEOUS SOLUTIONS

SHRUTI PADHEE 27 NOV 2008
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History

• 1881 A.G Bell proposed spectrophone ,to examine
  absorption spectra of bodies in those portions
  of the spectrum that are invisible.
• 1970s Allan Rosencwaig and Allen Gersho laid
  the basis for photo acoustic effect in solids
  called R-G theory

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What is photo acoustic (PA) spectroscopy?

• PA spectroscopy is an indirect absorption
  measurement.
• Here the absorption of light causes a change in
  the temperature or a parameter related to
  temperature (pressure or density) .
• The heating of the sample produces a PA signal
  correlated directly to absorbed electromagnetic
  energy.
• When temperature rise in the sample occurs faster
  than its volume can expand local pressure wave
  is generated (considered as sound signal)

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The Main principle
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Photo acoustic laser spectra vs Absorption
spectra

• PA is useful to measure extremely weak
  absorptions (forbidden transitions of rare earth
  ions in liquid).
• PA can detect radiation less decay processes.
• Spectra for a turbid or a solid sample can be
  obtained.
• Background noise can be decreased in PA
  spectroscopy.
• PA has better detection limit than Absorption
  spectroscopy.

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Experimental reagents

• Transition earth metals (Pr, Nd, Eu, Ho, Er)
• Rare earth nitrates were prepared by dissolving
  the oxides(99.9) in nitric acid.
• Then adjusted to a certain concentration with
  distilled water.

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Apparatus
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Apparatus contd

• Tunable dye laser pumped by argon ion laser
  (modulated at 200 Hz by light chopper)
• Dyes used Rhodamine(110,6G,B)-?-540-600,
  570-630, 600-670nm)
• Band width of dye laser-1cm-1
• Power of laser dye 300 mW.
• Pressure fluctuation detected by piezoelectric
  ceramic.
• 90mm cell length, all measurements at room
  temperature .

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Praseodymium ion(Pr3) _at_ 590nm

• E.configuration4f3 6s2
• Ion 4f2
• Transition from 3H4 to 1D2.
• Fine structure obtained as compared to absorption
  spectra.
• Splitting due to stark effect.
• the 1D2 splits ,µ _2,_1,0

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Neodymium ion(Nd3)_at_575 and 625 nm

• E.configuration4f4 6s2
• Ion 4f3
• Transitions4I9/2 to 2G ,4G(mixed) and 2H11/2 .
• Fine structures were seen in PA and not in
  absorption spectra.
• In aqueous solutions these ions undergo rapid
  radiation less relaxation.

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Europium ion(Eu3)_at_579nm

• E.configuration4f7 6s2
• Ion4f6
• Transition4F0 to 5D0
• Europium fluoresces a
• lot at 613nm therefore
• if conc. below 0.1 M are considered then PA
• spectrum cannot be clearly observed.

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Holmium ion (Ho3)_at_540 nm

• E.configuration 4f11 6s2
• Ion 4f10
• Transition 5I8 to 5F4 and 5S2
• Due to the limiting wavelength of dyes used PA
  could not be observed over a whole wavelength
  range.
• Only slight difference between the absorption and
  the PA spectra due to different cascade processes.

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Erbium ion(Er3)_at_542nm and 652nm

• E.configuration4f14 6s2
• Ion4f13
• Transition
• 542nm4I15/2 to 4S
• 652nm4I15/2 to 4F9/2
• Both spectra were not clear due to limiting
  lasing wavelength.

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Conclusions

• PA is a simple spectroscopy method .
• PA spectra can give spectral measurements of
  extremely weak absorption.
• Can be used for variety of samples in gases or
  in solid phase.
• Useful for substances with very high molar
  absorptivities.

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Advantages of having lasers as light source

• The PA signal is proportional to temperature rise
  in sample ,thus proportional to absorbed energy
  (pulse energy)
• Selectivity of a PA analysis depends on the
  bandwidth of excitation energy.

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Disadvantages

• Compounds that fluoresce a lot cannot be detected
  by the photo acoustic spectroscopy as the PA
  signal is directly proportional to the amount of
  heat released by the sample. (if there is more
  fluorescence then there is less heat released by
  the molecule, hence PA signal is not detected)

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Applications

• Quantification of soot particles in diesel engine
  exhaust gas.
• Analysis of highly concentrated textile dyes.
• Atmospheric pollution monitoring.

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Expected exam Question

• What are the advantages of PA spectroscopy over
  absorption spectroscopy?
• Look at slide 5

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References

• Sawada ,T. Oda, S. Shimizu,H. and Kamada, H.
  , Analytical chemistry (1979),vol 51 no.6.
• Haish,C. and Niessner ,Institute of
  Hydrochemistry, Technical university of Munich
  ,Germany.

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Thank you DR.CEDENO and the Audience

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Any questions???